Drake G.W.F. (editor) Handbook of Atomic, Molecular, and Optical Physics

Подождите немного. Документ загружается.

Conduction of Electricity in Gases References 1333

87.5 Conclusions

The study of the conduction of electricity in gases

has had several renaissances because of application

incentives. Discharge lighting technology science and

development are described by Waymouth [87.70]. At-

mospheric plasma physics and cosmic plasma physics

are covered by Rees [87.71]andAlfvén [87.72], respec-

tively. The role of plasma physics in gas discharge laser

development was recently reviewed [87.73], and the fun-

damental interactions in laser generated plasmas are

detailed by Hughes [87.74]. The scaling of discharge

volumes to many liters is again of interest because of

applications to large area surface treatments and thin

ﬁlms. The discharge enhanced chemistry of complex

gas mixtures is of particular relevance to etch process-

ing used for most microelectronics fabrication. Work on

discharge-generated and -trapped particulates in micro-

electronics processing plasmas has re-established links

with cluster physics and space plasmas [87.75]. The

physics of many of the effects was formulated earlier

by Emeleus and Breslin [87.76]. Future prospects for

further exploitation of non-equilibrium gas discharge

physics include waste/toxic hazard decontamination,

fabrication of deﬁned complex molecules, some with

strain energy, selected cluster morphologies, and the

further development of large area plasma ﬂat panel

displays.

References

87.1 P. Lenard: Ann. Physik 12, 714 (1903)

87.2 M. Hayashi: Tech. Rep. No.1PPJ-AM-19. Ph.D. Thesis

(Institute of Plasma Physics, Nagoya 1981) (unpub-

lished)

87.3 H. S. W. Massey: Handbuch der Physik,Vol.36

(Springer, Berlin, Heidelberg 1956) p. 232

87.4 W. J. Pollock: Trans. Farad. Soc. 64, 2919 (1968)

87.5 R. Nagpal, A. Garscadden: J. Appl. Phys. 75, 703

(1994)

87.6 G. J. Schulz: Rev. Mod. Phys. 45, 423 (1973)

87.7 P. Kleban, H. T. Davis: Phys. Rev. Lett. 39, 456 (1977)

87.8 W. P. Allis: Handbuch der Physik, Vol. 21 (Springer,

Berlin, Heidelberg 1956) p. 383

87.9 G. Cavaleri, G. Sesta: Phys. Rev. 170, 286 (1968)

87.10 G. L. Braglia: Physica 92C, 91 (1979)

87.11 W. L. Nighan: Phys. Fluids 12, 162 (1969)

87.12 L. Spitzer: Physics of Fully Ionized Gases (Wiley–

Interscience, New York 1956)

87.13 H. Dreicer: Phys. Rev. 117,343(1960)

87.14 S. T. Butler, M. J. Buckingham: Phys. Rev. 126,1

(1962)

87.15 I. P. Shkarofsky, T. W. Johnston, M. P. Bachynski:

The Particle Kinetics of Plasmas (Addison-Wesley,

Reading 1966)

87.16 A. M. Weinberg, E. P. Wigner: The Physical Theory

of Neutron Chain Reactors (Univ. Chicago Press,

Chicago 1958)

87.17 V. L. Ginzburg: The Propagation of Electromagnetic

Waves in Plasmas (Addison-Wesley, Reading 1964)

87.18 L. Pekarek: Uspeki Fiz. Nauka 94,463(1968)

87.19 T. L. Cottrell, I. C. Walker: Trans. Farad. Soc. 61,1585

(1965)

87.20 M. R. Hallada, P. Bletzinger, W. F. Bailey: IEEE Trans.

Plasma Sc. PS-10, 218 (1982)

87.21 W. H. Long Jr.W. F. Bailey, A. Garscadden: Phys.

Rev. A 13,471(1976)

87.22 J. Dutton: J. Phys. Chem. Ref. Data 4,577(1975)

87.23 S. R. Hunter, L. G. Christophorou: Electron-Molecule

Interactions and Their Applications, Vol. II, ed.

by L. G. Christophorou (Academic, New York 1984)

p. 89

87.24 L. G. H. Huxley, R. W. Crompton: The Diffusion and

Drift of Electrons in Gases (Wiley–Interscience, New

York 1974)

87.25 A. L. Gilardini: Low Energy Electron Collisions in

Gases (Wiley– Interscience, New York 1972)

87.26 J. A. Hornbeck: Phys. Rev. 84, 615 (1951)

87.27 S. H. Pullins, R. A. Dressler, R. Torrents, D. Gerlich:

Z. Phys. Chem. 214, 1279 (2000)

87.28 S. Sakabe, I. Izawa: At. Data Nucl. Data 49,257

(1991)

87.29 J. S. E. Townsend: Electricity in Gases (Clarendon,

Oxford 1915)

87.30 E. B. Wagner, F. J. Davis, G. S. Hurst: J. Chem. Phys.

47, 3138 (1967)

87.31 J. H. Parker, J. J. Lowke: Phys. Rev. 181, 290 (1969)

87.32 H. R. Skullerud: J. Phys. B: At. Mol. Opt. Phys. 2,

696 (1969)

87.33 R. N. Franklin: J. Phys. D. 36, 828 (2003)

87.34 W. P. Allis, D. J. Rose: Phys. Rev. 93, 84 (1954)

87.35 F. C. Hoh: Rev. Mod. Phys. 34,267(1962)

87.36 F. F. Chen: Introduction to Plasma Physics (Plenum,

New York 1974)

87.37 L. S. Frost, A. V. Phelps: Phys. Rev. 127, 1621 (1962)

87.38 A. G. Englehardt, A. V. Phelps, C. G. Risk: Phys. Rev.

135, 1566 (1964)

87.39 R. Nagpal, A. Garscadden: Phys. Rev. Lett. 73,1598

(1994)

87.40 M. Kurachi, Y. Nakamura: J. Phys. D: Appl. Phys. 21,

602 (1988)

87.41 R. D. White, R. E. Robson, B. Schmidt, M. A. Morri-

son: J. Phys. D 36, 3125 (2003)

Part G 87

1334 Part G Applications

87.42 B. Schmidt, K. Berkan, B. Goetz, M. Mueller: Phys.

Scr. 53, 30 (1994)

87.43 W. L. Morgan, B. M. Penetrante: Comput. Phys.

Commun. 58, 127 (1990)

87.44 http://www.sni.net/siglo/database/x-sect/siglo.sec

87.45 D. Bohm: Characteristics of Electrical Discharges in

Magnetic Fields, ed. by A. Guthrie, R. K. Wakerling

(McGraw-Hill, New York 1949)

87.46 I. Langmuir: Gen. Elect. Rev. 26, 731 (1923)

87.47 K-U. Riemann: J. Phys. D 24, 493 (1991)

87.48 R. N. Franklin: J. Phys. D 36, 823 (2003)

87.49 A. Von Engel: Ionized Gases (Clarendon Press, Ox-

ford 1965)

87.50 A. Garscadden, W. F. Bailey: Progress in Astronau-

tics and Aeronautics,Vol.74,ed.byS.S.Fisher

(AIAA, New York 1981) p. 74

87.51 B. E. Cherrington: IEEE Trans. Elec. Devices ED 26,

148 (1979)

87.52 L. Vriens: J. Appl. Phys. 45, 1191 (1974)

87.53 M. Cacciatore, M. Capitelli, C. Gorse: J. Phys. D:

Appl. Phys. 13, 575 (1980)

87.54 C. E. Treanor, J. W. Rich, R. G. Rehm: J. Chem. Phys.

48, 1798 (1968)

87.55 E. W. McDaniel, W. L. Nighan: Gas Lasers,Vol.3

(Academic, New York 1982)

87.56 A. Garscadden, R. Nagpal: Plasma Sources Sci.

Technol. 4, 268 (1995)

87.57 L. C. Pitchford, S. V. O. NeilJ. R. Rumble Jr.: Phys.

Rev. A 23, 294 (1981)

87.58 H. S. W. Massey: Negative Ions, 3rd edn. (Cambridge

Univ. Press, Cambridge 1976)

87.59 P. J. Chantry: Gas Lasers, Vol. 3 (Academic, New York

1982)

87.60 H. Conrads, M. Schmidt: Plasma Sources Sci. Tech-

nol. 9, 441 (2000)

87.61 Y. Raizer: Gas Discharge Physics (Springer, Berlin,

Heidelberg 1997)

87.62 A. D. MacDonald: Microwave Breakdown in Gases

(Wiley Interscience, New York 1969)

87.63 A. T. Bell: In: Engineering, Chemistry and Use of

Plasma Reactors, ed. by J. E. Flinn (American Insti-

tute of Chemical Engineering, New York 1971)

87.64 M. A. Lieberman, A. J. Lichtenberg: Principles of

Plasma Discharges and Materials Processing (Wiley

Interscience, New York 1994)

87.65 F. F. Young, C. J. Wu: IEEE Trans. Plasma Sci. 21,312

(1993)

87.66 R. Krimke, H. M. Urbassek, D. Korzec, J. Engemann:

J. Phys. D: Appl. Phys. 27, 1653 (1994)

87.67 G. Gozadinos, D. Vender, M. M. Turner, M. A. Lieber-

man: Plasma Sources Sci. Technol. 10, 117 (2001)

87.68 U. Kogelschatz: Plasma Chem. Plasma Process 23,1

(2003)

87.69 J. P. Boeuf: J. Phys. D 36, R53 (2003)

87.70 J. F. Waymouth: Electric Discharge Lamps (MIT

Press, Cambridge 1971)

87.71 M. H. Rees: Physics and Chemistry of the Upper At-

mosphere (Cambridge Univ. Press, Cambridge 1989)

87.72 H. Alfvén: Cosmical Electrodynamics (Clarendon,

Oxford 1950)

87.73 A. Garscadden, M. J. Kushner, J. G. Eden: IEEE Trans.

Plasma Sci. 19, 1013 (1991)

87.74 T. P. Hughes: Plasmas and Laser Light (Wiley–

Interscience, New York 1975)

87.75 A. Evans: The Dusty Universe (Horwood, New York

1993)

87.76 K. G. Emeleus, A. C. Breslin: Int. J. Elect. 29, 1 (1970)

Part G 87

1335

Applications to

88. Applications to Combustion

However, there have been two major advances

in combustion research which contain new ideas

and directions in detailed chemistry and physics.

The ﬁrst is the use of powerful computers for

the numerical solutions to combustion problems,

so that a predictive description can be built

beginning with fundamental physical principles.

The second is the use of laser diagnostic techniques

for the determination of the detailed properties of

the combustion system, especially temperature,

velocity, and composition, including both major

components and trace chemical intermediates.

These have not only provided tests of the

computational models, but have also furnished

new insights and approaches to an understanding

of combustion. This has occurred by measuring

properties of the system not previously available,

or via improved scales of spatial and temporal

resolution.

88.1 Combustion Chemistry .........................1336

88.2 Laser Combustion Diagnostics ...............1337

88.2.1 Coherent Anti-Stokes

Raman Scattering ...................... 1338

88.2.2 Laser-Induced

Fluorescence ............................ 1339

88.2.3 Degenerate

Four-Wave Mixing.....................1341

88.3 Recent Developments........................... 1342

References .................................................. 1342

This chapter concentrates on these two areas

of physical models and laser diagnostics, and

the outstanding physical questions that remain.

Each is greatly inﬂuencing the development of

the two important combustion science issues:

describing turbulent ﬂows and incorporating

realistic chemistry.

Combustion processes are vital to the operation of

present-day society. Although there exist alternative

energy sources, combustion of a variety of fuels is

worldwide a major mode of energy production, and

will remain so for many years to come. Despite a well-

established technology, combustion is not without need

of improvement. The pollutant emissions associated

with burning pose major barriers to more widespread

use. Increasingly stringent environmental regulations

make this a current focus of combustion research and

development.

Both scientiﬁc inquiry and empirical solutions will

be important in advancing combustion in the future.

Most of the progress in combustion technology has been

made without beneﬁt of an understanding of the science

involved; rather, it has been accomplished through an

approach of trial and error , which has led to many in-

genious, innovative solutions to problems. In fact, as

witnessed by the early development of thermodynam-

ics, the technology has often driven the science rather

than the other way around. Nonetheless, future advances

in combustion are expected to rely more and more on

a planned implementation of fundamental knowledge of

physics and chemistry.

Combustion may be thought of as self-sustaining

reactive ﬂow, in which chemical energy is converted

into extractable, useful heat. This is usually accompa-

nied by an abrupt change in properties of the system in

space and/or time, particularly the chemical composi-

tion and the temperature. The description of combustion

processes involves the subdisciplines of thermochem-

istry, chemical kinetics, ﬂuid mechanics, and transport.

The major challenge is to apply the known principles

from these to a description and understanding of the en-

tire combustion process. This endeavor involves three

approaches: experiment, computation, and theory, the

latter two differentiated via the characteristic of numer-

ical versus analytical solutions. Each part now plays an

important role in combustion science.

The efﬁciency of combustion processes, be they

steady state such as a burner ﬂame or rocket mo-

tor, or transitory such as an explosion or internal

combustion engine, is generally governed by the ove r-

all thermochemistry, ﬂuid dynamics, and heat transfer

Part G 88

1336 Part G Applications

characteristics of the system. Details of the chemical

reactions and the role of trace intermediates are usu-

ally unimportant for this question. The latter do play

a key part, however, when considering problems of pol-

lutant formation (emission of NO

,SO

, soot, toxic

orga nic compounds), and ﬂame ignition and inhibition.

The current challenges in combustion science are so-

lutions to these problems of efﬁciency and pollution,

with advancement likely to occur through two major

thrusts: a better understanding of turbulent reactive

ﬂow, and the incorporation of detailed chemical kinet-

ics into combustion models (as contrasted to simpliﬁed

versions such as one- or two-step reaction schemes).

Additionally, new areas of science involve combustion

under unusual conditions (e.g., microgravity combus-

tion aboard the space shuttle) and new applications (e.g.,

ﬂame synthesis of exotic materials such as diamond

ﬁlms).

From the viewpoint of the basic science, most of

the general, qualitative understanding of behavior at

the molecular scale needed to describe combustion is

at hand. In many engineering applications, an empirical

approach is taken in which a quantitative description

of that behavior is condensed and parametrized, using

previously acquired experimental data acquired directly

on the system of interest. As models become more de-

tailed (often through computational advances), there is

a need for further quantitative supporting molecular

data, such as improved values of transport properties

of molecules.

88.1 Combustion Chemistry

The addition of detailed chemical kinetics to ﬂame

models will greatly advance our understanding of

combustion, at least for nonturbulent ﬂows. Detailed

chemistry is becoming a part of codes describing sys-

tems of greater and greater complexity. Numerical

integration packages, including full chemistry for a one-

dimensional ﬂow, are routinely available [88.1], and

progress is being made for the two-dimensional case.

A sensitivity analysis enables the modeler to examine

the value predicted for some variable (e.g., concentra-

tion of a transient species) at a given point in the ﬂame

as a function of the rate coefﬁcients and other input vari-

ables. This facilitates interpreting the output in terms of

physically meaningful variables, and extrapolating the

model predictions to other applications.

The quality of the predictions of a combustion model

is a strong function of the quality of the reaction rate co-

efﬁcients for important elementary reaction steps, and

the thermochemical properties of reactive species, espe-

cially free radicals. A ﬂame chemical mechanism can be

quite complex: a current methane/air model [88.2] con-

taining reactions involving only hydrogen, oxygen and

carbon (i. e., no nitrogen chemistry) contains 30 species

and 177 reactions (plus their reverses). Although it is

necessary to include most of these steps in the reaction

scheme, a much smaller number play dominant roles in

determining any given parameter such as ﬂame speed.

The determination of sufﬁciently accurate rate coefﬁ-

cients for those important reactions is made difﬁcult by

the complexity of the reactions, and the difﬁcult con-

ditions (high temperature) and reactants (usually free

radicals) that are involved.

There are many unimolecular reactions and their re-

verses that play important roles in combustion chemistry,

including the recombination of fuel-derived radicals

such as CH

+H+M→ CH

+ M, and low temperature

decomposition/recombination reactions important in ig-

nition, e.g., H + O

+M→ HO

+ M, compared with

chain-branching steps. Under most combustion condi-

tions, the important reactions in this class are either in

the low pressure limit, where the rate constant is pro-

portional to total pressure P of collider M, or in the

fall-off region between that limit and the P-independent

high pressure limit. Knowledge of the collisional energy

transfer characteristics for highly excited vibrational lev-

els of polyatomic molecules is apropos to a fundamental

understanding of these reactions. For purposes of com-

bustion models, these rate constants can be formulated

in a modiﬁed Arrhenius mode, requiring as many as nine

parameters to describe correctly the combined P, T de-

pendence needed for a realistic range of combustion

conditions.

A further complication is posed by the multiple po-

tential energy surfaces that can be accessed by many

reactions. For example, two methyl radicals can combine

to form not only C

,butalsoC

+ H. Measure-

ments are often made of the overall rate coefﬁcient (in

this case, disappearance of CH

) but seldom are deter-

minations made of branching ratios into these multiple

paths. Especially, the temperature dependence of that

branching is rarely known accurately enough. Since

reaction rate coefﬁcients can change orders of mag-

nitude over the range of temperatures encountered in

combustion, simple extrapolations are seldom sufﬁcient.

Part G 88.1

Applications to Combustion 88.2 Laser Combustion Diagnostics 1337

Theoretical chemistry has begun addressing these

problems. Electronic structure calculations provide ther-

mochemical properties of transient, reactive species.

Using theories which include semi-empirical adjust-

ments, enthalpies of formation of free radicals and

potential barriers on reactive surfaces can be calcu-

lated with accuracies of about 2 kcal/mole, sufﬁcient

for modeling purposes. Multidimensional potential sur-

faces provide starting points for conventional transition

state theory calculation of rate coefﬁcients, as well as

variational transition state approaches. Theoretical cal-

culations may be useful for understanding the energy

transfer questions of unimolecular and recombination

rate theory. Also useful are recent laser experiments

which examine the foundations of rate theory. These

include laser photolysis investigations of the reaction

rate coefﬁcient as a function of energy just above

threshold [88.3], and the formation of molecules from

dissociating complexes on subpicosecond scales [88.4],

as well as stimulated emission pumping experiments

which probe the density of states and energy transfer

in high vibrational levels of diatomic and polyatomic

molecules [88.5].

We conclude this section with brief comments on

some individual combustion systems. Flames of H

and

CO burning in (moist) O

are relatively simple, and

their mechanisms are important subsets of reaction net-

works for more complex systems. The chemistry of

these ﬂames is relatively well understood on a funda-

mental basis. So, generally, is that of methane, although

there remain crucial questions, for example, the rates

for CH

+ OH, a multisurface reaction producing sin-

glet and triplet CH

O, as well as CH

OH. For

hydrocarbon fuels more complex than methane, the

complexity of the mechanism rapidly escalates, due

to the many possible ways to combine radicals con-

taining multiple carbon-carbon bonds. For a fuel such

as isooctane (where few rate coefﬁcients are kno wn)

it is necessary to use a computer algorithm simply to

write down the mechanism without making mistakes or

omissions [88.6].

Many pollutant emissions are produced by reactions

of trace intermediate species. Nitric oxide has received

much recent attention due to increasingly stringent en-

vironmental standards. NO is formed in combustion

processes in three ways: (i) so-called thermal (or Zel-

dovich) NO, formed by reaction of O + N

,N+O

,and

OH + N

, where the atoms and OH are usually present

at nonequilibrium, kinetically controlled concentrations;

(ii) prompt NO, which begins with the CH + N

reac-

tion breaking the strong N-N bond to form N atoms

which react with O

and HCN, which is oxidized to NO

in a series of steps; (iii) and fuel-nitrogen NO, which

is formed from the nitrogen present in coal and coal-

derived fuels, usually entering the combustion process

as HCN. NO can be dealt with through catalytic reduc-

tion, staged combustion (burning at a series of mixing

ratios), reburning at a later stage with additional fuel,

and injection of compounds such as NH

or cyanuric

acid, (HOCN)

. Progress has been made in the de-

termination of the rate coefﬁcients of many pertinent

elementary reactions for each of these processes, but

signiﬁcant gaps remain. Examples are the full T, P de-

pendence of the complex-forming reaction CH + N

and the complicated reaction pathways in the NH

NO reaction important in de-NO

through ammonia

injection.

The mechanism of soot formation is a matter of

dispute; unsaturated chains containing 4 or 5 carbons,

aromatic rings, ions, and C

radicals have all been

suggested as precursors. Ignition (and engine knock)

chemistry occurs at low temperatures and probably

involves hydroperoxy (HO

) and alkylperoxy (RO

)

radicals. The mechanism of production of toxic organic

compounds such as aldehydes is poorly known, and few

relevant rate coefﬁcients are at hand.

88.2 Laser Combustion Diagnostics

The advent of the laser has altered much of experimental

combustion science. Laser Doppler velocimetry, which

employs particle scattering to measure directed ﬂow

velocity, is available in the form of commercial sys-

tems. Coherent anti-Stokes Raman scattering (CARS)

is widely used in engineering laboratories to study

practical combustors, including jet engines, industrial

furnaces, internal combustion engines, and burning pro-

pellants. Laser-induced ﬂuorescence (LIF) has come

into prominence for the study of combustion systems

and related chemical kinetics.

The spectroscopically based techniques of most in-

terest in atomic, molecular, and optical physics are

CARS, LIF, and the related recently emerging method

of degenerate four wave mixing (DFWM). Each may

be used to determine individual molecular species or

temperature. Each is highly selective, due to its spectro-

scopic nature; they provide good spatial resolution, often

Part G 88.2

1338 Part G Applications

on the order of a mm

or less; the use of pulsed lasers

yields excellent time resolution, ≈ 10 ns. When used

properly, they are nonintrusive, perturbing neither the

ﬂow dynamics nor ﬂame chemistry. They can be used in

very hostile environments (corrosive atmospheres, high

temperatures) where physical probes would not survive.

LIF and CARS also have separate, complementary

features. CARS is suitable for the determination of major

species (fuel, O

,CO

O, and, in air-breathing com-

bustion, N

). It may be used for accurate temperature

measurements, including measurements on a single laser

shot when operated in a broadband mode. This makes

CARS especially useful for problems in which ﬂuid dy-

namics and ﬂow patterns are most important. Because

CARS produces a coherent, laser-like beam, spatial ﬁl-

tering can be used to discriminate against background

radiation such as the intense thermal emission from hot

combustor walls. LIF is highly sensitive and is useful for

measurements on reaction intermediates present at low

concentrations. It may be used in an imaging mode to

obtain instantaneous two-dimensional patterns of radical

species in rapidly time-varying ﬂows.

Despite their utility, LIF and CARS are not user-

friendly technologies. At present, considerable nurturing

of the equipment, knowledge of the theory, and care

in the data analysis are needed for accurate measure-

ments, and nearly all such measurements are made

by scientists trained in physics, chemistry, or physics-

oriented engineering departments. Both methods (and

DFWM) are examined here with an emphasis on the

physics questions in their continued development and

application.

88.2.1 Coherent Anti-Stokes Raman

Scattering

CARS depends on the nonlinear polarization induced in

molecules by the intense electric ﬁelds of lasers, ex-

ploiting resonant effects in the third order nonlinear

susceptibility χ

(3)

. χ

(3)

mixes the ﬁelds from a pump

laser at frequency ω

and a Stokes beam at frequency ω

to form a third beam at ω

= 2ω

− ω

. χ

(3)

is strongly

enhanced when the difference ω

− ω

is tuned to a vi-

brationally or rotationally resonant frequency in the

ground state of a molecule. This is usually accomplished

using the intense green beam of a frequency doubled

Nd:YAG laser for ω

, and a dye laser Stokes shifted by

the vibrational frequency of the molecule of interest for

. In general, the signal in the visible region of the spec-

trum contains a combination of a nonresonant χ

(3)

due

to the electronic polarizability of the medium (mostly

for air-breathing combustion) and the resonant part.

For high concentrations c

(e.g., ∼ 70% N

in air based

ﬂames), the signal is a Lorentzian-shaped resonance on

top of the ﬂat nonresonant term, proportional to c



where Γ is the linewidth. For lower concentrations (e.g.,

a few percent CO

in exhaust gases) the resonant and

nonresonant amplitudes interfere, so that the signal is

dispersion shaped with a magnitude ∝ c



Γ .

A very useful and popular application of CARS oper-

ates the Stokes laser in a broadband mode. This generates

the entire coherent anti-Stokes spectrum at once, which

is then dispersed through a spectrometer and detected

with an array. T may be determined from the rota-

tional structure and the appearance of vibrational hot

bands. These measurements are generally performed on

the N

, almost always present in large quantity in any

combustion system of practical interest. This is the most

reliable, accurate method for determination of pointwise

temperatures with a single laser shot.

The nonlinear, coherent nature of CARS,inwhich

interfering amplitudes are added, means that spectral

details can have a signiﬁcant inﬂuence on the observed

spectrum. These include the concentration of all con-

tributing species (including nonresonant background)

and the widths of individual Raman lines. The linewidths

must be dealt with correctly in order to deduce tempera-

tures accurately from the spectral shapes. It has only

been in the past ﬁve years that linewidths as a func-

tion of rotational level have been available for N

at the accuracy needed for application of the spectral

ﬁtting codes used for T determination. As P is in-

creased, the lines broaden enough to begin to overlap;

at high enough P (several atm for N

) collisions oc-

cur so rapidly that the spectrum collapses to a single

broad line. To make T measurements at high pressure,

one must deal with both these P broadening and colli-

sional narrowing effects; this requires an understanding

of the underlying collision dynamics and energy trans-

fer, generally on a rotational level state speciﬁc basis.

Both energy transfer and dephasing collisions contribute

to the linewidths. For molecules other than N

,there

is scant information on Raman linewidths even at at-

mospheric pressure, at accuracies needed for modeling

CARS spectra. Molecules of particular interest are O

O, CO, and, in the future, hydrocarbon fuels. In

some cases, especially H

O, further spectral information

for high rotational lines (populated at high temperature)

is also needed.

Most of the treatments of CARS spectra assume

a monochromatic laser. Finite bandwidth effects of real

lasers must be taken into account for accurate theoret-

Part G 88.2

Applications to Combustion 88.2 Laser Combustion Diagnostics 1339

ical treatments (and thus accurate T and concentration

determinations). There are several nonequivalent theo-

retical treatments to convolute the frequency dependence

of χ

(3)

with the bandwidths of the pump and Stokes

lasers.

88.2.2 Laser-Induced Fluorescence

LIF is much more sensitive than CARS, relying on reso-

nant excitation to a real, emitting electronically excited

state. The use of a tunable laser to scan the molecu-

lar excitation spectrum makes LIF also highly selective

for small species that have characteristic, identiﬁable

line spectra. Thus LIF is suitable for measurement

of reactive intermediates of importance in understand-

ing ﬂame chemistry. Concentrations determined by LIF

range from parts per billion to parts per thousand. De-

tectable species are diatomics such as OH and CH,

triatomics including HCO, NCO, and NH

,andafew

larger molecules including CH

O. Most of the pertinent

molecular electronic transitions are in the UV or blue

region of the spectrum. Because VUV radiation cannot

penetrate ﬂame gases, atoms (such as H and O) must be

detected via two-photon absorption to a high-lying elec-

tronic state of the same symmetry as the ground state,

which then radiates in the visible or near IR to another

excited state.

A list of all molecules through teratomics composed

of the ﬁve main atoms naturally occurring in com-

bustion (H, C, O, N, and S), and detected to date by

LIF,isgiveninTable88.1. This forms a large frac-

tion of the small reactive intermediates important in

combustion chemistry, so that LIF has the ability to

characterize fairly completely a combustion reaction

mechanism. It would be particularly desirable to add

certain other species; noteworthy are HO

H, CH

and triplet CH

, although suitable ﬂuorescing electronic

transitions for these molecules are not currently known.

In addition to the species listed in Table 88.1,there

are many other combustion-related compounds that can

be seen with LIF. These include metal atoms and their

compounds, species present in specialized combustion

situations such as boron or chlorine containing radicals,

and some polyatomic, partially oxidized hydrocarbon

molecules.

LIF may be employed in several ways to study

ﬂames. With a pulsed laser, very rapid time resolu-

tion is achievable. Pointwise, single shot measurements

(usually of OH) in turbulent ﬂames may be interpreted

on a statistical basis for comparison with ﬂame mod-

els incorporating simpliﬁed chemistry. Two dimensional

planar imaging (usually of OH, CH, or NO) is accom-

plished by focusing the laser into a sheet of radiation

which passes through the ﬂame. Imaging the illuminated

region at right angles onto a two-dimensional array pro-

duces an instantaneous snapshot of the distribution of

the radical throughout the ﬂame. This may be used to

understand ﬂow patterns under conditions of rapid time

variation, such as turbulence or ﬂame spread following

ignition.

On the other hand, turbulent ﬂows are far too com-

plex for an investigation of ﬁne details of the combustion

chemistry. This is best accomplished via experiments

in laminar ﬂames, where LIF is used to obtain spatial

proﬁles (one- or two- dimensional) of reactive species,

made as a function of height above a burner surface.

Absolute or relative proﬁles provide highly constraining

tests of detailed models of ﬂame chemistry. Operation at

reduced P spreads out the active ﬂame front region, fur-

nishing excellent spatial resolution for these proﬁles.

The T proﬁle, used as input to the model, is meas-

ured using rotational excitation scans. The OH radical,

present throughout much of any gi ven ﬂame contain-

Table 88.1 Combustion chemistry intermediates detect-

able by laser-induced ﬂuorescence

Excitation Excitation

wavelength wavelength

Molecule

(nm) Molecule (nm)

∗

205 NCO

∗

440

∗

280 HCO

∗

245

∗

226 HCN 189

∗

211 HNO 640

S 311 NH

∗

598

∗

309 C

405

∗

413 C

O 665

∗

336 S

O 340

∗

324 SO

∗

320

∗

388 NO

∗

590

∗

280 HSO 585

CS 258 CS

320

∗

226 N

272

∗

231 NCN 329

∗

267 CCN 470

∗

217 NH

∗

305

∗

308 NO

570

∗

516 C

∗

220

∗

537 CH

∗

320

An asterisk denotes that LIF detection has been performed in

aﬂame

Part G 88.2

1340 Part G Applications

ing hydrogen and oxygen, is an ideal LIF thermometer.

LIF may also be used in a semiquantitative way, so

that simply the detection of some species at an approxi-

mate concentration can reveal new information about the

chemical mechanism [88.7]. An example is LIF detec-

tion of NS in ﬂames containing minor amounts of N and

S compounds, indicating the importance of this radical

in linking the chemistry of NO

and SO

formation.

LIF measurements involve quantitative knowledge

of molecular spectroscopic characteristics and colli-

sional behavior. Identiﬁcation of the absorbing molecule

requires assignment of the vibrational and rotational

structure of the electronic transition in question. For

weak signals, particularly in the presence of a strong

absorber/ﬂuorescor (e.g., CH

O in the presence of OH

near 310 nm), it may be necessary to scan many lines

to ensure identiﬁcation. The ﬂuorescence spectrum is

also valuable since selective detection of ﬂuorescence

may be used to discriminate between two molecules

absorbing at the same wavelength. In many cases, the

detailed spectroscopy of molecules of interest is well

established in the literature; in other cases, it may be

necessary to make studies via ﬂow cells. A recent ex-

ample is an LIF study of the B–X ultraviolet system

of the HCO molecule [88.8], a radical whose reactions

are important in controlling the hydrogen atom con-

centration in hydrocarbon ﬂames. In some cases (e.g.,

NCO) LIF spectroscopic studies in ﬂames themselves

have been useful, in that high rotational and vibrational

levels are populated and thus readily accessed. In addi-

tion to spectral line and band identiﬁcation, rotational

line strengths and vibrational band transition probabili-

ties (including possible effects of an electronic transition

moment that varies with internuclear distance) provide

the Einstein A and B coefﬁcients needed for the data

analysis. Accurate values are especially important in the

determination of T via excitation scans, since system-

atic errors in oscillator strengths can lead to errors of

hundreds of degrees, but are not discernible through sta-

tistical goodness-of-ﬁt criteria. Such large errors in T

render meaningless any attempts to compare measure-

ments with predictions that include detailed chemical

kinetics.

Of considerable consequence for LIF measurements

is an understanding of the collisional effects govern-

ing the ﬂuorescence quantum yield. For example, in

a ﬂame at atmospheric pressure, approximately three of

each thousand OH molecules excited by the laser will

ﬂuoresce; the remainder is removed nonradiatively by

collisions with the ambient ﬂame gases. Furthermore, to

reduce interference or background, ﬂuorescence detec-

tion is often accomplished using a ﬁlter or spectrometer

with a bandpass encompassing only a portion of the total

ﬂuorescence. Vibrational and rotational energy transfer

collisions of the excited radical with the surrounding

gases may determine the fraction of emission into that

particular bandpass.

Because of the extreme importance of the OH radical

to combustion chemistry (and also in the atmosphere), its

collisional behavior has been investigated extensively,

although not yet completely. The results show deﬁnite

quantum state and translational energy dependence of

energy transfer and quenching cross sections. These are

not only crucial to quantitative measurements of this

radical, b ut they are also important in understanding

molecular collision dynamics. OH is small enough that

its interactions are amenable to quality ab initio calcu-

lations, at least with simple colliders like noble gases

and H

. Quantum scattering calculations may then be

compared with experimental results; the state-to-state

detail provided by this open shell radical furnishes valu-

able tests of those calculations. Furthermore, they may

be compared with spectroscopic characteristics and half-

collision dynamics of van der Waals complexes of OH

with noble gases (and, in the future, molecular partners).

Quenching of the A

excited state of OH is

the major factor determining the ﬂuorescence quantum

yield. Measurements have been made over a wide range

of temperatures, from 200–2500 K, using cooled and

room temperature ﬂow cells, laser ﬂash heated cells,

ﬂames, and shock tubes [88.9]. For nearly all colli-

sion partners the quenching cross section decreases with

increasing T, i . e., increasing collision velocity. This

shows that attractive forces are involved in the colli-

sion, and the form of the variation for most colliders can

be explained in terms of a collision complex formation

mechanism. The A

state of NO behaves similarly,

but quenching of the A

∆ state of CH increases with T,

showing that a barrier exists on the potential surfaces;

the A

Π state of NH exhibits more varied behavior.

Quenching also varies with rotational level for OH

and NH, as does vibrational transfer (v = 1 → v



= 0)

for OH. This appears to be a dynamic, not energetic,

effect, and has been ascribed to a rotational averaging

of the effects of the highly anisotropic surface on which

these polar hydrides interact with colliders. Vibrational

transfer in OH has been studied in both the excited and

ground electronic states. It is found to be much faster

in the upper A

state for all colliders, by factors

of 100–1000. This is perhaps related to the fact that,

according to ab initio calculations on OH–Ar, there is

a much deeper well for interaction of the excited radical

Part G 88.2

Applications to Combustion 88.2 Laser Combustion Diagnostics 1341

than in the ground state. Rotational transfer in OH has

been shown to have some deﬁnite propensities, including

parity conservation for some collisions with He that is

reproduced in quantum scattering calculations.

Such a wealth of detail is not yet available for other

free radical combustion intermediates, although they are

beginning to be investigated. Of particular current inter-

est is NO and the major precursor to its formation, CH.

The details of quenching by a variety of colliders are

probably known well enough that quantum yields for

OH, NO, and CH may be calculated reasonably accu-

rately, given the local temperature and major species

ﬂame composition at the point of measurement. There

are enough data to compute quenching rates to within

about 30–50% for many ﬂames; this is partly because

O, when present, accounts for the majority of the

quenching for all three radicals. When the local environ-

ment (especially T) is not known, the situation is worse.

This would arise for an instantaneous two-dimensional

image of a radical in a ﬂame. Even so, the net quenching

rate for OH rarely varies more than threefold through

the entire ﬂame, so useful semiquantitative information

can still be obtained.

If absolute LIF measurements are desired, some

means of calibration is necessary. Stable compounds

(NO, CO) can simply be introduced into the ﬂames or

corresponding cold ﬂows. Free radicals pose a greater

problem. OH is often present in sufﬁcient quantity

that absorption measurements in a stable, laminar one-

dimensional ﬂame can be performed, but this is generally

not possible for other species. The recently introduced

technique of cavity ring down spectroscopy may alter

this situation [88.10]. Burnt gases, in thermal equilib-

rium at a measured T, furnish a reliable source of known

concentrations of OH, H, and O but generally not other

radicals (which are consumed earlier in the ﬂame).

Two-photon excitation is used for detection of

atomic species and the CO molecule. The high laser ﬂux

needed for two-photon absorption, together with the uv

wavelengths required for the pertinent transitions, of-

ten leads to problems with photochemical interferences.

Sometimes these produce the same species that is be-

ing measured, e.g., dissociation of vibrationally excited

at the wavelength of atomic oxygen detection, with

subsequent excitation of the spurious O atom. In other

cases, especially in complex hydrocarbon fuels, a par-

ent compound or partially oxidized fragment may be

excited directly or photolyzed to yield an emitting prod-

uct. Excitation scans and measurements as a function

of laser power are needed to discern and avoid these

interferences.

88.2.3 Degenerate Four-Wave Mixing

DFWM is a nonlinear process like CARS; but like LIF,

it operates on real transitions. It combines some of the

attributes of both methods: production of a coherent

signal beam that can be spatially ﬁltered to discrim-

inate against background, and high sensitivity so that

trace radical species may be detected [88.11]. It depends

on χ

(3)

,asdoesCARS, and can be used in a broadband

mode. Like LIF, it can produce two-dimensional im-

ages, with the proper laser sheet arrangement. DFWM

adds some of its own advantages, especially a Doppler-

free nature enhancing the spectral resolution and thus

the molecular selectivity. Three laser beams are used

to generate the DFWM signal, all the same frequency,

and tuned to an absorption transition of the species of

interest. Two pump beams create an interference pat-

tern in the medium, and a probe beam scatters off the

resulting grating to form the signal. The interference pat-

tern may be in the ground/excited state populations or

may exploit polarization phenomena. Rapid quenching

of the excited state may produce a thermal grating in

the medium; the probe beam can then scatter from the

resulting variation in the index of refraction. DFWM de-

pends only on absorption and may be used for sensitive

detection in a nonﬂuorescing or poorly ﬂuorescing case

(e.g., a predissociative state or ir vibrational transition).

The interpretation of DFWM signals to obtain

molecular concentrations or temperatures requires

knowledge of the underlying physical phenomena. Be-

cause they are governed by χ

(3)

,theDFWM amplitudes

interfere, and an understanding of molecular collisions,

motion, and the effects of the laser power are needed

for accurate modeling of DFWM spectra. Collisional

effects are of particular interest due to the Doppler-

free character of the technique. Signal intensities can

be dramatically inﬂuenced by line broadening due to

quenching, energy transfer, and dephasing collisions.

The magnitude of the inﬂuence depends sensitively on

the degree of optical saturation. A cohesive picture of

these effects is currently being sought through a combi-

nation of measurement and theory. Furthermore, nearly

all current theoretical treatments assume single-mode

lasers; however, real lasers contain intensity and fre-

quency ﬂuctuations within each laser pulse. The effects

of these ﬂuctuations on the nonlinear wave-mixing pro-

cess must be accounted for in a proper description

of DFWM. Furthermore, because collisions affect the

signal only during the laser pulse, rapid (ps) measure-

ments may provide excellent sensitivity under very high

P conditions.

Part G 88.2

1342 Part G Applications

The articles and books [88.7,9,11–18]areofareview

or overview nature. In addition, archival articles illus-

trating the state of the combustion ﬁeld can be found in

the biennial International Symposia volumes published

by the Combustion Institute, Pittsburgh, now referred to

as the Proceedings of the Combustion Institute.

88.3 Recent Developments

In the last eight years, there have been but two important

advances in fundamental combustion laser diagnostics.

The ﬁrst of these is the publication of a book [88.19]

containing an in-depth discussion of state of the art laser

diagnostics. It includes eight chapters on basic experi-

mental methods, nine chapters on applications, which is

presently the largest growing area, and nine chapters on

perspectives, future needs, and emerging applications.

The interested reader is referred to this comprehensive

treatise on the topic.

In recent years, there has indeed been a num-

ber of new applications of LIF and CARS to

practical combustors (see the Applications chapters

of [88.19]). LIF has been extended to only a few

new molecules but has been reﬁned for several

(see Chapt. 2 of [88.19] for a listing of all pertinent

LIF molecules). DFWM has not found signiﬁcant new

applications, and efforts for nonﬂuorescing molecules

have diverted to cavity ringdown spectroscopy. New

laser sources are important, especially those in the

infrared.

The major experimental advance has been the ad-

vent of cavity ring-down spectroscopy (see Sect. 43.2).

This topic is covered in [88.20], and its particular ap-

plications to ﬂame diagnostics are discussed in Chap. 4

of [88.19]. This method can be used on nonﬂuorescing

molecules which do absorb available laser wavelengths.

A recent article on CH in ﬂames [88.21] discusses ex-

perimental methodology, references earlier papers, and

compares this technique with LIF.

References

88.1 R. J. Kee, F. M. Rupley, J. A. Miller: Sandia National

Laboratory Report SAND89-8009 (1989)

88.2 W. Gardiner, M. Frenklach, H. Wang, M. Gold-

enberg, C. Bowman, R. Hanson, D. Davidson,

G. Smith, D. Golden, W. Gardiner, V. Lissianski:

Proceedings of the 1995 International Gas Research

Conference (Gas Research Institute, Chicago 1995)

88.3 A. H. Zewail: Science 242, 1645 (1988)

88.4 H. Reisler, M. Noble, C. Wittig: Photodissocia-

tion processes in NO-containing molecules. In:

Molecular Photodissociation Dynamics,ed.by

M. N. R. Ashfold, J. E. Baggot (Royal Society of

Chemistry, London 1987) p. 139

88.5 X. Yang, J. M. Price, J. A. Mack, C. G. Morgan,

C. A. Rogaski, D. McGuire, E. H. Kim, A. M. Wodtke:

J. Phys. Chem. 97, 3944 (1993)

88.6 J. Warnatz: In: Twenty-Fourth Symposium (In-

ternational) on Combustion (The Combustion

Institute, Pittsburgh 1992) p. 553

88.7 D. R. Crosley: Comb. Flame 78, 153 (1989)

88.8 A. D. Sappey, D. R. Crosley: J. Chem. Phys. 93,7601

(1990)

88.9 D. R. Crosley: J. Phys. Chem. 93, 6273 (1989)

88.10 G. Meijer, M. G. H. Boogaarts, R. T. Jongma,

D. H. Parker, A. M. Wodtke: Chem. Phys. Lett. 217,

112 (1994)

88.11 R. L. Farrow, D. J. Rakestraw: Science 257,1894

(1992)

88.12 D. R. Crosley: Laser measurement of chemically

reactive intermediates, in combustion. In: Tur-

bulence and Molecular Processes in Combustion,

ed. by T. Takeno (Elsevier, New York 1993)

p. 221

88.13 A. C. Eckbreth: Laser Diagnostics for Combustion

Temperature and Species (Abacus, Cambridge 1988)

88.14 W. C. Gardiner (Ed.): Combustion Chemistry

(Springer, New York 1984)

88.15 L. P. Goss: CARS instrumentation for combustion

applications. In: Instrumentation for Flows with

Combustion, ed. by A. M. K. P. Taylor (Academic,

New York 1993) p. 251

88.16 K. Kohse-Höinghaus: Prog. Energy Combust. Sci.

20, 203 (1994)

88.17 A. Linan, F. A. Williams: Fundamental Aspects of

Combustion (Oxford Univ. Press, Oxford, New York

1993)

88.18 J. A. Miller, R. J. Kee, C. K. Westbrook: Ann. Rev.

Phys. Chem. 41, 345 (1990)

88.19 K. Kohse-Höinghaus, J. B. Jeffries(Eds.): Applied

Combustion Diagnostics (Taylor and Francis, New

York 2002)

88.20 B. A. Paldus, A. A. Kachanov: Can. J. Phys. 83 (2005),

in press

88.21 J. Luque, P. A. Berg, J. B. Jeffries, G. P. Smith,

D. R. Crosley, J. J. Scherer: Appl. Phys. B 78,93

(2004)

Part G 88