Vij D.R. Handbook of Applied Solid State Spectroscopy

Подождите немного. Документ загружается.

12.8 Direction of Future Efforts

571

12.8 DIRECTION OF FUTURE EFFORTS

12.8.1 Why Luminescence?

Before projecting into the future, it seems worthwhile to ask Why has

luminescence spectroscopy been such a fruitful field? Some possible responses

to this question are:

1. The basic study of luminescent systems has been a precursor to the

development of the laser (laser diodes and doped insulator-based lasers)

and several other devices/materials that enjoy widespread application.

2. The study is good in itself, in that it acts as a probe to the understanding

of very complex systems, lending insight into their static and dynamic

properties.

Static: Luminescence allows us to (1) study the energy levels that cannot be

identified solely by absorption measurements, (2) help probe the symmetry of

the host material, and (3) reveal the existence of excitonic levels and to provide

the means to study them.

Dynamic: Through luminescence, we (1) probe collective excitations of solids,

such as phonons, magnons, etc. and (2) track the energy as the system decays

from an excited level to the ground level, revealing the existence of processes

such as energy transfer, upconversion, and non-radiative decay.

From the essentially inexhaustible source of luminescent systems, there

emerges a continuous stream of new data and with it a deeper understanding of

the processes relevant to those systems.

Luminescence spectroscopy remains a vibrant field of research, even 400

years since the investigation into the Bologna stone, over 150 years since

Stokes’s observations on luminescence from fluorspar, and after more than a half

century of modern, scientific inquiry. The body of past work has revealed only

some of the dynamics, complexity, and possibilities of luminescent systems. As

testament to the validity of past investigations, we have been educated in the

workings of these systems, and presented with more questions and challenges as

we move forward.

12.8.2 Challenges and Future Work

Though luminescence spectroscopy has much to offer, the complexity of these

systems renders the task of establishing general criteria for luminescent systems

very difficult, posing continuoal challenges to workers in the field. The ultimate

goal, of course, is to improve the predictive value of our knowledge so that we

can accurately predict the response of a system to an impulse of excitation.

The realization of this goal is an extremely difficult task. The luminescence

properties depend not just on the general features of the solid, but also on its

12. Luminescence Spectroscopy

572

more nuanced features—breaks in symmetry, impurities, clustering of ions,

local charge distributions, phonon properties of the solid, etc. Indeed, an accurate

prediction of even the most fundamental process of luminescence, the radiative

transition probability, requires precise knowledge of the wavefunctions of the

system, something that has heretofore proven elusive.

This complexity, which provides the challenges of the field, is also the source

of the usefulness of luminescent materials. These systems are so varied and

complex that they can be designed for a wide variety of applications. Even after

several decades of successes, developments of optical materials and their

applications continues to proceed rapidly.

Advances in the field of luminescence spectroscopy will continue to be aided

by the following:

1. The emergence of new materials: These include the historically important

single crystals, glasses, and thin films, as well as novel materials such as

ceramics, nanoparticles, nano-layered systems, quantum wires, and quantum

dots.

2. Advances in technology: This includes femtosecond lasers, high intensity

lasers, and more sophisticated electronics. Advances in computing have allowed

for easier manipulation of data and made theoretical calculations accessible to

those with even modest computing facilities by modern standards.

3. The demands of the market: For numerous applications, there is a desire

for light sources—lasers, in particular—that are more efficient, of higher

intensity, more compact, cheaper, faster, and that operate over a wide bandwidth.

Recently, for example, the areas of reading, storing, and transmitting data have

been particularly instrumental in providing impetus for the development of new

materials and devices.

The following list presents a few areas of research that should be particularly

fruitful to the field of luminescence, both in the development of luminescence-

related technologies and in the understanding of basic physical phenomena.

1. The understanding and control of non-radiative losses: Following

excitation, a luminescent system often releases at least part of its energy as heat.

By controlling these losses one can, in principle increase or otherwise tailor the

radiative emission.

One system in which the control over non-radiative processes can be

exercised is nanomaterials. Because of the small particle size involved, the

phonon spectrum is no longer a continuum, but is discrete. The lower frequency

modes, in particular, are not present [48]. Thus, phonon-related processes may

be suppressed in these systems, making radiative processes more probable.

Another possibility for controlling non-radiative decay is through the use of

femtosecond laser pulse-shaping techniques. For example, a method called

coherent wavepacket excitation can create a superposition state of vibrational

modes in an upper electronic state. The ensuing motion directs the excitation

energy to the opposite side of the parabola (in a configurational coordinate

model), instead of simply decaying via phonon emission to the base of the

parabola, offering the possibility of altering the normal decay process [49].

Acknowledgement

573

Also, Auzel et al. [10] have shown that under condition of high excitation

density, the multiphonon decay rate in rare earth-doped glasses is inhibited. Such

an effect could have important consequences in the development of certain lasers

where high excitation densities can be achieved.

2. Extension to higher energy regions: To date, work on rare earth ion-doped

systems has concentrated mainly on energy levels roughly below 40,000 cm

–1

Though these represent only a small fraction of the 4f-levels, they have been

utilized for many applications. A continuation into the higher 4f-and 5d energy

levels seems promising for both scientific and technological reasons.

3. Shorter time scales: The use of femtosecond lasers has yet to be exploited

in luminescent materials, particularly in the investigation of the decay of an

excited system by phonon emission. With the period of lattice vibrations on the

order of 50 femtoseconds, and phonon emission occurring over several

vibrational periods, femtosecond laser pulses could be used to study phonon

emission processes while they are in progress. The pump-probe technique seems

especially suitable for such investigations.

4. Nonlinear processes: The importance of nonlinear processes, such as

upconversion and two-photon excitation, will continue to expand, particularly as

high power lasers become more accessible. UV lasers pumped by visible lasers

and relying on upconversion could provide an application for nonlinear effects.

Using upconversion or two-photon absorption, one can even access the vacuum

ultraviolet region without the need for special optics or an evacuated excitation

system.

From the current vantage point, we see two major societal factors that will

drive research in the field. First, the scarcity of nonrenewable energy resources,

the growing demand for these resources, and concerns about the environment

will provide an impetus for the development of more energy efficient lighting.

Second, the rapid a growth in communications will require improved optical

systems, one important component of which will be solid state lasers.

The response to these and other forces will be the continued research into the

fundamental processes of luminescence, resulting in more answers and, equally

likely, more challenges. Using the past as an example, our scientific curiosity

and imagination will lead us to reconfirm, time and again, the observation

attributed to van Vleck: “Every crystal, a surprise.”

ACKNOWLEDGEMENT

The authors would like to thank Professor Claus Klingshirn for very helpful

discussions and clarifications.

REFERENCES

[1] Goethe, J.W. The Sorrows of Young Werther, trans Mayer, E. & Brogan, L. New York: The

Modern Library.

[2] Reif, F. (1965) Statistical Thermal Physics. New York McGraw.

[3] DiBartolo, B. & Peccei, R. (1965) Phys. Rev. A 137, 1770.

[4] Dieke, G.H. (1968) In (eds.) Spectra and Energy Levels of Rare Earth Ions in Crystals,

Crosswhite, H.M. & Crosswhite, H., (New York: Interscience.)

[5] Carnall, W.T., Goodman, G.L., Rajnak, K. & Rana, R.S. (1988) A systematic analysis of the

spectra of the lanthanides doped into single crystal LaF3, Argonne National Laboratory

Report AN-88-8.

[6] Morrison, C.A., & Leavitt, R.P. (1982) Handbook on the Physics and Chemistry of Rare

Earths. Gschneidner, K.A., & Eyring, L. (eds.) (Amsterdam: North-Holland).

[7] Wegh, R.T., Meijerink, A., Lamminmaki, R.-J., Holsa, J. (2000) J. Luminescence 87-89

1002-4

[8]

101(7), 1047-56.

[9] Wang, X., Ganem, J., Dennis, W.M. & Yen, W.M. (1991) Phys. Rev. B, 44(2), 900-2.

[10]

[11]

Academic) p 31,.

[12] Moulton, P.F. (1986) J. Opt. Soc. Amer. B 3, 125-32.

[13]

[14]

[15] Struck, C.W. & Fonger, W.H. (1991) Understanding Luminescence Spectra and Efficiency

Using Wp and Related Functions Berlin Heidelberg: Springer-Verlag.

[16]

Plenum).

[17] Toyazawa, Y. & Kamimura, H. (1967) Dynamical Processes in Solid State Lasers Kubo R.

ed (New York: W.A. Benjamin) p 90.

[18] Struck, C.W. & Fonger, W.H. (1991) Understanding Luminescence Spectra and Efficiency

Using Wp and Related Functions Berlin Heidelberg: Springer-Verlag.

[19] Fowler, W.B. (1968) in Physics of Color Centers, Fowler, W.B., ed (New York: Academic)

p 55.

[20] Gebhardt, W. & Kuhnert, H. (1964) Phys. Letters 11, 15.

[21] Baldacchini, G. (2001) Advances in Energy Transfer Processes Di Bartolo, B., ed (New

Jersey London Singapore Hong Kong: World Scientific).

[22] Henderson, B. (1987) Spectroscopy of Solid State Laser Materials Di Bartolo, B., ed (New

York: Plenum).

[23] Judd, B.R. (1962) Phys. Rev. 127, 750-761.

[24] Ofelt, G.S. (1962) J. Chem. Phys. 37, 511-20.

[25] Carnall, W.T., Fields, P.R. & Rajnak, K. (1968) J. Chem. Phys. 49, 4424.

[26]

[27] Imbusch, G.F. (1978) Luminescence Spectroscopy Lumb, M.D., ed (London, New York:

Academic).

[28] Weber, M.J. (1973) Phys. Rev. B 8, 54-64.

[29] Auzel, F. (1978) Luminescence of Inorganic Solids, Di Bartolo B. ed (New York: Plenum).

[30] Moos, H.W. (1970) J. Luminescence 1,2 106.

[31] Struck, C.W. & Fonger, W.H. (1991) Understanding Luminescence Spectra and Efficiency

Using Wp and Related Functions (Berlin Heidelberg: Springer-Verlag).

[32] Auzel, F. (1980) Radiationless Processes

Di Bartolo B. ed (New York: Plenum).

[33] de Mello Donega, C., Meijerink, A. & Blasse, G. (1992) J. Phys. Condens. Matter 4, 8889-

902.

12. Luminescence Spectroscopy

574

Auzel, F. & Pelle, F. (1996) J. Luminescence 69, 249-55.

Rebane, K. (1970) Impurity Spectra of Solids (New York: Plenum).

Wegh, R.T., Van Loef, E.V., Burdick, D., G.W. & Meijerink, A. (2003) Molecular Physics

Di Bartolo, B. (1978) In Luminescence of Inorganic Solids Di Bartolo D. ed (New York:

Imbusch, G.F. (1978) In Luminescence Spectroscopy Lumb, M.D., ed. (London, New York:

Tanabe, Y. & Sugano, S. (1954) J. Phys. Soc. Japan 9, 753-766.

Walsh, B., Barnes, N. & Di Bartolo, B. (1998) J. Appl. Phys. 83 8 2772-87.

Bibliography

[34] Förster, T. (1948) Ann. Phys. 21, 55.

[35] Dexter, D.L. (1953) J. Chem. Phys. 21, 836-850.

[36] Inokuti, M. & Hirayama, F. (1965) J. Chem. Phys. 43, 1978.

[37] Förster, T. (1949) Z. Naturforch. 4a, 321.

[38] Yamada, N., Shionoya, S. & Kushida, T. (1972) J. Phys. Soc. Japan 32, 1577.

[39] Collins, J.M. & Di Bartolo, B. (1996) J. Luminescence 69, 335-341.

[40] Auzel, F. (2004) Chem. Rev. 104, 139-173.

[41] Di Bartolo, B. (1968) Optical Interactions in Solids (New York: Wiley.

[42] Chen, X., Di Bartolo, B., Barnes, N.P. & Walsh, B.M. (2004) Physica Status Solidi (b) 241

(8) 1957-76.

[43] Dekker, A.J. (1957) Solid State Physics (Englewood Cliffs, NJ: Prentice Hall) p 312.

[44] Elliott, R.J. & Gibson, A.F. (1974) An Introduction to Solid State Physics and Its

Applications (London and Basingstoke: MacMillan.

[45] Flaherty, J.M. & Di Bartolo, B. (1973) Phys. Rev. B 8, 5232.

[46] Baumesteir, P.W. (1961) Phys. Rev. 121, 359.

[47] Elliott, R.J. & Gibson, A.F. (1974) An Introduction to Solid State Physics and Its

Applications (London and Basingstoke: MacMillan).

[48] Liu, G.K., Zhuang, H.Z. & Chen, X.Y. (2002) Nano Letters 2, (5) 535-9.

[49] Collins, J.M. (1998) Ultrafast Dynamics of Quantum Systems Di Bartolo B. editor (New

York: Plenum).

BIBLIOGRAPHY

Dekker, A.J. (1957) Solid State Physics Englewood Cliffs, NJ: Prentice Hall.

Di Bartolo, B. (1968) Optical Interactions in Solids New York, London, Sydney, Toronto: John

Wiley & Sons.

Elliott, R.J., & Gibbon, A.F. (1974) An Introduction to Solid State Physics and Its Applica- tions,

London and Basingstoke: Macmillan.

Lumb, M.D. ed (1978) Luminescence Spectroscopy New York: Academic.

Kaplianskii, A.A. & Macfarlane, R.M. (1987) Spectroscopy of Solids Containing Rare Earth Ions

Amsterdam Oxford New York: North-Holland.

Henderson, B. & Imbusch, G.F. (1989) Optical Spectroscopy of Inorganic Solids Oxford:

Clarendon Press.

Rebane, K. (1970) Impurity Spectra of Solids New York: Plenum.

Liu, G.K. & Jacquier, B. eds (2005) Spectroscopic Properties of Rare Earths in Optical Materials

Berlin Heidelberg New York: Springer.

Yu, P.Y. & Cardonna M., Fundamentals of Semiconductors Berlin, Heidelberg, New York:

Springer

575

Klingshirn, C.F. (1997) Semiconductor Optics Berlin, Heidelberg, New York: Springer.

CHAPTER 13

LASER-INDUCED FLUORESCENCE

SPECTROSCOPY

G. Geipel

Institute of Radiochemistry, Forschungszentrum Rossendorf, P.O. Box 510119, D-01314

Dresden

13.1 INTRODUCTION

The emission of so-called cold light is named “luminescence”. According to

the origin of the luminescence, four categories can be distinguished:

photoluminescence, electroluminescence, chemiluminescence, and biolumi-

nescence. Photoluminescence is generated by absorption of radiation. This

category is divided into two phenomena, fluorescence and phosphorescence.

Formerly, lifetime was used to discriminate between fluorescence and

phosphorescence. Fluorescence is generated by a spin-allowed transition,

where no change in multiplicity occurs. In contrast to this, the radiative decay

to the ground state responsible for phosphorescence properties is a spin-

forbidden intercombination transition.

As one can see on the periodic table of elements, many of the lanthanides

and actinides show photoluminescence properties in solution and in solid

state. In addition, photoluminescence can be observed in complex systems

containing point defects, like color centers in alkali halides, single crystals,

and many others.

Laser-induced spectroscopic methods were also used in actinide chemistry

as a tool to study the speciation of the actinide elements in aquatic

environments. The main goal of the application of time-resolved laser-

induced fluorescence spectroscopy (TRLFS) and laser-induced photoacoustic

spectroscopy (LIPAS) is to achieve detection limits of the actinide species as

low as possible or in concentration ranges expected under environmental

conditions.

With laser sources it is possible to provide any wavelength from the near

UV to the NIR (Near Infrared) wavelength range. The development of tunable

solid-state lasers overcame some disadvantages of the dye laser systems with

respect to short tunable wavelength range and the use of hazardous chemicals.

13. Laser-Induced Fluorescence Spectroscopy

The application of low temperatures for samples measured with fluorescence

spectroscopic methods brought success, especially in the detection of

carbonate species.

It is not possible to include all literature in this field, therefore this

contribution includes only a selection of papers. The focus here will be mainly

on the fluorescence spectroscopy of the lower actinides like uranium (VI),

americium (III) and curium (III).

13.2 EXPERIMENTAL SETUP

A typical setup for fluorescence measurements is shown in Figure 13.1.

Fluorescence measurements of solid samples are mainly performed by use of

intensified camera systems and diode arrays. In the case of time-resolved

measurements this results in a time-domain setup. An overview of used

excitation wavelength for fluorescence spectroscopy of actinides is given in

Table 13.1. According to the monochromatic radiation of laser sources no

excitation monochromator for the excitation of samples is necessary.

Figure 13.1 Typical scheme for time-resolved laser-induced fluorescence spectroscopy with

solid samples.

Solid samples generate a lot of stray light off the exciting laser beam. To

minimize this stray light two different methods are applicable. First, the

emitted fluorescence photons may be focused in other than a rectangular setup

into the spectrograph. However, for samples that do not have planar surfaces,

stray light may be generated at each surface plane, so this leads only to a

reduction of the stray light. Second, a filter between the excited sample and

the entrance of the spectrograph can be used. The edge of this filter should

block all wavelengths below the emission spectrum. Some additional remarks

on stray light can be found in the literature [1].

The spectrograph should be a conventional imaging monochromator

system with no exit slit. The width of the entrance slit influences the spectral

578

13.3 Fluorescence Spectroscopy of Minerals

resolution. It should be selected as small as possible. The ICCD normally

consists of a multichannel plate (an intensifier with a factor of about 1:10

an optical connector, and a CCD camera system. The optical connector

usually is a fiberoptical taper or an imaging telescope. Telescopes do not

generate interferences in the image, therefore their use should be preferred.

The delay generator shifts the time of the measurements against the laser

pulse. The experiment can be triggered either by a TTL pulse provided by the

laser system or by the delay generator if the laser can be fired by an external

trigger pulse. The PC controls delay, spectrograph, and ICCD, and it stores

the received spectra.

In the case of uranium (VI) as a detected fluorescent species, often a laser

wavelength of 266 nm is used as excitation source. It is necessary to point out

that in such an arrangement the selection of the filter has to be made very

carefully. Normal glass filters absorb the UV laser light completely and

therefore they are a nearly ideal filter material. Nevertheless these glass filters

generate so-called “white light.” This increases the background of the

spectrum, and the calculation of fluorescence decay times becomes more

difficult. Therefore, for each application of laser-induced fluorescence

measurements, the laser stray light has to be looked for and its second order

diffraction in the spectrograph.

Table 13.1 Excitation wavelength applicable to fluorescent oxidation states of the actinide

series from protactinium to curium.

Ion Excitation Wavelength (nm) Reference

[[ [[[ [[

278 (308) [2]

248 [3]

266

337

355

380–440

[4, 5]

[6]

[7]

[8]

504 (aqueous solution)

337/355 (solid sample)

[9]

[10]

395 [11]

13.3 FLUORESCENCE SPECTROSCOPY OF MINERALS

Uranium as well as many other heavy metals, form a wide variety of minerals.

It is known that minerals containing uranium (VI) show more or less intense

fluorescence emission after excitation with UV light. Therefore, fluorescence

measurements on these minerals are a possible tool to determine their

chemical composition. However, fluorescence spectroscopy is not often used

in mineralogy. The description of minerals remains only in remarks on

579

13. Laser-Induced Fluorescence Spectroscopy

fluorescent or nonfluorescent minerals and sometimes information on the

fluorescence color can also be found [12]. A systematic study of the

fluorescence of uranium minerals and its comparison to that of synthetic

compounds can contribute to better precision in the determination of natural

samples. Therefore, from the point of view of actinide chemistry the uranium

minerals are of the greatest of interest. About 180 uranium minerals are

known. However, not all of them show fluorescence properties.

A publication on the selective detection of uranium in geologic samples

was contributed in the 1980s by deNeufville [13]. Unfortunately no spectral

information about the emission maxima of the uranyl species was supplied.

Some fluorescence decay times are summarized. For most of the studied

minerals decay times in the s range were detected.

Among uranium (VI) minerals the phosphates show a very intense

fluorescence emission. Therefore, it is not surprising that the fluorescence

spectrum of autunite is one of the first published fluorescence spectra of a

uranium mineral [14]. Later, a series of fluorescence spectra of several

uranium phosphates and arsenates were measured [15]. From data on the

222 42 2

identify a solution species. In addition to this, the synthesis of solid arsenate

phases UO

AsO

)

× 1H

O and UO

HAsO

× 4H

O brought success in the

determination of the aqueous species. From these fluorescence spectra of

minerals and solid phases, the fluorescence spectra in solution could be

deconvoluted and the concentration of the species determined. This led last

but not least to the determination of complex stability constants [16] already

reviewed in the thermodynamic database of uranium [17].

The problem of false determination of species from a natural mineral

sample may be shown by the example of the mineral chernikovite,

(UO

)

(PO

)

× 6H

O. The mineral used from the minerals collection

of the Technical University Mining Academy, Freiberg (collections number

72753) did not show any fluorescence spectrum. However, it could be

expected from extrapolations of other phosphate minerals and by comparison

with real arsenate mineral troegerite that the mineral should show intense

fluorescence properties. The synthetic sample of this mineral did show a very

intense fluorescence spectrum as expected [18]. The fluorescence spectrum of

this sample is shown in Figure 13.2.

The centers of the fluorescence maxima are located at 502.3, 523.6, 547.4,

573.1, and 603.2 nm, respectively. The fluorescence decay time was found to

be multiexponential. However, no spectral shift with increasing delay time

was observed. Therefore this behavior can only be explained by the existence

of species with a different number of water molecules in the crystal lattice or

by a loss of water during the measurement. The longest detected fluorescence

decay time was determined to be 27.6 ± 0.05 s.

From solution chemistry it is known that uranyl di- and tri-carbonate do not

show fluorescence properties at room temperature. However, in seepage

waters from a mining area in Saxony/Germany, fluorescence spectra could be

580

mineral troegerite, H (UO ) (AsO ) × 8H O researchers were later able to

13.3 Fluorescence Spectroscopy of Minerals

obtained [19]. This unexpected result led to several studies in the alkaline

earth uranium (VI) carbonates. At first in could be shown that the fluo-

rescence spectrum results from a dissolved Ca

(CO

)

(aq.) species

[20–22]. The fluorescence spectrum of this solution species agrees very well

with the fluorescence spectrum of the mineral liebigite [23]. They differ only

in the determined fluorescence decay times. For the aqueous species the

uranyl carbonates are summarized [23–25].

Figure 13.2 Fluorescence spectrum of synthetic chernikovite.

Many other uranium minerals show also fluorescence properties. About

150 have been tested for their fluorescence spectra. For example, the spectrum

of johannite, a copper-containing uranium mineral, is shown in Figure 13.3.

The data obtained from TRLFS measurements at room temperature for

this and some selected other uranium bearing minerals are summarized in

Table 13.3 [26].

Figure 13.3 Time-resolved fluorescence spectrum of the mineral johannite

Cu(UO

)

(SO

)

(OH)

8 H

species 320 s. In Table 13.2 spectroscopic parameters of alkaline earth

fluorescence decay time was determined to be about 50 ns and for the mineral

500 550 600

0.0

2.0x10

4.0x10

6.0x10

8.0x10

1.0x10

Intensity at a delay of 3000 ns (A.U.)

Wavelength (nm)

450 500 550 600

100

201

451

2501

5001

7501

10001

Delay (ns)

Wavelen

(

)

Intensity (A.U.)

581