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805

Chemical Evolution

onInterstellar

Grains

atLow

Temperatures

Kenzo Hiraoka

University of Yamanashi

Kofu,

Japan

29.1 introduCtion

Dense interstellar clouds are the birthplaces of stars of all masses and their planetary systems.

Interstellar molecules and dust become the building blocks for protostellar disks, from which

planets, comets, asteroids, and other macroscopic bodies eventually form. It has been recog-

nized that dust is ubiquitous in the Galaxy. Light essentially cannot penetrate dense interstellar

clouds, and temperature inside dark clouds remains at ∼10 K. About 120 interstellar molecules have

been observed by millimeter and submillimeter spectroscopy in such dark clouds. It is becoming

Contents

29.1 Introduction ..........................................................................................................................805

29.2 Simulation of the Chemical Evolution Taking Place on the Dust

GrainsintheDarkClouds...............................................................................................806

29.3 Reactions of H and D Atoms with Solid C

, C

, and C

at CryogenicTemperatures ..............................................................................................808

29.3.1 In

Situ and Real-Time Observation of Products Formed from Reactions

HAtoms with Solid C

and C

.....................................................................808

29.3.2 Temperature

Dependence on the Yield of Reaction Products.................................. 810

29.3.3

Reactions

of D Atoms with Solid C

, C

, and C

: Isotope Effect ................ 811

29.4 Formation

of Formaldehyde by the Tunneling Reactions of H with Solid CO at 10

K ........ 815

29.4.1 Formation

of H

CO from the Reaction of H with Solid CO .................................... 815

29.4.2 Temperature

Dependence of the Yield of H

CO from the Reaction H + CO........... 816

29.4.3 Does

the van der Waals Solid Film Erode during the H-Atom Spray?.................... 816

29.5

Reaction

of H and D Atoms with Solid Formaldehyde and Methanol

CryogenicTemperatures................................................................................................... 817

29.5.1

Reaction

of H with Solid H

CO................................................................................ 818

29.5.2 Reaction

of D with Solid H

CO................................................................................824

29.5.3 Reactions

of H and D with Solid CH

OH.................................................................825

29.6 Methanol

Formation from Electron-Irradiated Mixed H

O/CH

Ice at 10 K .......................827

29.7 Solid-Phase Reactions of D with CN to Form DNC and DCN

at CryogenicTemperatures .............................................................................................. 832

29.8 Is

Formed in Solid-Phase Reactions? ........................................................................... 835

29.9 Conclusion ............................................................................................................................837

References...................................................................................................................................... 837

806 Charged Particle and Photon Interactions with Matter

increasingly clear that gas-phase ion–molecule reactions—the dominant processes in diffuse inter-

stellar clouds—are less important in the dense clouds. Instead, chemical reactions on cosmic dust

grains play an important role (Millar and Williams, 1993). Interstellar ices, which grow on solid

dust particles in cold, dense clouds, become the substrates for various chemical reactions. The

Infrared Space Observatory (ISO) has provided much information about interstellar and cometary

ices (Ehrenfreund and Schutte, 2000).

Unfortunately, direct observations of the solid-phase chemistry of dense clouds are difcult to come

by. In the absence of direct evidence, we have to resort to laboratory studies. A wealth of data has been

gathered on gas-phase ion–molecule reactions, but investigations on the solid-phase reactions of various

molecules remain scarce (Greenberg and Pirronello, 1991), although there have been experimental stud-

ies of molecular hydrogen formation on the surface of cosmic dust analogues (Pirronello et al., 2000).

Hydrogen atoms are believed to play a particularly important role in the chemical evolution of

dense clouds because of their high mobility in solids. Because of their wave nature and low atomic

weight, hydrogen atoms can “tunnel” through seemingly insurmountable barriers (Yang and Rabitz,

1994). In a solid, a hydrogen atom can be visualized as a quantum liquid that creeps quickly from

one site to another and participates in tunneling reactions whenever it encounters suitable reactions.

In the following sections, the role of dust grains for the formation of hydrocarbons, formaldehyde,

methanol, HCN/HNC, and ammonia will be described. The importance of the H-atom tunneling

reactions and radiation chemistry induced by the cosmic rays in the dark clouds will be underlined.

29.2 simulation oF the ChemiCal evolution takingplaCe

the d

ust

rains

in the d

ark

Clouds

The conceptual idea of the apparatus is shown in Figure 29.1 (Hiraoka et al., 2002a). The cryocooler

(Iwatani Plantech, type D310) and a quadrupole mass spectrometer (Leda Mass, Microvision 300D)

are housed in a vacuum manifold. The vacuum chamber was evacuated by two turbomolecular

pumps (ULVAC, UTM-500, 500L s

−1

and Seiko Seiki, STP-H200, 200L s

−1

) connected in tandem.

Wafer

Movable

lid

Sample

gas

Colloidal

graphite

–6 kV

Bottleneck

discharge tube

Cold head

100 V

Figure 29.1 Schematic diagram of the experimental system (not to scale). (From Hiraoka, K. et al.,

Astrophys. J.,

577, 265, 2002a. Reproduced with permission of the AAS.)

Chemical Evolution onInterstellar Grains atLow Temperatures 807

The base pressure of the vacuum system under the current experimental conditions was ∼5 × 10

−10

Torr, after baking the vacuum manifold at about 130°C for 72h. The major residual gas components

are H

O, N

, O

, and H

. The pressure rise over the lifetime of the experiment was ∼1 × 10

−9

Torr.

The

original base pressure was resumed after overnight evacuation at room temperature.

The

sample gas was deposited on the silicon substrate ([100] surface with the size of 30 × 50 ×

0.5mm

), which was rmly pressed to the cold head of the cryocooler using indium foil between

the mating surfaces. The sample gas was introduced through a calibrated stainless steel capillary

(internal diameter of 0.1mm and 1 m long) onto the cooled silicon substrate in the vacuum chamber.

After the deposition of the sample, H (or D) atoms produced by the dc discharge of H

(or D

) were

sprayed

over the sample lm. The base pressure rose to ∼1 × 10

−6

Torr by the H

gas introduction.

The dc discharge was generated by applying −6kV to the stainless steel capillary in a bottleneck

discharge tube. The sample lm was prevented from being bombarded by the charged particles

and UV photons produced by the plasma. The electron current effusing out of the dc discharge

tube over the silicon substrate was measured by an electrometer. By adjusting the voltage applied

to the coil electrodes inside the optical traps of the bottleneck discharge tube, the electron current

was suppressed below the minimum current that could be measured by the electrometer (<10

−12

A).

That is, the ux of electrons sprayed over the lm was lower than ∼10

electrons cm

−2

−1

. Thus, the

chemical reactions induced by the electron bombardment of the sample lm should be negligible.

The discharge tube that was held tight by the copper sleeve connected to the cold head was kept at

∼27K when the cold head was cooled to 10K. Thus, the temperature of H atoms sprayed over the

sample

lm may have been about 27

In order to prevent the contamination of the gas line of the H-atom spray system, the exit of the

bottleneck discharge tube for the H-atom spray was plugged by using a movable lid every time for

gas introduction and for the thermal desorption experiment. In order to sputter the residual mol-

ecules adsorbed on the inner wall of the discharge tube, H

-plasma was always generated for 30 min

before and after the experiment. These procedures are necessary in order to perform the experi-

ments on the low-temperature solid-phase reactions in which the amounts of reaction products are

only

limited to less than an equivalent monolayer.

The

introduction rate of H

into the discharge tube was about 10

molecules s

−1

. The measure-

ment of the ux of H atoms sprayed over the solid lm was not made in this experiment. Cherigier

et al. (1999) reported that a few percent of reagent gas is decomposed into atoms in the ordinary

glow discharge plasma. The H-atom ux was estimated to be ≤10

atoms cm

−2

−1

under the pres-

ent experimental conditions (Hiraoka et al., 2002a). To a crude approximation, the irradiation of

Hatoms with the ux of ∼10

−2

−1

for 300min spray may correspond to ∼5 × 10

years of H dose

in the dark cloud, assuming that the H-atom number density is ∼1cm

−3

, the H-atom temperature is

70K,

and the dust grain size is 0.1

μm

in the dark cloud.

The

electrical power necessary to decompose H

molecules completely to H atoms, with the

ow rate of ∼1 × 10

−1

, is less than 0.1 W. In a separate experiment, we used a microwave

plasma with the power of ∼10W as the H-atom source. We found that the wall of the discharge tube

was seriously corroded by plasma etching (sputtering) and various radiation products such as solid

particulates (amorphous silicon), SiH

, H

O, CO, etc., were deposited on the cold silicon substrate.

The contamination from the discharge products will totally destroy the very delicate experiments

because the present experiments dealt with a few to tens of monolayers (ML) sample lms. The

present dc discharge with the electric power of 0.2W (Figure 29.1) is almost free from contamina-

tion. Besides, the dc discharge tube was maintained at about 27K. This must suppress the etching

of the Pyrex discharge tube.

In our work, the quantitative analysis of reaction products was performed by thermal desorp-

tion spectrometry (TDS) with the programming rate of 6 K min

−1

, taking the mass cracking

patterns and the relative ionization cross sections into account for all gaseous products. The

yields of the products were calculated from the ion intensities of the gaseous products and those

of the reactant recovered.

808 Charged Particle and Photon Interactions with Matter

To perform in situ and real-time product analysis in the low-temperature solid-phase reactions,

the infrared absorption spectra of the deposited lm reacted with H (or D) were measured using

a FT-IR spectrometer (Nicolet, Magna−IR 760), with a resolution of 4 cm

−1

in combination with a

KBr beam splitter and a liquid N

-cooled MCT (HgCdTe) detector. The quantitative analyses of

reaction products were made by FT-IR spectra, but the reliability is less than those obtained by TDS

due

to weak absorption intensities.

gaseous products were detected by the mass spectrometer during the H-atom spray over the

solid sample in the temperature range investigated. This indicates that the desorption of reactants

and products during the H-atom spray was negligible, suggesting that the reactions proceed via

the Langmuir–Hinshellwood mechanism. This is reasonable because the temperature of H atoms

sprayed

over the lm was as low as 27

29.3 reaCtions oF h and d atoms with solid C

,C

and

at CryogeniC temperatures

29.3.1 in Situ and real-tiMe obServation of productS forMedfroMreactionS

of h atoMS with Solid c

and c

It is well known that the formation of saturated hydrocarbons in the interstellar medium is difcult

to explain by the gas-phase ion–molecule reactions, because the carbenium ions cease to react with

in the unsaturated forms (Ikezoe et al., 1987). Consequently, the observation of saturated hydro-

carbons, either frozen on grains or as gases, would provide strong evidence that reactions on the dust

grains

play important roles in the interstellar chemical evolution.

Cometary

nuclei are the least modied solar system bodies extant from the time of planetary

formation. Their current compositions are thought to be representative of the materials from which

they formed (Mumma et al., 1996). The native ices are thought to have originated either in theinter-

stellar dense cloud core that preceded the solar system or at distances beyond Jupiter in the nebula

surrounding the protosun. In comet C/1996 B2 Hyakutake, hydrocarbons C

, C

, and CH

along with CO and H

O were detected with the use of a high-resolution infrared telescope on Mauna

Kea (Mumma et al., 1996). The abundances of C

, CH

, CO, and H

O were in the proportions

:CH

:CO:H

O = 0.4:0.7:5.8:100. Mason et al. reported the detection of C

in Hyakutake at an

abundance

of 0.1%–0.4% relative to H

O (Mason et al., 1996).

Comet C/1996 01 Hale–Bopp was observed at wavelengths from 2.4 to 195μm with ISO when

the comet was about 2.9 AU from the sun (Crovisier et al., 1997; Crovisier, 1998). The gas-phase

abundance of C

and C

were found to be about the same and their ratios with H

O were

about 0.5%.

In the observation of comets Hyakutake and Hale–Bopp, it should be noted that C

had not

been detected by infrared spectroscopy despite the reasonably strong intensities of C

hydrocarbons,

, and C

The abundances of C

are generally observed to be low in the solar system; for example, C

is missing in Saturn and its moon Titan, where other hydrocarbons with two carbon atoms are com-

mon.

To nd out why, we must consider the channels through which these molecules form.

order to perform in situ and real-time product analysis in the low-temperature solid-phase

reactions, we measured the infrared absorption spectra of C

and C

lms being reacted with

H atoms at cryogenic temperatures (Hiraoka et al., 2000). Figure 29.2a shows the FT-IR spectra

for a 20 monolayers (ML) thick C

sample lm sprayed by H atoms for 3h at 10 K. The bottom

spectrum is that for the 20ML thick C

before the reaction. The spectra from the bottom to the

top were measured with a time interval of 15min. Steady growth in the absorption of the ν

band

(2974cm

−1

) and ν

band (2881cm

−1

) of C

can be seen with the reaction time. This clearly indi-

cates the occurrence of the reaction of H with solid C

to form C

at 10 K. However, no C

radicals could be detected by FT-IR. The H atom being sprayed over the solid C

lm is itself

Chemical Evolution onInterstellar Grains atLow Temperatures 809

aradical scavenger. Thus, the steady-state concentration of C

in the C

matrix must be kept

low during the H-atom spray over the C

solid lm because of the occurrence of the efcient

recombination or abstraction reaction between H and C

. No radical species could be detected by

FT-IR

in the reactions of H atoms with either C

or C

Figure 29.2b displays the FT-IR spectra for a 10ML thick C

lm reacted with H atoms for

5h at 10K. The bottom spectrum is that for the 10ML thick C

before reaction. The spectra from

the bottom to the top were measured with the time interval of about 25min. Steady growths in the

absorption of the ν

band (2974cm

−1

) and ν

band (2881cm

−1

) of C

can be seen with the H-atom

spray time. In the reaction of H with C

to form C

, C

must be formed as an intermediate

product. However, no absorption due to C

is observed in Figure 29.2b, nor was C

detected

by the much more sensitive TDS. The fact that no intermediate products could be detected except

3100

H+C

–0.0026

–0.0024

–0.0022

–0.0020

–0.0018

–0.0016

–0.0014

–0.0012

–0.0010

–0.0008

–0.0006

–0.0004

–0.0002

–0.0000

0.0002

0.0004

0.0006

0.0008

3050 3000 2950

Wavenumbers (cm

–1

)(a)

Absorbance

2900

3250

H+C

–0.0008

–0.0006

–0.0004

–0.0002

–0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

0.0014

0.0016

0.0018

0.0020

0.0022

0.0024

0.0026

0.0028

3200 3150 3100

Wavenumbers (cm

–1

)(b)

Absorbance

3050 3000 2950 2900 2850

Figure 29.2 (a) Change of the FT-IR spectra when the 20ML thick C

lm was sprayed by H atoms for 3h

at 10K. The bottom spectrum is that for the 20ML thick C

before reaction. The spectra from the bottom to

the top were measured with the time interval of 15min. (b) Change of the FT-IR spectra when the 10ML thick

lm was reacted with H atoms for 5h at 10K. The bottom spectrum is that for the 10ML thick C

lm

before reaction. The spectra from the bottom to the top were measured with the time interval of about 30min.

(From Hiraoka, K. et al., Astrophys. J., 532, 1029, 2000. Reproduced with permission of the AAS.)