Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces

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830 Charged Particle and Photon Interactions with Matter

to 300eV. The steady increase of the yield is likely due to the occurrence of more frequent cascade

scattering of primary electrons, resulting in the denser track formation in the solid sample.

In order to obtain more detailed information on the reaction mechanisms for the formation of

methanol, CD

was used instead of CH

as the mixing partner with H

O. The TDS spectra obtained

for 100eV e

−

-irradiated H

O/CD

(10/1), with a ux of 100nA cm

−2

, are shown in Figure 29.17. The

appearance of the peak with m/z 35 at 162K clearly indicates the formation of CD

OH. In Figure

29.17, the peak with m/z 34 is approximately 70% of that with m/z 35, which is much larger than the

expected value for the fragment ion from CD

OH. This strongly suggests the formation of the less

deuterated methanol, that is, CHD

OH. Though the cracking pattern of CHD

OH is not available,

it is reasonable to assume that the ion with m/z 34 is a fragment originating from CHD

OH. This

is because the molecular ion with m/z 35 is strongly observed for CD

OH, and it should hold for

CHD

OH also. The relative abundances of the products, CD

OH and CHD

OH, were about the same.

In this work, the electron ux was measured by using a Faraday cup and thus the quantitative

determination of the abundance of methanol could be made. As shown in Figure 29.15, the yield

of methanol increases with the electron ux. The initial slope may make it possible to estimate the

yield of methanol molecules with respect to the electron numbers irradiated. By the method of least

squares applied to Figure 29.15, the yield of one CH

OH molecule per 60 incident electrons (100 eV)

could

be derived. This corresponds to a G-value of 0.017.

The

methanol formation may proceed along the following pathways: ionization of H

O and CH

followed by rapid ion–molecule reactions, and neutralization with excess electrons to form reactive

radicals

(Roser et al., 2003). The net reactions may be simplied as follows.

H O H OH

→ + (29.23)

CH CH H

→ + (29.24)

CH OH CH OH

+ → (29.25)

The above mechanism is supported by the experiment using mixed H

O/CD

ice in which CD

has

been detected as one of the major deuterated methanol products.

CD OH CD OH

+ →

(29.26)

100 150

m/z 33

200

Temperature (K)

5×10

–10

1×10

–9

O/CD

(10/1), 10 ML, 10 K

100 eV, 350 nA cm

–2

, 10 min

Pressure (Pa)

Figure 29.17 TPD spectra for e

−

-irradiated H

O/CD

(10/1). Simultaneous 100eV electron irradiation with

the ux of 350nA cm

−2

(8eV molecule

−1

) at 10K. The total sample thickness, 10ML. (From Wada,A. et al.,

Astrophys. J.,

644, 300, 2006. Reproduced

with permission of the AAS.)

Chemical Evolution onInterstellar Grains atLow Temperatures 831

The mechanism (29.26) is consistent with the previous work of the proton irradiation experiment

(Moore and Hudson, 1998). In addition to CD

OH, CHD

OH was formed in about equal amounts.

This

suggests the occurrence of the methylene radical insertion reaction (29.27),

CD H O CHD OH

2 22

+ → (29.27)

Kaiser and Roesller (1998) suggested that the CH

radicals are formed very efciently in pure

solid CH

irradiated by protons and alpha particles. They found that the long-chain hydrocarbons

dominate the reaction products. In contrast, limited formation of hydrocarbons larger than C2 could

be detected in this work. This is reasonable because the polymerization reactions induced by the

hydrocarbon radicals are largely suppressed by the primary radicals of OH and H formed from

the major component, H

O, which act as radical scavengers. About equal amounts of CD

OH and

CHD

OH formed from the e

−

-irradiated H

O/CD

(10/1) suggest that the relative contribution of

reactions

(29.26) and (29.27) for the formation of methanol is about equal.

The rather facile formation of H

CO found in this work is noticeable. As shown in Figure 29.15,

the yield of H

CO shows a rather steep increase in the range of low-electron dose, 30–300 nA cm

−2

This

trend is similar to that observed for methanol. This suggests that H

CO is not a secondary but

a primary product. The C-atom addition to H

O, followed by the intramolecular isomerization, may

explain

the direct formation of H

CO.

C HOH HCOH (formation of carbene)

+ →

(29.28)

HCOH H CO (intramolecular isomerization)

→ (29.29)

In fact, the mechanism proposed above is conrmed by the experiment using mixed CD

O ice.

That

is, only H

CO but no HDCO or D

CO could be detected from this system.

It would be meaningful to consider the relative importance of the possible sources for methanol

in ISM, for example, cosmic-ray induced radiation chemistry taking place in the mantles of dust

grains. The methanol was found to be formed from the mixed H

O/CO (10/1) ice irradiated by the

0.8 MeV protons at a dose of 22 eV molecule

−1

(Hudson and Moore, 1999). The column density

of CH

OH molecules formed was 1.23 × 10

molecule cm

−2

. That is, 6.4 methanol molecules

were formed per one proton bombardment. The components of cosmic rays consist of 97%–98%

protons and 2%–3% alpha particles. In the energy range of 1–10 MeV, the cosmic-ray ux is 10

particles cm

−2

−1

. For the average size of the dust grain with 0.1 μm in diameter (cross section

of ∼10

−10

), the frequency of cosmic irradiation is 1 particle per 10,000 days. Thus, the rates

of methanol generation in ISM can be estimated as 6.4 for every 10,000 days. In the interstellar

environment, the sticking probability of a H atom on the dust grain becomes increasingly smaller

above 20 K (Hiraoka et al., 2006). Therefore, the contribution to methanol formation by H-atom

addition reactions to CO on the dust grain may be much smaller than that induced by cosmic rays.

Discussing the routes of formation of CH

OH in quantitative terms is not an easy task, because

the parameters appearing in rate equations, such as the uxes of cosmic rays, Hatoms, and UV

photons and rate constants for various possible reactions, have large uncertainties. Moreover,

the interstellar media are spatially not uniform even in molecular clouds and diffuse clouds.

Although reliable estimation on the relative contribution of various sources for the formation of

methanol is difcult at present, we believe that the cosmic-ray induced radiation chemistry plays

an important role for the formation of methanol because dirty H

O-base ice mantles on the dust

grains can survive up to ∼100 K in the diffuse clouds where they must suffer from intense irradia-

tion by cosmic rays and UV photons. In the diffuse clouds, H-atom grain-surface reactions may

not play important roles because the sticking probability of H atoms on the dust grains is much

less than that in the dark clouds at ∼10 K.

832 Charged Particle and Photon Interactions with Matter

29.7 solid-phase reaCtions oF d with Cn to FormdnC

and

at Cryogeni

C temperatures

HNC was rst observed in the gas phase by radio astronomers (Snyder and Hollis, 1976; Irvine

et al., 1998). The HNC/HCN ratio, which would be almost zero at thermal equilibrium, can be

greater than unity in cold molecular clouds, and it exhibits a striking inverse temperature depen-

dence. Hirota et al. (1998) determined the abundance ratio of HNC/HCN to be 0.54–4.5 in dark

cloud cores. The large value of the HNC/HCN abundance ratio in cold molecular clouds has been

considered to be due to gas-phase reactions (29.30) through (29.32), (Watson, 1974, 1980; Irvine

et al.,

1996, 1998),

H HCN HCNH H

3 2

+ +

+ → +

(29.30)

HCNH e HNC H

+ −

+ → + (29.31)

HCNH e HCN H

+ −

+ → + (29.32)

and

due to reactions (29.33) and (29.34) (Green and Herbst, 1979)

C NH H NC H

+ +

+ → +

(29.33)

H NC e HNC H

+ −

+ → + (29.34)

It has been shown that the HNC and HCN are produced with nearly equal abundances from reac-

tion,

HCNH

+ e

−

(Watson, 1974; Herbst, 1978; Taketsugu et al., 2004).

In the diffuse interstellar clouds, the rst interstellar molecules, CN, CH, and CH

, were detected

via their characteristic absorption spectra in the visible region of the spectrum by using ground-

based telescopes. Prasad et al. (1987) discussed the chemical evolution in the interstellar clouds.

They pointed out that CN is the terminal product for gas-phase nitrogen chemistry and its coupling

with carbon chemistry. In cold molecular clouds, CN radicals formed in the gas phase will be

adsorbed on the dust grains and they might be involved in the grain-surface reactions, for example,

the

association of CN with H to form HNC and HCN.

We studied the association reactions (29.35) and (29.36) in solid phase (Hiraoka et al., 2006).

D CN DNC ( = 456 kJ mol )

o 1

+ → −

−

∆H ∼ (29.35)

D CN DCN ( = 518 kJ mol )

o 1

+ → −

−

∆H ∼ (29.36)

In this work, CN radicals were generated by decomposing HCN in a dc discharge plasma. The HCN

reagent gas was synthesized by mixing NaCN with sulfuric acid (100%). The HCN gas was mixed

with

major gas with the ratio of HCN/N

= 1/50.

The N

gas containing 2% of HCN was excited by dc discharge plasma generated in a straight-

type

bottleneck discharge tube shown in Figure 29.1.

During

the deposition, D atoms produced by the dc discharge of D

gas using another discharge

tube shown in Figure 29.1 were simultaneously sprayed over the substrate for 60min. The dose of

sprayed on the metal target is estimated to be about 10

ML.

Figure 29.18a shows the FT-IR spectrum for N

containing 2% of HCN deposited on the metal

substrate (not discharge ignited). Only the absorption peaks at 2096 and 3286cm

−1

due to the reac-

tant HCN are observed. Figure 29.18b through d show the FT-IR spectra for the dc-plasma activated

/HCN (50/1) deposited on the gold-plated copper substrate being reacted with D simultaneously

at 10, 15, and 20K. The absorption intensities of the reactant HCN at 3286cm

−1

in Figure 29.18b

Chemical Evolution onInterstellar Grains atLow Temperatures 833

through d become about 10% of that in Figure 29.18a. This indicates that a greater amount of HCN

supplied into the discharge tube has been decomposed and converted to the discharge products. In

fact, the inside of the discharge tube was found to be coated by yellow lm products (tholin) after

hours of operation. In order to obtain reproducible data, laborious effort was necessary to clean the

discharge tube from time to time. In Figure 29.18b, absorptions due to N

−

, HNC, CN, (CN)

−

, DCN, and DNC have been observed as major product peaks. The appearance of negative ions,

−

and CN

−

, suggests some leakage of electrons from the straight-type bottleneck discharge tube

shown in Figure 29.1. The high electron afnity (EA) of CN (3.7eV, (Lias et al., 1988)) explains the

formation of CN

−

. The EA of N

has not been measured. The appearance of

−

in Figure 29.18

suggests

that N

has a positive EA.

3800

0.000

0.005

0.010

3600 3400 3200

Wavenumbers (cm

–1

)

No discharge ×0.1 (a)

10 K ×1 (b)

15 K

×1 (c)

20 K

No discharge

10 K

15 K

20 K

HNC

3565 cm

–1

HNC

3286 cm

–1

HNC

2034 cm

–1

HCN

2096 cm

–1

2092 cm

–1

–

2004 cm

–1

2040 cm

–1

(CN)

2059 cm

–1

1656 cm

–1

DNC

2728 cm

–1

DCN

2616 cm

–1

×1 (d)

×0.1 (a)

×0.1 (b)

×0.1 (c)

×0.1

×1

×1 (d)

Absorbance

0.000

0.005

0.010

Absorbance

3000 2800 2600

2100 2050 2000

Wavenumbers (cm

–1

)

1650 1600

–

2083 cm

–1

Figure 29.18 (a) FT-IR spectrum for the N

containing 2% of HCN deposited on the metal substrate with

no discharge ignited. Only a strong peak of HCN is observed. Figures (b), (c), and (d) show the FT-IR spectra

for the dc-plasma activated mixed gas N

/HCN (50/1) deposited on the gold-plated copper substrate at 10, 15,

and 20K, respectively. During the deposition, D atoms were sprayed simultaneously over the deposited lm

for 60min. The dose of N

molecules sprayed over the metal substrate is about 10

monolayers (ML). (From

Hiraoka,

K. et al., Astrophys. J, 643, 917, 2006. Reproduced

with permission of the AAS.)

834 Charged Particle and Photon Interactions with Matter

The appearance of strong DNC and DCN in Figure 29.18b at 10K is likely due to the occurrence

of the association reactions of CN with D to form DNC and DCN on the surface of the lm. The ratio

DNC/DCN is about 3 at 10K in Figure 29.18b. With increase of temperature 10 → 15 → 20K, the

peak intensities of DNC and DCN decreased rapidly and became negligible at 20K. This is mainly

due to the decrease of the sticking probability of D atoms on the solid surface. The total absence of

DNC and DCN at 20K suggests that the residence time of D atoms on the solid surface is too short

to encounter with the trapped CN radicals. The grain-surface reactions in which H atoms participate

would become increasingly less important with increase of temperature above 20K in ISM.

The bond energies of D–NC (∼456kJ mol

−1

) and D–CN (∼518kJ mol

−1

) are larger than that of

(443kJ mol

−1

). Thus, D-atom abstraction reactions are both endothermic, that is, these reactions

must

be negligible at cryogenic temperatures.

D DNC D CN ( 13 kJ mol )

o 1

+ → + ≈ +

−

∆H (29.37)

D DCN D CN ( 75 kJ mol )

o 1

+ → + ≈ +

−

∆H (29.38)

In Figure 29.18, a small but a distinct peak of CN

−

at 2083cm

−1

was observed. In the gas phase, an

associative

electron detachment reaction is known to take place with a collision rate.

CN H or D HCN or DCN e ( 159 kJ mol )

o 1− − −

+ → + = −( ) ( ) ∆H (29.39)

→ + = −

− −

HNC or DNC e ( 92 kJ mol )

o 1

( ) ∆H (29.40)

The branching ratio of these reactions has not been determined. DNC and DCN observed in Figure

29.18

may partly be formed by these reactions.

The

isomerization via reaction (29.41) could take place during the D-atom spray over the DNC

and HNC molecules trapped in the N

matrix.

D CND D CND DCN D

+ → − → +[ ] (29.41a)

D CNH D CNH DCN H

+ → − → +[ ] (29.41b)

The associated complex, HCNH, is predicted to locate 110kJ mol

−1

lower in energy than the isolated

H and CNH (Talbi et al., 1996). Since the energy barrier for H + CNH → [H−CNH]

→ HCNH is

calculated to be only 17.6kJ mol

−1

, DCND and/or DCNH may form under the present experimental

conditions through tunneling processes. However, we could not identify any probable candidates for

DCND and DCNH in the infrared absorption spectra. This suggests either that the formation of the

species DCND (or DCNH) is largely suppressed by the presence of the energy barrier (17.6kJmol

−1

)

or that the species DCND and DCNH are quickly converted to CND (or CNH) and DCN by the

D-atom

abstraction reactions below:

D DCND or DCNH CND or CNH D ( 159 kJ mol )

o 1

+ → + ≈ −

−

( ) ( )

∆H (29.42)

→ + ≈ −

−

DCN D or HD ( 322 kJ mol )

o 1

( ) ∆H (29.43)

Since the latter reaction is much more exothermic than the former, the formation of DCN may be

favorable than DNC.

The

intensities of the absorption peaks of products in Figure 29.18 were found to decrease with

increase of temperature but they persisted to appear at 20K except for DNC and DCN. This is

Chemical Evolution onInterstellar Grains atLow Temperatures 835

reasonable because these products, other than DNC and DCN, are mainly formed in the gas-phase

plasma of mixed N

/HCN gas and are deposit–frozen in the N

matrix. Gradual decrease in the

absorption intensities for these products is likely to decrease in the sticking probability of N

molecules

(i.e., matrix gas) on the solid surface with increase of temperature.

Although

the present result doesn’t give any quantitative information whether HCN and HNC pro-

duction favorably competes with other routes of HCN/HNC formation, the grain-surface reactions of

H atoms with CN radicals trapped in the dust grains may explain some part of the abundance of HNC

and HCN in the cold molecular clouds since CN radicals, as the precursors for the formation of these

cyanide compounds, are present with rather high abundance in ISM (Prasad et al., 1987).

29.8 is nh

Formed in solid-phase reaCtions?

In Figure 29.18b through d, N

is observed as the strongest absorption peak. Interestingly, however,

neither DN

nor ND

was observed in Figure 29.18. This suggests that the N atom and also the N

radi-

cal do not play important roles for the formation of ND

and DN

. This is rather puzzling because both

N and N

have unpaired electrons and association reactions (29.44) and (29.45) are highly exothermic.

H (or D) N NH or ND ( 314 kJ mol )

o 1

+ → = −

−

( ) ∆H (29.44)

H (or D) N NH or DN ( 397 kJ mol )

3 3 3

o 1

+ → = −

−

( ) ∆H (29.45)

In low-temperature solid-phase reactions, the rate of reaction is strongly dependent on the energy

dissipation processes. The energy relaxation of vibrational energy to phonon energy is quite inef-

cient because vibrational quanta are more than one order of magnitude larger than phonon quanta

as already mentioned. In this regard, high exothermic reactions do not necessarily take place with

high

efciencies in the low-temperature solid-phase reactions.

The

N-atom abstraction reaction (29.46) may be another channel for reaction of H (or D) with N

H (or D) N NH or ND N ( 259 kJ mol )

o 1

+ → + = −

−

( )

∆H (29.46)

Reaction (29.46) might be more favorable than reaction (29.45) in solid-phase reactions because

reaction (29.46) is less exothermic than reaction (29.45) and the heat of reaction (29.46) can be dis-

sipated

as the kinetic energies of the products, NH and N

The N

radical may be formed by the reaction of electronically excited N atoms, for example,

D), with a N

molecule (ground state

or excited states, e.g.,

N( D N N X ( 284 kJ mol for N X )

2 3 g

o 1

) ( ) :+ → ≈ −

− +2

Π ∆ ΣH

(29.47)

The standard heat of formation of N

was calculated to be

∆H

≈

418kJ mol

−1

by ab initio calcu-

lations with CCSD(T)/cc-pVTZ basis set (Hiraoka et al., 2006). This value leads to the enthalpy

changes of reactions (29.46) and (29.47) to be −259 and −284kJ mol

−1

, respectively. If NH is formed

by reactions (29.44) and/or (29.46), NH

may be formed as a nal product. Strangely, however, ND

could not be detected by FT-IR as already pointed out. One possible reason for the absence of ND

is the existence of the energy barrier that prohibits the reaction (29.46) to proceed at cryogenic

temperatures. Another reason is that even if NH radical is formed in reactions (29.44) and (29.46), it

does not lead to the formation of the nal product ammonia due the occurrence of H-atom abstrac-

tion

reactions (29.48) and (29.49).

H (or D) HN (or DN) H or D N ( kJ mol )

o 1

+ → + = −

−

( )

121∆H

(29.48)

836 Charged Particle and Photon Interactions with Matter

H (or D) H N (or D N) H or D NH (or ND) ( kJ mol )

2 2 2

o 1

+ → + = −

−

( )

29∆H (29.49)

H (or D) H N (or D N) H or D NH (or ND ) ( kJ mol )

3 3 2 2 2

+ → + = +

−

( )

17∆

H (29.50)

In our previous work (Hiraoka et al., 1995), the reaction of H atoms with N atoms trapped in an N

matrix was studied. Ammonia was formed but its yield was found to be very low. Besides, only N

and H atoms were observed but no radicals such as NH and NH

could be detected by the electron

spin resonance spectroscopy when the solid N

(100/5) was irradiated by x-ray at 4.2K, and the

sample temperature was increased up to 30K. The absence of NH and NH

in the electron spin

resonance

spectra suggests that sequential association reactions (29.44) and (29.51) are less likely.

H NH NH ( mol )

o 1

+ → = −

−

405∆H kJ

(29.51)

The absence of NH

may be due to the inefcient association reactions (29.44) and (29.51) and/or the

favorable H-atom abstraction reactions (29.48) and (29.49). These predictions are not very unreason-

able because the highly exothermic reactions (29.44) and (29.51) must experience appreciable centrifu-

gal barriers when two reactants come close to each other. In contrast, abstraction reactions (29.48) and

(29.49) are less exothermic and may proceed more efciently because the heats of reactions can be

carried away as kinetic energies of two separating products. Actually, the energy dissipation plays a

crucial role in the solid-phase low-temperature reactions. Tielens (1989) pointed out that heat transfer

in the amorphous materials changes abruptly in character at about 25K. Below that temperature, the

energy is carried by low-frequency phonons (<10

−12

s), which have a long mean free path (∼10

Å). With

increasing temperature, the heat is carried by high-frequency phonons with a mean free path of about

10Å. Thus, the energy relaxation becomes increasingly less favorable at lower temperatures. This may

result in the less exothermic H-atom abstraction reactions being more favorable than the more exother-

mic association reactions at lower temperatures. Besides, the conservation of angular momentum for

reactions (29.48) and (29.49) is much more easily realized than that for reactions (29.44) and (29.51).

In the separate experiment, using the apparatus shown in Figure 29.13, OCN

−

, CO, CO

, and

were formed as major reaction products, but no NH

could be detected by FT-IR when the

mixed CH

O/N

(1/1/9) lm was irradiated by 100eV electrons at 10K. In order to obtain more

direct information on the formation of NH

in the solid-phase reactions, the H atoms and N atoms

were sprayed simultaneously over the metal substrate at 10K. The H and N atoms were generated

by owing the H

and N

gases through two discharge tubes, respectively, as shown in Figure 29.1.

Surprisingly, no NH

could be detected by FT-IR though a strong absorption due to N

radicals

could be detected in the deposited sample. These results again indicate that the consecutive hydro-

genation

reactions, N → NH → NH

→ NH

, are quite inefcient in the solid phase.

In low-temperature solid-phase reactions, the reactants basically interact with each other by the

remote-encounter fashion because the close encounter is largely prohibited by the presence of matrix

molecules. In such a case, the H-atom tunneling reactions such as reactions (29.48) and (29.49) may

become more favorable than reactions (29.44) and (29.51). As was described, the deuterated ethane

6−n

with n up to 6 were formed. This clearly indicates the occurrence of the H-atom abstrac-

tion reaction, ethyl radical + D → ethylene + HD (or D

). This result is only an example demonstrat-

ing that H-atom abstraction reactions play important roles in solid–phase reactions. Such H-atom

abstraction reactions taking place on the dust grains may be one possible source for the formation of

hydrogen

molecules in the universe as was already pointed out by Tielens (1989).

The

less efcient NH

formation in solid-phase reactions underlines the importance of the gas-

phase synthesis of NH

in ISM. Herbst and Klemperer (1973) proposed the consecutive ion–molecule

reactions of

(n = 0 → 3) with H

to form the nal product ion

, followed by the recombina-

tion of

with an electron to form NH

. The bottleneck for these consecutive reactions is that with n

=3, that is,

NH H NH H

3 4

+ +

+ → +

, the rate constant was measured to be 1.8 × 10

−13

molecule

−1

Chemical Evolution onInterstellar Grains atLow Temperatures 837

at 85K (Adams and Smith, 1984).This value is about four orders of magnitude smaller than the col-

lision rate. The gas-phase ion–molecule routes for the formation of

may be reconciled by the

alternative gas-phase reactions of

with ubiquitous H

O and CH

NH H O NH OH

2 4

+ → + (29.52)

NH CH NH CH

4 4

+ → + (29.53)

The rate constants for reactions (29.52) and (29.53) have been measured to be 4 × 10

−10

and 5 × 10

−10

molecule

−1

, respectively (Ikezoe et al., 1987), which are close to the collision rates. We conjecture that

O and CH

play an important role as reactants not only in the solid phase but also in the gas phase.

29.9 ConClusion

Because of their high mobility in solid, H atoms play an important role in the chemical evolution

of dense clouds at 10K via tunneling processes. In all the reactions investigated, the product yields

increased drastically with decrease of temperature from 50 to 10 K. This nding led us to con-

clude that the chemical evolution, taking place on the dust grains via H-atom tunneling reactions,

becomes efcient only at cryogenic temperatures. If the temperature of the molecular clouds is

higher than 10K, the rate of the chemical evolution would become slower due to the short residence

time

of H atoms on the dust grains (Hiraoka et al., 2001b).

the reactions of D with solid C

and C

, the fully deuterated product, C

, is formed.

This indicates the occurrence of the solid-phase abstraction reactions of D atoms with C

and

to form C

HD and C

D, respectively. In the reaction of D with H

CO, H-atom abstraction

reaction D + H

CO → HD + HCO is much more favored than the addition reaction, D + H

CO →

DCO (or CH

OD). In the reaction of H with N atoms seeded in the solid matrix, no NH

was

detected as a reaction product. This indicates that the major processes for the reaction of H with

with n = 1 and 2 are the H-atom abstraction reactions, H + NH

→ NH

n−1

+ H

. As such, the

abstraction reactions are found to be one of the major processes in the solid-phase reactions of H

with relevant molecules. The favorable abstraction reactions may be due to the efcient dissipation

of internal energies of the intermediate products as kinetic energies of the separating products and

the

presence of large channels for the conservation of angular momentum of the reaction system.

While

the consecutive hydrogenation reactions CO → HCO → H

CO → CH

CO(CH

COH) →

OH are less likely to explain the ubiquitous methanol in the interstellar medium, it was found

that methanol is the major product from the electron-irradiated water ice containing carbon source

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