Release of hydrogen molecules from the photodissociation of amorphous solid water
and polycrystalline ice at 157 and 193 nm
Akihiro Yabushita, Tetsuya Hama, Daisuke Iida, Noboru Kawanaka, Masahiro Kawasaki, Naoki Watanabe,
Michael N. R. Ashfold, and Hans-Peter Loock
Citation: The Journal of Chemical Physics 129, 044501 (2008); doi: 10.1063/1.2953714
View online: http://dx.doi.org/10.1063/1.2953714
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/129/4?ver=pdfcov
Published by the AIP Publishing
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THE JOURNAL OF CHEMICAL PHYSICS 129, 044501 共2008兲
Release of hydrogen molecules from the photodissociation of amorphous
solid water and polycrystalline ice at 157 and 193 nm
Akihiro Yabushita,1 Tetsuya Hama,1 Daisuke Iida,1 Noboru Kawanaka,1
Masahiro Kawasaki,1,a兲 Naoki Watanabe,2 Michael N. R. Ashfold,3 and Hans-Peter Loock4
1
Department of Molecular Engineering, Kyoto University, Kyoto 615-8510, Japan
Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan
3
School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom
4
Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
2
共Received 29 January 2008; accepted 10 June 2008; published online 22 July 2008兲
The production of H2 in highly excited vibrational and rotational states 共v = 0 – 5, J = 0 – 17兲 from the
157 nm photodissociation of amorphous solid water ice films at 100 K was observed directly using
resonance-enhanced multiphoton ionization. Weaker signals from H2共v = 2 , 3 and 4兲 were obtained
from 157 nm photolysis of polycrystalline ice, but H2共v = 0 and 1兲 populations in this case were
below the detection limit. The H2 products show two distinct formation mechanisms. Endothermic
abstraction of a hydrogen atom from H2O by a photolytically produced H atom yields vibrationally
cold H2 products, whereas exothermic recombination of two H-atom photoproducts yields H2
molecules with a highly excited vibrational distribution and non-Boltzmann rotational population
distributions as has been predicted previously by both quantum-mechanical and molecular dynamics
calculations. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2953714兴
I. INTRODUCTION
Hydrogen is the most abundant molecule in the interstellar medium. To account for this abundance, astronomers
have proposed that H2 forms by reaction on the surface of
the interstellar dust grains present in this region.1–3 Dust
grains in molecular clouds at temperatures below ⬃130 K
are covered with an amorphous water ice mantle. Photolysis
of water ice thus plays an important role in grain chemistry.
One process arising during ice photolysis is photodissociation of water molecules and the subsequent formation of molecular hydrogen.
When water ice is exposed to vacuum-ultraviolet 共vuv兲
radiation, the H–O bond breaks. This process is a photolytic
source of hot hydrogen atoms at the surfaces of comets and
dust grains in the interstellar medium.1 As shown schematically in Fig. 1, translationally and internally hot H2 molecules can arise from hot H atoms formed in the photolysis
of amorphous solid water 共ASW兲 by two distinct mechanisms: hydrogen abstraction 关HAB, reaction 共1兲兴, an endothermic process whereby a photolytically produced H atom
abstracts a H atom from H2O to yield vibrationally cold H2
products, and hydrogen recombination 关HR, reaction 共2兲兴, a
highly exothermic process where two H-atom photoproducts
combine to yield H2 molecules with a highly excited vibrational distribution,
HAB:
HR:
H + HOH → H2 + OH,
H + H → H 2,
⌬H = 0.6 eV,
⌬H = − 4.5 eV.
共1兲
共2兲
There have been several experiments on the formation of H2
molecules by photolysis of amorphous water ice.4–6 The
a兲
Author to whom correspondence should be addressed. Electronic mail:
[email protected].
0021-9606/2008/129共4兲/044501/8/$23.00
studies of Westley et al.5 and Watanabe et al.6 both led to
estimates of H2 formation efficiencies, but did not reveal
details such as the reaction mechanism共s兲 or the energy partitioning in the reaction products. The quantum-state distributions of H2 produced during electron beam excitation of
ASW were reported by Kimmel et al.7 In this study the formation of hydrogen also involved a third mechanism, i.e. the
molecular elimination step H2 + O. When H2 is produced, the
excess energies resulting from the enthalpy of reaction,
which varies with reaction channel, are partitioned into internal and kinetic energies of the H2 products and into the surface. The latter may induce desorption of molecules existing
in the vicinity of the reaction site. H2 product kinetic energy
would contribute to interstellar heating. The rovibrational energy of the nascent H2 products will affect their subsequent
gas phase reactivity. For example, the availability of vibrationally excited H2 from vuv photodissociation of ice could
lead to a significant acceleration of oxygen chemistry in the
FIG. 1. 共Color online兲 Schematic illustrations of the H-atom abstraction
共HAB兲 and H-atom recombination 共HR兲 mechanisms for forming H2 during
vuv irradiation of ASW. White balls represent photolytically produced H
atoms.
129, 044501-1
© 2008 American Institute of Physics
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044501-2
J. Chem. Phys. 129, 044501 共2008兲
Yabushita et al.
interstellar medium.8 Thus the energy partitioning accompanying H2 formation is crucial in understanding chemical evolution. The ortho/para ratio of H2, gOPR, is another important
parameter since the gOPR of H2 is considered a useful indicator for the physical and chemical history of H2 in interstellar clouds.
The purpose of the present study is to reveal details of
the H2 formation mechanism共s兲 during vuv photolysis of water ice at 100 K and to report the first simultaneous measurements of the kinetic and rovibrational energy distributions of
H2 products formed as a result of vuv photoirradiation of
water ice.
II. EXPERIMENTAL
A. Time-of-flight „TOF… apparatus and preparation of
ice films
The experimental apparatus used in the present study has
been described elsewhere.9 A vacuum chamber was evacuated to a base pressure of 10−8 Torr using two turbomolecular pumps in tandem 共Mitsubishi Heavy Industry, 800
and 50 l s−1兲. An optically flat sapphire substrate, sputter
coated with a thin film of Au, was supported in the center of
the chamber by a liquid-nitrogen-cooled manipulator connected to an X-Y-Z stage.10 The substrate temperature can be
controlled in the range of 130– 700 K, with cooling provided
by liquid nitrogen, heating from a 0.35 mm diameter tantalum filament attached to the substrate, and a controller composed of an alumel-chromel resistance thermometer. To prepare the ice film, H2O vapor was injected through a pulsed
nozzle 共General Valve兲 at a rate of 10 Hz and an incidence
angle of 45°, for 60 min onto the cooled substrate at 100 K.
This procedure resulted in a low-density ASW film. Polycrystalline ice 共PCI兲 films were prepared by deposition of
background water vapor at 100 K for 60 min, and then maintained at 130 K for 30 min for annealing purposes. The total
exposure of H2O was typically 1800 L 共1 L = 1
⫻ 10−6 Torr s兲, which resulted in the formation of ice films
to a depth of ⬃600 ML on the Au substrate.11 UV photoirradiation and the subsequent multiphoton ionization detection of gas phase H2 molecules 共and H atoms兲 were performed with the substrate temperature maintained at 100 K.
To photolyze water ice, unfocused 157 nm 共or 193 nm兲
laser radiation was directed onto the ice surface at an angle
of either ⬃80° or 45° to the surface normal. The incident
fluence F was typically ⬍0.1 mJ cm−2 pulse−1 at 157 nm, in
a 15 ns pulse duration, corresponding to an incident intensity
of ⬃7 kW cm−2. The corresponding values at 193 nm were
F ⬃ 0.5 mJ cm−2 pulse−1 共⬃30 kW cm−2兲. Multiphoton processes at such low incident intensities are considered very
unlikely, and we found no evidence for formation of any
atomic or molecular fragments with total energies ⬎7.8 eV
共at 157 nm兲. H2 products were subsequently ionized at a distance r = 2 mm from the substrate surface by 共2 + 1兲
resonance-enhanced multiphoton ionization 共REMPI兲 on the
H2 E / F 1⌺+g ← X 1⌺+g 共0 , v⬙兲 transition and collected with a
small mass spectrometer aligned perpendicular to the ice surface. Radiation at the requisite wavelengths 共201– 245 nm兲
was produced by a Nd3+ : YAG pumped dye laser, with sub-
sequent frequency doubling and mixing. The delay t between
the photolysis and REMPI laser pulses was varied with a
delay generator to allow investigation of the flight times of
the H2 products. The measured TOF spectra were fitted with
one or more flux-weighted Maxwell–Boltzmann 共MB兲 distributions defined by a translational temperature Ttrans. To obtain the H2 rovibrational populations, the raw REMPI signals
measured at a constant laser power were corrected by the
respective line strength factors.12 All rotational data for
H2共v = 0 – 4兲 were consistent only with the statistical gOPR
= 3. Use of gOPR = 1.5 关as would be appropriate for H2 molecules in thermal equilibrium at 100 K 共Ref. 13兲兴 resulted in
ragged rotational distributions that cannot be described by
smooth functions.
B. Simulation of H2 TOF spectra
The TOF spectrum, S共ai , t , Ti兲, was fitted with a sum of
the flux-weighted MB distributions, SMB共t , r , Ttrans兲, defined
by the translational temperatures Ti. r is the distance traveled
by the H2 photofragment between creation and ionization,
S共ai,t,Ti兲 = a1SMB共t,T1兲 + a2SMB共t,T2兲 + a3SMB共t,T3兲,
共3兲
SMB共t,r,Ttrans兲 = r3t−4 exp关− mr2/共2kBTtranst2兲兴,
共4兲
PMB共Et,Ttrans兲 = 共kBTtrans兲−2Et exp关− Et/共kBTtrans兲兴.
共5兲
The MB distribution in the energy frame, PMB共Et , Ttrans兲, is
characterized by an average translational energy 具Et典
= 2kBTtrans, where kB is the Boltzmann constant. Conversion
between the TOF and energy distributions employed the
Jacobian given by Zimmerman and Ho.14 Since the surface
morphologies of the ice films studied in these experiments
were polycrystalline 共for PCI兲 or amorphous 共for ASW兲, the
H2 product angular distributions were all assumed to be isotropic. The reliability of the conversion procedure was confirmed experimentally by changing r from 2 to 5 mm with
the aid of the X-Y-Z stage. Details of the TOF spectral simulation have been described previously.9
III. RESULTS
A. Photodissociation of ASW at 157 nm
H2 photoproducts from photodissociation of ASW at ␭
= 157 nm were detected in vibrational levels v = 0 – 5 and in
rotational levels as high as J = 17 共Figs. 2 and 3兲. Results for
v = 1 were similar to those for v = 0, while results for v = 2
and 4 were almost identical to those for v = 3. In what follows, we focus on describing the state distributions for products with v = 0 and 3 since these clearly illustrate two different formation mechanisms.
H2共v = 0兲 photoproducts were observed in rotational
states with J 艋 5 as shown in Fig. 2. The REMPI signal intensities for J = 2 – 5 products were much weaker than those
for J = 0 and 1. The TOF spectrum for H2共v = 0 , J = 0兲 is well
reproduced by a MB distribution with Ttrans = 100⫾ 10 K,
whereas that of the H2共v = 0 , J = 3兲 products is described by
Ttrans = 1500 K 共Fig. 4, upper panel兲. Figure 2 shows Boltz-
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044501-3
H2 from the photodissociation of ice
FIG. 2. 共Color online兲 Left panel: 2 + 1 REMPI spectra of the E / F ← X共v⬘
= 0 , v⬙ = 0兲 transition of H2 following 157 nm photolysis of ASW at 100 K.
The upper and lower traces are spectra recorded at t = 3 ␮s 共slow H2兲 and
0.55 ␮s 共fast H2兲, respectively. Rotational quantum numbers J⬙ are shown
above selected peaks. The inset shows the corresponding E / F ← X共v⬘
= 0 , v⬙ = 1兲 REMPI transition measured at t = 3 ␮s. Right panel: H2共␯ = 0兲
product rotational distributions. The solid lines are best fits to Boltzmann
distributions, yielding rotational temperatures Trot = 100 and 1300 K, respectively, for the slow 共filled circles兲 and fast 共open circles兲 TOF components.
mann plots for the rotational distributions measured for the
slow and fast TOF components, with the relevant peak intensities 共and thus relative populations兲 taken from the excitation spectra. A linear fit to the slow H2共J = 0 – 2兲 data yields
Trot = 100⫾ 10 K, with gOPR = 3. The rotational state populations for fast H2共J = 1 – 5兲 products also require gOPR = 3 and
are fitted well by Trot = 1300 K. Thus the slow H2 products
appear to be thermally equilibrated with the ice film temperature 共Ttrans = Trot = 100 K兲, while the fast H2 products are not.
The average H2共v = 0兲 photoproduct energies are determined
from the present data as 具Erot典 = 13 meV and 具Etrans典
= 28 meV.
H2共v 艌 2兲 products were detected in a much wider range
of rotational levels, up to J = 17 in the case of v = 3 as illustrated in Fig. 3. The lower panel of Fig. 4 shows the measured TOF spectrum of H2共v = 3 , J = 3兲 products, which can
be fitted to a sum of two MB distributions with Ttrans
= 110⫾ 10 and 1800⫾ 300 K. The relative contribution from
the fast H2 products declines with increasing J. Figure 3
shows Boltzmann plots for the rotational distributions measured for the slow and fast TOF components again with
FIG. 3. 共Color online兲 Left panels: 2 + 1 REMPI spectra of the E / F
← X共v⬘ = 0 , v⬙ = 2 , 3 , 4兲 transitions of H2 following 157 nm photolysis of
ASW at 100 K. The upper and lower traces in each case show spectra
recorded at t = 3 ␮s 共slow H2兲 and t = 0.55 ␮s 共fast H2兲, respectively. Right
panels: H2共v , J兲 product rotational distributions 共plotted in Boltzmann form兲
for the slow 共filled circle兲 and fast components 共open circle兲. The superposed curves in these plots are intended simply to guide the eye.
J. Chem. Phys. 129, 044501 共2008兲
FIG. 4. 共Color online兲 TOF spectra of 共a兲 H2共v = 0 , J = 0兲, 共b兲 H2共v = 0 , J
= 3兲, 共c兲 H2共v = 3 , J = 3兲, and 共d兲 H2共v = 3 , J = 13兲 products from 157 nm photolysis of ASW at 100 K. The solid curves are best fits to MB distributions
with Ttrans = 110 and 1800 K.
gOPR = 3. The rotational distribution of the slow H2 products
with low J共艋4兲 is cold and not described well by a Boltzmann temperature. However, the higher J component 共6 艋 J
艋 12兲 of the slow H2 products fits reasonably to Trot
⬃ 16 000 K. Fast H2 products with J = 0 – 6 and 8–13 are
associated with Trot = 1800 and ⬃6000 K, respectively. All
data are consistent with the statistical gOPR = 3, but not with
gOPR = 1.5. The average energies derived for the H2共v = 3兲
products are Evib = 1480 meV, 具Erot典 = 540 meV, and 具Etrans典
= 28 meV.
TOF spectra of H2共v = 1 , 2 , 4 , and 5兲 products were also
measured; the comparative weakness of H2共v = 1 and v = 2兲
and H2共v = 4 and v = 5兲 is a reflection both, the H2 product
vibrational distribution and the relative REMPI line strength
factors, respectively.12 The translational and rotational distributions of are comparable to those observed for v = 0, while
those for v = 2 and 4 products appear similar to those for v
= 3. REMPI spectra for the E / F ← X 共0,2兲 and 共0,4兲 transitions following photolysis of ASW at ␭ = 157 nm are also
included in Fig. 3, along with the derived rotational energy
distributions of the fast and slow H2共v = 2 and 4兲 products.
H2共v = 5兲 signals are also shown for J = 1 and 3 in Fig. 3.
The H2 vibrational population distribution was estimated
as follows. The TOF spectrum of products in each of the
levels was measured for v = 2 – 4, J = 1 – 13 and integrated to
obtain the total intensities associated with the v, J level. The
measured signal intensity was then calibrated using the reported intensity factor,12 and the resulting ratios then normalized to the v = 3 population, as shown in Table I. The relative
integrated intensities for odd rotational levels 共J = 1 – 13兲 are
0.58/ 1 / 1.34 for v = 2 / 3 / 4. The v = 4 level is more populated
for J 艋 5, while the v = 2 level is less populated—as illustrated in Fig. 5. This may indicate that the measured population distributions in the most highly internally excited levels are closest to the true nascent distributions, whereas the
less highly internally excited levels contain some contributions from population that has already experienced some relaxation. The vibrational population distributions derived
here are in at least qualitative agreement with the results of a
recent molecular dynamics 共MD兲 simulation of H2 formation
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J. Chem. Phys. 129, 044501 共2008兲
Yabushita et al.
TABLE I. Vibrational population distributions of H2 products formed in the
157 nm photolysis of ASW at T = 100 K. The tabulated data show the relative populations of products with v = 2, 3, and 4, and their variation with
rotational quantum number J.
J
v = 2a
v = 3a
v = 4a
1
3
5
7
9
11
13
0.9⫾ 0.1
0.6⫾ 0.1
0.4⫾ 0.1
0.7⫾ 0.1
0.7⫾ 0.1
0.4⫾ 0.1
0.5⫾ 0.1
1
1
1
1
1
1
1
0.9⫾ 0.2
1.1⫾ 0.2
1.6⫾ 0.2
1.7⫾ 0.2
1.6⫾ 0.2
1.1⫾ 0.2
1.1⫾ 0.2
The relative integrated intensities for the odd rotational levels 共J = 1 – 13兲
are 0.58/ 1 / 1.34 for v = 2 / 3 / 4.
a
by the HR mechanism on an ASW surface at 70 K, which
predicted the formation of translationally excited H2 molecules in a broad range of vibrational states with v 艌 3 and
peaking at v = 9.15
B. Photodissociation of PCI at 157 nm
Weak signals from H2共v = 2 , 3 and 4兲 products were obtained following 157 nm photolysis of a PCI film, but spectra
corresponding to the H2共v = 0 and 1兲 populations were below
the detection limit. Figure 6 shows the TOF spectrum of
H2共v = 3 , J = 1兲 products from 157 nm photolysis of PCI at
100 K, which can be fitted well as a sum of three MB distributions, with Ttrans = 110, 500, and 1800 K.
C. Photodissociation of ASW at 193 nm
Figure 7 shows the TOF spectrum of H2共v = 0 , J = 1兲
products from 193 nm photolysis of ASW at 100 K, which is
dominated by a component with Ttrans = 100 K. No H2共v = 3兲
signals were observed from photolysis of ASW at this wavelength.
IV. DISCUSSION
By way of background, it is worth first summarizing the
findings of previous studies16 of H atoms formed following
157 and 193 nm photodissociation of ASW and PCI. The
H-atom TOF spectrum from 157 nm photolysis of PCI was
reproduced by a combination of three MB energy distributions with Ttrans = 4750, 625, and 110 K, with relative weightings of 41%, 44% and 15%, respectively. The Ttrans
= 4750 K component was attributed to H atoms desorbed
from the ice surface, while the other components are associated with H atoms originating from deeper into the bulk. The
significant yield of slow H atoms was attributed to the small
absorption coefficient 共and consequent large optical penetration depth, ⬃100 nm兲 at 157 nm.17 In contrast, the H-atom
TOF spectrum measured following 193 nm photolysis of PCI
is dominated by one component with Ttrans = 2400 K. These
fast H atoms are attributed to photodissociation of water species on the top layer of ice surface, which escape from the
flat PCI surface without suffering significant collisional relaxation.
FIG. 5. Rotational population distributions associated with different H2共v兲
from 157 nm photolysis of ASW at 100 K obtained from integrated TOF
spectra: v = 2 共䊏兲, 3 共䊊兲, 4 共쎲兲.
The TOF spectra of H atoms measured from both 157
and 193 nm photodissociation of ASW films are dominated
by a slow component 共Ttrans = 110 K兲, indicating that the majority of the H photofragments accommodate to the substrate
temperature of 100 K 共by collisions within the micropores兲
prior to escaping into the vacuum and subsequent REMPI
detection.
A. Mechanisms of H2 formation from the
photodissociation of water ice films
1. H2 from 157 nm photodissociation of ASW
Formation of H2 from photolysis of ASW is deduced to
involve two distinct mechanisms: either a photolytically produced H atom induces abstraction 共HAB兲 of a H atom from a
nearby H2O molecule via reaction 共1兲 or H-atom recombination 共HR兲 via reaction 共2兲. Before summarizing either
mechanism in any detail, it is worth reviewing the environment in which they occur. ASW presents a porous structure,
with high surface to volume ratio. Based on the absorption
coefficient, the optical penetration depth of a water ice film
at 157 nm is ⬃100 nm. Thus there is ample opportunity for
H-atom formation at depths far below the ice surfacevacuum interface. The maximum translational energy of a H
atom formed by 157 nm photodissociation of H2O is
2.75 eV, but many H atoms formed in the bulk will inevitably collide with surrounding H2O molecules and progres-
FIG. 6. 共Color online兲 TOF spectrum of H2共v = 3 , J = 1兲 products from
157 nm photolysis of PCI at 100 K. The curves are fits to the data derived
assuming MB distributions with Ttrans = 110, 500, and 1800 K.
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044501-5
H2 from the photodissociation of ice
sively thermalize before, or instead of, reacting. Andersson et
al.18 reported MD simulations of the ␭ ⬃ 165 nm photodissociation of ASW at 10 K. Desorption of H atoms was found to
dominate for molecules in the first bilayer but, by the second
bilayer, H-atom desorption and trapping, and H-atom recombination with OH, were found to be of comparable importance. The distribution of distances traveled by H atoms in
the ice was calculated to peak at ⬃6 Å, with a tail extending
to ⬃60 Å. These predictions are broadly consistent with
available experimental results for the 157 nm photolysis of
ASW at 100 K, e.g., the H-atom translational energy distribution is dominated by a MB component with
Ttrans = 110 K,16 consistent with their being formed at subsurface locations.
HAB mechanism. The rotational energy disposals and the
lack of vibrational excitation of the H2共v = 0兲 products are
consistent with their formation via the HAB mechanism, occurring at a range of depths beneath the vacuum ice interface. The activation energy for the HAB reaction is 1.0 eV.19
A simple kinematical hard-sphere model suggests that the
efficiency of translational energy transfer from H to a single
H2O molecule is 艋20% per collision.18 Thus the HAB
mechanism should still be energetically possible even after a
photolytically produced H atom has experienced a number of
subsurface collisions. The dynamics of this reaction favors
formation of vibrationally cold H2 products 共v = 0 and 1兲,20,21
but the extent of product rotational and translational excitation should be expected to depend on the depth at which
reaction occurs. The rough, porous surface and large surface
area of ASW exposes many surface OH groups to direct
attack by photoproduced H atoms 共Fig. 1兲. The HAB mechanism for such surface species can account for the translationally and rotationally excited H2共v = 0兲 products that constitute a minor channel compared to the thermalized, cold H2
photoproducts. The majority of cold H2共v = 0 , Trot = Ttrans
= 100 K兲 products are likely to originate from HAB reactions
occurring at greater depths, within micropores. The product
formed in these cases will progressively thermalize to the
bulk temperature as a result of collisions. The H2 products
percolate through the micropores and/or the extensive network of hydrogen bonded H2O molecules that constitute the
ASW surface, en route to the vacuum.
HR mechanism. The alternative mechanism depicted in
Fig. 1 involves a photolytically produced H-atom scattering
onto 共and combining with兲 a previously adsorbed hydrogen
atom, yielding H2. The large exothermicity in reaction 共2兲
means that H2 products formed by this route can have much
higher internal energies. Quantum-mechanical calculations
reported as long ago as 1970 predicted that the H2 products
of the H + H reaction on ice would be formed with substantial
translational and internal excitation,22 while more recent MD
simulations of HR on an ASW surface at 70 K predicted
formation of translationally excited H2 molecules in a broad
range of vibrational states with v 艌 3 and peaking at v = 9.15
In related experimental studies, Price and co-workers23–25
demonstrated the formation of vibrationally excited H2
共v 艋 4兲 关and HD 共v 艋 7兲兴 products from HR on a graphite
surface at 15 and 23 K and determined the H2 rotational
temperature as Trot = 309– 364 K.
J. Chem. Phys. 129, 044501 共2008兲
FIG. 7. TOF spectrum of H2共v = 0 , J = 1兲 products from 193 nm photolysis
of ASW. The solid curve is a fit to the data derived assuming MB distributions with Ttrans = 100.
As Fig. 3 showed, the H2共v 艌 2兲 products formed in the
present study also exhibit two distinct translational energy
distributions. The fast H2共v 艌 2 , Ttrans = 1800 K兲 products are
attributed to an Eley–Rideal-type HR mechanism, whereby a
photoproduced H atom recombines with a previously adsorbed 共and thus thermalized兲 H atom trapped close to the
ice surface-vacuum interface—as shown schematically in
Fig. 1. However, given the optical penetration depth at
157 nm, many more H atoms will be formed at sites beneath
the microporous ice surface26 and have to migrate some distance in the bulk before colliding with a H atom trapped at or
very close to the ASW surface. The key to forming fast H2
molecules is that they subsequently desorb directly into the
vacuum. The nonthermal vibrational energy distributions of
the observed H2 products reflect the dynamics of the HR
process. The considerable energy release can also be expected to heat the immediate environment of the reaction and
thereby lead to the simultaneous release of intact water molecules. The majority of the H2共v 艌 2兲 photoproducts are detected with near-thermal translational energies. These slow
H2共v 艌 2 , Ttrans = 110 K兲 products are most plausibly attributed to the same HR mechanism but occurring in micropores
deeper within the bulk; escape of these H2 products will
involve multiple collisions with the ASW surface as a result
of which their translational energies thermalize to that of the
bulk.
The present observations are in accord with the results of
Hornekær et al.,27 who found that the translational energy
distribution of the H2 products formed when dosing atomic
H onto a porous ASW film was almost thermalized to the
temperature of the surface. Both data sets imply that 共a兲 H2 is
produced by the HR mechanism in the micropores and in the
bulk phase very near the ice surface-vacuum interface and
共b兲 the translational energy of the H2 products formed subsurface is lost to the bulk before the H2 desorbs into the
vacuum. Micropores have been similarly implicated in the
electron-stimulated production of H2 from low temperature
ice.28
The measured rotational energy distributions of the
H2共v = 2 , 3 , 4兲 products are clearly nonthermal as shown in
Fig. 3. The underlying source of the rotational excitation
must be the exoergicity of the HR reaction and the range of
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044501-6
Yabushita et al.
possible impact parameters with which the photoproduced H
atom can approach its preadsorbed partner. A simple impact
parameter model indicates that a H2 molecule formed by a
glancing collision between one H atom with 1 eV kinetic
energy 共i.e., with velocity ⬃104 m s−1兲 and a preadsorbed H
atom at an impact parameter b = 1 Å would have J ⬃ 16. A
theoretical study reported that the Eley–Rideal-type HR
mechanism on a porous carbon particle surface is expected to
produce H2 products in highly excited rotational states, while
the maximum J level depends on the collision energy.29 The
persistence of this excitation, even in H2共v 艌 2兲 products that
have thermalized translationally, may be understood as follows: vibrational quenching of H2 is generally rather inefficient but can show interesting resonance effects30 that may
well contribute to the unusual rotational population distributions. The H2 rotational constant is large 共Be = 60.85 cm−1兲.
This ensures large energy separations between successive
high J levels of a given nuclear spin symmetry, e.g., the J
= 11 and 13 ortho-levels are separated by ⬃3000 cm−1. As a
result, the more closely spaced, low J levels collisionally
relax more efficiently than do the higher J levels. The
quenching rate of any given rovibrational state is determined
not only by its v and J quantum numbers but also depends on
the availability of quasiresonant vibration-rotation 共QRVR兲
relaxation pathways.30 The general propensity rule for QRVR
relaxation is Erot共⌬J = even兲 ⬃ Evib共⌬v兲. For H2共v 艌 1兲, the
efficiency of this QRVR pathway shows a local minimum in
the range 7 艋 J 艋 12, i.e., for the J levels we observe to be
highly populated in fast H2共v 艌 2兲. As a result, 共a兲 the higher
J levels retain significant population and 共b兲 the population
distribution among the J = 0 – 17 rotational states cannot be
described by a single temperature. MD simulations of HR on
an ASW surface at 70 K returned a translational energy of
1.3 eV 共7500 K兲 for H2共v = 3兲 products, i.e., a value that is
significantly higher than the value observed here, Ttrans
= 1800 K, even for just the fast H2共v = 3兲 component.15 The
theoretically calculated rotational energy 共0.52 eV兲 is comparable to the observed value in the present experiment,
具Erot典 = 0.54 eV.
The present rotational data analysis assumes gOPR = 3
throughout. All attempts to use a smaller gOPR value result in
unphysically ragged rotational state population distributions.
gOPR = 3 has been widely assumed elsewhere, e.g., in theoretical studies of the HR reaction on an ice surface15 and in
experimental studies of HR on the surface of porous carbon
grains.23–25 We rationalize the use of this same high temperature limiting value by noting that the HR reaction is exothermic by 4.5 eV and that the HAB is effectively exoergic 共by
⬃0.4 eV, because the activation energy for the HAB reaction
is 1.0 eV19兲 once past the transition state. In both mechanisms, therefore, any excess energy released to the bulk
would result in localized heating of the reaction site and
enable gOPR to adopt its high temperature limiting value.13
2. H2 from 193 nm photodissociation of ASW
The observation of H2共v = 0兲, but not H2共v = 3兲, product
formation following photolysis of ASW films at ␭ = 193 nm
lends further support to the above mechanistic interpreta-
J. Chem. Phys. 129, 044501 共2008兲
tions. In our previous TOF measurements of the H atoms
from the 193 nm photolysis of PCI at 100 K, only the high
translational energy component was observed.31 This fact
suggests that the source of the fast H atoms is the photodissociation of a dimerlike structured surface water molecule
that exists only at the very top layer of the ice surface, not
available in the bulk. On the other hand, the ice bulk has
strong photoabsorption at 157 nm while it is transparent at
193 nm, and as a consequence we observe a considerable
fraction of H atom formed subsurface when photolyzing PCI
and ASW at 157 nm.16 Both behaviors are consistent with
our present experimental observations. There is thus far less
opportunity for trapping H atoms formed in the photolysis
step at 193 nm. Photoproduced H atoms from ASW may
form H2共v = 0兲 products via the HAB mechanism, as at
157 nm, but otherwise will desorb as H atoms with Ttrans
values close to that of the substrate. Given the low density of
trapped H atoms, HR is unlikely—consistent with the absence of H2共v = 3兲 products at 193 nm.
3. H2 from 157 nm photodissociation of PCI
In contrast to ASW, PCI presents a much smoother surface and smaller surface area. Zheng et al.32 reported D2
formation following irradiation of a D2O ice film at 12 K
with 5 keV electrons and subsequent heating to 293 K. D2
formation rates were found to be consistently higher from
ASW samples than from PCI films. This observation was
attributed to the higher recombination rate of D atoms that
diffuse into voids within the pores of an ASW film. D atoms
from the surface layer were assumed to escape into the voids
and then diffuse around the boundary of the cavity until encountering a second D atom, forming D2. A polycrystalline
D2O ice film, in contrast, has a much denser structure. The
absence of micropores greatly reduces the chance of trapping
D atoms and increases the relative importance of D + OD
surface recombination reactions.32
The present observation that the H2共v = 3兲 REMPI signals following 157 nm photoexcitation of PCI are significantly weaker than those from ASW is fully consistent with
FIG. 8. 共Color online兲 Schematic illustrations of HR reaction pathways for
forming H2 following vuv irradiation of PCI. Vibrationally excited H2 products are formed by HR events occurring within a few bilayers of the surface
and diffuse through 共and are translationally relaxed by collisions with兲 the
walls of channels within the PCI crystal structure. White balls represent
photolytically produced H atoms.
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044501-7
J. Chem. Phys. 129, 044501 共2008兲
H2 from the photodissociation of ice
the above picture. The H2共v 艌 2兲 Ttrans = 1800 and 110 K
components are attributed to H2 molecules formed via the
HR mechanism at, and just below, the surface, respectively,
as illustrated schematically in Fig. 8. The latter diffuse
through, and are translationally relaxed by collisions with the
walls of, channels within the PCI crystal structure. The
H2共v 艌 2兲 Ttrans = 500 K component appears to be unique to
PCI. We speculate that H2 molecules formed 共and partially
accommodated兲 at grain boundaries in the PCI structure may
contribute to both this and the Ttrans = 110 K component. As
in the case of ASW, the relative contribution of the fast
H2共v 艌 2兲 products from 157 nm photolysis of PCI is found
to decline with increasing J. Negligible H2共v = 0兲 signal was
collision
collision
detected, indicating a minimal role for the HAB mechanism
in the case of PCI—consistent with the foregoing discussion
of this mechanism.
4. Summary of H2 formation mechanisms
The reaction mechanisms outlined above and illustrated
schematically in Figs. 1 and 8 may be summarized as follows. H atoms are produced by
H2O + vuv共␭ = 157 nm兲 → OH + H**共2.75 eV兲,
where H** represents a highly translationally excited H-atom
photoproduct. Collisions with the bulk lead to a progressive
reduction in the H-atom kinetic energies,
collision
H**共2.75 eV兲 ——→ H**共⬎1 eV兲 ——→ H*共⬍1 eV兲 ——→ H共trapped兲.
The HAB reaction is calculated to have a potential barrier of ⬃1 eV;19 only the H** species are thus able to react according
to
H** + HOH共interfacial兲 → H2*共v = 0, high Ttrans and Trot兲
共minor兲,
H** + HOH共on micropore surface兲 → H2* → H2共v = 0 and 1, low Ttrans and Trot兲
共major兲.
The HR reaction, in contrast, is barrierless and can thus involve H* species as follows:
H*/H** + H共trapped near interface兲 → fast H2共v 艌 2兲
共minor兲,
H*/H** + H共trapped in micropores and bulk phase兲 → slow H2共v 艌 2兲
Before concluding this section it is prudent to consider possible alternative routes to forming H2 products in the 157 nm
photolysis of water ice. A 157 nm photon provides sufficient
energy to form the molecular elimination products O共 1D兲
+ H2, which could be a source of H2共v 艋 1兲. Early photodissociation studies of gas phase H2O molecules concluded that
the quantum yield, ⌽O1D, for forming O共 1D兲 atoms at photon
energies 艋8.5 eV is ⬍0.1%,33 but a recent theoretical study
of H2O dissociation as a function of photon energy
共艌8.5 eV兲 suggests ⌽O1D ⬃ 0.05 and that the H2 products
will be rotationally excited.34 Molecular hydrogen elimination has been reported following electron beam excitation of
water ice,7,35,36 although the reported yield at 7.8 eV
共157 nm兲 was small H2共v = 0 , 1兲 produced via the dissociation process of H2+O in the electron beam irradiation on
water ice at 80 K showed gOPR=3. These products were variously attributed to the decay of a single-centered exciton
and/or to ion-molecule reactions triggered by excitation to
the conduction band of water ice. The 157 nm photons in the
present work could also excite the conduction band edge.37
However, the consensus view is that molecular elimination
yielding O共 1D兲 and O共 3 P兲 atoms in the condensed phase
have much lower probability than the homolytic dissociation
channels 共1兲 and 共2兲,38 and our nonobservation of H2共v 艋 1兲
products from 157 nm photolysis of PCI leads us to conclude
共major兲.
that such channels make 共at most兲 a minor contribution to the
total H2 yield observed in the present experiments.
B. Astrophysical implications
The photon energy at 157 nm is just above the threshold
for the first electronic excitation band of water ice. Excitation leads to O–H bond fission and the formation of H and
OH photofragments. The Lyman-␣ wavelength 共121.6 nm兲 is
the most intense wavelength of interstellar relevance. Since it
falls within the same absorption band of water ice, it is
tempting to expect similarities between the reactions induced
by absorption of 157 nm and Lyman-␣ photons. One point to
note is that a comparatively high concentration of H atoms
needs to be present after each vuv laser pulse in order to
enable HR reactions. We have shown previously that most of
the H atoms from 157 nm photolysis of ASW are thermalized at the ice temperature of 100 K, i.e., that they are produced in the bulk and undergo collisions with the ice bulk
water molecules prior to escaping.16 Similarly the present
TOF spectra of thermalized H2 molecules imply that the HR
mechanism contributes in the bulk phase. Surface morphology will obviously be important—H atoms may be trapped
more effectively in pores and cracks, and vibrational relaxation of H2共v兲 has been suggested to occur in the defects.6
The likely importance of micropores was also shown in the
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2016 22:53:33
044501-8
electron stimulated production of H2 in low temperature
ice.28 Since the residence time of H atoms in water ice at
100 K could be in the order of microseconds, the H-atom
concentration in the present experiments decays completely
between laser pulses. Nevertheless it is high enough during,
and immediately after each 20 ns laser pulse for the HR reaction to occur at the ice surface-vacuum interface, which
resulted in the production of translationally and vibrationally
hot H2 with a relatively small yield.
Hence, even under continuous irradiation with a weaker
light source, if the ice temperature is low enough for H atoms to be trapped, hot H2 production via the HR mechanism
may be anticipated. The photolytic formation of hot H2 molecules in the interstellar medium could thus be appreciable.
Based on the present measurements, we predict that direct
photolysis of ice particles will yield odd hydrogen species
and hot H2 molecules in the gas phase. Reaction of the latter
with O共 3 P兲 atoms could have an appreciable effect on the
oxygen chemistry in the interstellar medium since the rate
constant for the O共 3 P兲 + H2共v = 3兲 reaction at 100 K is predicted to be ⬃11 orders of magnitude larger than that for
reaction with H2共v = 0兲 molecules.8
The ortho/para ratio of H2 is frequently used as an indicator for the physical and chemical history of H2 formation.
The equilibrium gOPR for H2 at low temperatures is low and
approaches the statistical limit value, 3, at high
temperatures.13 The spin temperature is often found to be
different from translational and rovibrational temperatures.
Ortho-para conversion can occur by spin exchange reactions
with protons and ions, but the conversion time scale in space
is comparable to the lifetime of typical molecular clouds. In
all REMPI spectra of H2 in this study gOPR = 3, corresponding
to the statistical, high temperature limit. We argue that
gOPR = 3 should be considered the norm for H2 molecules
produced by vuv photolysis of water ice—by both the HR
and HAB mechanisms.
ACKNOWLEDGMENTS
The collaboration between Kyoto, Bristol, and Kingston
benefited from support from both the Daiwa Anglo-Japanese
Foundation and the Japan Society for Promotion of Sciences.
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