Direct Determination of the Activation Energy for Diffusion of OH Radicals on Water Ice A. Miyazaki1 M. Tsuge1 H. Hidaka1 Y. Nakai2 N. Watanabe1

2025-05-03 0 0 949.32KB 21 页 10玖币

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Direct Determination of the Activation Energy for Diffusion of OH Radicals on Water

Ice

A. Miyazaki1, M. Tsuge1, H. Hidaka1, Y. Nakai2, N. Watanabe1

1Institute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido 060-

0819, Japan; watanabe@lowtem.hokudai.ac.jp

2Radiative Isotope Physics Laboratory, RIKEN Nishina Center, Saitama 351-0198,

Japan

Abstract

Using a combination of photostimulated desorption and resonance-enhanced

multiphoton ionization methods, the behaviors of OH radicals on the surface of

interstellar ice analog was monitored at temperatures between 54 and 80 K. The OH

number density on the surface of ultraviolet (UV) -irradiated compact amorphous solid

water gradually decreased at temperatures above 60 K. Analyzing the temperature

dependence of OH intensities with the Arrhenius equation, the decrease can be explained

by recombination of two OH radicals, which is rate-limited by thermal diffusion of OH.

The activation energy for surface diffusion was experimentally determined for the first

time to be 0.14 ± 0.01 eV, which is larger than or equivalent to those assumed in

theoretical models. This value implies that the diffusive reaction of OH radicals starts

to be activated at approximately 36 K on interstellar ice.

1. Introduction

Physicochemical processes like surface reactions, UV photolysis, and ion

bombardment on interstellar ice are indispensable for promoting chemical evolution in

the early stage of star formation. Chemical evolution on interstellar ice can be activated

in a dense core of molecular clouds at temperatures as low as 10 K even without external

energy inputs such as ultraviolet (UV) photons (as a review, e.g. Watanabe & Kouchi

2008; Hama & Watanabe 2013). In the first step, the hydrogenation of primordial atomic

and molecular species, including H2 formation, has an important role because hydrogen

atoms, as light particles, can migrate and encounter the reaction partners on the dust

surface even at ~10 K. Notably, molecules abundantly observed on ice, such as H2O and

CH3OH, have been theoretically proposed to form by the hydrogenation of oxygen and

CO molecules (e.g., d’Hendecourt et al. 1985; Hasegawa et al. 1992; Cuppen & Herbst

2007; Charnley et al. 1997). Following these theoretical predictions, the formation of H2O,

NH3, H2CO, and CH3OH on the ice surface was confirmed at approximately 10 K in

laboratory experiments where O, O2, N, and CO reacted with H or H2 (e.g., Dulieu et al.

2010; Miyauchi et al. 2008; Ioppolo et al. 2008; Hidaka et al. 2011; Watanabe & Kouchi

2002). When the temperature of ice is elevated in star-forming regions, heavier species

start to move and trigger another type of reaction on the surface. Many kinds of complex

organic molecules (COMs) in addition to methanol and their precursors are expected to

be efficiently produced by reactions among heavier species. In particular, radical

reactions are key for chemical evolution, including COMs formation, as simulated in

chemical models (e.g., Hollis & Churchwell 2001). Garrod et al. (2008) explicitly

demonstrated the importance of reactions of radicals such as OH, CH3, and HCO for

chemical complexity in hot cores and corinos. In addition, simulation experiments

observed signatures for the formation of COMs through radical reactions (Butscher et al.

2017; Fedoseev et al. 2015; He et al. 2022; Ioppolo et al. 2021; Santos et al. 2022). In

these experiments, the COM products were detected by infrared spectroscopy and/or

temperature-programmed desorption after UV photolysis of ice mixtures or codeposition

of H atoms and CO, where many radicals accumulated in/on solids. However, because of

experimental difficulty in directly detecting radicals on the surface, the detailed behavior

of each radical is still unknown. Since reactive radicals can cause barrierless reactions,

their surface diffusion often becomes a rate-limiting process for molecular formation.

Therefore, the activation energy for diffusion,

diff, of radicals is an essential parameter

for evaluating chemical evolution through radical reactions. In theoretical models, even

for stable small molecules,

diff was often assumed to be a universal fixed fraction of

desorption energy,

des (Cuppen et al. 2017). However, a recent experiment clearly

showed that the ratios of

diff to

des are not universal but dependent on species (Furuya

et al. 2022). It is not easy to quantum-chemically calculate the

diff for long-distance

diffusion on rough amorphous surfaces relevant to realistic interstellar dust. Therefore,

experimental determination of

diff is highly desirable.

Among various radicals, OH radicals are considered one of the most abundant

radicals on ice because they can be easily produced by the surface reaction of O + H and

photolysis of H2O. Thus, the behaviors of OH radicals on ice would be closely related to

various phenomena occurring on ice. Because the reactivity of OH radicals is very high,

preparing the surface density of OH on ice enough for detection with conventional

experimental methods is difficult. Therefore, methods often used for solids, such as

Raman, infrared, and electron spin resonance spectroscopies, are not applicable.

Although microscopic methods, such as scanning tunneling microscopy and field-

emission microscopy, can detect adsorbates on the surface (e.g., Zangwill 1988; Gomer

1990; Laufon & Ho 2002), these methods are inappropriate for distinguishing between

OH and H2O. Furthermore, ice is not an electric conductor. The OH radicals in bulk ice

were detected by near-edge, X-ray absorption, fine structure spectroscopy, but

unfortunately, this method is not surface-sensitive (Laffon et al. 2006; Lacombe et al.

2006). In the present study, we apply a combination of photostimulated desorption (PSD)

and resonance-enhanced multiphoton ionization (REMPI), also known as the PSD-

REMPI method, which was previously developed for the detection of H (D) and H2 (D2)

on amorphous solid water (ASW) (Kuwahata et al. 2015; Hama et al. 2012; Watanabe et

al. 2010) and recently applied for detecting OH radicals on ASW (Miyazaki et al. 2020;

Tsuge & Watanabe 2021; Kitajima et al. 2021). We determined

diff for OH radicals on

compact amorphous solid water (c-ASW) by the direct detection of OH radicals.

2. Experiments

The detailed experimental setup was previously described (Miyazaki et al. 2020).

The compact amorphous solid water (c-ASW) samples with approximately 100

monolayers were deposited on a sapphire disk at 100 K by introducing the vapor of

freeze-pump-thaw cycled ultrapure water into an ultrahigh vacuum chamber (~10−8 Pa).

After the preparation of samples, OH radicals were produced by photolysis of H2O with

UV irradiation from a conventional deuterium lamp (“H2D2” light source unit,

Hamamatsu Photonics K. K.) at temperatures in the range from 54 to 80 K. The UV flux

was measured by a photodiode (AXUV-100G, IRD Inc.) to be approximately 1 × 1013

photons cm−2 s−1 above the sample surface. The photons from the lamp photodissociate

H2O mainly into H + OH with minor channels, H2 + O and 2H + O (Slanger & Black

1982). In addition, as secondary products, H2 and O2 are produced on the surface.

However, these volatile photofragments and products on the surface immediately desorb

when the sample temperature is well above their desorption temperatures. To avoid the

possible effect of these species other than OH, the experiments were performed at

temperatures from 54 K.

The OH radicals on the sample surface were detected by the PSD-REMPI

method. The details of the procedure are described in Miyazaki et al. (2020). Briefly, the

OH radicals on c-ASW were photodesorbed by unfocused pulsed-weak laser radiation

(typically 40 μJ per pulse with an approximate 3 mm2 spot on the surface) at 532 nm

from a Nd:YAG laser (hereafter denoted as a PSD laser); the photon energy at 532 nm

(2.33 eV) is below the dissociation energy of H2O (5.17 eV). Subsequently, photodesorbed

OH radicals were selectively ionized by the (2+1) REMPI process via the transition of

2Σ−

(

′ = 0) ←

2Π (

″ = 0) at approximately 1 mm above the c-ASW sample and detected

by a time-of-flight mass spectrometer. The repetition rate of the laser shots was 10 Hz.

Because the power density of the PSD laser shot was very weak, no heating effect on ice

was observed as confirmed in the previous works (Watanabe et al. 2010; Hama et al.

2012). The detected OH intensities per shot of the PSD laser were independent of the

total number of laser shots, indicating that the PSD laser shots had a minimal effect on

the surface number densities of OH. In addition, to further examine the effect of PSD

laser on the surface OH radicals, we performed two kinds of measurements. In the first

measurement, the OH intensities were continuously monitored during UV exposure for

a given period of time (the continuous PSD laser irradiation). In the other measurement,

the OH intensities were measured only after the UV exposure for the same exposure

time (the PSD laser irradiation only after terminating UV exposure). The intensities of

OH radicals were equivalent between two measurements, showing that the PSD laser

shots should have a negligible effect on the surface OH densities. Isolated neither H2O

nor OH absorbs a photon at 532 nm, whereas our previous work (Miyazaki et al. 2020)

revealed that OH-(H2O)n complex having three hydrogen bonds with neighboring H2O

molecules can absorb a photon at approximately 532 nm, leading to photodesorption.

According to quantum chemical calculations, the binding energies of OH on the ASW

surface range from 0.06 to 0.74 eV depending on the number of dangling-H or dangling-

O atoms on the binding site (Miyazaki et al. 2020). Notably, the OH radicals adsorbing

to the surface through three hydrogen bonds should have nearly the strongest binding

energy. In the present experiments, using PSD laser radiation at 532 nm, we can

selectively monitor OH radicals trapped in deep binding sites, and the detected OH

intensities,

OH, are proportional to the surface densities, [OH], of these deeply bound

OH radicals. It should be noted that the PSD of OH radical is caused by one-photon

absorption process but not by thermal process.

3. Results and Discussion

Figure 1 shows the (2+1) REMPI spectrum for OH desorbed from c-ASW during

UV irradiation at 70 K. The observed spectrum was well reproduced with rotational

temperatures of OH in the range of 120 to 200 K by using the PGOPHER program

(Western 2017) regardless of the c-ASW temperature, indicating that the rotational

distribution is determined by photodesorption dynamics rather than the sample

temperature. Although determination of the mechanism is ambiguous, this kind of

independence of rotational temperature has also been reported in experiments on H2O

photodesorption from ice (Hama et al. 2016). Hereafter, the OH intensities are

represented by the area of the strongest peak at 244.164 nm. The OH intensities

immediately increased upon turning the UV lamp on and reached a steady state after

approximately several minutes of irradiation. The OH intensities did not change during

the measurements, typically greater than 1 hour of UV irradiation. We monitored the

OH intensities on the surface of c-ASW at temperatures between 54 and 80 K in the

steady state during UV irradiation, as shown in Figure 2(a). The OH intensities

gradually decrease with the temperature of c-ASW from approximately 60 K to 80 K.

This result was conserved regardless of procedure of heating or cooling the sample.

Reasonably assuming that the surface composition is dominated by H2O and OH only

(Miyazaki et al. 2020), the OH surface density, [OH], in deeply bound sites under steady

state conditions at a given temperature can be expressed by

 

  



,

(1)

where

, σ, c,

OH-OH, and

des are the UV flux, dissociation cross-section of H2O to H +

OH, a factor for remaining OH on the surface at photodissociation, rate constant of OH–

OH recombination, and desorption rate of OH radical, respectively. Some fraction of

products from recombination would desorb by so-called chemical desorption (Williams

1968; Garrod et al. 2007). Because the dissociation cross-section and the factor “c” should

be independent from the sample temperature, the decrease in OH intensities with

temperature in Figure 2(a) can be attributed to recombination or desorption processes,

whose rate strongly depends on temperature. To evaluate the dominant process for the

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DirectDeterminationoftheActivationEnergyforDiffusionofOHRadicalsonWaterIceA.Miyazaki1,M.Tsuge1,H.Hidaka1,Y.Nakai2,N.Watanabe11InstituteofLowTemperatureScience,HokkaidoUniversity,Sapporo,Hokkaido060-0819,Japan;watanabe@lowtem.hokudai.ac.jp2RadiativeIsotopePhysicsLaboratory,RIKENNishinaCenter,Saitama3...

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Direct Determination of the Activation Energy for Diffusion of OH Radicals on Water Ice A. Miyazaki1 M. Tsuge1 H. Hidaka1 Y. Nakai2 N. Watanabe1

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