Ⅱ-7. Astrophysics and Astro-Glaciology

RIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Direct determination of the activation energy for diﬀusion of OH
radicals on water ice†
A. Miyazaki,∗1 M. Tsuge,∗1 H. Hidaka,∗1 Y. Nakai,∗2 and N. Watanabe∗1
Physicochemical processes such as surface reactionsof radicals, photolysis by ultraviolet (UV) irradiation,and ion bombardment on interstellar icy grains areindispensable for chemical evolution during star formation. The chemical evolution begins in molecular clouds where the temperature is as low as 10 K.
In molecular clouds, the hydrogenation of primordial
atomic and molecular species is crucial because hydrogen atoms can migrate and encounter reaction partners
on the grain surface even at ∼10 K.1–7) During temperature elevation in star-forming regions, heavier species
can diﬀuse on the ice and many types of complex organic molecules (COMs) are considered to be produced
through reactions among heavier species, particularly
radicals. Thus, the behavior of radicals on the ice surface should be investigated to understand the formation pathways of COMs. OH radical would be one of
the most abundant radicals on ice surface as they canbe produced through photolysis of H2 O and hydrogenation of O atoms on the ice surface. Therefore, OH radicals are expected to participate in various physicochemical processes on ice surface. However, conventional
methods for detecting OH radicals, such as Raman, infrared, and electron spin resonance spectroscopies, are
not applicable because of their low detection eﬃciencies
and non-surface selectivity. In this study, we applied acombination of photostimulated desorption (PSD) andresonance-enhanced multiphoton ionization (REMPI)
to facilitate direct detection of OH radicals on the icesurface.8) Using this PSD-REMPI method, we determined the activation energy for diﬀusion (Ediﬀ ) of OH
radicals on compact amorphous solid water (c-ASW),
because the surface diﬀusion of radicals often becomesa rate-limiting process in reactions involving radicals.
The c-ASW samples were deposited on a sapphiredisk at 100 K by introducing the water vapor into an ultrahigh vacuum chamber (∼10−8 Pa). After the preparation of ASW sample, OH radicals were produced via
photolysis of water molecules using a deuterium lampin the temperature range of 54–80 K. In this photolysis, H2 O is dissociated mainly into H + OH with minorchannels, H2 + O and 2H + O, and as secondary products, H2 and O2 are also produced on the surface. To
avoid the undesired eﬀect of these species other thanOH, the experiments were performed at temperatures
above 54 K where hydrogen and oxygen cannot stayon the surface. The OH radicals on the sample surface
were detected with the PSD-REMPI method.8) The OH†∗1∗2Condensed from the article in Astrophys. J. Lett. 940, L2(2022)
Institute of Low Temperature Science, Hokkaido UniversityRIKEN Nishina Center- 64 -radicals were photodesorbed by weak-pulsed laser radiation at 532 nm from an Nd:YAG laser. The photon energy at 532 nm is below the dissociation energy of H2 O.
Photodesorbed OH radicals were selectively ionized bythe (2 + 1) REMPI process above the c-ASW surface
and detected with a time-of-ﬂight mass spectrometer.
We performed two types of measurements. In the ﬁrst
measurement, the OH intensities were monitored during UV exposure. We monitored the OH intensities inthe steady state during UV irradiation. In the othermeasurement, the OH intensities were measured following a particular period of UV exposure. Note that
we can selectively monitor OH radicals based on onephoton chemical process.8)
When monitoring OH during UV exposure, we foundthat the OH intensities in steady state gradually decreased with the temperature of c-ASW in the range ofapproximately 60–80 K. We experimentally conﬁrmedthat OH radicals were lost through OH–OH recombination rather than thermal desorption of OH. The recombination rate should be limited by the diﬀusion ofOH radicals on the c-ASW surface because the radicalradical reaction itself is barrierless. By analyzing therecombination rate with the Arrhenius type formula,
the activation barrier of surface diﬀusion, Ediﬀ , wasevaluated to be 0.14 ± 0.01 eV (1650 ± 60 K). Furthermore, in the other measurement of OH monitoring after
UV exposure, the OH intensities decreased with timeafter UV termination. Again, considering that this decrease is dominated by OH–OH recombination, we derived Ediﬀ to be 0.13 ± 0.01 eV (1540 ± 80 K). Thisvalue is consistent with the previously obtained value.
Under a simple assumption that the activation energyof diﬀusion has the single value of 1650 K, the OH radicals can migrate over 100 nm on the icy dust within 105
years at temperatures above 36 ± 1 K. The present results suggest that OH reactions start at approximately
36 K and become important in the warming-up of starforming regions. Note that we do not exclude thepossibility that the OH radicals drift to a certain extent through shallower sites even at temperatures below
36 K.
References
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3) N. Watanabe et al., Astrophys. J. Lett. 571, L173
(2002).
4) S. Ioppolo et al., Astrophys. J. 686, 1474 (2008).
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7) H. Hidaka et al., Chem. Phys. 13, 15798 (2011).
8) A. Miyazaki et al., Phys. Rev. A 102, 052822 (2020).