H2O2 photoproduction inside H2O and H2O:O2 ices at 20–140 K

We report the results of laboratory measurements of H2O2 production inside thin (50 nm thickness) H2O and H2O:O2 ice samples irradiated by 121.6 nm photons at different temperatures. In the case of H2O ice, H2O2 is formed at the temperatures below 60 К. In the case of H2O:O2 ice, H2O2 is formed in the 20–140 К range. For H2O:O2 = 9:1 ice, we derived H2O2 photochemical quantum yield as a function of sample irradiation temperature. The obtained data can be used for evaluation of H2O2 photoproduction at the surface of astrophysical water ice bodies and inside the particles of Noctilucent Clouds in the Earth’s atmosphere.

It is well known that surfaces of most icy bodies in the outer Solar System and interstellar space consist mainly of water and are regularly bombarded with energetic particles and photons. This irradiation triggers a spectrum of physicochemical processes inside solid phase [1][2][3][4] : the formation of primary products (H, OH, H 2 , and О); their fast recombination or leaving from the initial position with subsequent diffusion inside ice; reactions between them, with H 2 O or impurities; appearance of secondary products (HO 2 , HO 3 , H 2 O 2 , O 2 , O 3 ); trapping of primary and secondary products by the ice matrix and their accumulation; and the flow of products into gas phase. The characteristics of these processes greatly depend on the characteristics of irradiation, ice type, its thickness, temperature, additives, and others. The end products such as H 2 O 2 , O 2 , O 3 , etc., both in solid and gas phase are of considerable interest for astrophysics as they are all oxidizing agents and may provide a source of chemical energy as a fuel for extraterrestrial life 5 .
The established detection of H 2 O 2 on Europa's surface 6 and the discussed existence or absence of H 2 O 2 on Enceladus, Ganymede, and Callisto 7 stimulated extensive laboratory studies of the mechanisms of formation and measurement of the parameters of producing concentrations of this component in high-purity H 2 O ice and H 2 O ice with different additives irradiated by energetic particles [8][9][10][11][12][13][14] . In particular, it was shown that, as compared to H 2 O ice, the presence of О 2 greatly increases the production of H 2 O 2 and other products (HO 2 , HO 3 , and O 3 ), especially at relatively high irradiation temperatures of 80-120 К. At the same time, except for a few works, significantly less attention was paid to investigation of H 2 O 2 production by VUV photons. In particular, Gerakines et al. 15 and Schriver et al. 16 reported about H 2 O 2 formation in H 2 O ice at 10 K irradiated by microwave discharge hydrogen flow lamps. Yabushita et al. 17

noctilucent clouds
There exists at least one analog of such water icy bodies regularly irradiated by VUV photons in the Earth's atmosphere. Each summer at polar and middle latitudes, one can observe the highest atmospheric clouds called Noctilucent Clouds (NLCs). They appear in mesopause region (altitudes range of 80-90 km) at the temperatures of 120-150 K [19][20][21] . Since the clouds discovery 22 , there were many discussions about their nature (see reviews by Gadsden & Schröder 19 and by Thomas 20 ). Only recently, the infrared spectra of clouds showed 23 that NLCs consisted mainly of water ice. Thus, it is not doubt now that clouds form by condensation of water vapour and can influence on gas-phase chemistry of this region due to water vapour is its key parameter.
In the conditions of daytime mesopause, water vapour is subjected to intensive solar VUV radiation (121.6 nm, so called the Lyman-α line) and the reaction H 2 O + hv → H + OH provides the main chemical source of the family of odd hydrogen (HO x : H, OH, and HO 2 ) 24 27 this hypothesis was verified by laboratory measurements of the photodesorption rate from thin water ice samples irradiated by 121.6 nm photons in the temperature range of 120-150 K. It was found that most photoproducts did not leave the solid phase and tended to recombine in water molecules back. Basing on the results of ice irradiation by energetic particles at relatively high temperatures of 80-120 К 8 15 . The temporal evolution of S H O 2 2 can be divided into two parts: growth stage when S H O 2 2 increases monotonically, and saturation stage. In both cases, S H O 2 2 is saturated after ~1 hour irradiation. In the case of H 2 O ice, the growth stage continues for ~20-30 min and can be described by a quadratic function of irradiation time (VUV fluence). This corresponds to the results of irradiation of H 2 O ice by Lyman-α photons at 10 K obtained by Gerakines et al. 15 . In the case of H 2 O:O 2 ice, the growth stage continues for ~10 min and can be described by a linear function of irradiation time (VUV fluence). Such behavior of S H O 2 2 in the case of H 2 O:O 2 ice was found at other photon flux intensities. It means that, in the case of H 2 O:O 2 ice, the H 2 O 2 production during the growth stage can be fit successfully to a (pseudo) first-order reaction. We can conclude that the rate of H 2 O 2 production is proportional to I α and can determine the H 2 O 2 photochemical quantum yield (γ H O 2 2 , the number of molecules of H 2 O 2 generated per a Lyman-α photon absorbed by ice) as a function of T ir following, for example, Cooper et al. 12 Fig. 3a,b). In particular, Loeffler et al. 8 data were interpolated into the temperature regions of 20-80 K and 80-110 K, and extrapolated to the temperature region of 110-140 K.
One can see from Fig. 3a

Discussion and conclusion
The possible mechanism of H 2 O 2 formation during VUV irradiation of H 2 O ice was discussed by Gerakines et al. 15 and pointed by Loeffler et al. 10 10 and Hand and Carlson 14 found out the same behavior of S H O 2 2 in H 2 O ice irradiated by high-energy ions and electrons correspondingly. It was proposed for explanation of this, in particular, that two OH could be produced in an ion track caused by an ion 10 . Evidently, this mechanism cannot be transferred on our situation. But, following Loeffler et al. 10    shown in Fig. 3a 2 2 inside such ice can be used for assessing the impact of Lyman-α photons on water ice and its contribution to H 2 O 2 production in different applications. It was found that G-value (defined as the number of H 2 O 2 molecules created per unit of absorbed energy) varied in the range of 0.2-0.4 molecules/100 eV at 50-100 K, that is close to G-value of Lyman-α photons at 50-100 K obtained in our paper. Thus, contribution of Lyman-α photons to H 2 O 2 production is defined by the ratio between energy fluxes of photons (EF ph ) and energetic particles (EF ep ). In the case of EF ph~E F ep , photons produce approximately the same amount of H 2 O 2 inside ice as particles. At that, as it was noted by Gerakines et al. 40 , the penetration depth for Lyman-α photons (~45 nm 41 ) in water ice is essentially less, than for protons which depends on its energy (for example, 1-2 μm for 0.1 MeV protons 10 and 22 μm for 1 MeV protons 42 ). So, one would expect the high relative H 2 O 2 concentration in the top few tens of nm caused by Lyman-α photons. It means that H 2 O 2 production by VUV photons can be important at EF ph /EF ep ≥ 10 −2 .
In the mesopause region of the Earth's atmosphere, typically EF ph  EF ep . But, at this moment, there is no information about containing of O 2 inside water ice of Noctilucent Clouds. It is well-known that NLCs are formed as a result of gas-kinetic collision of H 2 O molecules with the surface of mesospheric aerosols, including adsorption and desorption properties. It is, generally, a relatively slow process, with the characteristic time (2-20 hours) depending on temperature 43 . In the real conditions of a summer mesopause, H 2 O concentration in gas phase is more than 4 orders of magnitude less than the concentration of O 2 in ground (triplet) state and less than daytime concentration of O 2 in singlet state ((2-4)·10 9 cm −3 at 80-85 km 44  , where I α is the local flux intensity of Lyman-α photons and S Mie is the Mie absorption cross-section of NLCs, S Mie ≈ S NLC /4, where S NLC is the NLC surface density. According to the data of the long-term (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) measurements with the ALOMAR RMR-lidar in Northern Norway (69 ○ N, 16 ○ E), at the altitudes of 81-86 km S NLC varies within the (3-6)·10 −8 cm 2 / cm 3 range 45 . Taking into consideration that at these heights in the conditions of average solar activity I α~3 · 10 11 photons/(cm 2 ·s) 24   To conclude, we have demonstrated for the first time that, if NLCs particles contain ≥0.1% O 2 , the physicochemical processes occurring in them may remarkably affect the chemical composition of the mesopause region. On the one hand, it may be a possible explanation of the results of early rocket mass-spectrometer measurements 28,29 indicating increased H 2 O 2 concentration in the clouds. On the other hand, H 2 O 2 , product of its UV photodissociation (OH), H 2 O, O 2 and other impurities (for example, CO 2 ) can participate in subsequent reactions producing more complex chemical compounds inside NLCs as it takes place, for example, in the bulk of supercooled water particles 46 . We hope that this research stimulates further experimental and theoretical investigations of the chemical composition of cloud particles. Note also that the obtained results are interesting for astrophysical applications, for example, for assessing the contribution of VUV irradiation to H 2 O 2 production in the outer Solar System and interstellar space depending on temperature.

Methods
Apparatus. The experimental set-up was the same as we used under laboratory measurements of the photodesorption rate from water ice. As described by Kulikov et al. 27 , the apparatus consisted of a Fourier Transform Infrared Spectrometer (Bruker IFS 66 v)), a closed-cycle He refrigerator (Leybold ROK 10-300), a gas preparation and inlet system (further briefly GPIS), and a high-vacuum chamber with a volume of about 1000 cm 3 pumped continuously by a turbomolecular pump system (Leybold-Heraeus) securing a high vacuum in the chamber down to the 10 −8 mbar range. Inside the chamber, at the cold end of the cryostat there was a vertically mounted aluminium mirror (2.5 × 4 cm in size) as a substrate whose temperature was precisely regulated by a temperature controller (Lake Shore, model 340). The mirror temperature could be selected in the 10-300 K range. The GPIS was equipped with baratrons and needle valves. The upper part of the high vacuum chamber had two ports, one of which was equipped with a MgF 2 (5 mm thick) input window for a VUV lamp. The second port had a KBr window for the IR beam of the FTIR spectrometer. The input for the VUV lamp made an angle of incidence of ~45 0 to the mirror surface and, according to the estimates of the manufacturer, MgF 2 transmitted about 60% of the quantum flux at the wavelength of 121.6 nm. As a VUV source (Lyman-α) we used a resonance hydrogen lamp (Opthos Instruments) containing a mixture of 10% H 2 and 90% Ar excited by a microwave generator (Opthos Instruments, model MPG-4M) with a frequency of 2450 MHz. The lamp intensity was determined by the power supplied by the microwave generator (about the lamp calibration see below). The FTIR spectrometer was placed on rails allowing precise positioning of the instrument with respect to the cryostat with the sample. This was important for achieving a good overlap of the areas of the light spots from both, infrared (from spectrometer light source) and vacuum ultraviolet irradiation (from VUV lamp) of the ice film sample on the substrate. The operation of the FTIR spectrometer was PC controlled by means of software (OPUS) that permitted scanning spectra over a wide range (from 6000 to 500 cm −1 ) and analyzing the obtained spectra. The spectra were recorded with a spectral resolution of 0.2-2 cm −1 in the RAIRS mode (reflection absorption infrared spectroscopy) where the IR beam passes through the sample twice. experimental procedures. The experimental procedures were almost the same as we used under laboratory measurements of the photodesorption rate from water ice. As described by Kulikov et al. 27 , each experiment with a particular sample of ice was conducted in two stages. At the first stage, two background spectra with different resolution were recorded at a mirror temperature of 20 K and at a temperature of subsequent irradiation (T ir ). At that time, H 2 O or H 2 O + O 2 gas was got ready in GPIS. We used the oxygen (Air Liquide 5.5) with purity better than 99.9995 Vol% and triply distilled water with resistivity better than 10 7 ohm cm, additionally degassed by freeze/thaw cycles in vacuum conditions. As described in our previous study 27 48 . After the sample preparation, the mirror temperature was set at T ir (in the 20-140 K range) and several IR spectra of unirradiated ice were recorded. At T ir = 120 K and above, the IR spectra showed crystalline features of all ice samples that was in accordance with composition of NLCs 49 . Note also, Bartels-Rausch et al. 50 discussed recently the simulations of disorder on pure ice at different temperatures below melting point (T m ) and showed that, at the temperatures 10 K below T m , disorder affected the first molecular layer of ice only. In current study, the samples consisted of more than 100 layers of water and the highest temperature in our experiments (140 K) was about 20 K below than characteristic T m of water ice in our vacuum chamber. Thus, we can conclude that interface processes could not influence essentially on the studied processes inside ice samples.
As described in our previous study 27 , at the second stage, the vacuum ultraviolet lamp was switched on and the ice films were exposed to VUV radiation with intensity set by microwave generator. After each photolysis period, IR spectra of the irradiated ice films were recorded. For improving the signal-to-noise ratio we used the spectral resolution of 2 cm −1 and a large amount of scans (2000). Hydrogen peroxide was found by detecting the IR absorption band of 2850-2860 cm −1 in difference spectra (before and after irradiation). The band was exuded by subtracting the baseline from the spectrum in manner described, for example, in Hand & Carlson 14  calibration of the hydrogen discharge lamp. The calibration was carried out in the same manner as it was described by Kulikov et al. 27 . Before an experiment with specific H 2 O or H 2 O + O 2 ice sample, we performed series of measurements of the absolute magnitude of the flux of Lyman-α photons that reach the ice sample at different adjustments of the microwave generator power. The widely used "ozone method" 16,51 was applied for the procedure. The intensity of the lamp was determined by measuring the O 2 → O 3 conversion rate in a VUV photolyzed sample of solid O 2 at 16 K. The ozone formation as a function of photolysis time was monitored with the FTIR spectrometer via the O 3 absorption band at about 1040 cm −1 . More specifically, for finding the VUV intensity at the mirror for a specific generator output power (GOP) we made successive measurements of the integrated area of the 1040 cm −1 absorption band (S O 3 ) as a function of irradiation time (see Fig. 2 in Kulikov et al. 27 ). After that, the lamp intensity at this GOP was determined as = ⋅ ⋅ 3 3 , where the derivative dS dt / O 3 was found by the linear part of the function S t ( ) O 3 , Y was the quantum yield for the formation of O 3 from O 2 , and A O 3 was the strength of the band absorption. The value of ⋅ Y A O 3 was adopted from Cottin et al. 52 and was equal to 8.4 · 10 −18 cm·photon −1 . We obtained that, depending on GOP varied within the range of 4-120 W, the photon flux intensity varied within the range 5 · 10 12 -10 15 photons/(cm 2 •s). The stability of lamp intensity at the fixed GOP was checked by means of photodiode SXUV300 (International Radiation Detectors).