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Context. Ammonia is detected both in the gas phase and in the ices of protoplanetary disks. However, its gas phase abundances are still difficult to reproduce via chemical modelling when only the thermal-, UV photon-, and cosmic ray-induced processes are considered. Among other non-thermal mechanisms, X-ray photo-desorption is a relevant pathway to maintaining a budget of molecules in the gas of disks. However, no quantitative data are available to constrain its efficiency in the case of ammonia-containing ices. Aims. We studied the desorption induced by soft X-rays from ammonia ices to determine the photo-desorption yields of neutral molecules that can be applied to the conditions of protoplanetary disks. We also aim to identify the desorption mechanisms at play. Methods. Pure ammonia ices were deposited at 23 or 75 K and irradiated between 395 and 435 eV, with monochromatic synchrotron light. Desorption of neutral molecules and fragments was detected using a quadrupole mass spectrometer, calibrated to obtain des-orption yields. These values were extrapolated and the spectrum of a protoplanetary disk was used to extract average astrophysical desorption yields. Results. Photo-desorption from NH3 ices is dominated by the desorption of neutral NH3 and N2 molecules. The desorption mechanism mostly involves the thermalisation of Auger electrons in the ice, although resonant phenomena also contribute near the N 1s ionisation edge. The NH3 photo-desorption is independent of ice morphology and irradiation temperature. Contrary to NH3 desorption, N2 desorption is sensitive to the photon fluence received by the ice. Average photo-desorption yields derived using the TW Hya X-ray spectrum reveal that the NH3 photo-desorption would be four to six times more efficient than that of H2O. This could be at the origin of unexplained high NH3/H2O abundance ratios predicted in the disk around TW Hya. This result ought to be confirmed by the study of mixed water-ammonia ices. [ABSTRACT FROM AUTHOR]

X-ray photodesorption yields of N 2 15 and CO 13 are derived as a function of the incident photon energy near the N (∼400 eV) and O K-edge (∼500 eV) for pure N 2 15 ice and mixed CO 13 : N 2 15 ices. The photodesorption spectra from the mixed ices reveal an indirect desorption mechanism for which the desorption of N 2 15 and CO 13 is triggered by the photoabsorption of CO 13 and N 2 15 , respectively. This mechanism is confirmed by the x-ray photodesorption of CO 13 from a layered CO 13 / N 2 15 ice irradiated at 401 eV on the N 1s → π* transition of N 2 15 . This latter experiment enables us to quantify the relevant depth involved in the indirect desorption process, which is found to be 30–40 monolayers in that case. This value is further related to the energy transport of Auger electrons emitted from the photoabsorbing N 2 15 molecules that scatter toward the ice surface, inducing the desorption of CO 13 . The photodesorption yields corrected from the energy that can participate in the desorption process (expressed in molecules desorbed by eV deposited) do not depend on the photon energy; hence, they depend neither on the photoabsorbing molecule nor on its state after Auger decay. This demonstrates that x-ray induced electron stimulated desorption, mediated by Auger scattering, is the dominant process explaining the desorption of N 2 15 and CO 13 from the ices studied in this work. [ABSTRACT FROM AUTHOR]

Water molecules trapped in rare gas matrices exhibit conspicuous shifts in their far-infrared (FIR), rotranslational spectral features compared with the corresponding transitions observed in the gas phase. These confinement-induced perturbations have been related not only to the quantization of translational motion but also to the coupling between the orientational and positional degrees of freedom: the rotation–translation coupling (RTC). As the propensity displayed by the nuclear spin isomers (NSI) of water to undergo interconversion in confinement is intimately related to how its nuclear spin degrees of freedom are coupled with those for intra- and intermolecular motions, confinement-induced RTC should also strongly impact the NSI interconversion mechanisms and rates. Insight into the rotranslational dynamics for H 2 16 O , H 2 17 O , and H 2 18 O , confined in argon and krypton matrices, is provided here based on the evolution of rotranslational spectra induced by NSI interconversion while a definitive assignment is provided from the transition energies and intensities calculated using the confined rotor model [Paper I, Wespiser et al., J. Chem. Phys. 156, 074304 (2021)]. In order to build a complete rotranslational energy diagram of confined water, which is fundamental to understand the NSI interconversion rates, the energy difference between the ground ortho and para rotranslational states is derived from the temperature dependence of the intensity ratio of mid-infrared lines emerging from these states. These investigations should provide deeper insight of the factors that control NSI interconversion of water isotopologues under extreme confinement. [ABSTRACT FROM AUTHOR]

Core-excitation of water ice releases many different molecules and ions in the gas phase. Studying these desorbed species and the underlying desorption mechanisms can provide useful information on the effects of x-ray irradiation in ice. We report a detailed study of the x-ray induced desorption of a number of neutral, cationic, and anionic species from amorphous solid water. We discuss the desorption mechanisms and the relative contributions of Auger and secondary electrons (x-ray induced electron stimulated desorption) and initial excitation (direct desorption) as well as the role of photochemistry. Anions are shown to desorb not just through processes linked with secondary electrons but also through direct dissociation of the core-excited molecule. The desorption spectra of oxygen ions (O+, OH+, H2O+, O−, and OH−) give a new perspective on their previously reported very low desorption yields for most types of irradiations of water, showing that they mostly originate from the dissociation of photoproducts such as H2O2. [ABSTRACT FROM AUTHOR]

In this paper, we have studied adsorption and thermal desorption of methanol CH3OH on graphite surface, with the specific aim to derive from experimental data quantitative parameters that govern the desorption, namely, adsorption energy Eads and prefactor of the Polanyi-Wigner law. In low coverage regime, these two values are interconnected and usually the experiments can be reproduced with any couple (Eads), which makes intercomparison between studies difficult since the results depend on the extraction method. Here, we use a method for determining independently the average adsorption energy and a prefactor value that works over a large range of incident methanol coverage, from a limited set of desorption curves performed at different heating rates. In the low coverage regime the procedure is based on a first order kinetic law, and considers an adsorption energy distribution which is not expected to vary with the applied heating rate. In the case of CH3OH multilayers, Eads is determined as 430 meV with a prefactor of 5 × 1014 s-1. For CH3OH submonolayers on graphite, adsorption energy of 470 ± 30 meV and a prefactor of (8 ± 3) × 1016 s-1 have been found. These last values, which do not change between 0.09 ML and 1 ML initial coverage, suggest that the methanol molecules form island-like structure on the graphite even at low coverage. [ABSTRACT FROM AUTHOR]

We report an investigation of X-ray induced desorption of neutrals, cations and anions from CO ice. The desorption of neutral CO, by far the most abundant, is quantified and discussed within the context of its application to astrochemistry. The desorption of many different cations, including large cations up to the mass limit of the spectrometer, is observed. In contrast, the only desorbing anions detected are O− and C−. The desorption mechanisms of all these species are discussed with the aid of their photodesorption spectrum. The evolution of the X-ray absorption spectrum shows significant chemical modifications of the ice upon irradiation, which along with the desorption of large cations gives a new insight into X-ray induced photochemistry in CO ice. [ABSTRACT FROM AUTHOR]

Investigates the collisional parameters of pure rotational transitions of H[sub 2] self-perturbed and He perturbed. Use of stimulated gain Raman spectroscopy; Exhibition of a linear behavior by collisional frequency shift; Analysis of elastic and inelastic contributions as function of temperature and rotational quantum number.

The photochemical processing of a CH4 : D2O 1 : 3.3 ice mixture adsorbed on an HOPG surface in the XUV regime was investigated using pulses obtained from the Free-electron LASer in Hamburg (FLASH) facility. Ice films were exposed to femtosecond pulses with a photon energy of hν = 40.8 eV, consistent with the HeII resonance line. Cationic species desorbing directly from the ice films were detected using time-of-flight (ToF) mass spectrometry. Simple ions formed through the fragmentation of the parent molecules and subsequent recombination reactions were detected and are consistent with efficient D+ and H+ ejection from the parent species, similar to the case for low energy electron irradiation. The FEL fluence dependencies of these ions are linear or exhibit a non-linear order of up to 3. In addition, a series of Cn+ cluster ions (with n up to 12) were also identified. These ions display a highly non-linear desorption yield with respect to the FEL fluence, having an order of 6–10, suggesting a complex multi-step process involving the primary products of CH4 fragmentation. Two-pulse correlation measurements were performed to gain further insight into the underlying reaction dynamics of the photo-chemical reactions. The yield of the D2O derived products displayed a different temporal behaviour with respect to the Cn+ ions, indicating the presence of very different reaction pathways to the two families of ionic products. [ABSTRACT FROM AUTHOR]

Over the last four years we have illustrated the potential of a novel wavelength-dependent approach in determining molecular processes at work in the photodesorption of interstellar ice analogs. This method, utilizing the unique beam characteristics of the vacuum UV beamline DESIRS at the French synchrotron facility SOLEIL, has revealed an efficient indirect desorption mechanism that scales with the electronic excitations in molecular solids. This process, known as DIET – desorption induced by electronic transition – occurs efficiently in ices composed of very volatile species (CO, N2), for which photochemical processes can be neglected. In the present study, we investigate the photodesorption energy dependence of pure and pre-irradiated CO2 ices at 10–40 K and between 7 and 14 eV. The photodesorption from pure CO2 is limited to photon energies above 10.5 eV and is clearly initiated by CO2 excitation and by the contribution of dissociative and recombination channels. The photodesorption from “pre-irradiated” ices is shown to present an efficient additional desorption pathway below 10 eV, dominating the desorption depending on the UV-processing history of the ice film. This effect is identified as an indirect DIET process mediated by photoproduced CO, observed for the first time in the case of less volatile species. The results presented here pinpoint the importance of the interconnection between photodesorption and photochemical processes in interstellar ices driven by UV photons having different energies. [ABSTRACT FROM AUTHOR]

Context. Ultraviolet photodesorption of molecules from icy interstellar grains can explain observations of cold gas in regions where thermal desorption is negligible. This non-thermal desorption mechanism should be especially important where UV fluxes are high. Aims. N2 and O2 are expected to play key roles in astrochemical reaction networks, both in the solid state and in the gas phase. Measurements of the wavelength-dependent photodesorption rates of these two infrared-inactive molecules provide astronomical and physical-chemical insights into the conditions required for their photodesorption. Methods. Tunable radiation from the DESIRS beamline at the SOLEIL synchrotron in the astrophysically relevant 7 to 13.6 eV range is used to irradiate pure N2 and O2 thin ice films. Photodesorption of molecules is monitored through quadrupole mass spectrometry. Absolute rates are calculated by using the well-calibrated CO photodesorption rates. Strategic N2 and O2 isotopolog mixtures are used to investigate the importance of dissociation upon irradiation. Results. N2 photodesorption mainly occurs through excitation of the b1Πu state and subsequent desorption of surface molecules. The observed vibronic structure in the N2 photodesorption spectrum, together with the absence of N3 formation, supports that the photodesorption mechanism of N2 is similar to CO, i.e., an indirect DIET (Desorption Induced by Electronic Transition) process without dissociation of the desorbing molecule. In contrast, O2 photodesorption in the 7-13.6 eV range occurs through dissociation and presents no vibrational structure. Conclusions. Photodesorption rates of N2 and O2 integrated over the far-UV field from various star-forming environments are lower than for CO. Rates vary between 10-3 and 10-2 photodesorbed molecules per incoming photon. [ABSTRACT FROM AUTHOR]

We present a combined theoretical and experimental study of the adsorption of two pairs of organic isomers, (i) acetic acid AA (CH3COOH) and methyl formate MF (HCOOCH3), and (ii) ethanol EtOH (CH3CH2OH) and dimethyl ether DME (CH3OCH3), onto crystalline water ice surfaces at low temperatures. Both approaches show that, for each pair, the most stable isomer from a thermodynamical point of view, i.e. AA and EtOH, is also the one which is the more tightly bound to the water ice surface compared to the less stable isomers (MF and DME). This finding, which can be explained by the ability of AA or EtOH to efficiently interact with the ice surface via hydrogen bondings, may have important consequences in an astrophysical context, since it could explain why the most stable isomer is not the most abundant observed in the interstellar gas phase. [ABSTRACT FROM AUTHOR]

Numerous hydrogenated molecules observed in comets and the interstellar medium present nuclear spin isomers. The non-thermal ortho: para ratios measured in space are believed to be of high diagnostic value, allowing remote measurement of physical conditions prevailing in the past, such as the initial temperature associated with a molecular formation or condensation. The lack of laboratory studies on interactions at the gas-solid interface limits our current interpretation of observations. Recent progress in nuclear spin conversion (NSC) dynamics are discussed in this paper. Results on water and methane in cryogenic matrices illustrate the conversion mechanisms within the condensed phase at very low temperatures. NSC of molecular hydrogen at the surface of 10 K Amorphous Solid Water (ASW) shows surface mediated conversion processes for molecular hydrogen. NSC involving molecular ices are discussed in the light of few recent studies, including UV photodesorption investigations. [ABSTRACT FROM AUTHOR]

Nuclear spin conversion (NSC) of ortho- to para-H2and para- to ortho-D2has been investigated on an amorphous solid water (ASW) surface at 10 K, in the presence of co-adsorbed O2. The dynamics of the nuclear spin conversion could be revealed by combination of resonance enhanced multiphoton ionization spectroscopy (REMPI) with temperature programmed desorption (TPD) experiments. The conversion rates are consistent with a diffusion of molecular hydrogen inducing a nuclear spin conversion enhanced in the vicinity of molecular oxygen. The conversion times were found to increase with decreasing O2and H2coverage. Finally, on oxygen free ASW surface, the extremely long conversion characteristic times measured showed that such surface is not an efficient catalyst for NSC, in contradiction with hypothesis commonly made for interstellar medium. [ABSTRACT FROM AUTHOR]

Fourier-transform infrared spectroscopy in the frequency range 400–4000 cm[sup -1] has been used to investigate the absorption of H[sub 2]O and H[sub 2]O:CO[sub 2] complex isolated in solid argon. Thanks to the lowest temperature reached in our experiment, temperature effects and nuclear spin conversion studies allow us to propose a new assignment of the rovibrational lines in the bending band ν[sub 2] for the quasi-freely rotating H[sub 2]O. An additional wide structure observed in this band shows two maxima around 1657.4 and 1661.3 cm[sup -1], with nuclear spin conversion of the high-frequency part into the low-frequency one. This structure is tentatively attributed to a rotation–translation coupling of the molecule in the cage. However, the equivalent effect is not observed in the vibrational stretching bands ν[sub 1] and ν[sub 3]. Finally, in double doping experiments with CO[sub 2] important new structures appear, allowing us to unambiguously extract the frequencies of the lines of the H[sub 2]O:CO[sub 2] complex. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract. In order to improve the accuracy of H[sub 2] CARS thermometry, H[sub 2] Q-branch CARS spectra have been recorded for various H[sub 2]-N[sub 2] mixtures in a high-pressure cell at different pressures and temperatures (up to 40 bar and 875 K). Due to the low spectral resolution of broadband CARS experiments, the relevant spectral lineshape factor is the linewidth ratio GAMMA(Q(3))/GAMMA(Q(1)), since Q(1) and Q(3) are the most intense lines of the Q-branch spectrum in this temperature range. For the first time, the speed-inhomogeneous effects are accounted for in the simulation of the CARS profiles. The evaluated temperatures are in good agreement with reference values obtained by thermocouples. The specific role on the accuracy of H[sub 2] CARS thermometry of the speed inhomogeneity is carefully analyzed, in connection with the influence of the nitrogen concentration. [ABSTRACT FROM AUTHOR]

Over the last four years we have illustrated the potential of a novel wavelength-dependent approach in determining molecular processes at work in the photodesorption of interstellar ice analogs. This method, utilizing the unique beam characteristics of the vacuum UV beamline DESIRS at the French synchrotron facility SOLEIL, has revealed an efficient indirect desorption mechanism that scales with the electronic excitations in molecular solids. This process, known as DIET – desorption induced by electronic transition – occurs efficiently in ices composed of very volatile species (CO, N2), for which photochemical processes can be neglected. In the present study, we investigate the photodesorption energy dependence of pure and pre-irradiated CO2 ices at 10–40 K and between 7 and 14 eV. The photodesorption from pure CO2 is limited to photon energies above 10.5 eV and is clearly initiated by CO2 excitation and by the contribution of dissociative and recombination channels. The photodesorption from “pre-irradiated” ices is shown to present an efficient additional desorption pathway below 10 eV, dominating the desorption depending on the UV-processing history of the ice film. This effect is identified as an indirect DIET process mediated by photoproduced CO, observed for the first time in the case of less volatile species. The results presented here pinpoint the importance of the interconnection between photodesorption and photochemical processes in interstellar ices driven by UV photons having different energies., Astronomy

Context: Ultraviolet photodesorption of molecules from icy interstellar grains can explain observations of cold gas in regions where thermal desorption is negligible. This non-thermal desorption mechanism should be especially important where UV fluxes are high. Aims: \(N_2\) and \(O_2\) are expected to play key roles in astrochemical reaction networks, both in the solid state and in the gas phase. Measurements of the wavelength-dependent photodesorption rates of these two infrared-inactive molecules provide astronomical and physical-chemical insights into the conditions required for their photodesorption. Methods: Tunable radiation from the DESIRS beamline at the SOLEIL synchrotron in the astrophysically relevant 7 to 13.6 eV range is used to irradiate pure \(N_2\) and \(O_2\) thin ice films. Photodesorption of molecules is monitored through quadrupole mass spectrometry. Absolute rates are calculated by using the well-calibrated CO photodesorption rates. Strategic \(N_2\) and \(O_2\) isotopolog mixtures are used to investigate the importance of dissociation upon irradiation. Results: \(N_2\) photodesorption mainly occurs through excitation of the \(b^1\sqcap_u\) state and subsequent desorption of surface molecules. The observed vibronic structure in the \(N_2\) photodesorption spectrum, together with the absence of \(N_3\) formation, supports that the photodesorption mechanism of \(N_2\) is similar to CO, i.e., an indirect DIET (Desorption Induced by Electronic Transition) process without dissociation of the desorbing molecule. In contrast, \(O_2\) photodesorption in the 7−13.6 eV range occurs through dissociation and presents no vibrational structure. Conclusions: Photodesorption rates of \(N_2\) and \(O_2\) integrated over the far-UV field from various star-forming environments are lower than for CO. Rates vary between \(10^{-3}\) and \(10^{-2}\) photodesorbed molecules per incoming photon., Astronomy

Close-coupling calculations of broadening and shifting coefficients of Raman S[sub 0](j) lines of H[sub 2] perturbed by He are performed up to 3000 K. They are used to test two approximate methods, the quantum coupled states theory and the RB semiclassical model. They are further applied to study the use of simple functional forms for fitting and extrapolating experimental data, and some new fits of experimental broadening coefficients are given. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The shape of the ν[sub 1] Raman Q branch of CH[sub 4] perturbed by Ar and He at room temperature has been studied. Stimulated Raman spectroscopy (SRS) experiments have been made in the 2915-2918 cm-1 spectral region for total pressures from 0.4 to 70 atm and mixtures of approx. 5% CH[sub 4] with He and Ar. Analysis of the spectra demonstrates that the shape of the Q branch is significantly influenced by line mixing and much narrower than what is predicted by the addition of individual line profiles. For the first time, a model is proposed for the calculation and analysis of the effects of collisions on the considered spectra. In this approach, the rotational part of the relaxation matrix is constructed, with no adjustable parameter, starting from semiclassical state-to-state rates. Two empirical constants which account for the shift and broadening of the branch due to vibrational effects are introduced and their values are determined from fits of measured spectra. Comparisons between measurements and results computed with and without the inclusion of line mixing are made. Although not perfect, our model satisfactory accounts for most effects of pressure at low densities, where rotational transfers are dominant, as well as at high densities, where the profile is strongly influenced by vibrational contributions. It is shown that collisions with He and Ar lead to different behaviors at elevated pressure. The influence of the perturbation introduced by the Fermi coupling between the ν[sub 1] and ν[sub 2]+ν[sub 4] levels is discussed and the rotational and vibrational contributions to the spectral shape are pointed out. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The paper presents high resolution Raman investigations of the Q(1) line of H[sub 2] in Ar mixture from low density (Doppler regime) to high density (collisional regime) analyzed with a unique line shape profile. Measurements are performed by stimulated Raman gain spectroscopy between 300 and 1000 K in a wide density range (from 0.2 to 11 amagat). All the observed spectral features are accurately described by a unified model recently proposed by two of the authors. This model accounts for a velocity-memory process, not restricted to the usual hard and soft limits. It also includes correlation between velocity- and phase-changing collisions. An exhaustive analysis of various possible mechanisms on the line shape is achieved. These mechanisms are the Dicke narrowing, the radiator speed dependence of the collisional broadening and shifting parameters, the collisionally induced speed-class exchange and the nonimpact effect. The present test shows the high consistency of the unified model, since it allows one to get a remarkable agreement with all the data by using a unique set of parameters. This model should be useful for optical diagnostics at moderate density, as required for combusting media or atmospheric work. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: We have measured a nuclear spin conversion time of 100min for H2O highly diluted in solid xenon at 4.2K. Compared with argon matrices, the conversion is very probably enhanced by the high natural abundance of xenon magnetic isotopes. The ν 2 mode absorption spectrum of rare gas isolated H2O, with polymerization and rotation translation coupling effects, is still debated. In this work, the ν 2 IR absorption spectrum of H2O in xenon is compared to that observed in neon, argon and krypton matrices. A new assignment of the rovibrational lines is proposed, demonstrated using the nuclear spin conversion studies. [Copyright &y& Elsevier]

Abstract: We have observed the time evolution of (o-H2) n :H2O clusters, trapped in solid H2 at 4.2K over a period of 178h, using O2 to catalyse the ortho to para-nuclear spin conversion of solid H2. The clusters observed at time 0 of the experiment (n =11–1) are identified for the first time. Each cluster increases and decreases over time and in turn, to give eventually rotating H2O. The H2O:O2 complex is assigned at 1593.1cm−1 in solid para-hydrogen. [Copyright &y& Elsevier]

The recently proposed spectral shear interferometry and the well-known z-scan techniques were employed for the determination of the nonlinear refractive index n2 of CS2, toluene and fused silica. The determined n2 values by both techniques were found to be in very good agreement. In addition, the role of the repetition rate of the laser is also investigated revealing its importance for the correct determination of both the size and the sign of the nonlinearity. [Copyright &y& Elsevier]