Indirect X-ray photodesorption of 15N2 and 13CO from mixed and layered ices

X-ray photodesorption yields of 15N2 and 13CO are derived as a function of the incident photon energy near the N (~ 400 eV) and O K-edge (~ 500 eV) for pure 15N2 ice and mixed 13CO:15N2 ices. The photodesorption spectra from the mixed ices reveal an indirect desorption mechanism for which the desorption of 15N2 and 13CO is triggered by the photo-absorption of respectively 13CO and 15N2. This mechanism is confirmed by the X-ray photodesorption of 13CO from a layered 13CO/15N2 ice irradiated at 401 eV, on the N 1s -> π* transition of 15N2. This latter experiment enables to quantify the relevant depth involved in the indirect desorption process, which is found to be 30 - 40 ML in that case. This value is further related to the energy transport of Auger electrons emitted from the photo-absorbing 15N2molecules that scatter towards the ice surface, inducing the desorption of 13CO. The photodesorption yields corrected from the energy that can participate to the desorption process (expressed in molecules desorbed by eV deposited) do not depend on the photon energy hence neither on the photo-absorbing molecule nor on its state after Auger decay. This demonstrates that X-ray induced electron stimulated desorption (XESD), mediated by Auger scattering, is the dominant process explaining the desorption of 15N2 and 13CO from the ices studied in this work.

Mechanism of Indirect Photon-Induced Desorption at the Water Ice Surface

Electronic excitations near the surface of water ice lead to the desorption of adsorbed molecules, through a so far debated mechanism. A systematic study of photon-induced indirect desorption, revealed by the spectral dependence of the desorption (7-13 eV), is conducted for Ar, Kr, N_{2}, and CO adsorbed on H_{2}O or D_{2}O amorphous ices. The mass and isotopic dependence and the increase of intrinsic desorption efficiency with photon energy all point to a mechanism of desorption induced by collisions between adsorbates and energetic H/D atoms, produced by photodissociation of water. This constitutes a direct and unambiguous experimental demonstration of the mechanism of indirect desorption of weakly adsorbed species on water ice, and sheds new light on the possibility of this mechanism in other systems. It also has implications for the description of photon-induced desorption in astrochemical models.

Detection of two interstellar polycyclic aromatic hydrocarbons via spectral matched filtering

Unidentified infrared emission bands are ubiquitous in many astronomical sources. These bands are widely, if not unanimously, attributed to collective emissions from polycyclic aromatic hydrocarbon (PAH) molecules, yet no single species of this class has been identified in space. Using spectral matched filtering of radio data from the Green Bank Telescope, we detected two nitrile-group-functionalized PAHs, 1- and 2-cyanonaphthalene, in the interstellar medium. Both bicyclic ring molecules were observed in the TMC-1 molecular cloud. In this paper, we discuss potential in situ gas-phase PAH formation pathways from smaller organic precursor molecules.

A new technique for determining the refractive index of ices at cryogenic temperatures

A reflection-absorption optical (RAO) spectrometer, operating across the ultra-violet/visible (UV/visible) wavelength region, has been developed that allows simultaneous measurements of optical properties and thickness of thin solid films at cryogenic temperatures in ultrahigh vacuum. The RAO spectrometer enables such measurements to be made after ice deposition, as opposed to most current approaches which make measurements during deposition. This allows changes in the optical properties and in the thickness of the film to be determined subsequent to thermal, photon or charged particle processing. This is not possible with current techniques. A data analysis method is presented that allows the wavelength dependent n and k values for ices to be extracted from the reflection-absorption spectra. The validity of this analysis method is shown using model data from the literature. New data are presented for the reflection UV/visible spectra of amorphous and crystalline single component ices of benzene, methyl formate and water adsorbed on a graphite surface. These data show that, for benzene and methyl formate, the crystalline ice has a larger refractive index than amorphous ice, reflecting changes in the electronic environment occurring in the ice during crystallisation. For water, the refractive index does not vary with ice phase.

A fibre-coupled UHV-compatible variable angle reflection-absorption UV/visible spectrometer

We present a novel UV/visible reflection-absorption spectrometer for determining the refractive index, n, and thicknesses, d, of ice films. Knowledge of the refractive index of these films is of particular relevance to the astrochemical community, where they can be used to model radiative transfer and spectra of various regions of space. In order to make these models more accurate, values of n need to be recorded under astronomically relevant conditions, that is, under ultra-high vacuum (UHV) and cryogenic cooling. Several design considerations were taken into account to allow UHV compatibility combined with ease of use. The key design feature is a stainless steel rhombus coupled to an external linear drive (z-shift) allowing a variable reflection geometry to be achieved, which is necessary for our analysis. Test data for amorphous benzene ice are presented as a proof of concept, the film thickness, d, was found to vary linearly with surface exposure, and a value for n of 1.43 ± 0.07 was determined.

Molecular hydrogen production from amorphous solid water during low energy electron irradiation

This work investigates the production of molecular hydrogen isotopologues (H₂, HD, and D₂) during low energy electron irradiation of layered and isotopically labelled thin films of amorphous solid water (ASW) in ultrahigh vacuum. Experimentally, the production of these molecules with both irradiation time and incident electron energy in the range 400 to 500 eV is reported as a function of the depth of a buried D₂O layer in an H₂O film. H₂ is produced consistently in all measurements, reflecting the H₂O component of the film, though it does exhibit a modest reduction in intensity at the time corresponding to product escape from the buried D₂O layer. In contrast, HD and D₂ production exhibit peaks at times corresponding to product escape from the buried D₂O layer in the composite film. These features broaden the deeper the HD or D₂ is formed due to diffusion. A simple random-walk model is presented that can qualitatively explain the appearance profile of these peaks as a function of the incident electron penetration.

Non-covalent interaction of benzene with methanol and diethyl ether solid surfaces

We have investigated the interactions involved at the interface of binary, layered ices (benzene on methanol and on diethyl ether) by means of laboratory experiments and ab initio calculations on model clusters.

Electrons, excitons and hydrogen bonding: electron-promoted desorption from molecular ice surfaces

Desorption of benzene from methanol and diethyl ether ices during irradiation with 250 eV electrons is reported and compared with our previous work on benzene/water ices to highlight the role of hydrogen bonding in excitation transport.

Peeling the astronomical onion

This work presents a study of water mobility on interstellar dust grain analogues at temperatures as low as 18 K. The work indicates that water forms pure domains rather than covering the entire grain, thereby leaving bare dust grain surfaces available on which other molecules can adsorb as well as themselves providing surfaces for further adsorption from the interstellar gas.