Novel approach to distinguish between vacuum UV-induced ice photodesorption and photoconversion

Abstract

Context. In cold regions of the interstellar medium with intense ultraviolet radiation fields, photodesorption has been suggested as a nonthermal desorption mechanism promoting the transition of molecules from the solid state to the gas phase. Laboratory experiments measuring photodesorption rates are crucial in attempting to explain high molecular gas phase abundances of species that are expected to form in the solid state, such as methane, methanol, and acetonitrile, and to aid astrochemical modeling. Due to the convoluted competition between photodesorption and photoconversion, it is far from trivial to derive accurate photodesorption rates. Aims. The aim of this study is to apply a new methodology to discriminate between the two processes. The method has been validated using the well-studied case of CO and extended to CH4, CH3OH, and CH3CN. Methods. Vacuum ultraviolet (VUV; photon energy of 7–10.2 eV) irradiated ices at 20 K are studied, first as a pure CH4, CH3OH, or CH3CN ice and subsequently with an Ar coating on top. The latter is transparent to the VUV photons (wavelength below 200 nm), but it quenches the photodesorption process. Comparing the laser desorption post ionization time-of-flight mass spectrometry of the ices with and without the Ar coating provides information on the different interactions of the VUV photons with the ice. Results. The newly developed experimental technique allowed for a derivation of photodesorption rates for ices at 20 K of: CO (3.1 ± 0.3)×10−3 mol. photon−1, CH4 (3.1 ± 0.5)×10−2 mol. photon−1, and upper limits for CH3OH (< 6 × 10−5 mol. photon−1) and CH3CN (< 7.4 × 10−4 mol. photon−1); in the latter case, no literature values have been reported yet. The newly introduced approach provides more insight into the photodesorption process, in particular, for commonly observed complex organic molecules (COMs). Photoconversion cross sections are presented in the 7–10.2 eV range. The possible role of photodesorption and photoconversion in the formation of interstellar COMs is discussed.

Photodesorption of acetonitrile CH3CN in UV-irradiated regions of the Interstellar Medium: an experimental evidence

“…Finally, the VUV photodesorption yield of intact CH 3 CN is ∼ 2.5×10 −5 molecule/photon at 10.5 eV. Our results compare very well to the recent study of Bulak et al 2020 who could not measure directly the photodesorption yield of CH 3 CN from pure CH 3 CN ice but provided an upper limit of 7.4 × 10 −4 molecule/photon, compatible with our measurements.…”

Photolysis of acetonitrile in a water-rich ice as a source of complex organic molecules: CH3CN and H2O:CH3CN ices

“…Prior to deposition, atmospheric gases trapped in the samples are removed by several freeze-pump-thaw cycles. The growth rates are determined in advance by HeNe interference measurements as discussed in previous works (Baratta & Palumbo 1998;Bulak et al 2020).…”

“…For an overview of photo-induced processes, see Fig. 1 in Bulak et al (2020). The photon flux of the MDHL is measured at the location of the substrate with a NIST calibrated photodiode and found to be (2.5 ± 0.5) × 10 14 photons cm −2 s −1 .…”

Quantification of O2 formation during UV photolysis of water ice: H2O and H2O:CO2 ices

“…A continuous deposition is regulated by a high precision needle valve, which allows the gas-phase sample to go into the main chamber. The growth rate of the ice is determined in advance using He-Ne laser (wavelength 632.8 nm) interference measurements (Baratta & Palumbo 1998;Bulak et al 2020). To calculate the deposition rate for H 2 O (background deposition) and CO 2 (front deposition), the refractive indices (1.2, and 1.21) and densities (0.94 g cm −3 , 0.98 g cm −3 ), respectively, are taken from the literature (Kofman et al 2019;Satorre et al 2008;Jenniskens et al 1998).…”