UV photoprocessing of NH3 ice: photon-induced desorption mechanisms

Abstract

Ice mantles detected on the surface of dust grains toward the coldest regions of the interstellar medium can be photoprocessed by the secondary ultraviolet (UV) field present in dense cloud interiors. In this work, we present UV-irradiation experiments under astrophysically relevant conditions of pure NH 3 ice samples in an ultra-high vacuum chamber where solid samples were deposited onto a substrate at 8 K. The ice analogs were subsequently photoprocessed with a microwave-discharged hydrogen-flow lamp. The induced radiation and photochemistry led to the production of H 2 , N 2 and N 2 H 4 . In addition, photodesorption to the gas phase of the original ice component, NH 3 , and two of the three detected photoproducts, H 2 and N 2 , was observed thanks to a quadrupole mass spectrometer (QMS). Calibration of the QMS allowed quantification of the photodesorption yields, leading to Y pd (NH 3 ) = 2.1 +2.1 −1.0 x 10 −3 molecules incident photon , which remained constant during the whole experiments, while photodesorption of H 2 and N 2 increased with fluence, pointing toward an indirect photodesorption mechanism involving energy transfer for these species. Photodesorption yield of N 2 molecules after a fluence equivalent to that experienced by ice mantles in space was similar to that of the NH 3 molecules (Y pd (N 2 ) = 1.7 +1.7 −0.9 x 10 −3 molecules incident photon ).

Synthesis of the first nitrogen-heterocycles in interstellar ice analogs containing methylamine (CH$_3$NH$_2$) exposed to UV radiation: Formation of trimethylentriamine (TMT, c-(-CH$_2$-NH)$_3$) and hexamethylentetramine (HMT, (CH$_2$)$_6$N$_4$)

“…In the pure CH 3 NH 2 ice sample, radicals bearing NH groups appear as broad bands at both sides of the N-H stretching region of the IR spectrum. In line with Martín-Doménech et al (2018), radicals appear readily within the irradiation period, but they are readily consumed, keeping a constant formation and destruction rate for longer irradiation times.…”

“…As an example the high desorption temperature of pure NH 3 ice, 95 K, compared to pure CH 4 ice, 40 K, is due to H bonds in NH 3 ice (e.g. Carrascosa et al 2020;Martín-Doménech et al 2018).…”

Characterizing Interstellar Medium, Planetary Surface and Deep Environments by Spectroscopic Techniques Using Unique Simulation Chambers at Centro de Astrobiologia (CAB)

“…We measured the absorption cross-sections of several ice species for the first time, which allowed to estimate the efficiency of photodesorption per absorbed photon in the ice [11,12,13]. We extended this study of CO ice UV-photodesorption to other molecular ice components, and started to explore the effect of soft X-ray irradiation in the photodesorption of ice analogues [14,15,33,34,35,36,37,38,39,40,41,42].…”

Systematic investigation of CO2 : NH3 ice mixtures using mid-IR and VUV spectroscopy – part 2: electron irradiation and thermal processing

“…electrons, UV photons) of pure NH 3 (ref. [29][30][31][32] with a desorption temperature between 155-172 K. 30,32,33 The gas-phase VUV spectra of N 2 H 4 and N 2 D 4 are characterised by three absorption peaks with maxima near 144 nm, 168 nm & 194 nm. 34,35 The three absorption peak maxima in Fig.…”