Electron-Induced Radiolysis of Astrochemically Relevant Ammonia Ices

Abstract

We elucidate mechanisms of electron-induced radiolysis in cosmic (interstellar, planetary, and cometary) ice analogs of ammonia (NH3), likely the most abundant nitrogen-containing compound in the interstellar medium (ISM). Astrochemical processes were simulated under ultrahigh vacuum conditions by high-energy (1 keV) and low-energy (7 eV) electron-irradiation of nanoscale thin films of ammonia deposited on cryogenically cooled metal substrates. Irradiated films were analyzed by temperature-programmed desorption (TPD). Experiments with ammonia isotopologues provide convincing evidence for the electron-induced formation of hydrazine (N2H4) and diazene (N2H2) from condensed NH3. To understand the dynamics of ammonia radiolysis, the dependence of hydrazine and diazene yields on incident electron energy, electron flux, electron fluence, film thickness, and ice temperature were investigated. Radiolysis yield measurements versus (1) irradiation time and (2) film thickness are semiquantitatively consistent with a reaction mechanism that involves a bimolecular step for the formation of hydrazine and diazene from the dimerization of amidogen (NH2) and imine (NH) radicals, respectively. The apparent decrease in radiolysis yield of hydrazine and diazene with decreasing electron flux at constant fluence may be due to the competing desorption of these radicals at 90 K under low incident electron flux conditions. The production of hydrazine at electron energies as low as 7 eV and an ice temperature of 22 K is consistent with condensed phase radiolysis being mediated by low-energy secondary electrons produced by the interaction of high-energy radiation with matter. These results provide a basis from which we can begin to understand the mechanisms by which ammonia can form more complex species in cosmic ices.

Combined Ammonia and Electron Processing of a Carbon-Rich Ruthenium Nanomaterial Fabricated by Electron-Induced Deposition

“…Here, the presence of m / z 30 points to C 2 H 6 [ 32 ] in line with the reducing action of NH 3 . Intensity at m / z 30 could also be related to diazene (N 2 H 2 ) desorption, but it would be surprising to detect such an unstable species but not the more stable NH 3 irradiation product hydrazine (N 2 H 4 ) at m / z 32 [ 33 ].…”

“…Here, the presence of m/z 30 points to C 2 H 6 [32] in line with the reducing action of NH 3 . Intensity at m/z 30 could also be related to diazene (N 2 H 2 ) desorption, but it would be surprising to detect such an unstable species but not the more stable NH 3 irradiation product hydrazine (N 2 H 4 ) at m/z 32 [33]. In Figure 5, RAIR spectra are shown for a NH3 film that has been condensed on top of an (EtCp)2Ru model deposit at T = 110 K in the pristine state (top), after several stages of electron exposure at E0 = 31 eV (middle), and after a final thermal annealing step to T = 450 K in the course of a TDS run (bottom).…”

Photochemistry vs. radiation chemistry of cosmic ice analogs

“…After irradiation, the crystal is radiatively heated to cause desorption of products, which are monitored by a quadrupole mass spectrometer. Our results demonstrate that both radiation chemistry and photochemistry of condensed ammonia produce hydrazine (N 2 H 4 ), diazene (N 2 H 2 ), triazane (N 3 H 5 ), and one or more isomers of N 3 H 3 (Mullikin et al 2018;Shulenberger et al 2019). Semiquantitative trends in the dependence of product yield on film thickness and radiation fluence indicate that the formation mechanism for both hydrazine and diazene involve radical-radical reactions (of amidogen (•NH 2 ) and imidogen (•NH) radicals, respectively).…”

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.…”