Ultraviolet photolysis of H2S and its implications for SH radical production in the interstellar medium

Abstract

Hydrogen sulfide radicals in the ground state, SH(X), and hydrogen disulfide molecules, H2S, are both detected in the interstellar medium, but the returned SH(X)/H2S abundance ratios imply a depletion of the former relative to that predicted by current models (which assume that photon absorption by H2S at energies below the ionization limit results in H + SH photoproducts). Here we report that translational spectroscopy measurements of the H atoms and S(1D) atoms formed by photolysis of jet-cooled H2S molecules at many wavelengths in the range 122 ≤ λ ≤155 nm offer a rationale for this apparent depletion; the quantum yield for forming SH(X) products, Γ, decreases from unity (at the longest excitation wavelengths) to zero at short wavelengths. Convoluting the wavelength dependences of Γ, the H2S parent absorption and the interstellar radiation field implies that only ~26% of photoexcitation events result in SH(X) products. The findings suggest a need to revise the relevant astrochemical models.

Photolysis Production and Spectroscopic Investigation of the Highest Vibrational States in H2 (X1Σg+ v = 13, 14)

“…The energy required for complete dissociation of ground state H 2 S into an S( 1 D 2 ) atom and two H atoms is about 69935(25) cm –1 . 36 The corresponding two-photon energy at 291.5 nm would reach only to about 1300 cm –1 below the H 2 dissociation limit, insufficient to produce H 2 fragments in v = 13 and v = 14. A 2 + 1 REMPI spectrum of H 2 S shows a resonance at 281.8 nm, and when the dissociation laser wavelength was fixed to this resonance at 281.8 nm, the energetic region of 1000 cm –1 above the dissociation limit of H 2 can be probed and v = 13, 14 produced.…”

“…The two-photon UV-photolysis proceeds via the path: The wavelength of the dissociation laser is set at 281.8 nm, as opposed to 291 nm in the previous studies, since the energy required for complete dissociation of H 2 S to form an S( 1 D 2 ) atom and two H( 2 S) atoms is about 69935(25) cm –1 . 36 The two-photon energy for 281.8 nm dissociation lies about 1000 cm –1 above this limit, which is needed to produce H 2 in the highest vibrational levels close to the dissociation limit. Focused UV-pulses at energies of 4.5 mJ are used for the photolysis step.…”

Sulfur Ice Astrochemistry: A Review of Laboratory Studies

“…The UV irradiation of H 2 S ice has most recently been revisited by Zhou et al (2020), who wished to address the apparent depletion of HS/H 2 S ratios observed in the interstellar medium relative to those predicted by contemporary models. Their experimental results revealed a wavelength dependence for the quantum yield of HS formation via H 2 S photodissociation.…”

Sulfur Poisoning on Rh Nanoparticles but Sulfur Promotion on Its Single-Site Catalyst for Carbonylation

“…The in situ ow-tube free-electron laser time of ight mass spectrometry (In situ FEL/TOF MS) were conduct at Dalian Coherent Light Source [31][32][33][34] to detect the intermediates and the radicals during methanol gas phase carbonylation reaction. Laser desorption ionization/time of ight mass spectrometry (LDI-TOF MS) was conducted to detect the dissociative fragment of spent Rh 1 /AC catalyst in normal and S-feed and further identify its single complex structure.…”