Photodissociation of H₂S: A New Pathway for the Production of Vibrationally Excited Molecular Hydrogen in the Interstellar Medium

Abstract

Hydrogen sulfide (H 2 S) is the most abundant S-bearing molecule in the solar nebula. Although its photochemistry has been studied for decades, the H 2 fragment channel is still not well-understood. Herein, we describe the photodissociation dynamics of H 2 S + hv → S( 1 S) + H 2 (X 1 Σ g + ) with the excitation wavelength of 122 nm ≤ λ ≤ 136 nm. The results reveal that the H 2 (X) fragments formed are significantly vibrationally excited, with the quantum yields of ∼87% of H 2 (X) fragments populated in vibrational levels v″ = 3, 4, 5, and 6. Theoretical analysis suggest that these H 2 products are formed on the H 2 S 4 1 A′ state surface following a nonadiabatic transition via an avoided crossing from the 3 1 A′ to 4 1 A′ state. The estimated quantum yield of the S( 1 S) + H 2 channel is ∼0.05, implying this channel should be incorporated into the appropriate interstellar chemistry models.

The vibronic state dependent predissociation of H2S: determination of all fragmentation processes

“…products is a signifier of non-adiabatic coupling to the 3 1 A′ (as well as the 2 1 A′) PES. Such a conclusion would also be consistent with the rationale offered recently for the observed energy disposal in the S( 1 S) + H 2 products 58 , which shows parallels with that associated with the major contributor to the S( 1 D) + H 2 product channel. As Fig.…”

“…A wavelength-dependent study of S( 1 S) production in the photolysis of H 2 S at shorter wavelengths has recently appeared elsewhere. 58 As in the case of S( 1 D) products, attempts to detect S( 1 S) atoms following photolysis at λ ≥ 146.50 nm were unsuccessful. The threshold energy for process (6) lies well below that provided by a λ = 143.00 nm photon, so the structure in the image and the accompanying P ( E T ) distribution necessarily reflects population of different v ′′, J ′′ states of the H 2 partner fragment.…”

“…Estimating the quantum yields of fragmentation channels probed in different PTS experiments is complicated by hard to define detection efficiencies, but the current knowledge suggests that the S( 1 S) + H 2 yield is lower than that for S( 1 D) + H 2 products. 58 Both constitute a photochemical source of vibrationally excited H 2 molecules when H 2 S molecules are exposed to the general interstellar radiation field (ISRF).…”

“…= 136.13 nm photons. A wavelength-dependent study of S( 1 S) production in the photolysis of H 2 S at shorter wavelengths has recently appeared elsewhere 58. As in the case of S( 1 D) products, attempts to detect S( 1 S) atoms following photolysis at   146.50 nm were unsuccessful.…”

“…Downloaded on 2/20/2023 4:47:28 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.corresponding S( 1 D) + H 2 products, but the high v″ (v″ ~3-6), low J″ becomes dominant as the excitation wavelength is reduced 58.…”