Photochemistry of H2S

The photodissociation dynamics of H₂S, the heavier homologue of H₂O, have also received much attention in recent decades. Solar photodissociation is an important destruction route for interstellar H₂S molecules [63]. Astrochemical databases currently recommend S–H bond fission, yielding H + SH fragments, as the sole decay process following photoexcitation at all energies up to the first ionization potential [1,64]. Recent VUV-FEL-enabled photodissociation studies of H₂S have revealed shortcomings in this picture. Since the S atom and SH radical abundances in the ISM are strongly linked with H₂S, and its photodissociation by VUV photons, a comprehensive understanding of H₂S photochemistry is clearly desirable for improved astrochemical modeling.

As with H₂O, the electronic absorption spectrum of H₂S shows continuous absorption at long wavelengths and a structured region, associated with absorption to predissociated Rydberg states, at shorter wavelengths (Fig. 6A). The velocity distributions of the H atom and S(¹D) atoms formed by FEL-induced photolysis of jet-cooled H₂S molecules have been recorded at many wavelengths in the range 122 ≤ λ ≤ 155 nm [21]. S(¹S) photoproducts have also been detected, by TSVMI, following VUV-FEL excitation in the range 122 ≤ λ ≤ 136 nm [65]. The P(E_T) spectra derived from HRTOF measurements at three widely separated wavelengths, shown in Fig. 6B–D, display a clear evolution. Long-wavelength excitation yields ground-state SH(X) fragments in a very broad spread of rovibrational levels; the quantum yield for forming H + SH(X) products is unity at λ ≥ 157.6 nm. Electronically excited SH(A) products become increasingly important as the photolysis wavelength is reduced, however, and are the exclusive SH product at λ = 122.95 and 121.6 nm. Formation of SH(A) super-rotors is evident in Fig. 6C and D, and can be explained by non-adiabatic coupling to and dissociation on a PES with a topography reminiscent of that of the state of H₂O [21]. Since the TBD channel to S(¹D) + 2H products is evident in the P(E_T) spectra derived from both the H-atom TOF measurements and the S(¹D) ion images recorded at shorter wavelengths, this component serves as a reference when estimating branching ratios for the H and H₂ forming product channels. These experimental data returned a quantum yield for forming H₂ + S(¹D) products at λ = 139.11 nm of ≤0.12. The quantum yield for S(¹S) + H₂ production is estimated to be smaller still [65], implying that S–H bond fission is the dominant primary event at this wavelength [21].

(A) Wavelength dependences of the general interstellar radiation field (ISRF, black line) and the total absorption (σ_tot, navy line) and photoionization (σ_ion, blue line) cross sections of H₂S. The P(E_T) spectra derived from H-atom TOF spectra following photodissociation of H₂S at (B) 154.53 nm, (C) 139.11 nm and (D) 122.95 nm (i.e. when exciting on the features marked with red arrows in (A)), with the detection axis aligned at the magic angle (θ = 54.7°) to the polarization vector of the photolysis laser radiation, ϵ. The inset in (C) shows an expanded view of the low E_T part of the corresponding θ = 0° and 90° data. The combs on these spectra show the E_T values associated with formation of H atoms in conjunction with selected rovibrational levels of the primary SH(X) and SH(A) fragments and, in (C), with H atoms formed by predissociation of primary SH(A, v = 0, N) fragments. The maximum E_T values associated with the various primary fragmentation channels are shown by colored arrows. (E) The quantum yield, Γ, for forming SH(X) photoproducts (red dots). The sigmoidal function (black line) through these data is used to derive the overall SH(X) product quantum yield ([21]).

However, all SH(A) primary products will predissociate on a nanosecond (or shorter) timescale to yield another H atom (along with an S(³P_J) partner) [66]. This secondary dissociation contributes another progression in the P(E_T) spectrum derived from HRTOF measurements—as illustrated in the case of the λ = 139.11 nm data in the inset within Fig. 6C. Thus, only those H₂S photodissociation events that yield SH(X) radical products should contribute to the SH/H₂S abundance ratios observed in different regions of the ISM. Convoluting the wavelength-dependent H₂S absorption, the spectrum of the general interstellar radiation field (ISRF) (Fig. 6A) and the quantum yield for forming SH(X) radicals determined at each wavelength investigated (Fig. 6E) reveals that only ∼26% of H₂S molecules excited by the general ISRF would yield a stable SH radical [21]. This experimental result may explain the interstellar SH/H₂S abundance ratio of ∼13% reported from the star-forming region W49N [67] and is generally consistent with more recent model predictions that suggest an [SH]/[H₂S] ratio of ≤0.2–0.6 at the edge of PDR regions in the ISM [68]. This quantum yield estimate implies that atomic sulfur is the dominant S-containing species from H₂S processing by the ISRF—a result that accords well with the observed strong correlation between measured S and H₂S signals in the coma of comet 67P/Churyumov–Gerasimenko [69].

The S-atom abundances can also provide clues about star-formation history, connecting the local and distant universes [70]. There are several S-atom transitions available for observational studies in the stellar spectra, including the ‘forbidden’ S(¹D) → S(³P₂) transition at 1082 nm [71,72]. As one of the most abundant S-bearing molecules in many interstellar environments, H₂S photolysis might be an important source of the observed S(¹D) atoms—a proposal that might be amenable to testing by careful linewidth measurements.

This subsection provides a further striking illustration of the richness and complexity of the H₂S photodissociation process. Figure 6C displays the P(E_T) spectrum obtained from the HRTOF measurements following the FEL excitation of a jet-cooled H₂S sample at λ ∼139.1 nm, which lies in the middle of the VUV wavelength range where H, SH(X), SH(A), S(¹D) and H₂ fragments have all been observed [21]. The bandwidth of the FEL pulse ensured that the experiment sampled much of the origin (i.e. v = 0 ← v = 0) band of the ¹B₁ ← ¹A₁ transition. This ¹B₁ Rydberg state predissociates (by non-adiabatic coupling to dissociative valence excited states) at a rate that is sufficiently slow that many of its rotational (J_KaKc) levels contribute resolvable fine structure within the ¹B₁ ← ¹A₁ band.

This is illustrated in Fig. 7A, which shows the photofragment excitation (PHOFEX) spectra for forming H atoms and S(¹D) atoms recorded by the HRTOF and TSVMI techniques, respectively, using a much narrower bandwidth VUV FWM pump laser source [19]. The spectra show four well-resolved features, associated with transitions to specific rotational levels of the ¹B₁(v = 0) state, but the relative line intensities are obviously different [19]. Earlier spectroscopy studies [73] of this ¹B₁ state had identified both homogeneous (i.e. vibronic) and heterogeneous (i.e. Coriolis- or rotationally induced) predissociation mechanisms but were silent with regard to the products. The rate of Coriolis-driven predissociation was shown to scale with <J_b²> (i.e. with the expectation value of the square of the angular momentum about the b-inertial axes in the excited rotational level), indicating that this predissociation pathway involved coupling to a continuum of ¹A′ symmetry.

(A) The H-atom (upper curve) and S(¹D)-atom (lower curve) PHOFEX spectra obtained following photodissociation of a jet-cooled H₂S in Ar sample at wavelengths in the range 139.14 ≥ λ ≥ 138.99 nm, offset vertical by 0.5-arb. units for clarity. The dominant transitions contributing to the four features are indicated (revised from [19]). The 3D contour plots of the P(E_T) distributions of the H + SH products from the photodissociation of H₂S via (B) the 0₀₀ ← 1₁₀ and (C) the 1₁₀ ← 0₀₀ lines, with E_T ≤ 41 000 cm⁻¹. Analogous 3D contour plots of the S(¹D) + H₂ products when exciting on (D) the 0₀₀ ← 1₁₀ and (E) the 1₁₀ ← 0₀₀ lines at ∼139 nm, with E_T ≤ 35 000 cm⁻¹ (data in all cases from [19]). The double-headed arrow in panel B shows the alignment of ϵ. The outer rings in plots (B) and (C) are associated with the population of rovibrational levels of the ground-state SH(X) radical products, whereas the inner structures are mainly due to SH(A) and/or S(³P) + 2H products. The rings in plots (D) and (E) are associated with the formation of rovibrational states of the H₂(X) products.

The 3D contour plots shown in Fig. 7 highlight the very different P(E_T) distributions obtained from the HRTOF (panels B and C) and S(¹D) imaging (panels D and E) data when exciting at, respectively, λ = 139.125 nm (panels B and D) and λ = 139.051 nm (panels C and E). The former involves a single rovibronic transition (the 0₀₀ ← 1₁₀ line). The populated level has <J_b²> = 0 and decays by a pure homogeneous predissociation pathway yielding S(¹D) + H₂(X, high v, low J) and/or 2H fragments (Fig. 7D) and H + SH(X) fragments in a very broad range of v, N levels (Fig. 7B) products following vibronic coupling to a dissociative valence continuum of ¹A″ symmetry. Excitation at λ = 139.051 nm, in contrast, is dominated by the 1₁₀ ← 0₀₀ transition. The excited level in this case has <J_b²> = 1 and the additional heterogeneous decay route by Coriolis coupling to the ¹A′ continuum can contribute, resulting in S(¹D) + H₂(X, low v, high J) (Fig. 7E) and H + SH(A, low v, high N) (Fig. 7C) products. As noted above, the SH(A) products predissociate further, to H + S(³P) atoms. The two other peaks in the PHOFEX spectrum are blended features involving more than one rovibronic transition. The photoexcited levels populated when exciting either feature can each decay by both predissociation pathways, but with different relative rates, so the measured P(E_T) spectra are sensitively dependent on the exact excitation wavelength, the sample temperature and thus the relative populations of the various H₂S(, v = 0, J_KaKc) levels [19].

These studies reveal another important and fundamental detail that is not immediately evident in the P(E_T) spectra plotted with the compressed scale used in Fig. 7. H₂S molecules contain two identical H nuclei (fermions) and symmetry dictates that each rotational level of H₂S must satisfy either ortho- or para-nuclear spin statistics. In the case of H₂S, these are distinguished by whether the sum K_a + K_c in the ground state is, respectively, an odd or even number. Key to the current discussion, the 0₀₀ ← 1₁₀ (λ = 139.125 nm) and 1₁₀ ← 0₀₀ (λ = 139.051 nm) transitions sample, respectively, ortho- and para-H₂S molecules and analysis of the P(E_T) spectra of the resulting S(¹D) + H₂(X, v, J) photoproducts reveals rigorous conservation of nuclear spin symmetry in the dissociation process: ortho-H₂S molecules yield only ortho-H₂ (i.e. odd J) products and para-H₂S molecules yield only para-H₂ (i.e. even J) products.

These recent studies of H₂S photolysis provide some of the most complete experimental investigations of molecular photofragmentation processes reported to date, affording initial parent quantum state selection and detailed investigation of competing product channels. After H₂O, H₂S is probably currently the second most comprehensively studied molecule. Its photodissociation dynamics have been studied across the whole range of VUV wavelengths up to the first ionization limit and all spin-allowed dissociation channels have been explored. The recent experimental studies afford detailed views of different photofragmentation pathways in H₂S, but any complete interpretation of the dynamics still requires better knowledge of the topographies of, and non-adiabatic couplings between, the various excited-state PESs.