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

Photochemistry plays a significant role in shaping the chemical reaction network in the solar nebula and interstellar clouds. However, even in a simple triatomic molecule photodissociation, determination of all fragmentation processes is yet to be achieved. In this work, we present a comprehensive study of the photochemistry of H2S, derived from cutting-edge translational spectroscopy measurements of the H, S(1D) and S(1S) atom products formed by photolysis at wavelengths across the range 155–120 nm. The results provide detailed insights into the energy disposal in the SH(X), SH(A) and H2 co-fragments, and the atomisation routes leading to two H atoms along with S(3P) and S(1D) atoms. Theoretical calculations allow the dynamics of all fragmentation processes, especially the bimodal internal energy distributions in the diatomic products, to be rationalised in terms of non-adiabatic transitions between potential energy surfaces of both 1A′ and 1A′′ symmetry. The comprehensive picture of the wavelength-dependent (or vibronic state-dependent) photofragmentation behaviour of H2S will serve as a text-book example illustrating the importance of non-Born–Oppenheimer effects in molecular photochemistry, and the findings should be incorporated in future astrochemical modelling.


Experimental methods
The H atom product translational energy distributions were recorded using a tuneable vacuum ultraviolet (VUV) free-electron laser (FEL) along with the H-atom Rydberg tagging time-of-flight (HRTOF) probe technique, performed using the recently constructed end-station for molecular photochemistry around the VUV FEL beam line at the Dalian Coherent Light Source (DCLS). 1,2 The VUV FEL operates in the high gain harmonic generation mode, in which the seed laser (λ ~ 240-360 nm) generated from a Ti: sapphire laser is injected to interact with the electron beam in the modulator. The electron beam is generated from a photocathode RF gun and accelerated to a beam energy of ~300 MeV by 7 S-band accelerator structures, with a bunch charge of 500 pC. The micro-bunched beam is then sent through the radiator, which is tuned to the nth harmonic of the seed wavelength, and coherent FEL radiation with wavelength λ/n is emitted. The optimization of the linear accelerator yields a high-quality light beam with an emittance of ~1.5 mm· mrad, an energy spread of ~1‰, a pulse duration of ~1.5 ps and maximum pulse energies >100 μJ pulse -1 . In the present study, the VUV-FEL operated at 10 Hz, and a typical spectral bandwidth is ~50 cm −1 .
In the HRTOF detection method, the H atom products are promoted from the ground state to a high n Rydberg state via a two-step excitation.
Step one involves resonant excitation from the n = 1 to n = 2 state at the Lyman-α wavelength (λ = 121.6 nm), while step two uses UV laser excitation at λ ~365 nm to further excite the H atom from the n = 2 state to a high-n (n = 30-80) Rydberg state lying slightly below the ionization threshold. The coherent 121.6 nm radiation was generated by difference four-wave mixing (DFWM) involving two 212.556 nm photons and one 845.384 nm photon overlapped in a stainless-steel cell filled with a 3:1 ratio Ar/Kr gas mixture. The neutral Rydberg-tagged H atom photofragments flew a distance d ~280 mm before reaching a grounded mesh mounted close in front of Z-stack rotatable microchannel plate (MCP) detector, where they were immediately field ionized by the ~ 2000 V cm 1 electric field. The signal detected by the MCP was then amplified by a fast pre-amplifier and counted by a multichannel scaler. The recorded TOF data were converted to the corresponding H atom kinetic energy distributions based on momentum conservation, as shown in the main text. The sample beam was generated by expanding a mixture of 1% H2S and Ar at a stagnation pressure of 760 Torr through a 0.5 mm-diameter pulsed nozzle. The molecular beam crossed the VUV-FEL beam at right angles.
The S( 1 D) / S( 1 S) atom product translational energy distributions were recorded using the VUV FEL pump time-sliced velocity map imaging (TSVMI) probe technique 3,4 , which involves a molecular beam, photolysis and probe lasers, and the detection system. The pulsed supersonic beam was again generated by expanding a mixture of 1% H2S and Ar at a stagnation pressure of 760 Torr into the source chamber where it was skimmed before entering the ion optics assembly (IOA, 23-plate ion optics). The beam passed through a 2 mm hole in the first electrode and propagated along the centre axis of the IOA towards the centre of the front face of the detector. The molecular beam was intersected at 90° angles by the counter-propagating photolysis and probe laser beams between the second and the third plates of the IOA. The

Computation methods
Electronic Structure calculations for the five lowest-lying states of both 1 A′ and 1 A″ symmetry was performed at the internally contracted multi-reference configuration interaction corrected with Davidson correction (ic-MRCI+Q) level. To obtain appropriate reference wavefunctions for the MRCI calculation, a prior state-averaged completed active space selfconsistent field (SA-CASSCF) calculation containing 18 A′ states and 18 A″ states was implemented. To provide a precise description of the highly excited Rydberg states, an exceptionally large active space (8e,13o) composed of nine a′ and four a″ active orbitals in Cs symmetry was applied. In addition, quite large basis sets were used for this system. The basis set for the S atom was a correlation-consistent, polarized, valence, quadruple zeta (cc-pVQZ) basis extended by six diffuse s functions, six diffuse p functions and eight diffuse d functions.
For the H atoms, a cc-pVQZ basis set extended by two diffuse s functions and two diffuse p functions was used. All the aforementioned ab initio calculations were performed using the MOLPRO 2015.1 package.
Potential energy surfaces were fitted using the Gaussian Process Regression (GPR) method based on >700 points covering both the interaction region and the two dissociation channels.
The permutation symmetry of the two identical H atoms in H2S was accommodated using permutation invariant polynomials (PIPs).
Oscillator strengths for transitions from the 1 1 A′ ground state to the first four excited states of A′ symmetry and the first five excited states of A″ symmetry were obtained by equation-ofmotion coupled cluster single and double excitation (EOM-CCSD) calculations at the ground state equilibrium geometry (θHSH = 92.2°, rSH1 = rSH2 = 1.34 Å). Inspection of the results listed in Tables 1 and S1 confirms that the discrepancies between the VEEs calculated using the EOM-CCSD and MRCI methods are reassuringly small ( 0.1 eV in all cases).        S5. (a) The best-fit simulation (red) of the experimental (black) P(ET) spectrum derived from the H atom TOF spectrum obtained following photodissociation of H2S at  = 151.64 nm, measured along a detection axis aligned at  = 54.7 to the  vector of the photolysis laser radiation, together with (b) the SH(X) product vibrational and rotational level population distributions derived therefrom.       1 A″, (c) 2 1 A′ and (d) 3 1 A′ PESs plotted using Jacobi coordinates R (defining the distance between the S atom and the H2 centre of mass) and the H-H bond length (rHH) for an S-H2 Jacobi angle of 90. The vertical region is indicated by the white dot in each panel and the energy contours are labelled in eV, defined relative to the ground state minimum energy geometry at E = 0. for H2(v″ = 5) products), with the avoided crossing between the two PECs at E ~9 eV circled. The energies are defined relative to the ground state minimum.