A theoretical study on photodissociation of the A state of the H2S+ ion

Abstract

We have studied photodissociation of the A state of the H2S+ ion using the quantum-chemical CAS methods, and the 12A″ (X2B1) and 14A″ states are involved in photodissociation of the 12A′ (A2A1) state (the electronic states in dissociation were studied in the Cs symmetry). The CASPT2 S-loss dissociation potential energy curve (PEC) calculations indicate that the 12A″ and 12A′ states correlate with the second limit [H2 + S+(2D)] while the 14A″ state correlates with the first limit [H2 + S+(4S)] and that there are a transition state and a local minimum along the 12A′ PEC and the repulsive 14A″ PEC crosses the 12A″ and 12A′ PECs. The CASPT2 H-loss dissociation PEC calculations indicate that the 12A″ and 14A″ states correlate with the first limit [HS+(X3Σ−) + H] while the 12A′ state correlates with the second limit [HS+(a1Δ) + H] and that the repulsive 14A″ PEC crosses the 12A′ PEC. For the crossing doublet and quartet states in pairs, we performed CASSCF minimum energy crossing point (MECP) calculations, and the CASSCF spin-orbit couplings and CASPT2 energies at the MECP geometries were calculated. We examined the two previously proposed mechanisms (mechanisms I and II) for dissociation of the A state to the S+ ion, based on our calculation results. We suggest processes for dissociation of the A state to the S+ ion (processes I and II, based on mechanisms I and II, respectively) and to the SH+ ion (process III) and conclude that photodissociation of the A state mainly leads to the S+ ion via the most energetically favorable process II: A2A1 (12A′) (2.38 eV) → barrier at the linearity (2.96 eV) → X2B1 (12A″) (0.0 eV) → the 12A″/14A″ MECP (3.50 eV, large spin-orbit coupling) → H2\( (X^{ 1} \Upsigma_{\text{g}}^{ + } ) \) + S+(4S) (2.92 eV) (the CASPT2 relative energy values to X2B1 are given in parentheses and the largest value is 3.50 eV at the MECP).