A CAS study on S‐loss and O‐loss dissociation mechanisms of the SO₂⁺ ion in the C, D, and E states

Abstract

In the present work, we mainly study dissociation of the C (2) B(1) , D(2) A(1) , and E(2) B(2) states of the SO(2) (+) ion using the complete active-space self-consistent field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) methods. We first performed CASPT2 potential energy curve (PEC) calculations for S- and O-loss dissociation from the X, A, B, C, D, and E primarily ionization states and many quartet states. For studying S-loss predissociation of the C, D, and E states by the quartet states to the first, second, and third S-loss dissociation limits, the CASSCF minimum energy crossing point (MECP) calculations for the doublet/quartet state pairs were performed, and then the CASPT2 energies and CASSCF spin-orbit couplings were calculated at the MECPs. Our calculations predict eight S-loss predissociation processes (via MECPs and transition states) for the C, D, and E states and the energetics for these processes are reported. This study indicates that the C and D states can adiabatically dissociate to the first O-loss dissociation limit. Our calculations (PEC and MECP) predict a predissociation process for the E state to the first O-loss limit. Our calculations also predict that the E(2) B(2) state could dissociate to the first S- and O-loss limits via the A(2) B(2) ← E(2) B(2) transition. On the basis of the 13 predicted processes, we discussed the S- and O-loss dissociation mechanisms of the C, D, and E states proposed in the previous experimental studies.

Excited state dynamics in SO2. I. Bound state relaxation studied by time-resolved photoelectron-photoion coincidence spectroscopy

“…The latter process would be expected to produce an unstable (C) 2 B 1 cation state that would fragment on our ion collection time scale. 83 Consistent with the production of such ion states, weak and broad features are observed in the SO + TRPEPICO spectra that span 3.2-4.2 eV (see Fig. 8(c), peak (6)).…”

“…With this in mind, the ground neutral state, single photon ionisation transitions to the (C ) 2 B 1 state can be attributed to an intensity borrowing mechanism. The transition to the optically dark (C ) 2 B 1 state gains oscillator strength through the non-adiabatic interaction with the optically bright (C) 2 [81][82][83] When such high-lying ion states are accessed at energies above the dissociative ionisation limit, the resulting parent ions are not expected to be stable on our ion collection time scale. [81][82][83] Hence, photoelectrons associated with accessing the (C) 2 B 1 state (with >60 meV of internal energy) or higher-lying states would be expected to at least partially correlate with SO + fragment ions, diminishing the signal contribution to the SO + 2 TRPEPICO spectra.…”

“…Like the higher-lying doublet ion states, the accessible quartets are also expected to be unstable with respect to fragmentation following population at energies above 15.963 eV. 83 Furthermore, quartet states accessed below the first dissociative ionisation limit may also contribute to the SO + spectrum were they to absorb one or more probe photons following ionisation. As for the singlet state ionisation routes, intermediate valence resonances in the triplet ionisation pathways would likely have affected the Koopmans' correlation for triplet ionisation.…”

“…This state would be populated above the dissociative ionisation limit and would fragment on our ion collection time scale. 83 This would lead to the photoelectrons associated with [1+4 ] MPI of the (b) 3 A 2 or (c) 3 B 2 states partially correlating with the SO + fragments and the associated TRPEPICO spectrum. Given the expected uncertainties of the quartet state ionisation potential calculations (±0.3 eV), these observations are also somewhat consistent with ionisation of one or more of the three single-pump-photon accessible triplet states through absorption of four probe photons to produce the (b) 4 B 2 and/or (c) 4 A 2 cations (again, see Table IX).…”

Excited state dynamics in SO2. III. An ab initio quantum study of single- and multi-photon ionization

“…The third band (C,D, andẼ states), 2.5 eV higher in energy, has been already widely studied [11][12][13][14] and will not be treated in the present work.…”