Communication: Multi-state analysis of the OCS ultraviolet absorption including vibrational structure

Abstract

The first absorption band of OCS (carbonyl sulfide) is analyzed using potential energy surfaces and transition dipole moment functions of the lowest four singlet and the lowest four triplet states. Excitation of the 2 1A′ state is predominant except at very low photon energies. It is shown that the vibrational structures in the center of the band are due to excitation of the 2 3A′′ triplet state, whereas the structures at very low energies are caused by bending excitation in the potential wells of states 2 1A′ and 1 1A′′.

The ultraviolet spectrum of OCS from first principles: Electronic transitions, vibrational structure and temperature dependence

“…The parameter α was set to 1.5 a −1 0 . Table I gives a summary of key spectroscopic parameters for states A, B, C, and c. The calculated S( 1 D) ← S( 3 P) excitation energy of 1.11 eV agrees well with the experimental value averaged over the three spin-orbit states 3 P 2 , 3 P 1 , and 3 50 and as will be discussed below, transitions to these vibrational states give rise to the oscillations observed in the long-wavelength tail of the UV spectrum. 12 As seen in panel (a) of Figs.…”

“…While σ C and σ d are small, σ c is relatively large and actually leads to the vibrational structures of the total cross section. 50 The structures reflect strongly mixed OC-S stretching and bending motion. Figure 12 shows the initial-state resolved cross sections for dissociation in the A state.…”

“…This discrepancy could have several origins. The motions in R and γ are strongly mixed in the c state 50 and small changes of the c state potential could lead to significant changes of the spectral features and their intensities. Contributions from the singlet C state or the triplet d state also cannot be excluded; the intensities depend primarily on the TDMs and their accuracy is certainly not perfect.…”

“…Very recently Schmidt et al 50 presented a theoretical study which, for the first time, provided an accurate description of the 223 nm absorption band of OCS. In the present article we discuss the details of the electronic structure and the dynamics calculations.…”

“…Because the structures in the band center are, according to Ref. 50, due to excitation of the 2 3 A state, some details of the triplet states will also be presented in this publication. Their main discussion, however, will be presented in future papers together with the calculation of the singlet/triplet branching ratio, the isotope dependence of the absorption cross section, and the product state distributions.…”