Photo-absorption studies on carbonyl sulphide in 30,000-91,000 cm−1 region using synchrotron radiation

Photo-absorption spectrum of carbonyl sulphide (OCS) is recorded in 30,000-91,000 cm−1 (3300-1050 Å) region at an average resolution of 1.2 Å using Photo-physics beamline on the 450 MeV Indus-1 synchrotron radiation source at RRCAT Indore, India. Owing to significant absorption cross section dependence, spectra of OCS are recorded at various pressures (0.001-5 mbar) to optimize the S/N ratio for band systems appearing at different energy regions. The spectral region below 70,000 cm−1 has contributions from dissociation mechanism of the ground state of OCS and three valence band systems arising from promotion of a 3π electron to 4π and 10σ orbital. Improved S/N ratio helped in unambiguous assignment of the valence band progressions at 42,000-48,000 cm−1, 53,000-62,000 cm−1 and 63,500-70,000 cm−1 regions to the 1Δ←X1Σ+ transition, the relatively intense and sharp bands of 1Π←X1Σ+ transition and intense but broad bands of 1Σ+←X1Σ+ transition, respectively, and obtain the vibrational frequencies. Above 70,000 cm−1 Rydberg series arising from s, p, d and f orbitals converging to the ionic ground state X2Π of OCS+ (90,121 cm−1) are identified. Long progression in the first few members of the Rydberg series is suggestive of mixed valence character. Quantum defects are evaluated and used to discuss the nature of the molecular orbital. The present study provides a unifying picture of the VUV photo-absorption spectrum of OCS up to its first ionization limit.

► Unifying picture of experimental VUV photo-absorption spectrum of OCS in 30,000-91,000 cm−1: synchrotron radiation source. ► Excited valence states, their mixed valence/Rydberg nature and vibrational constants. ► Rydberg series (s, p, d and f orbitals), quantum defects and nature of the molecular orbital.