Decomposing the First Absorption Band of OCS Using Photofragment Excitation Spectroscopy

Abstract

Photofragment excitation spectra of carbonyl sulfide (OCS) have been recorded from 212−260 nm by state-selectively probing either electronically excited S( 1 D) or ground state S( 3 P) photolysis products via 2 + 1 resonance-enhanced multiphoton ionization. Probing the major S( 1 D) product results in a broad, unstructured action spectrum that reproduces the overall shape of the first absorption band. In contrast, spectra obtained probing S( 3 P) products display prominent resonances superimposed on a broad continuum; the resonances correspond to the diffuse vibrational structure observed in the conventional absorption spectrum. The vibrational structure is assigned to four progressions, each dominated by the C−S stretch, ν 1 , following direct excitation to quasi-bound singlet and triplet states. The S( 3 P J ) products are formed with a near-statistical population distribution over the J = 2, 1, and 0 spin−orbit levels across the wavelength range investigated. Although a minor contributor to the S atom yield near the peak of the absorption cross section, the relative yield of S( 3 P) increases significantly at longer wavelengths. The experimental measurements validate recent theoretical work characterizing the electronic states responsible for the first absorption band by Schmidt and co-workers.

Time-resolving the UV-initiated photodissociation dynamics of OCS

“…REMPI spectra of the S photoproducts showed a broad, unstructured spectrum in the major S( 1 D 2 ) channel. 55 This suggests that the absorption spectrum and correspondingly the observed products are dominated by excitation to the repulsive 2 1 A (1 1 ∆) and 1 1 A (1 1 Σ − ) states, which lead to prompt dissociation, see also Figure 2. The repulsive nature of these two states along the dissociation coordinate was confirmed by earlier calculations 47 However, these calculations also indicated that these states have shallow local minima supporting bound vibrational states around bending angles of θ ≈ 40 • .…”

“…29 For graphical representations of the energies of all electronic states involved we refer the reader to, e. g., Figures 1 in refs. 49,55. Although the dissociation process responsible for the formation of rotationally excited CO(X 1 Σ + ) and S( 1 D 2 ) products is well understood and mainly attributed to prompt dissociation following the excitation of the 2 1 A (1 1 ∆) and 1 1 A (1 1 Σ − ) states, the mechanisms responsible for the formation of S( 3 P J ) fragments remains to be elucidated. Strong vibrational resonances were recently observed in the 2+1 resonance-enhanced multi-photon ionisation (REMPI) spectra of the S( 3 P J ) photoproducts recorded in a wavelength range from 212 nm to 260 nm, which were assigned to vibrational progressions in the C-S stretching following direct excitation to the 2 1 A (1 1 ∆) and triplet 2 3 A (1 3 Σ − ) metastable states.…”

“…Strong vibrational resonances were recently observed in the 2+1 resonance-enhanced multi-photon ionisation (REMPI) spectra of the S( 3 P J ) photoproducts recorded in a wavelength range from 212 nm to 260 nm, which were assigned to vibrational progressions in the C-S stretching following direct excitation to the 2 1 A (1 1 ∆) and triplet 2 3 A (1 3 Σ − ) metastable states. 55 Molecules in the 2 3 A (1 3 Σ − ) state can rapidly predissociate through non-adiabatic transitions to the repulsive 1 3 A state, leading to S( 3 P J ) products. Alternatively, it has been predicted that the 2 1 A (1 1 ∆) and 2 3 A (1 3 Σ − ) states can predissociate through non-adiabatic coupling to the 1 1 A (1 1 Σ − ) state or through spin-orbit coupling to the 2 1 A (1 1 ∆) state, respectively, with subsequent intersystem crossing leading to the formation of S( 3 P J ) fragments.…”

“…Alternatively, it has been predicted that the 2 1 A (1 1 ∆) and 2 3 A (1 3 Σ − ) states can predissociate through non-adiabatic coupling to the 1 1 A (1 1 Σ − ) state or through spin-orbit coupling to the 2 1 A (1 1 ∆) state, respectively, with subsequent intersystem crossing leading to the formation of S( 3 P J ) fragments. 55 So far, the experimental studies of the photodissociation of OCS were limited to frequencyresolved spectroscopies, i. e., with nanosecond lasers. This includes laser-induced fluorescence (LIF) spectroscopy 27,31,56 and REMPI.…”

Photodissociation dynamics of OCS at 207 nm: S(1D2)+CO(X1Σ+) product channel

“…Meanwhile, the photolysis products orbital polarization effect of carbonyl sulfide has attracted enormous interest due to its important information of angular momentum [7][8][9][10][11]. In recent years, extensive experimental [12][13][14][15][16][17][18][19][20][21][22][23][24][25] and theoretical [26][27][28][29] studies on the photodissociation dynamics of OCS in ultraviolet (UV) region have been reported.…”