Direct Detection of S(³P) and S(¹D) Generated in the O(¹D) + OCS Reaction: Mechanism of the Formation of S₂(X³Σ_g^–  and  a¹Δ_g)

Abstract

Highly vibrationally excited disulfur S in the XΣ and aΔ states has been detected in the gaseous mixture of O and OCS irradiated with light at 266 nm. Generation of CO in the reaction system has been reported; however, no direct detection of sulfur atoms (S(P) and S(D)) has been made. In the present study, we have employed the two-photon laser-induced fluorescence (2P-LIF) technique to detect S(P) and S(D) directly and recorded the time profiles of the atoms at varying pressures of OCS. Kinetic analyses of the profiles show that (i) S(D) is generated in the O(D) + OCS reaction and consumed by the S(D) + OCS reaction, and (ii) S(P) is mainly generated in the O(D) + OCS reaction instead of quenching of S(D) by collisions with OCS and ambient gases. The vibrational levels v = 19 and 10 of the respective electronic states XΣ and aΔ of S were detected in the O/OCS/266 nm system. The two vibrational levels cannot be generated by the available energy of the S(P) + OCS reaction, giving evidence that S in the XΣ and aΔ states are generated by the S(D) + OCS reaction.

Nascent Vibrational Energy Distribution of CS(X1Σ+) Generated in the S(1D) + CS2 Reaction

“…The initial number density of S­( 1 D) was sufficiently small compared to those of CS 2 (3 × 10 15 cm –3 ) and OCS (8 × 10 14 cm –3 ), satisfying the pseudo-first-order condition [S­( 1 D)] ≪ [CS 2 ], [OCS]. The overall rate coefficients for S­( 1 D) + CS 2 and S­( 1 D) + OCS reactions are k 1 = (3.5 ± 2.0) × 10 –10 and k 3 = (3.0 ± 0.3) × 10 –10 cm 3 molecule –1 s –1 , respectively. ,,, Quenching of S­( 1 D) by He is inefficient (<4.4 × 10 –14 and <5 × 10 –14 cm 3 molecule –1 s –1 ) and made a negligible contribution to the kinetics of S­( 1 D). The sum of the pseudo-first-order rates with CS 2 (13 Pa) and OCS (3.3 Pa) was k 0 ≡ k 1 [CS 2 ] + k 3 [OCS] = 1.4 × 10 6 s –1 , corresponding to a time constant τ = 1/ k 0 = 0.7 μs.…”

“…The background reactions of S­( 1 D) with OCS proceed at gas kinetic rates: ,,, k 3 = k 3a + k 3b + k 3c ≈ 3.0 × 10 –10 cm 3 molecule –1 s –1 , and S 2 refers to highly vibrationally excited levels up to v = 19 and 10 of the X 3 Σ g – and a 1 Δ g states, respectively. − The vibrational bands of the and transitions of S 2 appear over a wide range of wavelengths (250–600 nm) and overlap with those of A 1 Π–X 1 Σ + transition of CS (257–276 nm). The vibrational bands for excitation and detection of the LIF of CS, the wavelengths of which do not overlap with those of S 2 , were selected carefully and are listed in Table .…”

“…The total pressure of a sample gas was monitored with a capacitance manometer (Baratron 122A). The calibration proceed at gas kinetic rates: 6,16,18,19…”