Elsevier

Chemical Physics Letters

Volume 657, 16 July 2016, Pages 95-101
Chemical Physics Letters

Research paper
Vibrational relaxation of S2(a1Δg, υ = 1–9) by collisions with He

https://doi.org/10.1016/j.cplett.2016.05.063Get rights and content

Highlights

  • The collision efficiencies of vibrational relaxation of S2(a1Δg) by He are 10−3–10−2.

  • Vibrational relaxation rates of S2(a1Δg, υ) by He increase superlinearly with υ.

  • Relaxation of S2(a1Δg) + He system is much faster than that of O2(X3Σg-) + He system.

  • The V–T processes of S2(a), O2(X), NO(X), and CO(X) by He are correlated by the gap law.

Abstract

Vibrationally excited S2(a1Δg, υ = 1–10) was generated by the S(1D) + OCS reaction and detected with dispersed laser-induced fluorescence (LIF) via the f1Δu–a1Δg transition. The time profiles of the vibrational levels of interest were recorded at varying pressures of He. A kinetic analysis made by the integrated profiles method has given the rate coefficients for vibrational relaxation of S2(a1Δg, υ = 1–9) by collisions with He. The energy gap law nicely correlates the probabilities of vibrational energy transfer per collision from S2(a1Δg), O2(X3Σg-), NO(X2Π), and CO(X1Σ+) to He.

Introduction

Sulfur is an important element in the cycles of earth’s matters and biochemicals. There are many (about 30) allotropes, and disulfur (S2) is the most basic molecule in the allotropes Sn [1], [2]. Sulfur and oxygen are congeners, and the lowest three electronic states of S2 and O2 are identical: X3Σg-, a1Δg, and b1Σg+. The small interactions between nonpolar molecules and noble gases lead to a low efficiency of vibrational energy transfer. Parker [3] has observed vibrational energy transfer from O2(X3Σg-,υ=1) to He with an acoustic resonance tube, reporting the rate coefficient to be 7.1 × 10−16 cm3 molecule−1 s−1 at 296 K. Klatt et al. [4] have detected the vibrational levels of υ = 8–10 of O2(X3Σg-) by the laser-induced fluorescence (LIF) technique and observed vibrational relaxation by collisions with He, determining the rate coefficients to be 1.4 ± 0.2, 1.0 ± 0.1, 1.3 ± 0.1 in units of 10−14 cm3 molecule−1 s−1 for υ = 8, 9, and 10, respectively. Hickson et al. [5] also have employed the LIF technique, giving the rate coefficients for relaxation of υ = 21 and 22 of O2(X3Σg-) by He to be 1.65 ± 0.1 and 2.6 ± 0.1 in units of 10−13 cm3 molecule−1 s−1, respectively. There have been few reports on the kinetics of the vibrational relaxation of O2(a1Δg) because the radiative lifetime of the a1Δg-X3Σg- transition is too long (≈72 min) [6] to detect and because there is no electronic state appropriate for being excited in the LIF technique. Pejaković et al. [7] have recently detected the a1Δg state by the 2 + 1 or 2 + 2 resonance-enhanced multiphoton ionization (REMPI) technique, reporting the rate coefficient for removal of the υ = 1 level of the a1Δg state by O2 and CO2.

On the other hand, there has been no kinetic study on the vibrational relaxation of S2. The vibrational constant ωe = 702.35 cm−1 of S2(a1Δg), which is the target in the present study, is about half of 1580.19 and 1483.5 cm−1 of O2(X3Σg-) and O2(a1Δg) [8], respectively, and consequently, relatively high efficiency of relaxation of S2(a1Δg) by He is expected. We have generated S2(a1Δg) in the following reaction:S(1D)+OCSS2(a1Δg)+CO,ΔrH298°=-175kJmol-1where ΔrH298° is the heat of reaction for generation of υ = 0 of S2(a1Δg). Unlike O2(a1Δg), S2(a1Δg) can be detected by the LIF technique via the f1Δu–a1Δg transition. We have recently found that highly vibrationally excited levels up to υ = 11 are formed by reaction (1) [9]. In the present study, the time profiles of a single vibrational level (υ = 1–10) were recorded at varying pressures of He and analyzed by an originally developed integrated profiles method (IPM), giving the bimolecular rate coefficients for vibrational relaxation of the υ = 1–9 levels of the S2(a1Δg) state by collisions with He. The resultant probabilities of relaxation per collision, ≈10−4–10−3 for υ = 1–9, are higher than 10−6–10−4 for υ = 1–22 of O2(X3Σg-) by He by more than an order of magnitude. Nevertheless the vibrational level dependence of the efficiency of relaxation of S2(a1Δg) by He is nicely correlated not only with that of O2(X3Σg-) by He but also with those of NO(X2Π) and CO(X1Σ+) by He on the basis of the energy gap law.

Section snippets

Experimental

The basic framework of the experimental apparatus has been described in the previous reports [10]. A gaseous mixture of OCS and He, typical pressures of which were pOCS = 40 mTorr and pHe = 10 Torr, at 295 ± 2 K in a flow cell was irradiated with the pulsed light at 248 nm from a KrF excimer laser (Lambda Physik LEXtra50). The photolysis of OCS at 248 nm yields an electronically excited sulfur atom S(1D) followed by the highly exothermic reaction (1). The initial concentration of S(1D), [S(1D)]0, was

Laser-induced fluorescence excitation spectra of S2 via the f1Δu–a1Δg transition

Fig. 1 shows the laser-induced fluorescence excitation spectra observed in the present study. The excited and detected vibrational bands together with the laser dyes used are listed in Table 1. The 1Δ–1Δ transition consists of two P, two Q, and two R branches according to the selection rule ΔJ = 0 and ±1. Sulfur atom 32S has no nuclear spin and one of the two P, Q, and R branches is missing. Moreover, the intensity of the Q branch decreases rapidly with J. As a result, the rotational structures

Summary

The integrated profiles method enabled us to make kinetic analysis of the long consecutive processes and determine the rate coefficients for the level-to-level vibrational relaxation of S2(a1Δg, υ = 1–9) by collisions with He. The probabilities of relaxation per collision of the S2(a1Δg) + He system, Pυ = k/σu = 10−4–10−3, are higher than those of the molecules with no or small dipole moment (O2(X3Σg-), NO(X2Π), and CO(X1Σ+)) by one to three orders of magnitude. The reduced probability, Pυ/υ (=kυ/υσu),

Acknowledgment

This work was supported by a Grant-in-Aid for JSPS KAKENHI Grant numbers 25410018 and 16K13937.

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  • Cited by (0)

    1

    Present address: Nippon Starch Chemical Co., Ltd., 3-3-29 Mitsuyakita, Yodogawa-ku, Osaka 532-0032, Japan.

    2

    Present address: Chugoku Regional Development Bureau, 3-20 Hatchobori, Naka-ku, Hiroshima, Hiroshima 730-0013, Japan.

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