Direct Detection of S(3P) and S(1D) Generated in the O(1D) + OCS

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Direct Detection of S(P) and S(D) Generated in the O(D) + OCS Reaction: Mechanism of the Formation of S (X # and a#) 2

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Haruka Tanimoto, Shogo Tendo, Kenichi Orimi, Hiroki Goto, Hiroshi Kohguchi, and Katsuyoshi Yamasaki J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b11375 • Publication Date (Web): 02 Feb 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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The Journal of Physical Chemistry

Direct Detection of S(3P) and S(1D) Generated in the O(1D) + OCS Reaction: Mechanism of the Formation of S 2 ( X 3 Σ g− and a1∆ g )

Haruka Tanimoto, Shogo Tendo, Kenichi Orimi, Hiroki Goto, Hiroshi Kohguchi, and Katsuyoshi Yamasaki*

Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan

* (K.Y.) Fax: +81-82-424-7405. E-mail: [email protected]

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ABSTRACT Highly vibrationally excited disulfur S2 in the X3 Σg− and a1∆ g states has been detected

in the gaseous mixture of O3 and OCS irradiated with light at 266 nm. Generation of CO2 in the reaction system has been reported; however, no direct detection of sulfur atoms (S(3P) and S(1D)) has been made. In the present study, we have employed two-photon laser-induced fluorescence (2P-LIF) technique to detect S(3P) and S(1D) directly and recorded the time profiles of the atoms at varying pressures of OCS. Kinetic analyses of the profiles show that (i) S(1D) is generated in the O(1D) + OCS reaction and consumed by the S(1D) + OCS reaction, and (ii) S(3P) is mainly generated in the O(1D) + OCS reaction instead of quenching of S(1D) by collisions with OCS and ambient gases. The vibrational levels v = 19 and 10 of the respective electronic states X3Σg− and a1∆ g of S2 were detected in the O3/OCS/266 nm system. The two vibrational levels cannot be generated of the available energy of the S(3P) + OCS reaction, giving evidence that S2 in the X 3 Σ g− and a1∆ g states are generated by the S(1D) + OCS reaction.

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INTRODUCTION Carbonyl sulfide (OCS) is the most abundant sulfur-containing compound in the atmosphere,1 attracting many researchers engaged in atmospheric chemistry. Sulfur compounds are main precursors of sulfate aerosol, and the heterogeneous reactions relevant to ozone depletion may occur on the surface of the aerosol.2 Nevertheless, there have been few reports on the kinetic study on the reaction of OCS. There are several exothermic channels of the title reaction.

O(1D) + OCS(X1Σ + ) → SO(X3 Σ − ) + CO(X1Σ + )

∆H 0o = −404 kJ mol−1

(1a)

→ SO(a1∆ ) + CO(X1Σ + )

∆H 0o = −334 kJ mol−1

(1b)

→ S(3 P) + CO 2 (X1Σ g+ )

∆H 0o = −413 kJ mol−1

(1c)

→ S(1D) + CO 2 (X1Σ g+ )

∆H 0o = −303 kJ mol−1

(1d)

→ CS(X1Σ + ) + O 2 (X 3 Σ g− )

∆H 0o = − 18 kJ mol−1

(1e)

→ O(3 P) + OCS(X1Σ + )

∆H 0o = −190 kJ mol−1

(1f)

The heats of reactions are calculated based on the heats of formation appearing in the database published by JPL/NASA.3 Gauthier and Snelling4 photolyzed a gaseous mixtures of O3, O2, and atmospheric gas at 253.7 nm and detected the emission from O 2 (b1Σ + ) generated by energy transfer from O(1D). Measurement of the intensity of the emission gave the relative rate coefficients of the reactions between O(1D) and atmospheric gases. Their rate coefficient for the O(1D) + OCS reaction 3.0 × 10−10 cm3 molecule−1 s−1 should be revised to be (1.6 ± 0.3) × 10−10 cm3 molecule−1 s−1 because the rate coefficient 7.4 × 10−11 cm3 molecule−1 s−1 of the

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reference reaction O(1D) + O2 → O(3P) + O2 at the time5 has been replaced with the latest recommended value 3.95 × 10−11 cm3 molecule−1 s−1.3 We6 irradiated the mixture of O3 and OCS in He at 266 nm and detected vibrationally excited SO(X 3 Σ − ) with laser-induced fluorescence (LIF) via the B3 Σ − − X3 Σ − transition. The rate of growth of SO(X 3 Σ − ) and its dependence on the number density of OCS gave the overall rate coefficient of the O(1D) + OCS reaction to be (2.1 ± 0.3) × 10−10 cm3 molecule−1 s−1. There have been some reports on the dynamics relevant to the mechanism of the O(1D) + OCS reaction. In their pioneering study, Shortridge and Lin7 photolyzed a gaseous mixture of O3, OCS and SF6 with light at λ > 215 nm and detected CO by CO laser resonant absorption technique. They found that the vibrational distributions of the levels v = 0 − 9 are Boltzmann with a temperature at 3300 K corresponding to 4−5 % of the

available energy of the reaction. Lin and colleagues8 employed the time-resolved FTIR emission technique and detected vibrationally excited CO2, OCS and CO generated in the photolysis of the mixture of O3 and OCS at 248 nm. They reported that ≈51 % of the available energy were deposited into highly excited ν3 vibration of CO2 and that of CO was 17−26 %. They also estimated the relative yield of S(3P)/S(1D) ≈ 81/19 on the assumption of the statistical distribution of the available energy. Their quantum chemical calculation showed that the O(1D) + OCS reaction goes not only to the singlet channels 1b and 1d but also to the triplet channels 1a and 1c via the several points for intersystem crossing. We have recently detected highly vibrationally excited S2 in two electronic states X3 Σ g− and a1∆ g in the mixture of O3 and OCS irradiated at 266 nm. Highly probable processes of the generation of S2 are the following exothermic reactions.

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S(1D) + OCS(X1Σ g+ ) → S2 (X3 Σg− ) + CO(X1Σ + ) → S2 (a1∆ g ) + CO(X1Σ + )

∆H 0o = −229 kJ mol−1

(2a)

∆H 0o = −177 kJ mol−1

(2b)

The difference in the energies between the X3 Σ g− and a1∆ g states of S2 has been reported to be 4394.25 cm−1.9 Although S(1D) may appear to be generated in the photolysis of OCS at 266 nm, neither of the two electronic states of S2 was observed without O3. This fact suggested that S(1D) was hardly generated in the photolysis of OCS but produced in the O(1D) + OCS reaction (channel 1d); however, there has been no report on the direct detection of S(1D) in the O3/OCS/266 nm system. In the present study, the two-photon LIF (2P-LIF) technique has been employed to detect S(3P) and S(1D), and the ordinary single-photon LIF was used to observe the X3 Σ g− and a1∆ g states of S2 in O3/OCS/266 nm and OCS/248 nm systems. A kinetic analysis and comparison of the O(1D) + OCS and S(1D) + OCS systems have drawn the following conclusions that (i) S(1D) is generated directly in the O(1D) + OCS reaction, (ii) the greater part of S(3P) is produced in the O(1D) + OCS reaction instead of quenching of S(1D) by collisions with OCS (S(1D) + OCS → S(3P) + OCS), and (iii) highly vibrationally excited S2 of both X3 Σ g− and a1∆ g states was generated by the S(1D) + OCS → S2 + CO reaction (channels 2a and 2b) subsequent to the O(1D) + OCS → S(1D) + CO2 reaction (channel 1d).

EXPERIMENTAL METHOD

The outline of the experimental apparatus has been described in the previous reports.10,11 Here, the details of the present experiments will be described. The photoabsorption cross section of O3 at 266 nm, σ 266 (O3 ) = 9.68 × 10−18 cm2,3 is larger

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than that of OCS, σ 266 (OCS) = 7.93 × 10−22 cm2,12 by about four orders of magnitude ( σ 266 (O3 ) σ 266 (OCS) ≈ 1.2 × 104 ), and consequently, selective photolysis of O3 in the mixture of O3 and OCS can be made with light at 266 nm. The large ratio of photoabsorption cross section of OCS12 at 248 and 266 nm σ 248 (OCS) σ 266 (OCS) ≈ 30 ( σ 248 (OCS) = 2.3 × 10−20 and σ 266 (OCS) = 7.93 × 10−22 cm2) allows us to photolyze OCS in the OCS/He mixture at 248 nm.

O3/OCS/266 nm system. A gaseous mixture of O3 and OCS in buffer He was irradiated with the light at 266 nm from a Nd3+:YAG laser (Spectra Physics GCR-130). Typical partial pressures of the sample gas were pO3 = 2.5 mTorr, pOCS = 5−40 mTorr, and pHe = 10 Torr at 295 ± 2 K. The initial concentration of O(1D), [O(1D)]0 , was

estimated to be ≈ 3.2 × 1012 atoms cm−3 from the following typical values: [O3 ] = 8.1 × 1013 molecules cm−3, the quantum yield of O(1D), 0.9 ± 0.1 ;1,3,13 the photoabsorption cross section of O3 at 266 nm,3 9.68 × 10−18 cm2 and the fluence of the photolysis laser 3.5 mJ cm−2. OCS/248 nm system. OCS in buffer He was irradiated with the light at 248 nm from a

KrF excimer laser (Lambda Physik LEXtra50). Typical partial pressures of the sample gas were pOCS = 15−60 mTorr and pHe = 10 Torr at 295 ± 2 K. The initial concentration of S(1D), [S(1D)]0 , was estimated to be ≈ (0.6 − 2.5) × 1011 atoms cm−3 from the following typical values: [OCS]0 = (0.5 − 1.9) ×1015 molecules cm−3, the quantum yield of S(1D), 0.95;14 the photoabsorption cross section of OCS at 248 nm, 2.3 × 10−20 cm2 and the fluence of the photolysis laser 4.6 mJ cm−2. Detection of S(3PJ) and S(1D). The sulfur atoms in the 3p 3PJ and 3p 1D states (3p

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is the subshell of the valence shell) were detected with a Nd3+:YAG laser (Spectra Physics GCR-130 or 170) pumped frequency doubled dye laser (Lambda Physik LPD3002 with SHG crystal (BBO III) or SCANmate2E coupled with INRAD Autotracker-III (BBO TST)). S(3 PJ ) is excited to the 4p 3PJ state with two photons at 308.15−308.18 nm, and the 4p 3PJ state transfers to the 4p 3S state with the infrared transition at 1046 nm followed by the VUV emission at 180−183 nm via the transition back to the 3p 3PJ state. S(1D) is excited to the 4p 1F state with two photons at 288.16 nm, and the 4p 1F state moves to 4p 1D state with radiative transition at 1064 nm followed by the VUV emission at 166.7 nm to the 3p 1D state. The VUV emission was collected with a MgF2 lens (f = 45 mm) and detected with a solar blind photomultiplier tube (PMT; Hamamatsu R10454) through a band-pass filter (Acton Research 180-B-1D). The VUV detection system was purged with dry nitrogen for eliminating oxygen. Figures 1 and 2 show the 2P-LIF excitation spectra of S(3 P2 ) and S(1D). Detection of S 2 ( X 3 Σ g− ) and S 2 (a1∆ g ) . The two states X3 Σ g− and a1∆ g of S2 were

excited via the B3 Σ −u − X3 Σ g− and f 1∆ u − a1∆ g transitions, respectively, with a Nd3+:YAG laser pumped frequency doubled dye laser also used in detection of sulfur atoms. Both electronic transitions with a large photoabsorption cross section are readily saturated even at the lowest pulse energy (100 µJ) of the present experiment. However, saturation does not lead to a problem in recording the time profiles of the population of a vibrational level because the LIF intensity is in proportion to the number density of the level of interest. The wavelengths of the vibrational band v′ − v′′ of both the electronic transition are nearly identical with that of (v′ + 3) − (v′′ + 2) , making the vibrational spectra congested. The LIF

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passing through an appropriate optical filter was detected with a PMT (Hamamatsu R1104). When a few vibrational levels are excited simultaneously with the probe laser, the filter was replaced with a monochromator (JEOL JSG-125S, f = 125 cm, ∆λ (full width at halfmaximum) = 3 nm) and a single vibrational band was observed with a PMT (Hamamatsu R928). To record the time profiles of the LIF intensity of the target species, the wavelength of the dye laser was tuned to that of the spectral line, and the delay times of the probe laser from the photolysis pulse were scanned automatically. The step size of the time scan was varied according to the time scales of the profiles. A single data point is an average of the signals from ten laser pulses and a single time profile consists of 1000 data points. The total pressure of a sample gas was monitored with a capacitance manometer (Baratron 122A). The calibration factors of mass flow controllers (STEC SEC-400 markIII and Tylan FC-260KZ) and sensors (STEC SEF-410 and KOFLOC 3810) for different gases were measured and used to convert the outputs of the controllers to the flow rates. The partial pressures of the reagents were calculated by proration of the total pressure with the flow rates. O3 was prepared by an electrical discharge in high grade O2 (Japan Fine Products, >99.99995 %) with a synthesizer made in-house and stored in a 4 dm3 glass bulb with He (5 % dilution). Highly pure grade OCS (Sumitomo Seika, 99.9 %) diluted with He (OCS : He = 60 : 40) and He (Japan Fine Products, >99.99995 %) were used without further purification.

RESULTS AND DISCUSSION

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Total Removal Rate Coefficient for the S(1D) + OCS Reaction. Figure 3 shows the

time-resolved 2P-LIF intensities of S(1D) generated in the OCS/248 nm system. The time profiles were recorded at varying pressures of OCS. The signals show instantaneous growth characteristic of photolysis followed by decay. The pseudo first-order conditions ([S(1D)]

Direct Detection of S(3P) and S(1D) Generated in the O(1D) + OCS

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