Photolysis Of OCS Research Articles

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

The internal energy distributions of reaction products are important information in clarifying the mechanism of chemical reactions. There are few reports of the nascent vibrational energy distribution of CS(X1Σ+) generated in the S(1D) + CS2 reaction. As long as S(1D) is produced by photodissociation of CS2, CS(X1Σ+), as a product of the chemical reaction and as a photoproduct of CS2 is indistinguishable. In this study, S(1D) was generated by the photolysis of OCS at 248 nm, where CS2 hardly dissociates, and CS(X1Σ+) was generated only by the S(1D) + CS2 reaction. The vibrational levels v″ = 0-6 of CS(X1Σ+) were detected with laser-induced fluorescence (LIF) via the A1Π-X1Σ+ transition. The identical time profiles of the LIF intensities showed that all the vibrational levels were produced by the S(1D) + CS2 reaction. The relative nascent vibrational populations of CS(X1Σ+) determined from the area intensities of the excitation spectra are 1.00 ± 0.11/0.58 ± 0.06/0.31 ± 0.03/0.078 ± 0.009/0.013 ± 0.001/<0.002/<0.002 (the values for v″ = 5 and 6 are the upper limits) for v″ = 0/1/2/3/4/5/6. The distribution agrees well with the statistical (prior) distribution.

Branching ratios of electronic quenching of atomic sulfur S(3p34p 3PJ) by collisions with N2

Atomic sulfur S(3p43P) was generated in the photolysis of OCS at 248 nm and exited to the 3p34p3P state by ultraviolet two-photon absorption at 308 nm. Transfer from the 3p34p3P state to the 3p34s3S state by infrared radiation and/or collisions with ambient gases was followed by the vacuum ultraviolet (VUV) fluorescence via the 3p34s3S → 3p43P transition at 180 nm. The time-resolved VUV fluorescence intensity was recorded at varying pressures of N2. Kinetic analyses have given not only the overall rate coefficient for quenching of the 3p34p3P state by N2 but also the branching ratio of the quenching from the 3p34p3P state to the 3p34s3S state.

Kinetics and dynamics on the formation of S2(X3Σg–,a1Δg) in the S(1D) + OCS reaction.

The reaction of electronically excited sulfur S((1)D) with OCS has exothermic channels generating S(2) in two electronic states X(3)Σ(g)– and a(1)Δ(g). The a(1)Δ(g) state is correlated directly to the reactants via the spin-allowed singlet surface; the X(3)Σ(g)– state, on the other hand, is a product of the spin-forbidden channel. There has been no report on kinetic evidence for the simultaneous generation of the two electronic states, although the two electronic states have been detected so far. The previous studies showed that little energy was released into rotation or vibration of the S(2) products despite large heats of reactions (228 and 175 kJ mol(-1) for generation of X(3)Σ(g)– and a(1)Δ(g), respectively). In the present study, S((1)D) was generated by the photolysis of OCS at 248 nm in a buffer He at 298 K, and the resulting two electronic states of S(2) (X(3)Σ(g)– and a(1)Δ(g)) were detected with dispersed laser-induced fluorescence (LIF) via the B(3)Σ(u)(-)–X(3)Σ(g)– and f(1)Δ(u)-a(1)Δ(g) transitions, respectively. Not only excitation but also dispersed fluorescence spectra made it possible to find a single rotational line of the vibrational level of interest. The time-resolved LIF intensities of the initial growth of the X(3)Σ(g)– and a(1)Δ(g) states showed identical OCS pressure dependences, giving the overall rate coefficient of the S(1D) + OCS reaction to be [3.2 ± 0.2(2σ)] × 10(-10) cm3 molecule(-1) s(-1). The simultaneous generation of the two electronic states indicates that the intersystem crossing plays a role in opening the spin-forbidden channel. As for the reaction dynamics, vibrational levels up to v = 19 of X(3)Σ(g)– and 11 of a(1)Δ(g) have been detected, which is distinctly different from the previous studies. The reaction mechanism has been discussed on the basis of the potential energies reported so far.

OCS photolytic isotope effects from first principles: sulfur and carbon isotopes, temperature dependence and implications for the stratosphere

Abstract. The isotopic fractionation in OCS photolysis is studied theoretically from first principles. UV absorption cross sections for OCS, OC33S, OC34S, OC36S and O13CS are calculated using the time-depedent quantum mechanical formalism and a recently developed ab-initio description of the photodissociation of OCS which takes into account the lowest four singlet and lowest four triplet electronic states. The calculated isotopic fractionations as a function of wavelength are in good agreement with recent measurements by Hattori et al. (2011) and indicate that photolysis leads to only a small enrichment of 34S in the remaining OCS. The photodissociation dynamics provide strong evidence that the photolysis quantum yield is unity at all wavelengths for atmospheric UV excitation, for all isotopologues. A simple stratospheric model is constructed taking into account the main sink reactions of OCS and it is found that overall stratospheric removal slightly favors light OCS in constrast to the findings of Leung et al. (2002). These results show, based on isotopic considerations, that OCS is an acceptable source of background stratosperic sulfate aerosol in agreement with a recent model study of of Brühl et al. (2012). The 13C isotopic fractionation due to photolysis of OCS in the upper stratosphere is significant and will leave a clear signal in the remaining OCS making it a candidate for tracing using the ACE-FTS and MIPAS data sets.

Ultraviolet absorption cross sections of carbonyl sulfide isotopologues OC&amp;lt;sup&amp;gt;32&amp;lt;/sup&amp;gt;S, OC&amp;lt;sup&amp;gt;33&amp;lt;/sup&amp;gt;S, OC&amp;lt;sup&amp;gt;34&amp;lt;/sup&amp;gt;S and O&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;CS: isotopic fractionation in photolysis and atmospheric implications

Abstract. We report measurements of the ultraviolet absorption cross sections of OC32S, OC33S, OC34S and O13CS from 195 to 260 nm. The OCS isotopologues were synthesized from isotopically-enriched elemental sulfur by reaction with carbon monoxide. The measured cross section of OC32S is consistent with literature spectra recorded using natural abundance samples. Relative to the spectrum of the most abundant isotopologue, substitution of heavier rare isotopes has two effects. First, as predicted by the reflection principle, the Gaussian-based absorption envelope becomes slightly narrower and blue-shifted. Second, as predicted by Franck-Condon considerations, the weak vibrational structure is red-shifted. Sulfur isotopic fractionation constants (33ε, 34ε) as a function of wavelength are not highly structured, and tend to be close to zero on average on the high energy side and negative on the low energy side. The integrated photolysis rate of each isotopologue at 20 km, the approximate altitude at which most OCS photolysis occurs, was calculated. Sulfur isotopic fractionation constants at 20 km altitude are (−3.7 ± 4.5)‰ and (1.1 ± 4.2)‰ for 33ε and 34ε, respectively, which is inconsistent with the previously estimated large fractionation of over 73‰ in 34ε. This demonstrates that OCS photolysis does not produce sulfur isotopic fractionation of more than ca. 5‰, suggesting OCS may indeed be a significant source of background stratospheric sulfate aerosols. Finally, the predicted isotopic fractionation constant for 33S excess (33E) in OCS photolysis is (−4.2 ± 6.6)‰, and thus photolysis of OCS is not expected to be the source of the non-mass-dependent signature observed in modern and Archaean samples.

Atmospherically-derived mass-independent sulfur isotope signatures, and incorporation into sediments

The discovery of sulfur mass-independent fractionation (S-MIF) in Archean sediments has motivated new work on atmospheric sulfur chemistry. Laboratory experiments showed that SO 2 photolysis produces S-MIF at wavelengths both shortward of and longward of the SO 2 photodissociation wavelength of 220 nm. It has been argued that the underlying S-MIF mechanism at wavelengths < 220 nm is SO 2 self-shielding. Additional S-MIF signatures associated with SO 2 photolysis are possible due to isotopologue-dependent variations in absorption intensity and dissociation probability, which must be evaluated through new spectral measurements. Here, I claim that SO 2 photoexcitation, near-UV CS 2 photolysis, OCS photolysis, non-statistical sulfur allotrope reactions, and surface reactions during thermochemical sulfate reduction are all unlikely sources of the largest Archean S-MIF, signatures with arguments presented for each proposed source. A potential problem with the theory proposed here is that large mass-dependent fractionation (MDF) accompanies S-MIF during SO 2 photolysis. The range of δ 34S values is ∼ 100‰ in photochemically produced elemental sulfur, which exceeds the δ 34S range observed in Archean rocks by a factor of ∼ 3–5, and represents a weakness of the photochemical theory for the origin of Archean S-MIF. A combination of chemical and biogenic MDF processes may have acted to reduce the δ 34S range of SO 2 photolysis products. MDF during reactions that form elemental sulfur in the atmosphere and during aqueous phase reaction of HS − with Fe 2+ and FeS to form FeS 2 may have reduced δ 34S values by ∼ 40‰ relative to atmospheric SO. A simple mixing model suggests that a mixture of FeS 2 in sediments from both elemental sulfur (yielding pyrite with δ 34S > 0 and Δ 33S > 0) and from bacterial sulfate reduction (BSR) of BaSO 4 (yielding pyrite with δ 34S < 0 and Δ 33S < 0) may contribute to reducing δ 34S from photochemical values to the observed range in Archean pyrites.

Masked Velocity Map Imaging: A One-Laser-Beam Doppler-Free Spectroscopic Technique

A novel spectroscopic technique has been developed which makes it possible to record Doppler-free resonance-enhanced multiphoton ionization (REMPI) spectra with just one laser beam. The approach simply involves masking the outer side of the phosphor screen under velocity map imaging conditions so that only those species having no velocity component parallel to the laser beam propagation direction are detected. The benefits of this method are demonstrated in spectroscopic characterization of highly translationally and rotationally excited CO fragments resulted from the 230 nm photolysis of OCS and acetone, yielding substantially improved values of the rotational constants for the B state (v'' = 0) of the CO molecule. The resolving power and the state distribution analysis of reaction products are also demonstrated for room-temperature H atoms generated by dissociation of background hydrogen molecules and oxygen atom detected from the 225.6 nm photolysis of ozone.

Experimental and theoretical investigation of rate coefficients of the reaction S(P3)+OCS in the temperature range of 298–985K

The reaction S(3P)+OCS in Ar was investigated over the pressure range of 50-710 Torr and the temperature range of 298-985 K with the laser photolysis technique. S atoms were generated by photolysis of OCS with light at 248 nm from a KrF excimer laser; their concentration was monitored via resonance fluorescence excited by atomic emission of S produced from microwave-discharged SO2. At pressures less than 250 Torr, our measurements give k(298 K)=(2.7+/-0.5)x10(-15) cm3 molecule-1 s-1, in satisfactory agreement with a previous report by Klemm and Davis [J. Phys. Chem. 78, 1137 (1974)]. New data determined for 407-985 K connect rate coefficients reported previously for T>or=860 and T<or=478 K and show a non-Arrhenius behavior. Combining our results with data reported at high temperatures, we derived an expression k(T)=(6.1+/-0.3)x10(-18) T1.97+/-0.24 exp[-(1560+/-170)/T] cm3 molecule-1 s-1 for 298<or=TK<or=1680. At 298 K and P>or=500 Torr, the reaction rate was enhanced. Theoretical calculations at the G2M(CC2) level, using geometries optimized with the B3LYP6-311+G(3df) method, yield energies of transition states and products relative to those of the reactants. Rate coefficients predicted with multichannel Rice-Ramsperger-Kassel-Marcus (RRKM) calculations agree satisfactorily with experimental observations. According to our calculations, the singlet channel involving formation of SSCO followed by direct dissociation into S2(a 1Deltag)+CO dominates below 2000 K; SSCO is formed via intersystem crossing from the triplet surface. At low temperature and under high pressure the stabilization of OCS2, formed via isomerization of SSCO, becomes important; its formation and further reaction with S atoms partially account for the observed increase in the rate coefficient under such conditions.

Experimental and theoretical investigations of rate coefficients of the reaction S(3P)+O2 in the temperature range 298–878 K

Rate coefficients of the reaction S+O(2) with Ar under 50 Torr in the temperature range 298-878 K were determined with the laser photolysis technique. S atoms were generated by photolysis of OCS with a KrF excimer laser at 248 nm; their concentration was monitored via resonance fluorescence excited by atomic emission of S produced from microwave-discharged SO(2). Our measurements show that k(298 K)=(1.92+/-0.29)x10(-12) cm(3) molecule(-1) s(-1), in satisfactory agreement with previous reports. New data determined for 505-878 K show non-Arrhenius behavior; combining our results with data reported at high temperatures, we derive an expression k(T)=(9.02+/-0.27)x10(-19)T(2.11+/-0.15) exp[(730+/-120)/T] cm(3) molecule(-1) s(-1) for 298< or =T< or =3460 K. Theoretical calculations at the G2M (RCC2) level, using geometries optimized with the B3LYP/6-311+G(3df) method, yield energies of transition states and products relative to those of the reactants. Rate coefficients predicted with multichannel RRKM calculations agree satisfactorily with experimental observations; the reaction channel via SOO(1A') dominates at T<500 K, whereas channels involving formation of SOO(3A") followed by isomerization to SO(2) before dissociation, and formation of SOO(1A") followed by direct dissociation, become important at high temperatures, accounting for the observed rapid increase in rate coefficient.

Electronic Spectra of Carbonyl Sulfide Sulfur Isotopologues

Environmental Context.Sulfate aerosols have been linked with ozone-depleting reactions, and their influence on cirrus cloud formation may affect climate. One source of sulfate aerosols is sulfur-containing gases such as carbonyl sulfide, which can rise up to the statosphere and become transformed into sulfate under the prevailing sunlight, but the importance and details of the phenomena involved are poorly understood. With a view to providing the fundamental information required to analyze the atmospheric processing of carbonyl sulfide, this paper reports the sulfur isotope effects on its ultraviolet absorption spectrum. Abstract.Carbonyl sulfide (OCS), a relatively inert tropospheric species that photolyzes into CO + S upon reaching the stratosphere, is deemed a significant contributor to background sulfate aerosol. A recent analysis of atmospheric infrared transmittance data has revealed that OC34S is preferentially depleted above ~10 km. We now report that the OC34S absorption band between 200 ≤ λ [nm] ≤ 260 at 298 K is broader and more intense than its OC32S counterpart. This finding is consistent with the faster photolysis of OC34S, and with a time-dependent formulation of electronic spectra. Since OCS photolysis in the lower stratosphere occurs in a spectral range in which its absorption cross-section is unaffected by temperature, we are able to estimate a 34S-enrichment factor &lt;34ϵ&gt; ~ (67 ± 7)‰ for this process that is commensurate with the value previously inferred from OC34S/OC32S ratio versus altitude profiles.

Infrared Spectrum of Matrix-Isolated CO and CO Photoproduct from OCS Photolysis

Experimental studies on the photodissociation of OCS in both crystalline argon and nitrogen are presented. OCS photodissociation is followed by high-resolution interferometric infrared spectroscopy of the CO photoproduct. In solid nitrogen, the photogenerated CO molecule is produced in a damaged site that can be partially healed by thermal annealing. In contrast, the photogenerated CO site in argon does not revert to the sites occupied by neat CO/Ar matrices, even when annealed at 40 K. Model calculations on the infrared spectral line shape and orientational motion of the matrix-isolated CO molecule support a structural interpretation of the experimental findings. The calculations use an Ar−CO intermolecular potential along with an Ar2 pair potential to minimize the energy of a finite section of the full matrix under two broad structural constraints and with both single- and double-substitutional site assumptions. The line shape is calculated via a CO center-of-mass motion weighted perturbation of the isolated CO intramolecular potential by the field of static Ar neighbors.

S 2 (a 1 Δ) production in the reaction of S 1D atoms with COS: Spectroscopy and dynamics

The reaction S(1D)+COS(X 1Σ+)→S2(a 1Δ)+CO(X 1Σ+) has been studied using pulsed photolytic production of S(1D) together with laser-induced fluorescence detection of S2(a 1Δ). S(1D) was produced by 212.8 nm (Nd:YAG fifth harmonic) photolysis of OCS. The nascent S2(a 1Δ) product has been observed under single collision conditions. Rotational and vibrational population distributions for excited S2(a 1Δ) have been obtained. The observed vibrational distribution was analyzed using surprisal analysis and was found to be much colder than predicted by purely statistical arguments, although it extends to at least v=6. A rotational population distribution was obtained for v=2 of the S2(a 1Δ) state. It shows a maximum around J=45, which is hotter than thermal but much colder than the statistical prediction. Resolved emission experiments were performed to verify calculated Franck–Condon factors for the S2 f 1Δ–a 1Δ transition. Ab initio calculations suggest several possible routes leading from reactants to products in the singlet reaction channel but predict significant reaction barriers for the triplet channel.

OCS photolysis and absorption in the 200‐ to 300‐nm region

A continuum source with three different interference filters and two different monochromatic sources were used to photolyze OCS in the region from 200 nm to 300 nm. The total quantum yield for the production of CO was measured, and the results indicate a primary quantum yield of 0.72±0.08. Absorption cross sections for OCS measured at 0.5‐nm intervals are reported as well as the percent change in absorption upon cooling to 195°K. The structured features in the absorption are reinvestigated and discussed.

High-intensity, laser photolysis of OCS at 157 NM: S(1S) production, photoionization, and loss

High-intensity, laser photolysis of OCS at 157 NM: S(¹S) production, photoionization, and loss

Relative rate data for the reactions of S(31 D 2) using the NS radical as a spectroscopic marker

The reactions of S(31D2), produced in the ultra-violet photolysis of OCS, have been studied with the gases, C2H6, C2H4, CH4, SF6, CO2, OCS, N2O, NO, CO, N2, H2, He, Ne, Ar, Kr and Xe. Relative rate data have been determined using the NS radical, formed in the reaction S(31D2)+ N2O → NS + NO as a spectroscopic marker.

Some Reactions of Ground-State (3P) and Electronically Excited (1D) Sulfur Atoms

Reactions of S(3P) and S(1D) have been studied by utilizing carbonyl sulfide photolysis as a source of both species. It has been shown that 46% ± 4% of the sulfur atoms generated in the photolysis of OCS in the 2250–2650-Å spectral region can react as S(3P), but that substantial portion of the S(3P) is likely produced by deactivating collisions of OCS molecules with S(1D). Both S(1D) and S(3P) react with CS2. The rate constant ratios for these reactions relative to the reactions with OCS have been determined as ∼ 1.9 and ∼ 2.4 × 102, respectively. Efficient isotopic exchange of S(3P) with CS2 is proposed to occur by means of CS3 formation in a reaction independent of pressure above 50 torr. The reaction of S(1D) with N2O yields N2, but kinetic complexities suggest the intermediate formation of a species such as N2OS. Measurement of the quenching of the N2 yield has led to the determination of the relative rates of collisional deactivation of S(1D) for several gases.