•The rate coefficients for energy transfer from S2(a1Δg) to SF6 have been determined.•The rate coefficients for energy transfer from S2(a1Δg) to CF4 have been determined.•SF6 is ≈5 times more efficient at vibrational energy transfer from S2(a1Δg) than CF4.•The ν4 vibrational mode of SF6 and CF4 accepts the vibrational energy from S2(a1Δg).•The quadrupole-transition dipole interaction governs the V–V energy transfer.Irradiation of the pulsed laser light at 248 nm to the gaseous mixture of OCS and He generated vibrationally excited S2(a1Δg,v=2-4) by the S(1D) + OCS reaction. A single vibrational level of S2(a1Δg) was detected with laser-induced fluorescence (LIF) via the f1Δu-a1Δg transition. The rate coefficients for energy transfer from vibrational levels v=2 and 3 of S2(a1Δg) to SF6 and CF4 have been determined from the time-resolved LIF intensities recorded at varying pressures of SF6 and CF4. The reduced probability of energy transfer per collision derived from the rate coefficients enabled us to identify the ν4 mode of SF6 and CF4 as the energy-accepting vibration.Download high-res image (89KB)Download full-size image

Versatile transformations of azo compounds are utilized not only in synthetic organic chemistry but also in materials science. In this study, a hitherto unknown stereoselectivity was observed by low-temperature in situ NMR spectroscopy for the photochemical denitrogenation of a cyclic azoalkane (2,3-diazabicyclo[2.2.1]heptane) derivative. Direct (singlet) photodenitrogenation at 188 K afforded two products, the configurationally retained ring-closed compound (ret-CP) and the inverted compound (inv-CP), in a ratio of 82/18 (±3) (ret-CP/inv-CP), with an overall yield of >95%. Triplet-sensitized denitrogenation at 199 K using benzophenone (3BP*) or xanthone (3Xan*) selectively produced inv-CP, with a ret-CP/inv-CP ratio of 7/93 (±3). Thermal isomerization of inv-CP into ret-CP was observed by low-temperature NMR spectroscopy. Transient absorption spectroscopy revealed that two distinct singlet diradicals are involved in the formation of CP during direct photodenitrogenation, that is, puckered puc-1DR and planar pl-1DR diradicals. The former produces ret-CP, whereas the latter affords inv-CP. Kinetic analysis using the integrated profiles method was used to determine the molecular absorption coefficient of pl-1DR (ε560 = 4900 ± 250 M–1 cm–1) for the first time. The involvement of the puckered singlet diradical resolves the mechanistic puzzle of stereoselective denitrogenation of diazabicycloheptane-type azoalkanes.

The reaction of electronically excited sulfur S(1D) with OCS has exothermic channels generating S2 in two electronic states X3Σg– and a1Δg. The a1Δg state is correlated directly to the reactants via the spin-allowed singlet surface; the X3Σg– 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 S2 products despite large heats of reactions (228 and 175 kJ mol–1 for generation of X3Σg– and a1Δg, respectively). In the present study, S(1D) was generated by the photolysis of OCS at 248 nm in a buffer He at 298 K, and the resulting two electronic states of S2 (X3Σg– and a1Δg) were detected with dispersed laser-induced fluorescence (LIF) via the B3Σu––X3Σg– and f1Δu–a1Δg 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 X3Σg– and a1Δg 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 X3Σg– and 11 of a1Δg 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.

The vibrational relaxation of OH(X2Π) by collisions with rare gases is very slow due to small molecular interactions. No measurement of the rate coefficients has been made for relaxation of relatively low vibrational levels v ≤ 4 of OH by He, and there is only one report of the upper limit for v = 2, <1 × 10–14 cm3 molecule–1 s–1. In this article, we have studied vibrational relaxation of the levels v = 1–4 of OH(X2Π) by collisions with He. A gaseous mixture of O3 and H2 in a carrier gas at 70–130 Torr of He was irradiated at 266 nm, and OH(X2Π, v ≤ 4) was generated in the reaction O(1D) + H2. A single vibrational level of OH was detected with laser-induced fluorescence (LIF) via the A2Σ+–X2Π transition. Time-resolved LIF intensities of OH(v) were recorded, and kinetic analysis was made by an originally developed integrated profiles method (IPM). On the basis of the evaluation of the pressure-dependent rate coefficients of diffusion loss and the effect of impurities on the kinetics, the rate coefficients of vibrational relaxation for OH(X2Π, v = 1–4) by He have been determined to be (2.9 ± 1.5) × 10–17, (1.4 ± 0.4) × 10–16, (5.2 ± 0.5) × 10–16, and (1.6 ± 0.2) × 10–15 cm3 molecule–1 s–1 for v = 1, 2, 3, and 4, respectively (the confidence limits are 2σ). The rate coefficients are larger at higher vibrational levels and smoothly correlate to those reported previously for v = 10–12.

The linear kinetic analysis called the integrated profiles method (IPM) makes it simple to analyze the multistep relaxation processes of vibrational manifold. The problem that plots for linear regression in the IPM analysis cannot be made, however, has been found in the study of self-relaxation of O2(X3Σg–, v = 6–8). The cause of the problem is the identical time-dependence of the populations of the adjacent vibrational levels. An addition of CF4 into the system made a difference in the time profiles and enabled us to make IPM analysis and determine the rate coefficients. In the experiments, a gaseous mixture of O3/O2/CF4 in an Ar carrier at 298 K was irradiated at 266 nm, and the direct photoproduct O2(X3Σg–, v = 6–9) from O3 was detected by laser-induced fluorescence (LIF) in the B3Σu––X3Σg– transition. Time-resolved LIF intensities of O2(X3Σg–, v) at various pressures of O2 and fixed pressure of CF4 were recorded. The resulting rate coefficients for v = 6–8 correlate smoothly with those for v ≤ 5 and v ≥ 8 reported previously. The vibrational-level dependence (v = 2–13) of the rate coefficients for relaxation of O2(X3Σg–, v) by O2 is accounted for by the balance between the harmonic transition probabilities and the energy defect in the V–V energy-transfer mechanism.

The collision complex formed from a vibrationally excited reactant undergoes redissociation to the reactant, intramolecular vibrational relaxation (randomization of vibrational energy), or chemical reaction to the products. If attractive interaction between the reactants is large, efficient vibrational relaxation in the complex prevents redissociation to the reactants with the initial vibrational energy, and the complex decomposes to the reactants with low vibrational energy or converts to the products. In this paper, we have studied the branching ratios between the intramolecular vibrational relaxation and chemical reaction of an adduct HO(v)–CO formed from OH(X2Πi) in different vibrational levels v = 0–4 and CO. OH(v = 0–4) generated in a gaseous mixture of O3/H2/CO/He irradiated at 266 nm was detected with laser-induced fluorescence (LIF) via the A2Σ+–X2Πi transition, and H atoms were probed by the two-photon excited LIF technique. From the kinetic analysis of the time-resolved LIF intensities of OH(v) and H, we have found that the intramolecular vibrational relaxation is mainly governed by a single quantum change, HO(v)–CO → HO(v–1)–CO, followed by redissociation to OH(v–1) and CO. With the vibrational quantum number v, chemical process from the adduct to H + CO2 is accelerated, and vibrational relaxation is decelerated. The countertrend is elucidated by the competition between chemical reaction and vibrational relaxation in the adduct HOCO.

The vibrational levels of O2(X3Σg−) generated in the ultraviolet photolysis of O3 at 266 nm were detected via laser-induced fluorescence (LIF) of the B3Σu− − X3Σg− system. The nascent vibrational energy distributions of O2(X3Σg−, v = 6−13) have been measured by two different methods. One is a kinetic analysis based on the originally developed integrated profiles method (IPM). The time-resolved LIF of a single vibrational level has been recorded in the presence of CF4 or O2 as a relaxation partner. The IPM analysis of the profiles gave the relative detectabilities of adjacent vibrational levels, and the initial relative populations of the vibrational levels have been determined from the intensities of LIF subsequent to the photolysis. The other is the analysis of the area intensities of the LIF of the vibrational levels of interest. The rotational levels with the identical quantum numbers of different vibrational levels in the X3Σg− state were excited to a common vibrational level v′ = 0 in the B3Σu− state. Correction for the LIF intensities with the Franck−Condon factors was made, and the initial relative populations have been obtained. The two different methods have given similar nascent vibrational energy distributions, and comparison to the previous reports has been made.

A gaseous mixture of O3/OCS/He was irradiated at 266 nm with a pulsed laser, and vibrationally excited SO(X3Σ−, v = 8 and 19) generated in the reaction of O(1D) with OCS was detected with the laser-induced fluorescence (LIF) via the B3Σ− – X3Σ− system. The apparent production rates of SO(v = 8) in the initial reaction time and their OCS pressure dependence have been measured, giving the absolute overall rate coefficient for the O(1D) + OCS reaction.O(1D) reacts with OCS, generating highly vibrationally excited SO.

A wide range of vibrational levels of O2(X3Σg−, v = 6−13) generated in the ultraviolet photolysis of O3 was selectively detected by the laser-induced fluorescence (LIF) technique. The time-resolved LIF-excited B3Σu−−X3Σg− system in the presence of CF4 has been recorded and analyzed by the integrated profiles method (IPM). The IPM permitted us to determine the rate coefficients kvCF4 for vibrational relaxation of O2(X3Σg−, v = 6−12) by collisions with CF4. Energy transfer from O2 (v = 6−12) to CF4 is surprisingly efficient compared to that of other polyatomic relaxation partners studied so far. The kvCF4 increases with vibrational quantum number v from [1.5 ± 0.2(2σ)] × 10−12 for v = 6 to [7.3 ± 1.5(2σ)] × 10−11 for v = 12, indicating that the infrared-active ν3 vibrational mode of CF4 mainly governs the energy transfer with O2(X3Σg−, v = 6−12). The correlation between the rate coefficients and fundamental infrared intensities has been discussed based on a comparison of the efficiency of energy transfer by several collision partners.

Vibrationally excited O2(X3Σ−g) was generated in the UV laser flash photolysis of O3 and single vibrational level was detected via laser-induced fluorescence (LIF) in the B3Σ−u−X3Σ−g system. The time-resolved LIF of adjacent vibrational levels has been analyzed by the integrated-profiles method and the rate coefficients for single-quantum relaxation, O2(X3Σ−g, v = 9–13) + O2(v = 0) → O2(X3Σ−g, v − 1) + O2(v = 1), have been determined. To the best of our knowledge, the rate coefficients for v = 12 and 13 are measured for the first time in the present study. The efficiency of relaxation is higher at lower vibrational levels, indicating that a small energy mismatch is suitable for the energy transfer. The vibrational level dependence of all the rate coefficients for the relaxation measured in the present study and previously reported by several groups can be rationalized by the energy gap law.

A laser flash photolysis–laser-induced fluorescence (LIF) technique has been employed to study the relaxation kinetics of vibrationally excited O2(X 3Σ−g). The time-resolved LIF excited B 3Σ−u–X 3Σ−g system has been recorded and analyzed by the integrated-profiles method. The rate coefficient for vibrational relaxation of O2(X 3Σ−g, ν = 8) by collisions with CF4, [1.4 ± 0.3(2σ)] × 10−11 cm3 molecule−1 s−1, indicates that CF4 is an efficient relaxant of O2(X 3Σ−g) and that the propensity rule for O2 relaxation suggested by Mack et al. (J. A. Mack, K. Mikulecky and A. M. Wodtke, J. Chem. Phys., 1996, 105, 4105) has been observed experimentally.

Vibrationally excited O2(X3Σ−g) was generated in the UV laser flash photolysis of O3 and single vibrational level was detected via laser-induced fluorescence (LIF) in the B3Σ−u−X3Σ−g system. The time-resolved LIF of adjacent vibrational levels has been analyzed by the integrated-profiles method and the rate coefficients for single-quantum relaxation, O2(X3Σ−g, v = 9–13) + O2(v = 0) → O2(X3Σ−g, v − 1) + O2(v = 1), have been determined. To the best of our knowledge, the rate coefficients for v = 12 and 13 are measured for the first time in the present study. The efficiency of relaxation is higher at lower vibrational levels, indicating that a small energy mismatch is suitable for the energy transfer. The vibrational level dependence of all the rate coefficients for the relaxation measured in the present study and previously reported by several groups can be rationalized by the energy gap law.