Tom F. Hunter

Photoacoustic measurements of photofragmentation in CH3I

Abstract Photoacoustic measurements are described giving branching ratios for the I( 2 P 1 2 ) and I( 2 P 3 2 ) atom production following vapour-phase photolysis of CH 3 I. A range of excitation wavelengths are used from the long wavelength tail up to 248 nm. The presence of three bands is shown within the σ * ← n continuum; in the strong-coupling model these are E ← A 1 (⊥), A * 1 ← A 1 (||) and E ← A 1 (⊥) with only the A * 1 ← A 1 transition giving excited iodine atoms.

The distribution of solubilisate molecules in micellar assemblies

Abstract The distribution law governing the statistics of solubilised molecules in micellar assemblies is commonly assumed to be the Poisson law. Other distribution laws are shown to be possible and cases where there is a limit to the number of solubilised molecules in any one micelle are examined. It is argued that for large, essentially water-insoluble solubilisates non-Poisson statistics are likely, with intermicellar migration taking place as clusters of a solubilised molecule plus a number of molecules (or ions) of the micellar compound.

Yield of I(2Psol:12) in the photodissociation of CH2I2

Abstract The production of I( 2 P 1 2 ) in the photolysis of CH 2 I 2 has been studied optoacoustically at excitation wavelengths between 365.5 and 247.5 nm. Bands found at 32200 and 47000 cm −1 correlate with I( 2 P 3 2 ) whilst those at 34700 and 40100 cm −1 , which correlate with I( 2 P 1 2 ), give final 2 P 3 2 / 2 P 1 2 ratios of 1.75 and 1.1, respectively, after curve crossing.

Vibrational-translational/rotational and vibrational-vibrational processes in methane: Optoacoustic measurements

Abstract Both V-T,R and V-V processes in methane have been studied optoacoustically following excitation of the ν3 level with a He-Ne laser at 2947.9 cm−1. The lifetime of the V-T,R process is 1.55 ± 0.05 μs atm. The rate constants for the fast equilibration between the bending modes is k(ν2 → ν4) = 60 μs−1 atm−1 and k(ν4 → ν2) = 13 μs−1 atm−1. The decay of the ν3 and ν2 stretching modes, which are in very rapid equilibrium, shows a rate constant of 0.23 ns−1 atm−1 and, within experimental error, produces exclusively the ν4 stretching mode. Part of this decay, 4.6%, is by a single-quantum process producing a large amount of translational/rotational energy; the dominant process, 95.4%, is double-quantum through the 2ν4 overtone. Both the yield of the single-quantum process and the exclusive production of the ν4 bending mode from the (ν3, ν2) level are in dispute with current theoretical models.

Intersystem crossing in anthracene

Abstract A temperature dependent intersystem crossing rate is observed below ca. 170°K for anthracene in methyl tetrahydrofuran. From the temperature dependence of the 0,0 bands in absorption and emission, this is interpreted as being due to slow lattice relaxation in the excited singlet state.

Triplet-state lifetime of vapour-phase benzene

Abstract A technique based on the optic-acoustic effect has been used to measure the triplet state lifetime of benzene over a range of pressure. The rate-constant for the deactivation of the triplet state by collisions with ground-state benzene is 1.1 × 10 −13 sec −1 cm 3 molecule −1 , which corresponds to a collisional efficiency of 1.6 × 10 −4 , and the pressure-independent radiationless rate-constant is approximately 1.7 × 10 3 sec −1 . The collisional efficiency is discussed in terms of triplet excimer formation.

Optoacoustic method of measuring reaction rates of the radicals CH3, CD3, C2H5 and CH2I with I and I2

It is shown how a modulating concentration of radicals, produced photolytically, can be used in an optoacoustic system to give kinetic information. The acoustic signal is the result of the enthalpy change present in the reaction. Results are reported for the rate constants of the reactions X + I2→ XI + I, where X is CH3, C2H5, CD3 and CH2I. Rate constants are also measured for the radical recombination of X with I in the presence of 100 Torr krypton.

Absolute fluorescence quantum yields for vapour-phase benzene and naphthalene, and comments on the non-radiative processes

Abstract Optic—acoustic measurements have been employed in the determination of absolute quantum yields for benzene and naphthalene. Heat yields are measured by a method using oxygen quenching of both triplet and singlet states. For vibrationally relaxed excited singlet states the fluorescence quantum yields, φBf, are 0.16 ± 0.02 and 0.79 ± 0.02 for benzene and naphthalene respectively. For 0.07 torr naphthalene at room temperature with 248 nm excitation, φf = 0.35 ± 0.03 and the quantum yield of internal conversion is less than 0.05. The decay of the highly vibrationally excited triplet state is dominated by vibrational relaxation for 0.07 torr naphthalene, but for benzene, even at high pressures, strong competition comes from an indirect coupling process to the ground state.

Vibrational-vibrational and vibrational-translational/rotational processes in methane, oxygen gas-phase mixtures

Various methane-oxygen mixtures of different compositions and total pressures were studied optoacoustically for vibrational-vibrational and vibrational-rotational/translational characteristics. V-R, T rates for CH4-CH4 and CH4-O2 were measured as respectively 0·645 and 0·105 μs-1 atm-1. The rate-constants for transfer between the CH4(v 2, v 4) bending modes and the oxygen fundamental, v, are 12±2 μs-1 atm-1 for v→(v 2, v 4) and 1.1 ± 0·2 μs-1 atm-1 for (v 2, v 4)→v. The excitation was to CH4(v 3) with a He-Ne laser and, following very fast equilibration with CH4(v 1), three routes of deactivation were followed in O2 collisions; (i) to the CH4(2v 4) level and thence by CH4 collisions to the v 4 fundamental, (ii) to the O2 overtone (2v), followed by the O2 intermolecular process to the fundamental, and (iii) to the O2 fundamental plus one CH4 bending mode. Rate constants for routes (i), (ii) and (iii) are given.

Non-radiative decay of the benzene 1B2u and 3B1u states

Abstract Optic-acoustic measurements on high pressure benzene are presented, and are used to analyse the nature of the decay channels form the highly vibrationally excited 3 B 1u state. The vibrationally relaxed benzene 3 B 1u state is deactivated by n -pentane with a collisional efficiency of 3 × 10 −5 . A model, introducing an intermediate state close in energy to the 3 B 1u state, is shown to be in good accord with the results.