The kinetics of the reactions of OH radicals with propene and isoprene in N 2 have been studied in the temperature range of 58-300 K in a Laval nozzle expansion. Laser-induced fluorescence of the OH radical that is formed in the photolysis of H 2 O 2 at 193 nm has been detected. The determined rate constants (2 × 10 -11 to 2 × 10 -10 cm 3 molecule -1 s -1 ) for the OH radicals reacting with excess propene and isoprene (2methyl-1,3-butadiene) have been found to increase when the temperature is decreased. The room temperature rate constants are in agreement with the literature data. Below 120 K, a saturation of the rate constant for the case of propene and a turnover to a formally positive temperature dependence for isoprene have been observed. The observed negative temperature dependence and the course of the temperature dependence at very low temperatures for both reactions are discussed within the framework of the loose transition-state theory and a simple two-transition-state model.

We have combined accurate quantum chemical calculations and light scattering measurements to obtain a reliable estimate of the electric dipole polarizability anisotropy of 1,3-butadiene. The theoretical investigation was based on finite-field many-body perturbation theory and coupled cluster techniques. An extensive study of basis set, electron correlation, and molecular geometry effects leads to a static value of R ) 54.04 e 2 a 0 2 E h -1 for the mean dipole polarizability of trans-butadiene, in very good agreement with the experimental result of 54.64 e 2 a 0 2 E h -1 obtained from an extrapolation to infinite wavelength of refractivity data [Hohm, U.; Tru ¨mper, U. Ber. Bunsenges. Phys. Chem. 1992, 96, 1061. The measured value of the dipole polarizability anisotropy of trans-butadiene is 42.5(1.7) e 2 a 0 2 E h -1 at 514.5 nm. To our knowledge, these are the first light-scattering experiments to be reported for 1,3-butadiene. The present investigation brings forth new possibilities for the extension of accurate determination of electric polarizabilities to molecules of some size. cis-Butadiene is less polarizable and less anisotropic than the trans isomer.

The rovibrational spectrum assigned to the low-lying (3)B(2) electronic state of ozone is measured with intracavity laser absorption spectroscopy. The experimental results are interpreted by means of quantum dynamical calculations on a global ab initio potential energy surface. The observed spectrum is shown to originate from the vibrational ground state in the local minimum of the (3)B(2) potential. The spectrum of short-lived resonance states in this local minimum is analyzed. Additionally, the global minimum of the surface is shown to lie in the dissociation channel in the van der Waals region. This region supports a short sequence of weakly bound vibrational states.

[1] Pulsed cavity ringdown spectroscopy in the near-infrared spectral range was employed to measure linewidths of the 3 A 2 (000) X 1 A 1 (000) band of ozone in direct comparison with intracavity laser absorption spectroscopy. It is demonstrated that also for a small ratio, V, which is the ratio of the absorption linewidth and the laser bandwidth, correct linewidths can be determined from a forward modeling of the experimental line profiles, taking explicitly into account the laser bandwidth and the cavity modes. The obtained linewidths have been compared with those measured by intracavity laser absorption spectroscopy and found to agree quantitatively within experimental error. Linewidths as a function of pressure have been analyzed to obtain pressure-broadening coefficients and the effective linewidth at zero pressure, which is a measure for the excited state lifetimes of the ozone in the excited triplet states. The observed linewidths are a direct observable for the intramolecular dynamics in the low-lying excited 3 A 2 triplet state of ozone. In any case, the observed lifetimes for the excited 3 A 2 (000) state in the range between 20 and 130 picoseconds (ps) rule out metastability of this state. Beyond linewidth measurements we have also refined the spectroscopic parameters of this electronically excited state and determined the integrated absorption of the corresponding 3 A 2 (000) X 1 A 1 (000) transition. X 1 A 1 (000) band of ozone by pulsed cavity ringdown spectroscopy,

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