Volume 214, Issue 10 - 10

Two types of pyrolysis nozzles have been constructed and coupled to a new Stark modulated microwave spectrometer. The nozzles were tested on their ability to generate rotationally cooled transient species through a supersonic expansion. The transients species vinylamine, thioketene and ketene were generated and detected using nozzle temperatures ranging from 400-800°C. Pyrolysis temperatures were generally lower than those used in normal flow pyrolysis experiments and rotational temperatures of ca. 10 K were achieved. A preliminary investigation of the jet nozzle pyrolysis of 3-methyl-4-hydroxy-iminoisoxaline-5-one was carried out and showed a different distribution of CHNO pyrolysis products to that observed in previous low pressure studies.

Assuming random, multiple collisions the `fall-off´ problem in chemical recombination reactions is reconsidered. Following this approximative model the `fall-off´ from third order reactions at low pressure to second order reactions at high pressure appears in a simple way as a consequence of the collision time considered as the lifetime of the unstable van der Waals complex formed initially in the recombination mechanism. Comparisons are presented for results from the `fall-off´ following the Lindemann type mechanism, and of the method of Troe, Gilbert et al . based on the RRKM theory, together with some experimental results for recombination reactions at low and intermediate pressures.

Some years ago Jay-Gerin and Ferradini attempted to establish a correlation between the optical absorption spectrum and the mobility of excess electrons in various polar solvents (J. Chem. Phys. 91 (1989) 3275). To explain this correlation they postulated two types of electrons, i.e., highly localized solvated electrons and so-called incompletely relaxed electrons. This was done, however, without any physical justification. According to this correlation and its interpretation just the stable ammoniated electron (surprisingly not considered by Jay-Gerin and Ferradini) should be a quite typical representative of these incompletely relaxed electrons. Therefore, we analyzed the theoretical relationship between the electron mobility and the zero-frequency absorption cross section with the aid of the experimentally observed absorption spectra. We found out that there is no support for a linear relationship between the mobility and the position of the maximum, hω max , of the optical absorption band and that the contribution of the cross section to the electron mobility is practically zero. Therefore, the proposed correlation has no physical background. We show, however, that there exists a rough correlation between the excess electron mobility and the self-diffusion coefficient of the polar solvent. Finally we critically consider the so-called incompletely relaxed electron state and the results of quantum statistical simulations of the diffusion coefficient of solvated electrons (e - solv ).

The decomposition rate constant of the CF 3 CO radicals was investigated as a function of pressure and temperature, using both experimental and theoretical approaches. The rate constant was measured experimentally by flash photolysis/UV absorption at one atmosphere pressure and in the 298-443 K temperature range. Experiments were complemented by RRKM calculations combined with DFT quantum calculations. At one atmosphere pressure (N 2 + O 2 ), the rate expression derived from experiments is: k = 10 (11.0±0.7) exp(-(4214±600)K/T) s -1 , in fairly good agreement with previous results obtained at different temperatures and using different experimental methods. The rate constant is in the falloff at one atmosphere pressure, near the low pressure limit at the highest temperature and thus, the above expression should not be used at other pressures. This explains the apparent low pre-exponential factor. RRKM calculations were performed and fitted to the present results as well as to the previous ones obtained at different pressures. Those calculations have resulted in a comprehensive and consistent description of the kinetic properties of this decomposition reaction. The detailed results are presented using the temperature dependent parameters of the Troe´s equation: k 0 (T) = 1.55×10 -8 exp(-4420 K/T) cm 3 molecule -1 s -1 (buffer gas N 2 + O 2 ) k ∞ (T) = 1.45×10 14 exp(-5878 K/T) s -1 for F c = 0.6.

Ultrafast pump-probe experiments with a time-resolution of 30 fs have been carried out to explore the non-radiative relaxation dynamics of electronically excited 1,8-dihydroxyanthraquinone (DHAQ) in polar liquid solution. The results are discussed in terms of a Lippincott-Schroeder double-minimum potential along the proton-transfer reaction coordinate for the ground (S 0 ) and first excited singlet states (S 1 ) of DHAQ. The 400-nm pump-visible probe data reveal a strongly Stokes-shifted stimulated emission due to the proton-transferred 1,10-quinone configuration in the S 1 -state. A dominant fraction of this stimulated emission appears instantaneously implying that cross-well excitation directly from the ground-state 9,10-quinone form into the excited-state 1,10-quinone configuration takes place. A smaller fraction of the stimulated emission appears delayed with a time constant of approximately 300 fs. This component may be due to proton-transfer in the S 1 -state following optical excitation. Further, a transient absorption is observed on the red edge of the linear absorption spectrum of ground-state DHAQ due to higher-lying electronic states presumably from the 1,10-configuration. Periodic modulations of the transient absorption due to wavepacket motion in the 9,10-quinone form are partially consistent with previous fluorescence excitation spectra of the molecule taken under jet-cooled conditions.

Potential reaction mechanisms of D + (D 2 O) n (n = 4-30) with chlorine nitrate (ClONO 2 ) have been investigated using a fast flow reactor operated under thermal conditions. Chemical reactions are not observed to occur between D + (D 2 O) n and chlorine nitrate. Instead, through studies employing the heavy isotope of hydrogen, we found that the nitric acid observed in the product ion spectrum arises from a nitric acid impurity that is usually present to some degree in the methods employed to synthesize ClONO 2 reactant gas. The results from this study serve to explain other conflicting reaction mechanisms reported in the literature, and which have been discussed in the context of atmospheric chemistry.

Exact numerical solutions of the time-dependent Schroedinger equation, TDSE of a 1-D H + 2 molecule are obtained for intense (I ≥ 10 14 W/cm 2 ) ultrashort (τ < 50 fs) laser pulses in order to examine laser control of the exact non-Born-Oppenheimer electron-nuclear dynamics in this simplest system. Such a non-perturbative treatment of the exact electron-nuclear dynamics allows us to examine the usefulness of simple two-colour (ω + 2 ω) coherent superpositions of laser pulses to control and manipulate electron and nuclear motion simultaneously. Three processes occur at these high intensities: molecular dissociation, electron ionization and Coulomb explosion. Varying the relative phase of the ω and 2 ω laser fields allows for the simplest control of the complete electron-nuclear dynamics of these three processes as shown by the present simulation. A major finding is that electron dynamics in the presence of intense ultrashort laser pulses is ´´counterintuitive´´, which can be explained in terms of a quasistatic tunnelling model. Control of the dissociative H+p channel can be explained in terms of ´´bond softening´´ occurring from laser-induced avoided crossings of dressed molecular potentials. Control of Coulomb explosion depends on a new nonperturbative phenomenon-Charge Resonance Enhanced Ionization, CREI, which occurs at large critical internuclear distances and can be explained in terms of ´´frontier´´ orbitals.