Volume 214, Issue 10 - 10

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.

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.