Effects of vibrational excitation on the F + H2O → HF + OH reaction: dissociative photodetachment of overtone-excited [F-H-OH].

The reaction F + H2O → HF + OH is a four-atom system that provides an important benchmark for reaction dynamics. Hydrogen atom transfer at the transition state for this reaction is expected to exhibit a strong dependence on reactant vibrational excitation. In the present study, the vibrational effects are examined by photodetachment of vibrationally excited F-(H2O) precursor anions using photoelectron-photofragment coincidence (PPC) spectroscopy and compared with full six-dimensional quantum dynamical calculations on ab initio potential energy surfaces. Prior to photodetachment at hνUV = 4.80 eV, the overtone of the ionic hydrogen bond mode in the precursor F-(H2O), 2νIHB at 2885 cm-1, was excited using a tunable IR laser. Experiment and theory show that vibrational energy in the anion can be effectively carried away by the photoelectron upon a Franck-Condon photodetachment, and also show evidence for an increase of branching into the F + H2O reactant channel. The experimental results suggest a greater role for product rotational excitation than theory. Improved potential energy surfaces and longer wavepacket propagation times would be helpful to further examine the nature of the discrepancy

Dissociation dynamics and stability of cyclopentoxy and cyclopentoxide

Abstract Cyclopentoxide c-C 5 H 9 O À undergoes photodetachment to stable cyclopentoxy or the ring-opened 5-oxo-pentan-1-yl radical and dissociative photodetachment, yielding C 3 H 5 O and C 2 H 4 photofragments, at both 532 and 355 nm. The adiabatic electron affinity of c-C 5 H 9 O À is estimated from the experimental results and ab intio calculations to be 1:5 AE 0:1 eV. The results show that c-C 5 H 9 O À is stable relative to dissociation into C 3 H 5 O À and C 2 H 4 by 1:23 AE 0:07 eV, whereas c-C 5 H 9 O is unstable relative to C 3 H 5 O and C 2 H 4 by À0:12 AE 0:12 eV. These results are discussed in terms of the factors affecting the stability of cyclic alkoxides and the corresponding alkoxy radicals

Photoelectron-Photofragment Coincidence Spectroscopy With Ions Prepared in a Cryogenic Octopole Accumulation Trap: Collisional Excitation and Buffer Gas Cooling

A cryogenic octopole accumulation trap (COAT) has been coupled to a photoelectron-photofragment coincidence (PPC) spectrometer allowing for improved control over anion vibrational excitation. The anions are heated and cooled via collisions with buffer gas <17 K. Shorter trapping times (500 μs) prevent thermalization and result in anions with high internal excitation while longer trapping times (80 ms) at cryogenic temperatures thermalize the ions to the temperature of the buffer gas. The capabilities of the COAT are demonstrated using PPC spectroscopy of O3- at 388 nm (Ehν = 3.20 eV). Cooling the precursor anions with COAT resulted in the elimination of the autodetachment of vibrationally excited O2- produced by the photodissociation O3- + hν → O + O2-(v ≥ 4). Under heating conditions, a lower limit temperature for the anions was determined to be 1,500 K through Franck-Condon simulations of the photodetachment spectrum of O3-, considering a significant fraction of the ions undergo photodissociation in competition with photodetachment. The ability to cool or heat ions by varying ion injection and trapping duration in COAT provides a new flexibility for studying the spectroscopy of cold ions as well as thermally activated processes

International Symposium on Free Radicals - August 12 - 17, 2007

This grant provided partial support for the 29th International Symposium on Free Radicals, held August 12-17, 2007 at the Big Sky Resort in Big Sky, Montana. This meeting dates back to 1956, and is thepremier international meeting focusing on the structure and dynamics of reactive free radicals. Free radicals and other reactive species remain topics of great interest today owing to the central role they play as reactive intermediates. Of particular relevance to the DOE mission in Basic Energy Sciences is the fact that nearly all combustion reactions are initiated by radicals, and a number of radical intermediates are important to almost any combusting system. The field has expanded to increasingly focus on the dynamics of radical reactions in addition to spectroscopy and kinetics. As we seek to understand complex environments in combustion, atmospheric chemistry, condensed phase phenomena and the interstellar medium in great detail, studies of free radicals remain essential. The meeting in Montana lived up to the long tradition of this meeting by bringing together leading researchers, including a large group of postdoctoral scholars and graduate students, from around the globe to advance this important area of science. The DOE grant helped to support participant costs (lodging and meals) for 24 invited speakers and 21 graduate students and postdoctoral fellows