The transition-state region for neutral hydrogen-transfer reactions can be probed by photodetaching the appropriate stable, hydrogen-bonded negative ion. This paper presents a detailed account of this method, in which the CI + HCI and Br + HBr reactions are investigated by photoelectron spectroscopy of CIHCI-, BrHBr-, and the corresponding deuterated species. The photoelectron spectra exhibit resolved vibrational structure attributed to the unstable neutral [CIHCI] or [BrHBr] complex. The peaks in the spectra are assigned to the antisymmetric stretch mode of the complex, and the peak widths are sensitive to the lifetime of the complex. The BrHBr-and BrDBr-spectra exhibit narrow (15-20 meV) peaks that are likely to result from reactive resonance states supported by the Br + HBr potential energy surface, as well as peaks that appear to be from an electronically excited state of the [BrHBr] complex. The BrHBr-and BrDBr-results have been analyzed to yield an "effective" collinear potential energy surface for the Br + HBr reaction. (24) Truhlar, D. G.; Kuppermann, A. J. Chem. Phys. 1970,52,384. Wu, (25) Bondi, D. K.; Connor, J. N. L.; Manz, J.; Romelt, J. Mol. Phys. 1983, (26) Manz, J.; Meyer, R.; Schor, H. H. R. (28) Neumark, D. M.; Wodtke, A. M.; Robinson, G . N.; Hayden, C. C.; S.-F.; Levine, R. D. Mol. Phys. 1971, 22, 991. 50, 467. Lee. Y . T. J. Chem. Phvs. 1985. 82. 3045. ~~ (29) Metz, R. B.; Kikopoulos; T.;~Weaver, A.; Neumark, D. M.

The reactions F + CH3OH -HF + CH30, F + C2H5OH -HF + C2H50, and F + OH -HF + 0(3P,'D) are studied by photoelectron spectroscopy of the negative ions CH3OHF~, C2H5OHP, and OHP. In each case, photodetachment accesses the transition-state region for direct hydrogen abstraction. The photoelectron spectra exhibit resolved vibrational structure which is sensitive to details of the potential surface in the transition-state region. To aid in the interpretation of the spectra, ab initio equilibrium structures, harmonic frequencies, and hydrogen bond dissociation energies are calculated for the ions CH3OHP and OHP. The anharmonic hydroxyl hydrogen stretching potential is also calculated for the two ions. Using the calculated ion properties and the fitted ab initio reaction surfaces of Sloan et al. (J. Chem. Phys. 1981, 75, 1190, a two-dimensional dynamical simulation of the photoelectron spectrum of OHP is presented and modifications to the reaction surfaces are discussed. The spectra of the alcohol complexes are discussed in light of this simulation, and the role of the "bath" degrees of freedom in these spectra is considered.

Photoelectron spectra of HCO 2Ϫ and DCO 2 Ϫ at 299 nm, 266 nm, and 213 nm are reported. Photodetachment accesses the 2 A 1 , 2 B 2 , and 2 A 2 states of the formlyoxyl radical, HCO 2 . The 2 A 1 state is assigned as the HCO 2 ground state, although it is nearly degenerate with the 2 B 2 state ͑T 0 ϭ0.027 eV͒, and the 2 A 2 state lies at T 0 ϭ0.536 eV. The electron affinity of HCO 2 is 3.498Ϯ0.015 eV. The spectra show partially resolved vibrational features, primarily involving progressions in the CO 2 bending mode. The irregular appearance of the spectra in some regions suggests vibronic coupling between the 2 A 1 and 2 B 2 states. The possible role of the HCO 2 radical as an intermediate in the OHϩCO→HϩCO 2 reaction and in HϩCO 2 inelastic scattering is discussed.

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Photoionization efficiency curves were measured for gas-phase FeO and CuO using tunable vacuum-ultraviolet radiation at the Advanced Light Source. The molecules are prepared using laser ablation of a metal-oxide powder in a novel high-repetition-rate source and are thermally moderated in a supersonic expansion. These measurements provide the first directly measured ionization energy for CuO, IE(CuO)=9.41 +/- 0.01 eV. The direct measurement also gives a greatly improved ionization energy for FeO, IE(FeO) = 8.56 +/- 0.01 eV. The ionization energy connects the dissociation energies of the neutral and cation, leading to a refined bond strength for the FeO cation: D0(Fe(+)-O)=3.52 +/- 0.02 eV. A dramatic increase in the photoionization cross section at energies of 0.36 eV above the threshold ionization energy is assigned to autoionization and direct ionization involving one or more low-lying quartet states of FeO+. The interaction between the sextet ground state and low-lying quartet states of FeO+ is key to understanding the oxidation of hydrogen and methane by FeO+, and these experiments provide the first experimental observation of the low-lying quartet states of FeO+.