R. B. Metz

Study of HCO2 and DCO2 by negative ion photoelectron spectroscopy

Photoelectron spectra of HCO−2 and DCO−2 at 299 nm, 266 nm, and 213 nm are reported. Photodetachment accesses the 2A1, 2B2, and 2A2 states of the formlyoxyl radical, HCO2. The 2A1 state is assigned as the HCO2 ground state, although it is nearly degenerate with the 2B2 state (T0=0.027 eV), and the 2A2 state lies at T0=0.536 eV. The electron affinity of HCO2 is 3.498±0.015 eV. The spectra show partially resolved vibrational features, primarily involving progressions in the CO2 bending mode. The irregular appearance of the spectra in some regions suggests vibronic coupling between the 2A1 and 2B2 states. The possible role of the HCO2 radical as an intermediate in the OH+CO→H+CO2 reaction and in H+CO2 inelastic scattering is discussed.

Examination of the Br+HI, Cl+HI, and F+HI hydrogen abstraction reactions by photoelectron spectroscopy of BrHI−, ClHI−, and FHI−

The photoelectron spectra of the ions BrHI−, ClHI−, and FHI−, along with their deuterated counterparts, are presented. These spectra provide information on the transition state region of the potential energy surfaces describing the exothermic neutral reactions X+HI→HX+I(X=Br, Cl, F). Vibrational structure is observed in the BrHI− and ClHI− spectra that corresponds to hydrogen atom motion in the dissociating neutral complex. Transitions to electronically excited potential energy surfaces that correlate to HX+I(2P3/2,2P1/2) products are also observed. A one‐dimensional analysis is used to understand the appearance of each spectrum. The BrHI− spectrum is compared to a two‐dimensional simulation performed using time‐dependent wave packet propagation on a model Br+HI potential energy surface.

Fast beam studies of N3 photodissociation

Abstract A new fast radical beam apparatus has been used to study the photodissociation cross section of N3 as a function of wavelength. Neutralization of a fast negative ion beam by photodetachment is used to create a beam of cold, mass-selected radicals, with subsequent photodissociation and efficient detection of the neutral fragments. N3 was observed to predissociate throughout the first electronic absorption near 270 nm. High-resolution predissociation spectra were obtained which show rotationally resolved structure. Time-of-flight measurements of the kinetics energy release confirm that the lowest energy dissociation pathway which occurs is the first spin-allowed channel producing (N(2D ) + N2(1Σ+g). The photoelectron spectrum of N−3 was also obtained, yielding an electron affinity of 2.68±0.01 eV for N3.

Adiabatic three‐dimensional simulations of the IHI−, BrHI−, and BrHBr− photoelectron spectra

In order to better characterize the transition‐state region for the I + HI, Br + HI, and Br + HBr reactions, the photoelectron spectra of IHI−, IDI−, BrHI−, BrHBr−, and BrDBr− have been simulated using a three‐dimensional adiabatic approach. This method of simulation uses a Born–Oppenheimer separation in time scales between the fast hydrogen‐atom motion and the slow halogen‐atom motion to greatly simplify the computation of the photoelectron spectrum. The resulting simulations are compared to the experimental photoelectron and threshold photodetachment spectra of these anions, and to ‘‘exact’’ simulations of the IHI− and IDI− spectra. The comparison with the exact simulations shows that the adiabatic method is reasonably accurate, and is a considerable improvement over previous approximate simulation schemes. Potential‐energy surfaces for the I + HI and Br + HI reactions are evaluated based on a comparison between the simulated and experimental spectra. A three‐dimensional surface for the Br + HBr reactio...

Fast beam studies of NCO free radical photodissociation

The spectroscopy and dissociation dynamics of the NCO radical have been investigated by applying fast radical beam photodissociation spectroscopy to the Bu20092Π←Xu20092 Π electronic transition. Measurements of the photodissociation cross section as a function of dissociation wavelength show that even the lowest vibrational levels of the Bu20092Π state predissociate. Analysis of fragment kinetic energy release reveals that the spin‐forbidden N(4S)+CO(1Σ+) products are produced exclusively until 20.3 kcal/mol above the origin, at which point, the spin‐allowed N(2D)+CO product channel becomes energetically accessible. The spin‐allowed channel dominates above this threshold. By determining the location of this threshold, we obtain a new ΔHf0 for NCO of 30.5±1 kcal/mol, several kcal/mol lower than the previously accepted value.

Investigation of the F+H2 transition state region via photoelectron spectroscopy of the FH−2 anion

The photoelectron spectrum of the FH−2 anion is reported. The spectrum provides a probe of the transition state region for the F+H2 reaction. The experimental spectrum is compared to the recent simulation by Zhang and Miller which assumes the T5a potential energy surface for the F+H2 reaction. The experimental spectrum is substantially broader. While this may be due to inaccuracies in the T5a surface, the possibility of additional transitions to low‐lying excited electronic surfaces not included in the simulation must also be considered.

Study of hydrogen abstraction reactions by negative ion photoelectron spectroscopy

Photoelectron spectra of the negative ions IHI−, BrHI− and (CH3OH)F− have been recorded at 266 nm and 213 nm. The vibrational structure observed in each spectrum is assigned to the corresponding unstable neutral complex for the bimolecular reaction X+HY→XH+Y. In the case of the centrosymmetric IHI− ion, the neutral complex formed lies near the collinear reaction transition state. For the asymmetric reactions, the geometry of the precursor anion determines whether the reactant or product valley of the reactive surface is probed. The peak positions and widths describe the potential energy surface for the neutral reaction, and in the case of I+HI have been interpreted in terms of a vibrationally adiabatic model.