Maria Tsiouris

OH vibrational activation and decay dynamics of CH4–OH entrance channel complexes

Infrared spectroscopy has been utilized to examine the structure and vibrational decay dynamics of CH4–OH complexes that have been stabilized in the entrance channel to the CH4+OH hydrogen abstraction reaction. Rotationally resolved infrared spectra of the CH4–OH complexes have been obtained in the OH fundamental and overtone regions using an IR-UV (infrared-ultraviolet) double-resonance technique. Pure OH stretching bands have been identified at 3563.45(5) and 6961.98(4) cm−1 (origins), along with combination bands involving the simultaneous excitation of OH stretching and intermolecular bending motions. The infrared spectra exhibit extensive homogeneous broadening arising from the rapid decay of vibrationally activated CH4–OH complexes due to vibrational relaxation and/or reaction. Lifetimes of 38(5) and 25(3) ps for CH4–OH prepared with one and two quanta of OH excitation, respectively, have been extracted from the infrared spectra. The nascent distribution of the OH products from vibrational predissoc...

INFRARED ACTION SPECTROSCOPY AND TIME-RESOLVED DYNAMICS OF THE OD-CO REACTANT COMPLEX

The infrared action spectrum of the linear OD–CO reactant complex has been recorded in the OD overtone region near 1.9 μm using an infrared pump-ultraviolet probe technique. The pure overtone band of OD–CO (2νOD) is observed at 5148.6 cm−1 and combination bands involving the simultaneous excitation of OD stretch and D-atom bend are identified 160.0 and 191.2 cm−1 to higher energy. Band assignments and spectroscopic constants are derived from the rotationally resolved structure of the spectra. The change in the ground state rotational constant upon deuteration demonstrates that the H/D-atom of the hydroxyl radical points toward CO in the OH/D-CO complex. Direct time-domain measurements yield a lifetime of 37(4) ns for OD–CO (2νOD) prior to decay via inelastic scattering or chemical reaction. This is significantly longer than the laser-limited lifetime of ⩽5 ns observed for OH–CO (2νOH), and is attributed in part to the closing of a near-resonant vibration to vibration energy transfer channel upon deuterati...

Activation of the CH stretching vibrations in CH4–OH entrance channel complexes: Spectroscopy and dynamics

The vibrational spectroscopy and decay dynamics of CH4–OH reactant complexes have been studied in the CH4 symmetric and antisymmetric stretching regions (ν1 and ν3). The vibrational spectra have been obtained using both infrared and stimulated Raman excitation with ultraviolet probe laser-induced fluorescence detection. Stimulated Raman excitation of CH4–OH in the symmetric stretching region reveals two blended Q branch features at 2912.5 and 2911.8 cm−1. An extremely weak infrared spectrum is also seen in the CH4 symmetric stretching region, which is induced by the presence of the nearby OH partner. Infrared excitation in the asymmetric stretching region results in an intense, yet enormously broad spectrum centered at 3020 cm−1 that extends over 40 cm−1. The appearance of the spectra in the ν1 and ν3 regions has been explained in terms of a model in which the CH4 unit undergoes internal rotation within the CH4–OH complex. The ν1 features are attributed to transitions involving two different nuclear spin ...

Stimulated Raman and electronic excitation of CH4–OH reactant complexes

Abstract Binary complexes composed of OH and CH4 reactants have been stabilized in a shallow well in the entrance channel to the hydrogen abstraction reaction. The CH4–OH reactant complexes are identified by laser-induced fluorescence in the OH A 2 Σ + –X 2 Π (1,0) and (0,0) spectral regions as well as by stimulated Raman excitation in the vicinity of the symmetric stretching mode (ν1) of CH4. The binding energy of CH4–OH is found to be 210±20 cm−1 in its ground electronic state and increases slightly upon vibrational excitation of the ν1 mode of methane.

OD–N2: Infrared spectroscopy, potential anisotropy, and predissociation dynamics from infared-ultraviolet double resonance studies

The infrared spectrum of the linear OD–N2 complex has been recorded in the OD overtone region near 1.9 μm using an infrared-ultraviolet double resonance technique. The pure overtone band of OD–N2(2νOD) was observed at 5173.99(1) cm−1, and combination bands involving the simultaneous excitation of OD stretch and geared bend were identified at 5209.02(1) cm−1 and 5214.59(2) cm−1. Assignments and spectroscopic constants have been derived from the rotationally resolved structure of each band, which are in good accord with model calculations based on an electrostatic interaction potential. Direct time–domain measurements yielded a vibrational predissociation lifetime of 150±16 ns for OD–N2 (2νOD) and a three fold decrease in lifetime upon intermolecular excitation of the lower-energy geared bending state. The OD (v=1) fragments of vibrational predissociation were found to be highly rotationally excited, indicating that predissociation proceeds by vibrational to rotational/translational energy transfer. The res...