Rebecca L. Schwartz

Electronic spectroscopy and quenching dynamics of OH–H2/D2 pre‐reactive complexes

Binary complexes of OH X 2Π and H2/D2 have been stabilized in the entrance valley to the hydrogen abstraction reaction and identified in the OH A 2Σ+–X 2Π 0–0 spectra region. Nearly all of the intermolecular vibrational levels supported by the OH A 2Σ+ (v′=0)+H2/D2 potential energy surface have been observed in fluorescence depletion experiments. Rapid electronic quenching precludes the observation of OH–H2/D2 prepared in these levels by laser‐induced fluorescence. A sharp onset of laser‐induced fluorescence occurs at the OH A 2Σ+ (v′=0)+H2/D2 dissociation limit. The binding energies for OH–H2/D2 in the ground state correlating with OH X 2Π (v″=0)+H2/D2 have been determined to be 54 cm−1 and more than 66 cm−1, respectively. The OH A 2Σ+ (v′=0)+H2/D2 excited state is found to be at least 577 cm−1 (H2) and 639 cm−1 (D2) more strongly bound than the ground state. The positions of observed features are compared with the corresponding intermolecular levels observed by laser‐induced fluorescence in the OH A–X 1...

Infrared spectroscopy and time-resolved dynamics of the ortho-H2–OH entrance channel complex

The rotationally resolved infrared spectrum of the prereactive o-H2–OH complex in its ground electronic state is obtained in the OH overtone region at ∼1.4 μm using an IR-UV double resonance fluorescence enhancement technique. The pure OH overtone band of o-H2–OH is observed as well as approximately 20 additional rovibrational transitions extending out to the OH (X 2Π,v=2)+o-H2(X 1Σg+) dissociation limit. These transitions are assigned as combination bands involving the simultaneous excitation of the OH vibrational overtone and intermolecular bending (internal rotor) states. The assignment of the experimental spectrum is aided by a detailed comparison with the bound states computed for the ab initio potential of Clary, Werner, and co-workers [Mol. Phys. 83, 405 (1994)]. The infrared spectroscopy results also verify the topology of this ab initio potential in the entrance channel to the OH+H2 hydrogen abstraction reaction. Direct time-resolved experiments indicate that the lifetime of the vibrationally act...

Infrared spectroscopy of OHH2 entrance channel complexes

Abstract The rotationally-resolved infrared spectrum of the ground electronic state of OHH2 has been observed near the OH gn = 2 ← 0 origin by an infrared-ultraviolet fluorescence depletion technique. The experimental spectrum agrees remarkably well with a fully ab initio infrared spectrum originating from the lowest intermolecular level of ortho-H2OH. The lack of measurable homogeneous line broadening in the spectrum indicates that neither vibrational predissociation nor chemical reaction is occurring faster than 45 ps. The magnitudes of the depletions, however, suggest that these processes are taking place on the nanosecond time scale.

Intermolecular vibrations and relaxation dynamics in complexes of OH A 2Σ+ (v′=0,1) with N2

The intermolecular vibrational energy levels supported by the OH A 2Σ+ (v′=0,1)+N2 potentials have been characterized spectroscopically through excitation of OH–N2 complexes in the OH A 2Σ+–X 2Π 0–0 and 1–0 spectral regions. At least 95 levels correlating with OH A 2Σ+ (v′=0)+N2 are observed in fluorescence depletion experiments. OH–N2 complexes prepared in these levels have lifetimes with lower limits ranging from 1.4 to 8 ps due to rapid electronic quenching which precludes their detection by laser‐induced fluorescence. An onset of OH–N2 laser‐induced fluorescence occurs at the OH A 2Σ+ (v′=0)+N2 dissociation limit, enabling determination of the ground and excited state binding energies at ∼250 and ⩾1372 cm−1, respectively. In the OH A–X 1–0 region, OH–N2 transitions originating from a common ground state level to single or groups of intermolecular vibrational levels correlating with OH A 2Σ+ (v′=1)+N2 are observed via laser‐induced fluorescence and fluorescence depletion measurements. Comparison of the...

Infrared spectroscopy of entrance channel complexes

The entrance channel to the OH + H2 yields H2O + H hydrogen abstraction reaction has been probed with unprecedented detail through IR overtone spectroscopy of binary OH-H2/D2 complexes. IR-UV double resonance techniques have been used to obtain rotationally resolved vibrational overtone spectra for ortho-H2-OH as well as ortho- and para-D2OH. Assignment of the spectra has been aided by bound state calculations based on the ab initio potential of Clary, Werner, and coworkers. The pure overtone stretch and combination bands involving intermolecular bending vibrations at energies up to the OH(v equals 2) + H2/D2 dissociation limit have been observed. The intermolecular excitations can drastically alter the relative orientation of the reactants within the complex, producing some that resemble the transition state structure. Direct time-domain measurements show that vibrationally activated OH-H2 complexes have a lifetime of 115(13) ns for the pure overtone stretch and an upper limit of 5 ns for OH-D2. The 20 times faster decay for OH-D2 arises form near-resonant vibrational energy transfer from OH to D2. Finally, IR optical pumping provides sufficient population transfer to enable the rotational distribution of the OH(v equals 1) products from inelastic scattering to be determined.