Kenneth S. Haber

Resonant ion‐dip infrared spectroscopy of benzene–H2O and benzene–HOD

Resonant ion‐dip infrared spectra of C6H6–H2O and C6H6–HOD have been recorded in the OH stretch fundamental region. The spectra provide further evidence for the unique, large‐amplitude motions present in these π hydrogen‐bonded complexes. In C6H6–H2O, transitions out of the lowest ortho (Π) and para (Σ) ground state levels are observed. A transition at 3634 cm−1 is assigned as an unresolved pair of parallel transitions (Σ→Σ and Π→Π) involving the symmetric stretch fundamental (at 3657 cm−1 in free H2O). In the antisymmetric stretch region, transitions at 3713, 3748, and 3774 cm−1 are assigned as Π→Σ, Σ→Π, and Π→Δ transitions, respectively. The spacing of the transitions is consistent with nearly free internal rotation of H2O about benzene’s sixfold axis in both ground and vibrationally excited states. The intensities of combination bands depends critically on the mixing of some local mode character into the symmetric and antisymmetric stretches at asymmetric positions of H2O on benzene. Surprisingly, in C6H6–HOD, five transitions are observed in the OH stretch region, all arising from the ground state zero point level. Even more unusual, the higher‐energy combination bands are many times stronger than the OH stretch fundamental. The local mode OH stretch has components both parallel and perpendicular to benzene’s sixfold axis, leading to strong parallel and perpendicular transitions in the spectrum. A two‐dimensional model involving free internal rotation and torsion of HOD in its plane is used to account for the qualitative appearance of the spectrum. The form of the OH(v=0) and OH(v=1) torsional potentials which reproduce the qualitative features of the spectrum are slightly asymmetric, double‐minimum potentials with large‐amplitude excursions for HOD over nearly 180°.

On the shape of C6H6

The benzene molecule serves as a benchmark among the aromatic hydrocarbons and has been the subject of numerous experimental and theoretical studies. Despite such intensive investigations, the precise structure of the benzene cation (C6H6+) is not known. Now, experiments measuring high-resolution state-to-state threshold photoionization spectra of benzene concretely establish the terms of vibronic levels in the distorted cation that are split by higher order Jahn-Teller coupling between its 2E1g electronic ground state and ν6 e2g in-plane ring-bending vibrational mode. This assignment, in turn, sets the absolute energy phase of the vibronic pseudorotation in this coordinate and thereby offers a definitive experimental determination of the shape of the benzene cation.

Near-infrared Raman imaging microscope based on fiber-bundle image compression

The design and performance of a near-infrared Raman imaging microscope (NIRIM) is described. This new instrument utilizes fiber-bundle image compression (FIC) to collect simultaneously a 3-D Raman spectral imaging data cube. Key NIRIM design features are discussed, including the FIC fiber-bundle, excitation laser, optical coupling to the microscope and fiber-bundle, holographic filtering, spectrograph imaging requirements, CCD parameters and chemical image processing. The theoretical collection efficiency and image quality of the NIRIM instrument are compared with those of tunable filter and line scanning Raman imaging methods. The performance of the NIRIM is demonstrated using a white light image of a bar-target and Raman chemical images of samples containing fructose–sucrose and Pb(NO3)2–K2SO4 microcrystalline mixtures. A Raman image collection time as fast as 1 s (total detector integration time) is demonstrated, for a 3-D data cube containing 322 image resolution elements and 900 Raman shift wavelengths. Copyright

Direct determination of the adiabatic ionization potential of NO2 by multiresonant optical absorption

Abstract The adiabatic ionization threshold of NO 2 is observed by direct optical excitation in a three-color, triply resonant, three-photon transition from vibrationless levels of the bent ground state through the vibrational ground state of the linear Rydberg state to the ground state of the ion. The overall energy for this process for rotationless molecules is estimated to be 77320 ± 20 cm −1 . The bent-to-linear two-photon transition to the Rydberg state is facilitated by a first-photon resonance with levels of NO 2 s mixed low-lying excited states. At higher energies, following intermediate selection of vibrationally excited 3pσ Rydberg states, transitions to series of high Rydberg states are observed uniformly converging to respective vertical ionization potentials. These transitions are marked by sharp bands below and in some cases above the adiabatic ionization threshold. Intensities suggest efficient autoionization.

Photoselection and the structure of highly excited states: Rotationally resolved spin–orbit autoionization spectrum of HCl

Ultraviolet two‐photon photoselection, followed by visible one‐photon absorption is applied to HCl to record the first double‐resonant spin–orbit autoionization spectrum of a hydrogen halide, and the first rotationally resolved such spectrum of HCl. The J=2 level of the F 1Δ2(v=0) Rydberg state serves as the intermediate two‐photon resonance. The ionization‐detected absorption spectrum from this initial state, scanned across the 634 cm−1 interval between the lower 2Π3/2 and upper 2Π1/2 thresholds, shows a complex system consisting of hundreds of sharp lines converging to the accessible rotational limits of the upper spin–orbit threshold. The complexity of the spectrum is attributed to the relaxed selection rules associated with dipole transitions from a state in Hund’s case (a) to a manifold approaching Hund’s case (e), in concert with the irregularities expected for angular momentum coupling intermediate between the limits of case (c) and case (e). A simple case (e) fit over the central portion of the sp...

Multiresonant spectroscopy and dynamics of molecular extravalent states: State-resolved intramolecular relaxation of NO2 above 9 eV

Abstract Multiresonant methods are described for the optical selection of individual rovibrational states at energies near and above the first ionization threshold of NO 2 . These techniques are applied to resolve distinct Rydberg series converging to respective vertical ionization potentials by optical absorption from assigned vibration-rotation levels of the low-lying sharp 3pσ 2 Σ u + state. Spectra of levels lying between adiabatic and vertical thresholds show evidence for vibrational autoionization. This process is found to be comparatively slow ( T ⪢ 30 ps) for all levels but those for which autoionization via a Δν=−1 transition in the symmetric stretch, ν 1 , is accessible. Such rates are fastest for levels just above this Δν=−1 threshold, exhibiting characteristic Fano lineshapes as broad as 11 cm −1 . Bands thereupon proceed to narrow with increasing energy, to first re-exhibit rotational fine structure, then evidence l uncoupling in patterns of transitions very close to vertical thresholds.

New Information on the Structure and Dynamics of Molecular Cations from Experiments on The Spectroscopy of Polyatomic Rydberg States

Historically, electronic spectroscopy has played a central role in the effort to structurally characterize small molecules, and, particularly, reactive intermediates.1 Increasingly, spectroscopic knowledge and analysis has also had a tremendous impact on our understanding of the radiationless dynamics of molecular excited states.2 The theoretical apparatus for dealing with such problems is well developed, and has been profitably extended to considerations of intramolecular relaxation in vibrationally excited electronic ground states.3