Jason D. Robinson

Double-resonance spectroscopy of the high Rydberg states of HCO. IV. Vibrational autoionization dynamics as a function of bending amplitude

Double-resonant ionization-detected absorption experiments extend an investigation of the autoionization dynamics of HCO to include the second overtone of the bend. In these experiments, first-photon transitions to the 3pπ 2Π Rydberg state select single rovibrational levels for second-photon scans of vibrationally autoionizing high Rydberg series that converge to specific rovibrational limits of HCO+. Line shapes reflect coupling widths that join discrete states built on vibrationally excited cores with underlying, vibrationally relaxed continua. Scans of series converging to HCO+ with one and two quanta of bend show narrow linewidths, reflecting relatively long autoionization lifetimes. However, for cores excited to the second harmonic of the bend, certain series abruptly broaden, indicating ultrafast decay. The implications of these results are discussed in terms of a qualitative extension of multichannel quantum defect theory to triatomic molecules.

An experimental measure of anharmonicity in the bending of DCO

Rydberg series of DCO have been rovibrationally isolated by double resonance and extrapolated to produce a map, with sub-wave-number accuracy, of individual rotational level positions in the bending fundamental and first overtone of DCO+. Analysis of this structure determines fundamental frequencies, rotational constants, and low-order anharmonicities associated with the bending mode of the cation. Rotational properties of excited DCO+ found by Rydberg extrapolation are observed to agree well with quantities measured by microwave spectroscopy. The CASSCF–MRCI potential energy surface of Puzzarini and co-workers, which yields fundamental frequencies that agree with experiment for HCO+, is shown to conform well with these first measurements of the bending frequencies of DCO+.

Double-resonance spectroscopy of the high Rydberg states of HCO. V. Rovibronic interactions and l-uncoupling in the (010) manifold

We report the ionization-detected absorption spectra of autoionizing Rydberg states converging to the (010) vibrational level of HCO+. Sharp second-photon resonances appear in transitions from first-photon-prepared originating states that have total angular momentum (less-spin) from N′=0 to 5, selected from the Σ+ and Σ− components of the (010) band of the 3pπ2Π Rydberg state. We systematically compare spectra in order to characterize observed resonances in terms of the good total angular momentum quantum number, N. Rydberg analysis establishes the convergence of series to detailed cation-core rotational quantum numbers, N+. Observed series are found to fit well with simulations employing a limited set of constant quantum defects (δ=1.062, 0.794, 0.606, 0.253, 0.015, 0.002, −0.027 and −0.076). The strengths of observed transitions as a function of initial and final total angular momentum provide a purely experimental indication of the appropriate assignment of the approximately good orbital angular moment...

Laser-assisted (1+1′)-photon ionization-detected absorption spectrum of the 3pπ 2Π state of HCO and DCO

We re-examine the ionization-detected ultraviolet absorption spectrum of the 3pπ 2Π←X 2A′ transition in HCO and DCO using a high-power visible laser to enhance the observation of first-photon resonant features. This technique, which we term here, assisted REMPI, significantly improves the signal-to-noise ratio of the spectrum, making many weak vibronic sub-bands visible for the first time. A comprehensive fit to the structure evident in a progression of bending levels from (000) to (040) refines the assignment of Song and Cool [X. M. Song and T. A. Cool, J. Chem. Phys. 96, 8664 (1992)] to yield a set of rotational constants that vary with K in relation to v2, together with a higher-order contribution to the Renner–Teller splitting in HCO, which is mirrored in DCO for all levels but (040). The (040) band falls at a frequency that is commensurate with that of CD stretch, and Fermi resonance between 3pπ 2Π(1000)Π and the higher-energy (040) K=1(Π) component gives rise to an added splitting that increases the...

Double-resonant photoionization efficiency spectroscopy: A precise determination of the adiabatic ionization potential of DCO

We report the first high-resolution measurement of the adiabatic ionization potential of DCO and the fundamental bending frequency of DCO+. Fixing a first-laser frequency on selected ultraviolet transitions to individual rotational levels in the (000) band of the 3pπ 2Π intermediate Rydberg state of DCO, we scan a second visible laser over the range from 20 000 to 20 300 cm−1 to record double resonance photoionization efficiency (DR/PIE) spectra. Intermediate resonance with this Rydberg state facilitates transitions to the threshold for producing ground-state cations by bridging the Franck–Condon gap between the bent neutral radical and linear cation. By selecting a single rotational state for ionization, double-resonant excitation eliminates thermal congestion. Spectroscopic features for first-photon resonance are identified by reference to a complete assignment of the 3pπ 2Π(000)−X 2A′(000) band system of DCO. Calibration with HCO, for which the adiabatic ionization threshold is accurately known, establ...

Bend–stretch Fermi resonance in DCO+

Rydberg series, rovibrationally isolated by double resonant excitation, are extrapolated to determine rotational-state detailed ionization thresholds to form DCO+ in the bending-excited levels (030) and (040). Analysis of the rovibrational structure derived from the positions of these thresholds for (030) agrees with a simple parameterization extended from a fit to lower vibrational levels. For (040), however, the vibrational angular momentum components (0400) and (0420) are observed to be inverted in energy, with the (0400) component displaced approximately 20 cm−1 to a position above that of (0420). This perturbation is interpreted to support a conjecture first made by Hirota and co-workers that the vibrational structure of DCO+ is perturbed by a 4:1 bend–stretch Fermi resonance. Extending the pattern by which vibrational angular momentum components are observed to be split in (020) and (030), we establish the unperturbed position of (0400), from which we estimate the matrix element for bend–stretch cou...