David W. Cullin

Rotational, fine, and hyperfine structure in the high‐resolution electronic spectrum of ArOH and ArOD

A number of vibrational bands of the A 2Σ+↔X 2Π electronic spectrum of both ArOH and ArOD have been investigated by laser induced fluorescence with a high‐resolution, pulsed laser system yielding linewidths ≲250 MHz in the UV. This spectrum not only displays completely resolved rotational structure, but also fine and hyperfine structure. The hyperfine constants and precise interatomic distances derived from the rotational constants provide a very interesting picture of the electronic and geometric structure of the complex. The bonding is incipiently chemical in the A state with clear evidence for at least some electronic reorganization between Ar and the open‐shell OH radical in the complex. Conversely, the X state appears to be bound almost solely by physical van der Waals interactions characteristic of systems containing only closed‐shell species.

High-resolution fluorescence excitation spectra of jet-cooled benzyl and p-methylbenzyl radicals

Abstract High-resolution, rotationally resolved, laser-induced, fluorescence excitation spectra of the A 1 and 6a 1 0 bands of benzyl and the 0 0 0 band of p -methylbenzyl radicals were obtained in supersonic expansions. All three spectra were assigned and fit, using the rigid rotor Hamiltonian as well as methyl group internal rotation theory. The results of the rotational analysis provide good rotation constants for benzyl and p -methylbenzyl and establish unambiguously that the symmetry of the excited electronic state in this transition of p -methylbenzyl is 2 A 2 (in C 2v ). The heights of torsional barriers that hinder the internal rotation of the methyl group in p -methylbenzyl also are determined. The torsional results are compared to those obtained previously for this radical in a vibrational analysis and to other open shell radicals.

High resolution laser spectroscopy of asymmetrically deuterated cyclopentadienyl radicals: A study of vibronic degeneracy resolution and Jahn–Teller distortion

The rotationally resolved, laser induced fluorescence, excitation spectra of the partially deuterated cyclopentadienyl radicals, C5H4D and C5HD4, have been observed at low temperature in a supersonic free jet expansion. The observed electronic transition in the uv region corresponds to the A 2A‘2■X 2E‘1 transition in the symmetric cyclopentadienyl isotopomers with D5h symmetry. In the reduced C2v symmetry of the C5HD4 and C5H4D isotopomers, this electronic transition splits into two distinct vibronic bands, separated by about 9 cm−1, which arise from the two vibronic components X1 and X2 into which the X state is resolved when the symmetry is lowered. In C5H4D the ground X1 state has 2A2 symmetry and a permanently distorted, elongated allyl‐like structure while the low‐lying X2 state has 2B2 symmetry and a compressed dienelike structure. The symmetries of the energy levels and the distortions are reversed for the C5HD4 species. A detailed theoretical model is developed to describe the splitting and...

High resolution electronic spectroscopy of Ne⋅OH

The high resolution electronic spectrum of Ne⋅OH has been recorded in a supersonic free jet expansion using the laser‐induced fluorescence technique. From an analysis of the spectrum which yields rotational constants, we are able to obtain Ne⋅OH bond lengths in several vibrational (hindered rotor) levels of the excited state and the vibrationless level of the ground state. We also measure the Fermi contact constant in the A state which is, unlike Ar⋅OH, insignificantly perturbed from the value in the OH monomer. However, we now measure a parity doubling of the X state rotational levels which is tenfold larger than the upper limit we established for such an interaction in Ar⋅OH. We interpret these latter measurements to imply weaker and more isotropic bonding in Ne⋅OH compared to Ar⋅OH in both electronic states.

High resolution electronic spectroscopy of Ar⋅OH and Ar⋅OD vibronic bands

Laser‐induced fluorescence spectra with resolved rotational, fine, and hyperfine structure have been observed for the U bands of the A←X electronic transition of the Ar⋅OH/D complex. The appearance of these spectra are very different from the previously analyzed A band system of the complex. Detailed analysis of the spectra reveals that their spectral differences are caused by changes in values of molecular parameters (due to the different regions of the potential surface sampled in the A state) and their resulting effect upon the coupling of the angular momenta in the complex. Numerical results are given for the molecular parameters in the A state levels examined in these experiments and the implication of these values for the geometry and the potential surface of the complex are discussed.

Rotationally resolved laser spectroscopy of the jet‐cooled methylcyclopentadienyl radical (CH3–C5H4 and CD3–C5H4)

The rotationally resolved, electronic, excitation spectra of supersonically cooled methylcyclopentadienyl radicals, CH3–C5H4 and CD3–C5H4, have been obtained. Analysis of these spectra characterizes both the ground and electronically excited states involved in this transition. The ground state barrier to internal rotation of the methyl group is very high compared to similar closed‐shell species. Additionally it shows a strong isotopic dependence and increases in magnitude further in the excited state. The doubly degenerate 2E‘1 ground state of C5H5 is split into two components, X 2B2 and A 2A2. The rotational analysis shows that the cyclopentadienyl ring undergoes a significant, static distortion from a regular pentagon. The measured distortion is consistent with and related to the dynamic Jahn–Teller distortion in C5H5 itself.