James M. Williamson

Near-Infrared Raman Spectroscopy with a 783-nm Diode Laser and CCD Array Detector:

A GaAlAs diode laser operating at 783 nm was combined with an unintensified charge coupled device (CCD) array detector and single grating spectrograph to obtain near-infrared (NIR) Raman spectra. The spectrometer has no moving parts and retains the high sensitivity expected for multichannel, shot-noise-limited detectors. Diode laser excitation permits high-sensitivity Raman spectroscopy with reduced fluorescence interference, in comparison to that produced with conventional visible lasers. The diode laser/CCD approach should exhibit much higher sensitivity than FT-Raman systems operating at 1064 nm, at much lower laser power. The sensitivity of the system was demonstrated by an S/N ratio of 17 for the 981-cm−1 band of 0.01 M (NH4)2SO4, obtained with 30 mW of 783 nm laser power.

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 electronic spectroscopy of ZnCH3 and CdCH3

ZnCH3 and CdCH3 radicals have been prepared in a cold supersonic free jet expansion and their laser‐induced‐fluorescence spectrum recorded for the A 2E←X 2A1 electronic transition. These spectra show well resolved rotational and spin structure, which has been completely analyzed. This analysis yields the rotational constants and the components of the spin–rotation tensors in the A and X states of both radicals. The observed constants are discussed in terms of the electronic structure of the radicals. It is demonstrated that the upper F2 spin–orbit component of the A 2E state of CdCH3 is strongly perturbed by another, dissociative electronic state. This leads to some predissociation of the A 2E3/2 component and a broadening of its lines. The rotational and fine structure in this state is also quite perturbed leading to an unusual, but still interpretable spectrum.

High resolution electronic spectroscopy of MgCH3

The MgCH3 radical was produced by a laser ablation/photolysis technique in a cold supersonic free‐jet expansion and probed by laser induced fluorescence. Rotationally resolved spectra for both spin‐orbit components of the A 2E←X 2A1 electronic transition have been recorded. The analysis of these spectra yields the rotational constants of MgCH3 and therefrom a structure for the radical is proposed. A comparison is made among a series of alkyl organometallic 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...

THE ELECTRONIC SPECTROSCOPY OF THE BA+-AR COMPLEX : POTENTIAL SURFACE AND DISSOCIATION ENERGIES

Ba+–Ar open‐shell ionic complexes were produced in a pulsed free‐jet expansion. The dispersed emission and both the low and high resolution A 2Π–X 2Σ+ excitation spectra of the Ba+–Ar complex are reported. The data obtained were used to construct potentials for the ground and excited states. A simple quantum mechanical model was introduced in order to simulate the experimentally measured potentials. The model potential is used to estimate the dissociation energy of the ground 2Σ+ state. This value, when combined with the spectral red shift, allows the dissociation energies of the two components of the excited 2Π state to be determined. The same electrostatic interaction model also explains the observed angular momentum coupling scheme as well as the much stronger binding in the excited 2Π state.

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.

The spin-rotation interactions in the methoxy radical

The spin-rotation coupling splitting in the A 2 A 1 state of CH3O has for the first time been spectrally resolved and measured using a very high resolution laser induced fluorescence apparatus. Both the spin-rotation parameters for the A state and the previously measured ones of the Xtilde; state are discussed in terms of first and second order contributions. A good understanding of all the measured constants is obtained, including the major Jahn-Teller induced contribution in the ground state.

Potential surface and dissociation energies from high-resolution electronic spectroscopy of Ne·OH

Abstract High-resolution electronic spectra have been obtained and analyzed for transitions terminating in levels of the A state of Ne·OH from the vibrationless level to near the dissociation limit. These results are used to construct a potential surface for the stretching motion in the A state and to obtain dissociation energies in both the A and X states.