Timothy M. Cerny

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.

H atom plasma diagnostics: A sensitive probe of temperature and purity

Two-photon absorption laser-induced fluorescence (TALIF) has proven to be a convenient diagnostic for reactive light atoms in plasmas. We have carried out a series of TALIF experiments and report the first temperature measurements of ground state H atoms in an rf discharge. With reasonable care, measurements of the H atom linewidths, broadened by the Doppler effect, provide detailed information about the translational energy, i.e., temperature of the atoms. It is found that in pure H2 plasmas, the H atom temperature is slighthy elevated with respect to ambient. In plasmas contaminated with the other H-containing molecules, Doppler-broadened linewidths corresponding to H atom temperatures in excess of 7000 K have been observed. The mechanisms leading to such high apparent temperatures are discussed.

Hyperfine structure in the electronic spectra of the CdH and CdCH3 radicals

Magnetic hyperfine interactions in the ground and first excited states of CdH and CdCH3 were studied using high resolution electronic spectroscopy. Hyperfine splittings associated with the H nucleus were observed in the X state of CdH; hyperfine splittings associated with 111Cd and 113Cd were observed in both the ground and first excited states of CdH and CdCH3. The hyperfine parameters of CdH were found to be similar to the corresponding parameters of CdCH3. Comparison of the ground state molecular hyperfine constants with values determined in an Ar matrix electron paramagnetic resonance (EPR) study [L. B. Knight, Jr. and W. Weltner, Jr., J. Chem. Phys. 55, 2061 (1971)] indicates clearly a shift of unpaired electron spin density towards Cd, in the matrix. The excited state molecular hyperfine constants indicate that the unpaired electron essentially resides in a Cd 5pπ orbital.

ROTATIONALLY RESOLVED ELECTRONIC SPECTRA OF THE HALF-SANDWICH ORGANOMETALLIC RADICAL, CAC5H5

Rotationally resolved spectra have been recorded and analyzed for both spin–orbit components of the 000 and 410 (ν4= Ca–C5H5 stretch) vibrational bands of the A 2E1←X 2A1 transition of the organometallic free radical CaC5H5. The radicals were prepared with a laser ablation/photolysis technique, then probed in a supersonic free‐jet expansion. Intensity variations from transitions originating from various K‘ levels confirm the C5v nature of the system with the Ca atom centered over the cyclopentadienyl ring. A complete structure could not be uniquely deduced from the rotational constants determined in the global fit of the two vibrational bands but reasonable assumptions for the C5H5 ring enable the Ca–C5H5 ring distance to be determined to be 2.333(+13−23) A in the ground state which shortens by 0.060(1) A in the excited state. Other fitted molecular constants indicate that Jahn–Teller distortion effects are unmeasurably small and that the electronic orbital angular momentum in the excited state is essen...