James R. Dunlop

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.

Absolute H-atom concentration profiles in continuous and pulsed rf discharges

Absolute concentrations of ground state atomic hydrogen are determined by two‐photon absorption laser‐induced fluorescence in a pure hydrogen rf discharge. Axial concentration profiles obtained across the interelectrode space show a relatively constant concentration (of the order of 1014 atoms/cm3) in the region around the center of the discharge until it decreases sharply within 3–4 mm of each metal electrode surface. In addition, the temporal evolution of the atomic concentration is monitored by means of a pulsed discharge from short times to the establishment of a steady‐state concentration. We present a model according to which the formation and decay of the distribution of atomic hydrogen is dominated by diffusion from the production regions towards both the center of the discharge and the metallic electrode surfaces where H atoms are absorbed.

Laser excitation and dispersed fluorescence spectra of the Ne·OH ×X̃ electronic transition

Abstract We report both laser-induced fluorescence and dispersed emission spectra for the Ne·OH complex. The present experiments significantly extend previous observations and lead to new results for dissociation limits and potential surface anisotropy parameters for the complex.

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.

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.

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.

Thioformaldehyde single rotational level photophysics: Longer than radiative lifetimes and reduced fluorescence yields in the isolated molecule

Single rotational level fluorescence lifetimes and relative fluorescence quantum yields have been measured under collision‐free conditions for a variety of rotational levels in the 41 vibrational state of S1 H2CS. About half of the rotational levels studied decay with a radiative lifetime of ≊170 μs. Other levels decay with lifetimes longer than the radiative lifetime and these states correlate with perturbations observed in the spectrum, as detected by Doppler‐limited laser‐induced fluorescence and sub‐Doppler intermodulated fluorescence techniques. The levels coupled to T1 and most of the levels coupled to S0 have relative fluorescence yields near unity. A few levels in S1 show reduced quantum yields of fluorescence which persist at pressures as low as 5×10−7 Torr. A sequential coupling model is proposed to qualitatively account for the observed excited state dynamics of these levels.

High-resolution electronic spectroscopy of the Ne·OH and Ne·OD A˜←X˜ bands

The high-resolution laser-induced fluorescence spectra of the Ne·OH A˜(0,11,1)←X˜(0,00,0) and Ne·OD A˜(0,00,0)←X˜(0,00,0),A˜(0,00,1)←X˜(0,00,0),A˜(0,10,1)←X˜(0,00,0),A˜(0,11,1)←X˜(0,00,0) transitions were obtained. Analyses of these spectra determine precise bond lengths and reveal interesting spin–rotation and parity structure. In the A (0, 1K=0,1, 1) levels of Ne·OH and Ne·OD, the spin–rotation structure is described with the parameter γ, whose magnitude is larger than in the other observed levels of Ne·OH, Ne·OD, and Ar·OH, and Ar·OD. For the K = 1 level an additional term κ is introduced and is found to be an order of magnitude larger than γ. A model based on perturbation theory is proposed and successfully explains the peculiar spin–rotation structure in the A (0, 1K, 1) levels. In addition, a parity-splitting (P-type doubling) for the X˜(0,00,0) level of Ne·OD was determined to be a factor of 10 larger than that in Ar·OD. The parity splitting (K-type doubling) for the A (0, 11, 1) level of Ne·OH is also observed and measured.

Effects of surfaces on H‐atom concentration in pulsed and continuous discharges

We report on the effect of Si and GaAs substrate materials on the hydrogen atom ground‐state concentration profiles in 10 MHz pulsed and continuous rf discharges. We measure absolute hydrogen atom concentrations in the discharge via two‐photon absorption laser‐induced fluorescence. The spatial profiles of the H atoms in an unloaded plasma reactor show a concentration on the order of 1014 atoms/cm3 at the center of the discharge, a concentration relatively constant until it sharply decreases at a 3–4 mm distance away from the electrode surfaces. However, when the discharge is loaded with semiconducting materials, the H‐atom concentration drastically increases at the region adjacent to the semiconducting surfaces. The evolution of these profiles obtained in a pulsed discharge suggests that the H atoms are produced in localized regions in the discharge, and they diffuse towards the center and the confining surfaces. We discuss the validity of a diffusion model which treats the metallic surfaces as sinks, and...

The laser-induced fluorescence spectra of jet-cooled arylmethyl radicals

Abstract Laser-induced fluorescence excitation and resolved emission spectra have been observed from the arylmethyl radicals, triphenylmethyl, diphenylmethyl, diphenylchloromethyl, and diphenylethyl. The free-jet-cooled radicals exhibit spectra with rich but well resolved vibrational structure, some of which is likely attributable to hindered twisting of the phenyl rings. Precise frequencies for the origins of the electronic transitions as well as fluorescence lifetimes for these radicals are reported.