J. M. Zellweger

The photolytic cage effect of iodine in gases and liquids

The photolytic cage effect is studied over a wide density range of several bath gases in the laser flash photolysis of iodine at 694.3 nm. Particular attention is paid to measurements in the ‘‘simple’’ solvents Xe and Kr, which are compared with molecular dynamics calculations. Measurements in polyatomic gases and liquids are also reported, as well as the variation of the cage effect with temperature and applied magnetic field. While measuring the cage effect under such a wide variety of conditions, we simultaneously obtain the second order rate constants for iodine atom combination, which help to establish the detailed mechanism of this elementary chemical reaction.

Infrared laser action on the spatial, velocity, and cluster-size distributions in an SF6 free jet

SF6 in a free jet was vibrationally excited by a continuous-wave CO2 laser. The subsequent energy transfer processes, in which vibrational energy is degraded, lead to changes in the beams spatial, velocity and cluster-size distributions. These changes were obsd. by mass-spectrometric techniques. The laser-induced perturbations of the free jet depend strongly on the location at which the laser beam is focused on the SF6 free jet. The largest perturbations were obsd. on irradn. close to the nozzle exit. Measurements were made for different stagnation pressures and temps., laser wavelengths and intensities. The main laser effects upon irradn. close to the nozzle are: (1) an increase in the mean velocity of the beam along the beam centroid, (2) a decrease in beam intensity measured near the center of the beam, and (3) an increase in the width of the velocity distribution (local temp.) of the beam. Effect 1 increases and effect 3 decrease the beams Mach no., with the latter dominating and thus a net decrease results. These effects are enhanced with increasing stagnation pressure in the range 0.5-2 bar and with decreasing stagnation temp. in the range 198 <T0 <358 K. Comparison of the time-of-flight spectra of the SF6 monomer and dimer shows both a higher mean velocity and a lower local temp. for the latter. Measurements on the mol. beam at a wavelength where only 34SF6 absorbs indicate fast scrambling of vibrational energy between the 2 isotopomers. Cluster formation could be inhibited by irradiating the SF6 monomer in the collisional region of the beam with laser power densities in the order of a few kW-cm-2. The wavelength dependence of the laser action changes drastically with the position at which the 2 beams intersect and with the stagnation conditions. This is due to the IR absorption of the van der Waals clusters.

The photolytic cage effect of iodine in the gas phase

The laser flash photolysis of iodine has been studied over a wide pressure range in several bath gases. At low pressures the observed quantum yield for photolysis at 6943 A is independent of the gas density, whereas at higher pressures the quantum yield decreases with increasing gas density. This phenomenon is discussed in terms of the cage effect, and the measured yields are compared with model calculations. The experiments also provide information on the second order rate constants for iodine atom combination.

Infrared vibrational predissociation spectroscopy of (SF6)mArn clusters: Implications for effective laser isotope separation

Selective infrared vibrational predissociation (IRVP) of Van der Waals clusters in a free jet is shown to be an effective way to separate isotopes. Absolute separation factors α in excess of 1.1 are observed upon irradiation of a beam of SF6 diluted in argon with a 20 W cw CO2 laser. Selection of the wavelength permits enrichment or depletion of any one of the sulfur isotopes in the mixture. In order to understand the observed wavelength dependence of the isotope enrichment we have measured the effective IRVP spectra of the individual Van der Waals clusters (SF6)mArn with 1 ⩽ m ⩽ 3 and n ⩽ 9. A systematic study of these spectra reveals a significant red-shift which occurs upon attaching Ar atoms to (SF6)m. Such shifts can attain as much as 10 cm−1, as in the case between SF6 and SF6Ar9.

Laser Probing of Cluster Formation and Dissociation in Molecular Beams

An SF6 molecular beam is irradiated with a cw CO2 laser. The multiple effects of the infrared radiation are analyzed with several mass spectrometric techniques. Detailed information is obtained on the extent of the collisional region of the adiabatic gas expansion. Infrared vibrational predissociation spectra of the van der Waals clusters are obtained.