No abstract

We have implemented the velocity map imaging technique to study clustering in the pulsed supersonic expansions of hydrogen bromide in helium, argon, and xenon. The expansions are characterized by direct imaging of the beam velocity distributions. We have investigated the cluster generation by means of UV photodissociation and photoionization of HBr molecules. Two distinct features appear in the hydrogen atom photofragment images in the clustering regime: (i) photofragments with near zero kinetic energies and (ii) "hot" photofragments originating from vibrationally excited HBr molecules. The origin of both features is attributed to the fragment caging by the cluster. We discuss the nature of the formed clusters based on the change of the photofragment images with the expansion parameters and on the photoionization mass spectra and conclude that single HBr molecule encompassed with rare gas "snowball" is consistent with the experimental observations.

Temperature dependence of the absorption spectrum of CH4 by high resolution spectroscopy at 81 K: (II) The icosad region (1.49-1.30 µm). AbstractThe high resolution absorption spectrum of methane has been recorded at liquid nitrogen temperature by direct absorption spectroscopy between 6717 and 7351 cm -1 (1.49 -1.36 m) using a cryogenic cell and a series of Distributed Feed Back (DFB) diode lasers.The investigated spectral region corresponds to the very congested low energy part of the icosad for which the HITRAN database provides neither rovibrational assignments nor the lower state energies. The positions and strengths at 81 K of 9389 transitions were obtained from the spectrum analysis. The minimum value of the measured line intensities (at 81 K) is on the order of 10 -26 cm/molecule. From the variation of the line strength between 81 K and 296 K, the low energy values of a total of 4646 transitions were determined. They represent 79.4 % and 68.4% of the total absorbance in the region at 81 and 296 K, respectively, and include 28 transitions assigned to the  2 +4 4 band near 6765 cm -1 . The reliability of the method based on the association of lines with coinciding centers in the 81K and 296 K spectra is discussed. The results of the present analysis have been combined with previously analyzed high energy part of the icosad dominated by the  2 +2 3 band near 7510 cm -1 . The line list for the whole icosad (6717-7655 cm -1 ) consists of 12865 transitions at 81 K.

High resolution IR overtone pumping with an injection seeded optical parametric oscillator (OPO) is used in conjunction with excimer laser photolysis to investigate the state-resolved dynamics of quantum state-selected van der Waals clusters in a slit supersonic expansion. The narrow band IR light source (160 MHz, 5 mJ) preselects a specific upper state via the internal rotor band of Ar–H2O which correlates to the ‖03〉−←‖00〉+, 000←101 transition in H2O monomer. At fixed UV photolysis and probe wavelength, scanning the high resolution OPO yields the ‖03〉−←‖00〉+, Σ(000)←Σ(101) overtone action spectrum of Ar–H2O complexes. Conversely, at fixed IR pump wavelength, the state distribution of the OH photoproduct from photolysis of quantum state selected Ar–H2O clusters can be probed by laser induced fluorescence on the A 2Σ+←X 2Π (0,0) band. The OH distributions from H2O monomer vs Ar–H2O photolysis from the same internal rotor state are remarkably similar, though significant anomalies are observed for specific K rotational levels.