James J. Valentini

The photodissociation cage effect in van der Waals complexes: Fluorescence spectra of I2B(3Π0+u) from the hindered photodissociation of I2Ar at 488 nm

The B→X fluorescence from I2 produced in the hindered photodissociation of I2Ar at 488 nm has been resolved. The spectra show several vibrational progressions with low rotational energy, the most prominent of these is a (v′,0) progression extending from v′ = 49 to at least v′ = 23. The I2Ar complexes were excited and the I2 fluorescence was observed under collision‐free conditions in a supersonic free‐jet expansion. We attribute these observation to one‐atom photodissociation cage effect in the I2Ar comples, in which the molecular iodine, although excited more than 400 cm−1 above B state dissociation limit, is prevented from dissociating by energy transfer to the argon atom, resulting in the breaking of the van der Waals bond: I2Ar→I2(B)+Ar, leaving the iodine molecule in a bound level of the B electronic state, 386–1889 cm−1 below the dissociation limit. (AIP)

Experimental study of the dynamics of the H+D2 → HD+D reaction at collision energies of 0.55 and 1.30 eV

We report here experimental measurement of the nascent HD product quantum state distributions for the H+D2u2009→u2009HD+D reaction. Pulsed laser photolysis of HI in an HI/D2 gas mixture produces hydrogen atoms giving H+D2 collision energies of 0.55 and 1.30 eV. Nanosecond‐time‐resolved coherent anti‐Stokes Raman scattering (CARS) spectroscopy is used to record rotationally and vibrationally resolved spectra of the HD reaction product under nearly single‐collision conditions. The spectra are analyzed to determine the nascent, single‐collision HD product quantum state distributions. These distributions are compared to the results of related experiments by E. E. Marinero, C. T. Rettner, and R. N. Zare and to the results of recent quasiclassical trajectory calculations of N. C. Blais and D. G. Truhlar. Our results are in qualitative agreement with those of Marinero et al., but there are some quantitative differences. The trajectory calculations yield HD quantum state distributions which are very close to those we hav...

Experimental determination of product quantum state distributions in the H+D2→HD+D reaction

Rotational and vibrational state distributions of HD formed in the reaction of translationally hot hydrogen atoms with D2 have been measured by CARS spectroscopy under nearly single‐collisions conditions. The rotational and vibrational excitation of the HD is small; most of the energy available to the products appears in translation.

High repetition rate pulsed nozzle beam source

A high repetition rate piezoelectric pulsed molecular beam source is described. Reptition rates as high as 750 Hz have been obtained with gas pulse widths from 100 μs to 10 ms. The measured expansion characteristics are very nearly those of an ideal isentropic expansion. The valve is highly reliable: We have obtained 107 pulses without failure.

Multiproperty empirical anisotropic intermolecular potentials for ArSF6 and KrSF6

Crossed molecular beam measurements of the total differential cross sections (DCS) for the scattering of He and Ne by SF6 are reported. Using anisotropic MSV potential forms and the IOS approximation to do the scattering dynamics, we determine anisotropic intermolecular potentials by simultaneously fitting the DCS, viscosities, virial, and diffusion coefficients. Damping of the diffraction oscillations in the DCS determines the anisotropy of the minimum position rm of the HeSF6 interaction; damping of the rainbow determines the anisotropy of the well depth e of the NeSF6 interaction. Consistency between the systems verifies the accuracy of the resulting HeSF6 and NeSF6 potentials.

Dynamics of the H+D2→HD+D reaction: Dependence of the product quantum state distributions on collision energy from 0.98 to 1.3 eV

Measurement of the nascent HD product rotational and vibrational state distributions for the H+D2→HD+D reaction is reported. Ultraviolet photolysis of HI using a pulsed laser at 291 or 280 nm is used to create H atoms in an HI/D2 gas mixture, giving H+D2 collisions with relative energies of 0.98 or 1.1 eV. Pulsed‐laser coherent anti‐Stokes Raman scattering (CARS) spectroscopy is used to record rotationally and vibrationally resolved Raman spectra of the HD product of the photolytically initiated H+D2 reaction, under effectively single‐collision conditions. The HD product quantum state distributions are extracted from these CARS spectra. The present data are combined with results we obtained previously at 1.3 eV collision energy to reveal the collision energy dependence of the product quantum state distributions. We find that at all three collision energies the product distributions can be quite accurately represented by a linear rotational and vibrational surprisal function. The rotational surprisal param...

Multiproperty empirical interatomic potentials for ArXe and KrXe

Crossed molecular beam measurements of elastic differential cross sections (DCS) for the scattering of Ar and Kr by Xe are reported. Empirical multiparameter M3SV interatomic potentials for the ArXe and KrXe interactions are determined by simultaneously fitting the DCS, the viscosities, and virial coefficients; the results are the most accurate potentials yet reported for these systems.

Use of a free-expansion jet in ultra-high-resolution inverse raman spectroscopy

Abstract Rotationally resolved, ultra-high-resolution inverse Raman spectra of CH 4 have been obtained in a free-expansion jet. Using relatively mild evpansion conditions a pure methane jet has been cooled to a rotational temperature of 13 K. The limiting rotational state populations observed, however, are not Boltzmann because of nuclear spin effects.

Anomalous rotational state distribution for the O2 photofragment in the UV photodissociation of ozone

Abstract The coherent anti-Stokes Raman scattering (CARS) spectrum of O 2 ( 1 Δ g ) formed in the photodissociation of ozone at 266 nm is presented. An anomalous propensity for even ( J = 2,4,6, …) retational states is observed. This propensity effect is discussed in terms of symmetry restrictions and dynamical bias in the photolysis.

The dynamics of infrared photodissociation of van der Waals molecules containing ethylene: An experimental study

Infrared (∼950 cm−1) predissociation of ethylene clusters has been studied using a crossed laser beam–molecular beam apparatus equipped with a moveable detector. van der Waals molecules undergo dissociation following absorption of a single infrared photon. Angular distributions, obtained for product molecules (C2H4)n, n=1–3, all show nearly exponentially decreasing product flux with increasing scattering angle. A product flux contour map has been generated for the photolysis reaction (C2H4)2u2009→u2009C2H4+C2H4. Two isotropic center‐of‐mass distribution functions yield excellent agreement with experimental results. One is a function of reaction kinetic energy E, with P(E)=exp(−E/80 cm−1); the other is a function of product velocity (momentum) u, with U(u) =uu2009exp(−u/9×103 cm/s). The latter distribution is characteristic of a dissociation pathway with a barrier in the exit channel. Such a barrier could result from centrigufal effects. It is argued that isotropic product scattering can be consistent with a direct di...