A. P. Hickman

Approximate scaling formula for ion–ion mutual neutralization rates

A complex potential model is used to treat the mutual neutralization of positive and negative ions. The model is used to examine neutralization by the charge transfer mechanism and also by internal excitation leading to capture. It is found that electron transfer is the dominant process for simple ions and small hydrated ions. The numerical results of the theory have been parameterized in terms of the reduced mass of the collision and the electron affinity of the electron donor. This procedure yields an approximate scaling formula that fits a wide range of experimental data to an accuracy of about ±30%.

Radiative lifetime and quenching of the 3p 4D0 state of atomic nitrogen

The radiative lifetime of nitrogen atoms in the 3p 4D0 state is determined to be 43±3 ns, and the total removal rate constants from the excited 3p 4D0 state of nitrogen atoms are measured for collisions with He, Ne, Ar, Kr, Xe, and N2. In a low pressure discharge flow reactor, the 3p 4D0 state is prepared by two‐photon excitation from the 4S0 ground state of atomic nitrogen. Time‐resolved fluorescence from the 3p 4D0→3s 4P transition monitors the temporal evolution of the population in the 3p 4D0 state. As the rare gases become heavier with a more complex electron cloud, the quenching rate constants increase from less than 0.6×10−11 cm3 s−1 for He to a value of 66±12×10−11 cm3 s−1 for Xe. Collision mechanisms which might account for such a dramatic increase are discussed.

Calculation of associative and Penning ionization of H and D by He(2 1S) and He(2 3S)

Associative and Penning ionization cross sections are calculated for collisions of H and D with He(2 1S) and He(2 3S) in the energy range 0.03–1.0 eV. Ab initio configuration–interaction potential curves V* and V+ for these systems are used. For the autoionization width Γ, the recent results of Hickman, Isaacson, and Miller for He(2 3S) ‐H are employed, and their calculations are extended to obtain the width for He(2 1S) –H. For the latter system, the calculated associative ionization fraction and the relative energy dependence of the total ionization cross section are in excellent agreement with the recent experiments of Fort, Laucagne, Pesnelle, and Watel. Comparison with the experiments of Morgner at thermal energies, and of Howard, Riola, Rundel, and Stebbings, and Neynaber and Magnuson at higher energies suggests that Γ (R) for He(2 3S) –H is fairly accurate, and that Γ (R) for He(2 1S) –H is low by about a factor of 2.

Efficient anti-Stokes Raman conversion in collimated beams

We have demonstrated 10% energy-conversion efficiency of 0.5-nsec FWHM laser pulses at 248 to 225 nm by an anti-Stokes process in hydrogen, using collimated (unfocused) laser beams. To accomplish this, we simultaneously seeded the Raman cell with a Stokes pulse that was 2–6% as intense as the pump at the phase-matching angle with respect to the pump beam axis. Under these conditions, a fully transient plane-wave calculation suggests that 44% energy-conversion efficiency is possible. Phase-front imperfections on the pump and Stokes beams are thought to limit the observed conversion efficiency. We present a simple model describing the effect of phase distortion on anti-Stokes production that agrees with our observations. Experimentally observed dynamic effects are in good agreement with theoretical predictions.

Interatomic potentials for excited states of XeHe and XeAr

Interatomic potentials for the interaction of low‐lying excited states of Xe(5p5nl) (nl=6s,6p,5d) with He and Ar have been calculated. A novel method has been developed that refines and extends the results of ab initio electronic structure calculations by incorporating available spectroscopic information using a model Hamiltonian. The ab initio calculations treat the heavy‐atom cores using relativistic effective potentials and include spin–orbit effects at the configuration‐interaction level. The model Hamiltonian depends on a small number of physically sensible parameters, some of which are extracted from the ab initio calculations, and others of which are determined more accurately from spectroscopy. The model Hamiltonian is then used to recalculate the potential curves and coupling matrix elements. The results obtained have significant implications for two classes of recent experiments. Recent measurements of rate constants for state‐to‐state transitions of Xe* induced by collisions with He or Ar have ...

High-resolution Brillouin gain spectroscopy in fused silica

We report what is to our knowledge the first high-resolution Brillouin gain spectrum in a solid. Resonances corresponding to longitudinal (compressional) and transverse (shear) acoustic waves in fused silica are observed with good resolution and a high signal-to-noise ratio. Absolute gain coefficients, linewidths, and Brillouin frequency shifts are measured. The agreement with previously measured values is good.

Calculations of inelastic collisions of excited states of Xe with He and Ar

Coupled channel calculations of thermal collisions of Xe(5p5nl) (nl=6s,6p,5d) with He and Ar have been performed using potential curves and coupling matrix elements that we previously calculated. Coupling among all of the closely spaced 6s’, 6p, and 5d fine structure levels of Xe is fully included. These excited levels lie in the range 76 000–84 000 cm−1 above the Xe ground state, and the spacing of adjacent levels ranges from 84 to 1460 cm−1. Rate constants for level‐to‐level transitions are calculated at T=300 and T=800 and compared with recent experiments; agreement is generally good. The rate constants are very sensitive to the specific initial and final levels and to the collision partner. This sensitivity can be explained by the pattern of crossings and avoided crossings of potential curves corresponding to individual fine structure levels. The calculations thereby illustrate the importance of spin–orbit effects in heavy atom systems and provide insight into a large amount of data.

Comparison of ion pair formation in the systems Ar*+I2 and K+I2

A simple model that has been used extensively by Los and co‐workers to treat ion pair formation in collisions of alkali atoms with diatomic molecules is extended to include continuum coupling via a competing Penning ionization channel. This extended model is then used to calculate the differential cross sections for ion pair formation for the system Ar*+I2 over the energy range 28–154 eV and to compare with a previous treatment of K+I2. In the absence of significant competition from continuum processes, Ar* is expected to behave in a manner similar to K, since the active electron is an unpaired 4s electron in both cases. We perform model calculations for Ar*+I2 to investigate the effects of varying the potential curves and charge exchange matrix elements and of including a continuum coupling function Γ (R). Comparison with previous calculations for K+I2 suggests increased repulsion on the Ar*–I2 surfaces relative to those of K–I2. The competing mechanisms of excitation transfer and Penning ionization may ...

Transient effects on stimulated Brillouin scattering

We present a detailed comparison of theory and experiment for transient stimulated Brillouin scattering for a pump pulse with Gaussian temporal profile. A new approach for measuring Brillouin linewidths is demon-strated, and an unexplained asymmetry is observed.

Model for fast, nonadiabatic collisions between alkali atoms and diatomic molecules

Equations for collisions involving two potential surfaces are presented in the impact parameter approximation. In this approximation, a rectilinear classical trajectory is assumed for the translational motion, leading to a time‐dependent Schroedinger’s equation for the remaining degrees of freedom. Model potentials are considered for collisions of alkali atoms with diatomic molecules that lead to a particularly simple form of the final equations. Using the Magnus approximation, these equations are solved for parameters chosen to model the process Cs+O2→Cs++O2−, and total cross sections for ion‐pair formation are obtained as a function of energy. The results exhibit oscillations that correspond qualitatively to those seen in recent measurements. In addition, the model predicts that the oscillations will become less pronounced as the initial vibrational level of O2 is increased.