Daniel H. Winicur

Electronic‐energy exchange cross sections for Ar* (3P) and Kr(1S)

Differential cross sections for Ar* scattered by Kr are obtained at four relative kinetic energies, 60.6–156 meV. Location and shape of rainbow maxima, observed at all the energies, are used to determine parameters for a double‐Lennard‐Jones potential. The parameters, e=9.11±0.25 meV and rm=5.06±0.84 A, are the same as those for K+Kr and are substantially different from those of Ar+Kr. A second set of maxima, visible at all energies and caused by exchange of electronic energy from Ar* to Kr, are analyzed to determine electronic‐energy exchange cross sections. The cross sections together with those obtained from thermal‐energy quenching measurements are consistent with an increase from a threshold of 25 meV. Possible mechanisms for the electronic‐energy exchange are discussed.

Co desorption and adsorption on Pt(111)

Abstract The kinetics of the desorption of CO from a Pt(111) crystal between 419 and 505 K is reported using a Low-Energy Molecular-Beam-Scattering (LEMS) technique with a helium probe beam and a CO dosing beam. The resulting first-order Arrhenius rate constant is k = 2.7 × 10 13 exp(−31.1 kcal mole · RT) s −1 . We also report a study of the equilibriumadsorbed CO between 400 and 600 K using LEMS. These results, fitted to a Temkin isotherm model, indicate that the adsorption energy decreases linearly with surface coverage with the average value equal to 31.1 + 1.2 kcal mole over the coverage range 0

Molecular beam investigation of the effect of inner‐shell electrons in molecular collisions

Differential elastic scattering cross sections were obtained at thermal energies using crossed molecular beams for metastable Kr*, Ar*, Ne*, and He* scattered by ground−state krypton. Potential well depths are calculated from the location of the observed rainbow maxima and are compared to those for the analogous alkali atoms Rb, K, Na, and Li scattered by krypton. The effect of the inner−shell electrons on molecular collisions is deduced. At high electron shell levels, the (n−1) electron decreases the strength of the interaction during a collision whereas at low shell levels, the interaction strength is increased by the (n−1) electron. s and p electrons are shown to affect the interaction strength differently at the lowest shell level.

Elastic and nonelastic cross sections for Ar* (3P)+HBr(X 1Σ)

The energy dependence of the differential scattering of metastable Ar* (3P) by ground state HBr(X 1Σ) has been studied at four relative kinetic energies from 60–160 meV over an angular range of 4–120 deg c.m. The position and curvature of rainbow maxima, which are observed at each energy, are used to obtain LJ(12,6) potential parameters. The position of the minimum rm=4.44±0.8 A is the same as that for K+HBr, and the well depth, e=25.2±1.1 meV, is about 4% larger than e (K+HBr). The scattered intensity shows a distinct falloff at large angles compared to that expected for elastically‐scattered Ar*. This depletion, caused by quenching of Ar*, is analyzed in terms of an optical model to determine the threshold and energy dependence of the quenching cross section. The nonelastic cross section increases from a threshold of 25 meV to a nearly constant value of 41 A2 at 160 meV. The optical analysis predicts a maximum of 42.5 A2 at 215 meV.

Low-energy differential elastic scattering of Ar* by HBr: Comparison with the scattering of K by HBr

Abstract Differential elastic cross section measurements of electronically excited, metastable argon atoms Ar* by HBr molecules a shallow rainbow. Parameters are deduced assuming an L-J (12,6) potential and are compared to those which have been reported for the scattering of K by HBr. The location of the potential minima for the two systems is identical but the potential well depth for Ar* + HBr is 10–20 percent lower than for K + HBr.

Low‐energy differential elastic scattering of Ne* (3P) by Kr(1S)

Differential elastic cross sections for Ne* (3P) scattered by Kr are obtained at three relative kinetic energies, 64 to 74 meV. Location and shape of rainbow maxima, observed at all energies, are used to determine the well depth and potential minimum for a double‐Lennard‐Jones potential. The results, e=8.05±0.49 meV, and rm=4.91±0.64 A are similar to those for Na+Kr and are substantially different from those of Ne+Kr. A value of 2.11 A for the atomic radius of Ne* is obtained which is close to that previously reported for Na.

Molecular asymmetry: A correlation between the anisotropy parameter of the polarizability and the shape of the electron–density contour diagram

An empirical relation is presented between the anisotropy parameter κ of the polarizability of diatomic molecules and a feature characterizing the shape of the 0.002 a.u. electron density contour, calculated to the Hartree–Fock limit. This correlation provides a means of estimating κ to within about 6% for a wide variety of diatomic types and also predicts the existence of negative values of κ.

Low‐energy elastic and electronic‐energy exchange scattering of He* by Kr

Differential cross sections for He* scattered by Kr are obtained at three relative energies, 63.6–82.3 meV. Location of rainbow maxima and ’’rapid’’ quantum oscillations, observed at all energies, are used to determine parameters for a double Lennard‐Jones potential. The parameters, e=8.0±0.6 meV and rm=4.8±0.25 A, are close to those for Li+Kr and are substantially different from those of He+Kr. The effects of direct electronic‐energy exchange scattering are observed as excess scattered intensities between 10° and 30° lab. These effects are compared to those arising from chemi‐ionization.

A method for the measurement of the electron-impact excitation cross section of argon

Abstract A method is described for the measurement of absolute values of the total electron-impact excitation cross sections of argon to metastable states ( 3 P). The method utilizes measurements of the ratio of the excitation to ionization cross section. Results are reported from 60 to 100 eV.

Energy transfer studies between Ne* and H2O

Translational to rotational energy trasfer between metastably excited Ne* and ground-state H 2 O using crossed molecular beams is reported at five collision energies between 113 and 260 mcV. The exchange is very efficient, and appears to proceed directly from translation to the rotational levels of the H 2 O molecule rather than via a vibrationally perturbed complexes as has been suggested for the Ar* plus CO 2 system.