John S. McKillop

Information on the impact parameter dependence of the Ba+HI → BaI(ν=8)+H reaction

Using selectively detected laser‐induced fluorescence, the rotational state distribution of the BaI product has been measured for the beam–gas reaction Ba+HI → BaI(ν=8)+H. Owing to the highly constrained kinematics for this system, these measurements can be used to derive the reaction probability as a function of the impact parameter for this channel, called the ‘‘specific’’ opacity function, once the reaction probability as a function of velocity has been determined. Unfortunately, lack of knowledge of the exoergicity and the height of any energy barrier prevents a conclusive determination of the specific opacity function for this reaction. Instead, various approximate opacity functions are estimated based on different models of the velocity dependence of the reaction channel studied. If the reaction probability is the same for all relative collision velocities, then the BaI(ν=8) specific opacity function peaks strongly near 2.6 A with a full width at half‐maximum of 1.0 A. However, the possible presence of a small energy barrier in the entrance channel causes a cutoff in the relative collision velocity distribution, and this type of velocity dependence would significantly affect the shape of the specific opacity function.

Hyperfine structure of the BaI X 2Σ+ and C 2Π states

Optical‐microwave double resonance measurements were carried out to find the hyperfine structure constants of the v=0 level of the BaI X 2Σ+ state. These were combined with sub‐Doppler optical measurements of the BaI C 2Π–X 2Σ+(0,0) band in order to derive the hyperfine structure constants of the excited state. We have determined the following molecular constants (in MHz) where the numbers in parentheses represent one standard deviation in a least squares fit: for the BaI X 2Σ+ state, γ‘=75.8501(33), b‘=93.117(19), c‘=52.170(54), and eQq‘=−33.62(12), and for the BaI C 2Π state, a’=263(53), b’+c’=−430(212), d’=−66.7(1.4), and eQq’=−214(11). The Fermi contact interaction and the electric quadrupole coupling constants for both the BaI X and C states appear to arise from the distortion of closed‐shell I− orbitals by the field of the Ba+ ion. In the BaI X state, the charge distribution on the Ba+ center is directed away from I− while in the C state toward I−.

Rotational analysis of the BaI C2Π-X2Σ+ (8,8) band

Abstract The BaI C2Π-X2Σ+ (8,8) band has been measured and rotationally assigned using techniques of population-labeling optical-optical double resonance (PLOODR) and selectively detected laser-induced fluoresence (SDLIF). A weighted nonlinear least-squares fit has been carried out to model the positions of 891 transitions with J″ ranging from 13.5 to 271.5 to a 2Π-2Σ+ Hamiltonian which has 10 spectroscopic constants. Despite the fact that most of our data is from 6 out of the possible 12 rotational branches and is biased in favor of the C 2 Π 1 2 -X 2 Σ + subband, we are able to assign J″ quantum numbers unambiguously for all the observed transitions as well as derive the principal spectroscopic constants of the BaI C2Π and X2Σ+ states for the (8,8) band.

Effect of intensity on fragment internal state distributions in the infrared multiphoton dissociation of vinyl cyanide

The technique of laser induced fluorescence has been used to observe the C2 and CN fragments produced in the infrared multiphoton dissociation of vinyl cyanide CH2 = CHCN in a low pressure flowing gas. The rotational distributions of the lowest vibrational level of the C2 fragment were found to be well characterized by temperatures. As was previously observed for the CN fragment of this parent molecule, a distinct decrease in the values of these temperatures is seen to occur during the photolysis laser pulse. This is interpreted as the result of an intensity effect in the later stages of the infrared pumping process. Additional information is also presented on the behavior of the CN radical. Modeling calculations within the framework of the energy‐grained master equation support this interpretation.

Effect of pulse intensity distributions on fragment internal energy in the infrared multiphoton dissociation of vinyl cyanide

A plasma shutter has been used to remove the less intense tail of a CO2 laser pulse without otherwise altering the pulse characteristics. It is found that variations in the pulse intensity distribution control the rotational distribution of CN fragments formed in the IR photolysis of vinyl cyanide H2C = CHCN.