No abstract

An expression is derived which relates the distribution of vibrational levels near the dissociation limit D of a given diatomic species to the nature of the long-range interatomic potential, in the region where the latter may be approximated by D − Cn / Rn. Fitting experimental energies directly to this relationship yields values of D, n, and Cn. This procedure requires a knowledge of the relative energies and relative vibrational numbering for at least four rotationless levels lying near the dissociation limit. However, it requires no information on the rotational constants or on the number and energies of the deeply bound levels. D can be evaluated with a much smaller uncertainty than heretofore obtainable from Birge–Sponer extrapolations. The formula predicts the energies of all vibrational levels lying above the highest one measured, with uncertainties no larger than that of the binding energy of the highest level. The validity of the method is tested with model potentials, and its usefulness is demonstrated by application to the precise data of Douglas, Mo/ller, and Stoicheff for the B 3Π0u+ state of Cl2.

No abstract

The electric six-pole field is employed for focusing of beams of symmetric-top molecules via the first-order Stark effect. Molecules capable of a specified degree of orientation of dipole moment with respect to an electric-field vector can be selected according to the voltage applied to the electrodes of the six-pole field. The rotational angular momentum of the focused molecules can also be well aligned with the field. Experiments with CH3I and CHI3, prolate and oblate tops, have been carried out to illustrate the method.

An experimental methodology is described for the real-time clocking of elementary bimolecular reactions, i.e., timing the process of formation and decay of the collision complex. The method takes advantage of the propinquity of the potential reagents in a binary van der Waals (vdW) ‘‘precursor’’ molecule. An ultrashort pump laser pulse initiates the reaction, establishing the zero-of-time (e.g., by photodissociating one of the component molecules in the vdW precursor, liberating a ‘‘hot’’ atom that attacks the nearby coreagent). A second ultrashort, suitably tuned, variably delayed probe laser pulse detects either the intermediate complex or the newly born product. From an analysis of this temporal data as a function of pump and probe wavelengths, the real-time dynamics of such a ‘‘van der Waals-impacted bimolecular (VIB)’’ reaction can be determined. Chosen as a demonstration example is the VIB reaction H+CO2→HOCO‡→HO+CO, using the HI⋅CO2 vdW precursor. The pump laser wavelength was varied over the range 231–263 nm; the probe laser detected OH in two different quantum states. The measured rates of formation and decay of the HOCO‡ complex are characterized by time constants τ1 and τ2; τ2 spanned the range 0.4–4.7 ps, varying with the available energy. The dynamics of the HOCO‡ decay are discussed.