The state-to-state vibrational predissociation dynamics of the hydrogen fluoride dimer has been investigated in detail using a newly developed instrument which gives both initial state selection and photofragment state determination. Results are reported for a wide variety of initial states associated with the ν1 and ν2 vibrations. The final state distributions universally indicate that the preferred dissociation channels correspond to the production of one HF fragment that is highly rotationally excited and another that is not. This is explained in terms of an impulsive dissociation mechanism which proceeds from a geometry close to that of the equilibrium structure of the dimer. We find that nearly degenerate initial states can have rather different final state distributions. In particular, there is a distinct difference between the upper and lower members of the tunneling doublet, which is most likely attributable to their related symmetries. The dissociation energy (D0) is determined to be 1062±1 cm−1.

The influence of laser polarization direction on the photofragment angular distributions resulting from vibrational predissociation of HF dimer has been studied in an effort to establish the correlation between the direction of the transition moment in the parent molecule (μ) and the recoil velocity vector of the fragments (ν). This is possible, even for the long-lived predissociative states considered here, since the dimer is initially excited to a single rotational level, with a well-defined time-independent, anisotropic rotational wave function. The mathematical framework appropriate for treating the photodissociation of this asymmetric rotor is presented and used to extract anisotropy parameters for the various initial states. The results clearly show that the recoil velocity vector of the fragments is preferentially aligned along the a axis of the dimer. The implications of this for the associated dynamics are discussed. Data of this type is directly related to the center-of-mass differential photodissociation cross section and thus can be used to establish further points of comparison with the theoretical calculations that are becoming possible for systems of this type.

Two new methods have been developed for studying the vibrational predissociation dynamics of weakly bound complexes at the state-to-state level. The first is based upon the use of two F-center lasers, one to photodissociate the complex by single quantum state excitation and the second to probe the final states of the resulting fragments. These pump–probe experiments are carried out with angular resolution of the photofragments, giving information on the associated vibrational, rotational and translational distributions. The second method makes use of a dc electric field to orient the parent molecules prior to photodissociation. In this way the two fragments recoil to different sides of the apparatus, allowing us to detect them independently. These experiments also provide us with valuable information on the nature of the photofragment angular distribution in the molecule fixed frame. The present application is to the study of the vibrational predissociation dynamics of N2–HF (v=1). A combination of the two methods has enabled us to determine that the complex dissociates via two distinctly different channels with similar probability, namely one that produces vN2= 1, jHF=7 and the other vN2= 0, jHF=12. The rotational distribution of the N2 fragment is also determined by fitting the angular distributions. Conservation of energy is used to determine the dissociation energy of the complex (D0) to be 398±2 cm−1.

A modified version of the optothermal technique has been used to measure photofragment angular distributions resulting from the vibrational predissociation of Ar–CO2 from both members of the (1001)/(0201) Fermi diad. These angular distributions show resolvable structure which can be assigned to individual final states of the resulting fragments. The emphasis of the present paper is on the role of the vibrational degrees of freedom of the CO2 fragment as depositories for the excess energy. The dissociation energy (D0) of the complex has been determined to be 166±1 cm−1. This result is used in conjunction with the spectroscopic data already available in the literature to refine two previously reported two-dimensional Ar–CO2 potential energy surfaces.

No abstract