Vibrational relaxation of hydrogen fluoride by HF dimers in a laser excited nozzle beam (original) (raw)

The Journal of Chemical Physics, 1986

Abstract

ABSTRACT An HF nozzle beam and a cw HF laser beam in a coaxial and antiparallel arrangement facilitated exciting HF molecules vibrationally in v=1,J up to the saturation limit. A beam consisting exclusively of monomeric HF was generated by heating the beam source to 600 K and was confirmed using a mass spectrometer. Energy‐flux measurements made using a bolometer indicate that the vibrational energy distribution initiated by the laser is conserved, confirming that V–T energy transfer to monomers is slow. The maximum measured flux of HF(v=1,J) molecules was 1.5×1017 molecules s−1 sr−1 at 670 mbar stagnation pressure. Beams consisting of a mixture of monomeric and polymeric HF were generated using appropriate combinations of source temperature and pressure. In the presence of dimers, the excited monomers relax rapidly by a fast V–V transfer and subsequent predissociation of the metastable dimers. The loss of beam energy due to energy transport by the departing dimer fragments was detected bolometrically. The laser‐induced depletion of dimers was determined over a wide source pressure range at T0=300 K using a mass spectrometer. From a kinetic model of these dimer‐depletion data, the product of cross section and transfer probability for V–V energy transfer from HF (v=1) monomers to dimers was deduced to be 14 Å2. The terminal‐dimer‐concentration equation for HF nozzle beams was refined. An analysis of the energy fluxes measured by the bolometer with no laser excitation of the beam revealed that, for an HF nozzle beam, the flow from the source to the detector is, in general, nonisenthalpic due to the onset of dimerization. Mach‐number focusing enriches the dimer mole fractions detected by the bolometer. An analysis of the excess enthalpy due to the presence of polymers in the beam results in a new method for the experimental determination of the dissociation energy of HF dimers, determined to be 0.36 eV.

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