Vibrational relaxation in seeded supersonic alkali halide beams

Publisher Website
Google Scholar
Abstract
A seeded supersonic beam source for alkali halides is described which can provide high intensity, a narrow velocity distribution, enhanced or reduced translational energy (by choice of diluent mass), low rotational temperature, and partially relaxed vibrational populations. Alkali halide vapor at ∼1200–1400 °K and ∼0.1 Torr is mixed with a diluent gas at ∼30–100 Torr and expanded through a pinhole nozzle 0.1 mm in diameter. Velocity analysis of a seeded CsF beam with Ar as the diluent indicates the beam properties conform approximately to the theory of isentropic expansions, with deviations attributable to the relatively low source pressure used. The vibrational populations of the v=0, 1, 2, 3 states in seeded beams of LiF with Ar, NH3, ND3, and SF6 as diluents were determined by electric resonance spectroscopy. For Ar as diluent, the LiF vibrational energy has a Boltzmann distribution with a temperature only ∼10% lower than the beam source. For the polyatomic diluents, the LiF vibrational distribution is markedly non‐Boltzmann. The nominal vibrational temperature Tv, defined by comparing the population of state v to that of the ground state, drops substantially for the higher v states. The vibrational relaxation becomes more pronounced as the source pressure of alkali halide or diluent gas is increased. For the experimental conditions used here, Tv for the v=3 state drops to ∼2/3 of the source temperature; much stronger relaxation can be expected at higher source pressures.