High sensitivity, high-resolution IR laser spectroscopy in slit supersonic jets: Application to N2HF ν1 and ν5+ν1−ν5

Abstract

A difference frequency IR spectrometer is combined with a slit supersonic expansion for high-resolution (≤50 MHz FWHM) direct absorption investigations of jet-cooled species. The 1.25 cm long nozzle provides a long path length and high densities suitable for synthesis and observation of van der Waals clusters, with a gradual spatial temperature gradient that permits experimental control of low frequency vibrational populations. Due to collisional quenching of velocity distributions, absorption linewidths are reduced and peak absorbance increased five- to sevenfold compared to pinhole expansions. Minimum detectable concentrations of HF containing complexes are 2×109 molecules/cm3/quantum state in a 2.5 cm path length. The combination of high sensitivity, sub-Doppler resolution, long path lengths, and temperature control make direct absorption in slit nozzle expansions a powerful and general technique for high-resolution study of jet-cooled species. The spectometer is used to obtain the near-infrared spectrum of N2HF. The ν1 (HF stretch) fundamental is observed at 3918.2434(2) cm−1, red shifted by 43.1795(2) cm−1 from the HF origin. In the warmer regions of the expansion close to the nozzle the ν5+ν1−ν5 Π←Π hot band is also observed, blue shifted by 2.7160(4) from the ν1 fundamental. Rotational analysis of these spectra reveals changes in vibrationally averaged molecular geometries upon excitation that are consistent with a near linear equilibrium geometry. The ν5 (N2 bend) frequency is estimated at 85±20 cm−1, based on the relative intensities of the two bands and on an analysis of the l doubling. The linewidths of the N2HF transitions show no increase over the HF monomer and are limited by instrumental resolution to 50 MHz FWHM; the lifetime of the upper level is therefore at least ≥3 ns.