Free-jet infrared absorption spectroscopy of stable and unstable molecular species

Abstract

High-resolution vibration–rotation spectra of stable and unstable molecular species have been studied at low rotational and vibrational temperatures. Infrared absorption in a pulsed supersonic free jet has been measured by a frequency-tunable diode laser in a single- or multi-pass configuration. The modulated infrared absorption is detected by a lock-in amplifier or a two-channel boxcar integrator. The ν₃ band of SiCl₄ at 624 cm^–1 is studied as one of the series of work on heavy non-polar inorganic compounds. Five spectroscopic constants are determined accurately for the ν₃ band of ²⁸Si³⁵Cl₄. Weakly bound molecular complexes are studied in the middle infrared region as unstable molecular species. Well resolved ro-vibrational spectra are observed in (N₂O)₂, C₂H₂·Ar, (C₂H₂)₂, (C₂H₂)₃, BF₃·X (X = Ne, Ar, Kr), C₆H₆·X (X = Ar, Kr), and (NO)₂ with Doppler-limited resolution. Infrared–microwave double resonance in a free jet is discussed for microwave spectroscopy of van der Waals complexes in vibrationally excited states.