Piezoelectric detection of vibrational overtones at cryogenic temperatures

Abstract

A resonant optoacoustic technique is described using continuous wave laser excitation and piezoelectric detection of overtone absorptions of liquefied gases at cryogenic temperatures. This technique uses a piezoelectric detector directly attached to a low temperature cell inside a cryostat. The liquefied sample inside the cell is vibrationally excited in the visible region with a continuous wave dye laser. The laser is modulated around 100 kHz with an acousto-optic modulator. The modulation frequency corresponds to an acoustic resonance frequency of the cylindrical cell which increases the sensitivity of the detection system. The pressure wave detected by the piezoelectric transducer is sent to a lock-in amplifier and processed with a microcomputer. This sensitive technique is used to obtain the visible spectra of pure liquid methane and ethane. Spectra of the fourth overtone of the C-H stretch vibration of methane, in solution with liquid argon at 94 K, and in solution with liquid nitrogen at 84 K, are also reported. Comparison is made with nonresonant and pulsed acoustic techniques.