High precision acoustic spectroscopy by laser excitation of resonator modes

Abstract

In a cylindrical cavity the longitudinal mode (002), the radial mode (100), and the combination mode (102) were excited with a chopped 3.39 μm He–Ne laser by vibrational excitation of CH₄. The recording of acoustic resonances was completely controlled by a microcomputer. This included repeated registration of the resonator temperature, stabilization and measurement of the modulation frequency, and integration of the acoustic signal detected with an electret microphone. The interaction of different standing waves in the cavity was taken into account. From the corrected line profiles, accurate values of the eigenfrequencies and half‐widths were determined as functions of pressure between 1–760 Torr. A quantitative theoretical description of these experimental results is given, which takes into account all important loss processes in the resonator. This analysis yields a value of ( pτ)=1.74±0.07 μs bar for the vibrational relaxation time and ( pτ)=1.18±0.24 ns bar for the rotational relaxation time of CH₄ at T=292 K.