Photoacoustic signal in a cylindrical resonator: Theory and laser experiments for CH4 and C2H6

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Abstract
The theory of the photoacoustic signal is developed for acoustic resonances in a cylindrical resonator. Measurements were performed to determine the signal strength in the pressure range between 1 and 760 Torr for CH4 and C2H6. For CH4 the optical power absorbed in the cavity is nearly constant but vibrational relaxation varies the signal in the investigated pressure region. C2H6 shows a strong pressure dependence of the optical power absorbed, however, V–R,T relaxation is so fast that in this case no relaxational variation of the signal is observed in the range of ν/p values studied. As expected, the strength of the acoustic signal is proportional to optical power deposition in this case. For CH4, the influence of V–V and V–R,T exchange processes on the photoacoustic signal is studied in detail. The ratio of heat generated by fast V–V processes to heat generated by slow V–R,T transfer is determined. Using the relaxation time 5.2 ns bar obtained by laser‐induced fluorescence experiments for double quantum V–V exchange between the stretching levels ν1,ν3 and the bending levels ν2,ν4 a value of 33 ns bar is derived for single quantum V–V exchange between these manifolds.