Photoacoustic Studies on Iodine.

A photoacoustic cavity was constructed which employs a temperature-controlled cylindrical cavity with optical windows at either end. It was operated in the lowest longitudinal mode using a small electret microphone for detecting the acoustic signal and a photomultiplier tube for detecting the optical signal. Molecular Iodine was used as the specimen gas and argon as the buffer gas. The photoacoustic characteristics of the system were studied. Iodine molecules, excited periodically by intensity modulated optical radiation (xenon discharge), de-excited by non-radiative processes which result in pressure waves having the same modulation frequency as that of the light. These pressure waves are detected as acoustical pulses by the microphone situated in the wall of the cavity. Studies were conducted for different pressures of buffer gas (100 torr to 800 torr) at several different Iodine pressures in the range between 0.3 and 1 torr. The longitudinal mode of excitation provides an opportunity to compare the response of the cavity under acoustical excitation with that under optical excitation. The relevant parameters in the investigation were: Q, the quality factor of the cavity; the resonant frequency, partial pressures of argon and Iodine; temperature; and the signal amplitude. It was found that the Q of the cavity was well -behaved following the theoretically predicted dependence on SQRT.(P and on T('- 3/4). The absorption coefficient of Iodine determined photometrically, increased with increasing argon pressure up to a limiting value of pressure that depended on Iodine concentration. The photoacoustic signal showed a similar increase with increasing argon pressure. This signal reached a limiting value at a pressure which corresponded closely with that found optically. This is taken to indicate that the extinction coefficient of Iodine in argon, at the level of dilution used in these studies, depends on the argon pressure. A method was developed for measuring the concentration of Iodine at low levels through application of the shift in the frequency of the longitudinal mode resonance of the cavity. Also, resonance technique was employed for determining the velocity of sound in argon. A value of 307.7 M/sec was established as compared with the value of 319 M/sec as reported in various standard handbooks.