Appl. Phys. B 43, 3541 (1987) Applied physics Physics B and Laser Chemistry Springer-Verlag 1987 A Theory for the Enhancement of the Photoacoustic Signals by Volatile Liquids J. Srinivasan Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560 012, India R. Kumar and K. S. Gandhi Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India Abstract. It has been shown experimentally that photoacoustic signals can be enhanced by introduction of a volatile liquid into the cell. Korpiun developed a theory which predicted the enhancement to be proportional to the mole fraction of the volatile component (Yo)in the gas phase. His theory is, however, based on a species conservation equation valid for dilute solutions only. We present a rigorous theory for predicting the enhancement of photoacoustic signal. Our theory shows the enhancement to be proportional to yo/(1 -Yo) which compares well with observations of Ganguly and Somasundaram. PACS: 78.20 Hp, 68.45 Da, 44.30, 43.35 S During the past two decades, the photoacoustic effect (PAE) has become an important tool for spectroscopic investigation of condensed matter. The PAE and its applications have recently been reviewed by Tam [t]. Ganguly and Somasundaram [2, 3] were the first to recognise the important role of adsorbed vapours on photoacoustic signal (PAS) and exploited this to enhance PAS of solids by introducing a non-absorbing volatile liquid into the cell. The enhancement here is due to vapours generated at the sample surface and it can be ten-fold or more at temperatures below, but close to, the boiling point of the liquid. Korpiun [4] showed that as the sample is cyclically heated by modulated light in the presence of a volatile liquid, the periodically varying heat flow from the sample to the gas would be accompanied by an oscillating mass transfer from the sample surface. He presented a theoretical calculation of the enhancement of PAS due to this oscillatory mass transfer. The recent measurements of Ganguly and Somasundaram [5] on nonporous samples using several volatile liquids show that predictions of Korpiun's theory [4] do not agree with experimentally observed enhancements. The like- ly sources for this disagreement are the following: (i) The apportioning of the absorbed energy into that part which propagates into the sample and that which heats the gas and evaporates the liquid at the interface was only alluded to by Korpiun but was not used in solving the temperature and concentration field equations, (ii) the effect of vaporization of the volatile liquid on the amplitude of temperature fluctu- ations at the interface of the sample has been ignored, (iii) the convection caused by diffusion has been neg- lected implicitly by the use of a species conservation equation valid only for dilute solutions i.e., for liquids with low vapour pressures, and, (iv) the changes in the thermophysical properties of the gas due to the presence of vapour in the gas phase have not been taken into account while making calculations. We present in this paper a more comprehensive theory without making the above assumptions to predict quantitatively the enhancement of PAS on account of the presence of volatile liquids in the cell. 1. Basic Equations Consider a photoacoustic cell containing a gas and a volatile liquid (Fig. 1). Let the temperature of the cell be