Cent. Eur. J. Phys. • 8(4) • 2010 • 634-638 DOI: 10.2478/s11534-009-0121-x Central European Journal of Physics On the modulation frequency dependence of the photoacoustic signal for a metal coated glass-liquid system Research Article Ernesto Marín ∗ , Gerardo Vera-Medina, Arturo García, Antonio Calderón Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada, Instituto Politécnico Nacional, Legaria 694, Col. Irrigación, C.P. 11500, México D.F., México Received 21 May 2009; accepted 20 July 2009 Abstract: We show that the front photoacoustic signal due to a sample consisting of a glass plate with a metal coated surface, at which thermal waves are generated by periodical light absorption, enhances for certain modulation frequencies when the other glass surface is covered with a liquid sample. This contradicts the intuitive expectation based on the assumption that the liquid provides a new channel for heat conduction thereby decreasing the substrate temperature. Experimental results are shown and the described effect is explained using a thermal wave interference model. PACS (2008): 42.25.Hz, 43.58.+z, 44.90.+c, 65.20.-w, 66.10.cd Keywords: photoacoustic • coatings • thermal waves • heat transfer © Versita Sp. z o.o. 1. Introduction The photoacoustic (PA) technique is a well established method for thermal characterization of condensed mat- ter samples and several experimental configurations have been proposed in the past. In one of them, an optically opaque sample covers one of the two openings of a PA cell, while the other is closed by a transparent glass window through which a modulated light beam impinges on the inner sample surface generating periodical heating (the so-called thermal waves) and hence a pressure fluctua- tion, which can be detected with a microphone already ∗ E-mail: emarin63@yahoo.es enclosed in the PA cell. This is an open cell configura- tion in the sense that the sample is placed in the outer part of the PA chamber. This kind of configuration has been used in the past for: i- thermal effusivity measure- ments in solids [1] and liquids [2] in good thermal contact with the non-illuminated surface of the sample, acting in this variant as a reference; ii- for specific heat capacity measurements in the sample itself using the liquid as a reference [3, 4]; and iii- for the monitoring of dynamic pro- cesses taking place at the sample-liquid interface, such as those described recently by Lima et al. [5]. To warrant full absorption of the light beam in the case of transparent samples, their inner surfaces are often coated by a thin opaque metal film, where heat will be thus gen- erated. For this configuration we have observed that the PA signal due to a liquid sample onto the glass metalized 634