Comparison of the Photoacoustic Effect in the IR and the UV Regions

Abstract

The photoacoustic effect has been studied intensively using light sources emitting in the infrared (IR) region. Corresponding photons lead to excitations of vibrational–rotational energy levels. Subsequent de-excitations by collisions with the surrounding molecules can increase the translational energy of these molecules. Consequently, by irradiating a gas sample with amplitude- or wavelength-modulated light, an acoustic wave can be created and recorded by means of microphones. In the case of light absorption in the ultraviolet (UV) region, however, the photon energy is too large for the before-mentioned transitions. Instead, photon absorption can induce electronic transitions or even molecule dissociation. While experiments reveal that these effects lead to a photoacoustic signal as well (see, e.g. Cvijin et al. in Appl Spectrosc 42:770, 1988; Dóka et al. in Appl Spectrosc 54:1405, 2000; Gondal and Mastromarino in Appl Opt 40:2010, 2001; Böttger et al. in Appl Phys B 113:227, 2013), investigations concerning the reason have been hardly published to date. By means of a 266 nm UV laser and an IR optical parametric oscillator (OPO) setup, photoacoustic measurements were carried out with a synthetic air–acetone gas mixture. In this paper, both measurements are compared with each other and the fundamental processes of photoacoustic signal production in the UV region are discussed.