Quantitative spectroscopy of micron-thick liquid films

We demonstrate a new sensitive spectroscopic technique for highly transparent micron-thick liquid films sandwiched between transparent substrates. A pulsed dye laser irradiates the film, producing (via the optoacoustic effect) a transient ultrasonic wave which is launched into the substrate and detected by a piezoelectric transducer bonded to the substrate. This technique is illustrated by measuring the absorption lines of films of aqueous solutions of rare-earth ion, and absorptions ∼10−5 can be detected presently. This technique should open up new opportunities to study the spectra of chemi- or physi-adsorbed species on surfaces.