A new angle into time-resolved photoacoustic spectroscopy: A layered prism cell increases experimental flexibility

Abstract

A new pulsed photoacoustic calorimetry cell that uses transmission of light through a pair of dovetail prisms is discussed. The layered prism cell (LPC) combines the enhanced time-resolution capabilities of the “layered” front-face irradiation geometry with the zero-background and broadband flexibility of the classical cuvette geometry. This work provides a phenomenological description of photoinduced pressure changes to yield an analytical expression to calculate the magnitude of the photoinduced acoustic pressure wave in a series of solvents. The mechanical to electrical conversion efficiency for an ultrasonic transducer coupled to the LPC is presented to provide a comparison of the experimentally observed photoinduced acoustic signal amplitudes to the empirically calculated acoustic signal amplitudes. An analysis of the background signals due to absorption and electrostriction of the media provides insight into the issues of sensitivity and limitations of pulsed photoacoustic experiments. The LPC provides several benefits to increase the flexibility of the photoacoustic spectroscopy: (1) greater sensitivity, (2) enhanced time resolution, and (3) the ability to obtain kinetic data in complex solvent mixtures. Under optically dilute conditions in the layered cell geometry, the acoustic transient time, $\mathrm{\tau a,}$ approaches zero because the photoinduced acoustic wave homogeneously expands against the walls of the photoacoustic cell. To demonstrate the unique capabilities of the LPC, rates of hydrogen abstraction by tert-butoxyl radical from solvent mixtures containing ethyl and methyl alcohol are presented.