Photoacoustic phase shift as a chemically selective spectroscopic parameter

The phase information obtained in photoacoustic experiments can be used to separate the signals originating from chemical species with overlapping absorption spectra. This approach was applied to quantify parts per million CO levels in propylene using quartz-enhanced photoacoustic spectroscopy and a quantum cascade laser as an excitation source. The experimental data were used to evaluate V –T relaxation rates of CO and N 2 O in propylene. Ultrasensitive chemical analysis of compounds in the gas phase based on molecular absorption in the mid-IR region is a well-established approach. A number of techniques have been developed to measure absorption coefficients as low as α = 10 −9 cm −1 or less, allowing in some cases quantification of chemical species at pptv concentration levels. However, absorption-based spectro-scopic methods have certain limitations. The availability of isolated absorption lines without significant background is an essential condition for high chemical selectivity and sensitivity of absorption spectroscopy. This requirement can usually be satisfied for simple molecules (5 atoms) but not for more complex organic compounds where a high density of vibrational states and line broadening often result in structured continuum absorption over a wide spectral region. In such cases, the detection of impurities for which the absorption lines are imbedded in the continuum of the major constituent is problematical. Then the knowledge of optical absorption is not sufficient, and additional parameters are necessary to ensure chemically selective concentration measurements. A similar problem occurs in micro-scopy of biological objects, where optical density alone can not reveal details of chemical composition. One of the techniques used to obtain more informative images is fluorescence lifetime imaging microscopy (FLIM) [1]. The fluores-cence lifetime can be derived either from the decay curve following the excitation pulse, or from the fluorescence intensity phase shift with respect to harmonically modulated excitation. In a conceptually similar manner, the relaxation rate of optically excited molecular vibrations can be used to enhance chemical selec-tivity of IR measurements. As was first pointed out by G. Gorelik in 1946 [2], the phase shift of the photoacoustic response (usually called " optoacoustic " , or " optic acoustic " in earlier publications) to the modulated excitation can be used as a means to measure the V – T relaxation rate. In the case of a two-level molecular system the photoacous-tic phase lag θ is determined by: tan θ = ωτ (C tr /C 0) (1) where ω is the …