Optical Generation and Detection of Acoustic Pulse Profiles in Gases for Novel Ultrasonic Absorption Spectroscopy

Abstract

We demonstrate a new all-optical, frequency-multiplexed, and fast technique for gas-phase ultrasonic absorption spectroscopy. A pulsed laser produces a short-duration acoustic pulse in the gas sample, and the acoustic pulse profiles at two distances are monitored by focused continuous probe beams. Fourier decompositions of these probe deflection signals provide the absorption spectrum, and examples for C0 2 and a C0 2 + H 2 0 mixture are given. Ultrasonic propagation speeds in the megahertz regime are also obtained for several gases. PACS numbers: 42.60.-v, 34.50.-s, 43.85.+fConventional acoustic absorption spectroscopy 1 " 5 in gases has been performed with transducers such as quartz plates, microphones, etc. The use of transducers for gases causes several limitations. The frequency response is usually limited to < 1 MHz. Also, the frequencies are scanned point by point, and the procedure is thus slow and unsuitable for transient conditions. Short-pulsed measurements are difficult because of transducer ringing. Furthermore, transducers cannot be used in hostile environments like flames. We describe here a new all-optical, pulsed, and multiplexed technique that avoids the above limitations, and is useful for acoustic absorption spectroscopy of gases. This technique relies on the use of a short-duration laser pulse to generate reliably a narrow acoustic pulse containing a broad Fourier frequency spectrum; as this pulse propagates, the various Fourier components are absorbed differently, resulting in pulse distortion that is probed by a focused cw laser beam. Fast Fourier transform of the transient probe deflection signal provides the acoustic absorption spectrum; this is much faster than the conventional pointwise frequency measurement. Our technique can be called "optoacoustic spectroscopy of the second kind" (OAS II) because it exploits optoacoustic pulse generation 6 " 8 for acoustic spectroscopy. This should be distinguished from the well-known optoacoustic spectroscopy 9 (understood to be the first kind), which exploits optoacoustic pulse generation for optical spectroscopy.Our experimental demonstration of OAS II is shown in Fig.

Direct Observation of Mass Transfer at Solid−Liquid Interface by Laser Flash Photolysis of the Interface Probe Molecules

“…(1) Weak cylindrical waves in a clean cell filled with a light absorbing medium: when a pulsed laser beam passes the light-absorbing medium, the thermal expansion of the illuminated volume generates cylindrical acoustic waves propagating in a lateral direction, shown schematically in Figure a. (2) Intense plane wave in a cell deposited with metallic film: the metallic film absorbs the laser light producing thermal elastic waves due to the thermal expansion of the constrained thin film. , A substantial enhancement in strain-wave amplitude has been observed with an increase factor of ca. 100 over that from a transparent substrate .…”

A Laser Interferometric Method for Small- and Finite-Amplitude Ultrasonic Waves’ Detection in Transparent Media

“…Due to the acoustic perturbations the solid medium becomes optically anisotropic. The resulting changes in the components of the dielectric tensor can be written as: (4) where EO is the dielectric constant of the initially isotropic solid, Pijkl (i, j k, 1 = 1, 2, 3) the photoelastic tensor and Ski the symmetric strain tensor. Here we deal only with guided waves whose displacements are located in the sagittal plane Oxy [8].…”

“…The effects of ultrasonic waves on the light transmitting through transparent media arise from the refractive index variations produced by ultrasonic waves. The index variations may be detected by optical deflection, diffraction or interference methods [1][2][3][4]. In Raman-Nath regime, the acoustic waves act as a moving phase grating and diffract the light into different orders.…”

“Dark” Excited States of Diphenylacetylene Studied by Nonresonant Two-Photon Excitation Optical-Probing Photoacoustic Spectroscopy

“…For nonfluorescent molecules like DPA, the calorimetric techniques are very useful. Improved temporal resolution has been achieved with the PAS technique using two light sources, one as excitation light and the other as a probe [32][33][34][35]. The PAS technique using probe light was called optical-probing PAS (OPPAS).…”