Optoacoustic effect

PAS is a technique based on the so-called optoacoustic effect When a short pulse of electromagnetic radiation interacts with condensed matter, the absorbed energy is converted into heat by fast nonradiative relaxation processes. Subsequently, the thermal expansion of the instantaneously heated medium causes an acoustic pulse, which is detected. 

H.J. Bauer, Son et lumiere or the optoacoustic effect in multilevel systems. J. Chem. Phys. 57,3130 (1972) (references to the historiceil development)... 

The optoacoustic effect, based on partial non-radiative dissipation of the absorbed light energy, followed by generation of an acoustic wave, was discovered about a hundred years ago. The effect is very weak and, before the advent of lasers, it was applied mainly for spectroscopic investigations of gases. Lasers and improvements in acoustic vibration detection have led to the development of laser optoacoustic spectroscopy (LOAS). Several physical processes are involved in LOAS (1)... 

Use is sometimes made of the optoacoustic effect in determining the efficiency of the pumping. This entails the use of a microphone in a vessel containing some of the (sub)mm laser vapour. This vessel might of course be the laser itself, as seen in Fig. 3.3. The technique is used as an aid in stabilizing the CO2 laser frequency on the absorption in the vapour or in the search for new absorption lines [5.1]. 

In some systems, triplet BET can occur, as deduced from time-resolved optical spectroscopy, magnetic field effects, CIDNP, or optoacoustic calorimetry. Triplet BET is governed by energetic factors, which determine rates, and by the relative topologies of the potential surfaces of parent molecule, radical ions, and of accessible triplet or biradical states. Divergent topologies for different states may cause rearrangements. 

The theory of rotation effects on prolate luminescent molecules in solution and its experimental verification have been developed and compared. Generalized diffusion equations for the rotational motion of an asymmetric rigid motor have been used to given an expression for steady-state fluorescence depolarization. " The radiationless transition from the first excited singlet state of Eosin has been measured by optoacoustic relaxation, and the absolute fluorescence quantum yields of organic dyes in poly(vinyl alcohol) have also been measured by the photoacoustic method. The accuracy of the method has been discussed in the latter paper. Actinometry in flash photolysis experiments has been assisted by new measurements on the extinction coefficient of triplet benzophenone. Matrix-isolation fluorescence spectrometry has been used to detect polycyclic aromatic hydrocarbons from gas chromatography. ... 

The sensitivity can be further enhanced by frequency modulation of the laser (Sect. 1.2.2) and by intracavity absorption techniques. With the spectraphone inside the laser cavity, the photoacoustic signal due to nonsaturating transitions is increased by a factor as a result of a -fold increase of the laser intensity inside the resonator (Sect. 1.2.3). The optoacoustic cell can be placed inside a multipath optical cell (Fig. 1.28) where an effective absorption pathlength of about 50 m can be readily realized [74]. 

The optoacoustic properties of plasmon-resonant gold nanoparticles originate from photoinduced cavitation effects. This process can be summarized as follows (i) thermalization of conduction electrons on the subpicosecond timescale/ (ii) electron-phonon relaxation on the picosecond timescale and thermalization of the phonon lattice, with a subsequent rise in temperature by hundreds to thousands of degrees (iii) transient microbubble expansion upon reaching the kinetic spinodal of the superheated medium, initiated on the nanosecond timescale (iv) microbubble collapse, resulting in shockwaves and other forms of acoustic emission. The expansion and collapse of a cavitation bubble takes place on a microsecond timescale, and are easily detected by ultrasonic transducers. 

Immersion of the spectraphone in a magnetic field gives an instrument that can be used to study the Zeeman effect of molecules by optoacoustic spectroscopy. This magnetospectraphone has been used to measure the Zeeman splittings of NO [8.46]. 

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