Photoacoustic Raman spectroscopy

Barrett J J and Berry M J 1979 Photoacoustic Raman spectroscopy (PARS) using cw laser sources Appl. Phys. Lett. 34 144-6... 

Siebert D R, West G A and Barrett J J 1980 Gaseous trace analysis using pulsed photoacoustic Raman spectroscopy Appl. Opt. 19 53-60... 

Here, E is the strength of the applied electric field (laser beam), a the polarizability and / and y the first and second hyper-polarizabilities, respectively. In the case of conventional Raman spectroscopy with CW lasers (E, 104 V cm-1), the contributions of the / and y terms to P are insignificant since a fi y. Their contributions become significant, however, when the sample is irradiated with extremely strong laser pulses ( 109 V cm-1) created by Q-switched ruby or Nd-YAG lasers (10-100 MW peak power). These giant pulses lead to novel spectroscopic phenomena such as the hyper-Raman effect, stimulated Raman effect, inverse Raman effect, coherent anti-Stokes Raman scattering (CARS), and photoacoustic Raman spectroscopy (PARS). Figure 3-40 shows transition schemes involved in each type of nonlinear Raman spectroscopy. (See Refs. 104-110.)... 

The principle of photoacoustic Raman spectroscopy (107) is similar to that of CARS. When two laser beams, vp (pump beam) and vs (Stokes beam), impinge on a gaseous sample contained in a cell (Fig. 3-43), these two beams interact when the resonance condition, vp — vs = vM, is met, where vM is a frequency of a Raman-active mode. This results in the amplification of the Stokes beam and the attenuation of the pump beam. Each Stokes photon thus... 

Figure 3.6-4 Schematic diagram for a few techniques in nonlinear (coherent) Raman spectroscopy (CSRS Coherent Stokes Raman Spectroscopy SRGS Stimulated Raman Gain Spectroscopy IRS Inverse Raman Spectroscopy (= SRLS Stimulated Raman Loss Spectroscopy) CARS Coherent anti-Stokes Raman Spectroscopy PARS Photoacoustic Raman Spectroscopy).

The methods of nonlinear Raman spectroscopy, i. e. spontaneous hyper Raman scattering (based on the hyperpolarizability) and coherent nonlinear Raman scattering (based on the third-order-nonlinear susceptibilities) are discussed in detail in Sec. 3.6.1. In Sec. 3.6.2 the instrumentation needed for these types of nonlinear spectroscopy is described. In this section we present some selected, typical examples of hyper Raman scattering (Sec. 6.1.4.1), coherent anti-Stokes Raman. scattering (Sec. 6.1.4.2), stimulated Raman gain and inverse Raman spectroscopy (Sec. 6.1.4.3), photoacoustic Raman spectroscopy (Sec. 6.1.4.4) and ionization detected stimulated Raman spectroscopy (Sec. 6.1.4.5). 

As discussed in Secs. 3.6.1.3 and 3.6.2.4 in photoacoustic Raman spectroscopy (PARS) the energy deposited in the sample by excitation of e. g. a vibration by the stimulated Raman process leads to pressure increases through relaxation to translational energy and can therefore be detected by a sensitive microphone. 

In another PARS study (Hopkins et al., 1985) it was shown that photoacoustic Raman spectroscopy is a sensitive technique for obtaining Raman spectra of hydrogen-bonded complexes in the gas phase. PARS spectra of the CN stretching region of HCN as a function of pressure revealed bands which could be assigned to HCN dimers and trimers. 

Photoacoustic Raman spectroscopy (PARS) Photoacoustic Raman spectroscopy (PARS) is again a nonlinear spectroscopic technique. In this technique, selective population of a given energy state of a system (transitions must involve change in polarizability) is amplified using coherent Raman amplification (also known as stimulated Raman scattering). In this process, it is also important that the frequency difference of the two incident laser beams must be adjusted to equal the frequency of Raman-active transition. 

It can sometimes be highly advantageous to detect the SRS signal indirectly. Thus in photoacoustic Raman spectroscopy (PARS), introduced by Barrett and Berry [37], a microphone or piezoelectric transducer is used to measure the acoustic wave amplitude generated in the sample when the vibrational or rotational excitation, created by the SRS process, relaxes non-radiatively into translational (heat) energy. Figure 5.8 shows the pure rotational PARS spectrum of CO2, taken from the article by J.J. Barrett, D.R. Siebert and G.A. West in Reference [4]. Although the resolution of -0.3 cm" is not remarkable, the... 

Fig. 7 Raman gain = stimulated Raman gain spectroscopy (SRGS), inverse Raman = inverse Raman spectroscopy (IRS) or stimulated Raman loss spectroscopy (SRLS), coherent anti-Stokes Raman spectroscopy (CARS), photoacoustic Raman spectroscopy (PARS), or ionization-detected stimulated Raman spectroscopy (IDSRS). In the following sections, the various methods are briefly described. More detailed information can be found in books [59-61], reviews [45,46,57,58,62,63] and conference reports [64-73].

Coherent Anti-Stokes and Photoacoustic Raman Spectroscopy... 

The S diads in the vibrational Raman spectrum of CO2 and its isotopic variants were measured and their intensity distribution simulated [48,164-167]. In Fig. 17, a more recent recording of the Raman spectrum of natural CO2 in the Fermi resonance region is presented [53]. The structure of the Q branch of the (10 02) component at 1285 cm has been resolved by CARS [168], stimulated Raman [169], and photoacoustic Raman spectroscopy [106] see Fig. 10. The Q branches of the overtones of the Fermi diad were also observed [170] and the evaluation of their intensities yielded mean polarizability derivatives. In supersonic jet experiments on CO2, the density, condensation, and translational, rotational, and vibrational temperatures were investigated [171]. 

GA West, DR Siebert, JJ Barrett. Gas phase photoacoustic Raman spectroscopy using pulsed laser excitation. J Appl Phys 51 2823-2828, 1980. 

M Rotger, B Lavorel, R Chaux. High-resolution photoacoustic Raman spectroscopy of gases. J Raman Spectrosc 23 303-309, 1992. 

In photoacoustic Raman spectroscopy (PARS), due to the interaction of the two input laser fields (coi, (o ) a population of a particular energy level (co ) of the sample is achieved. As the vibrationally (or rotation-ally) excited molecules relax by means of collisions, a pressure wave is generated in the sample and this acoustic signal is detected by a sensitive microphone. 

Figure 16 The pure rotational photoacoustic Raman (PARS) spectrum of CO2 gas at a pressure of 80 kPa (600 torr) pump laser wave length at 532 nm. Note the complete absence of any acoustical signal due to Rayleigh scattering (at 532 nm). Reproduced by permission of Academic Press from Barrett JJ (1981) Photoacoustic Raman Spectroscopy. In Harvey AB (ed) Chemical Applications of Nonlinear Raman Spectroscopy, pp 89-169. New York Academic Press.

In another PARS study it was shown that photoacoustic Raman spectroscopy is a sensitive technique for obtaining Raman spectra of hydrogen-bonded... 

Barrett JJ (1981) Photoacoustic Raman spectroscopy. In Harvey AB (ed) Chemical Applications of Nonlinear Raman Spectroscopy, pp 89-169. New York Academic Press. 

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