Raman Spectroscopy with Subnatural Linewidth

A very interesting method for achieving subnatural linewidths of optical transitions is based on induced resonant Raman spectroscopy, which may be regarded as a special case of optical-optical double resonance (Sect. 5.4). The pump laser Li is kept on the molecular transition 1 2) (Fig. 9.83), while the tunable probe laser L2 induces downward transitions. A double-resonance signal is obtained if the frequency co of the probe laser matches the transition 2) 3). This signal can be detected by monitoring the change in absorption or polarization of the transmitted probe laser. 

When both laser waves with the wave vectors and are sent collinearly through the sample, energy conservation for the absorption of a photon fuo and emission of a photon by a molecule moving with the velocity v demands 

Integration over the velocity distribution N(Vz) of the absorbing molecules yields with (9.79) the width of the double-resonance signal [1311] 

Of course, one cannot get accurate information on the level width y2 from (9.80) unless all other contributions to /s are sufficiently well known. The width /s is comparable to the width of the direct transition 1) 3. However, if 1) 2) and 12) 13) are allowed electric dipole transitions, the direct transition 1) 3) is dipole-forbidden. 

The small signal width of the OODR signal /s becomes essential if the levels 3) are high-lying rotation-vibration levels of heavy molecules just below the dissociation limit, where the level density may become very high. As an example, Eig. 9.84 shows such an OODR signal of a rotation-vibration transition D Eu(v = 50, J = 48) X Ug v = 125, J = 49) in the Cs2 molecule. A comparison with 

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Fig. 9.83 Stimulated resonant Raman transition spectroscopy with subnatural linewidth resolution (a) level scheme (b) experimental arrangement...

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