Resonant stimulated Raman scattering process

When the two beams of pump laser and probe laser in the double resonance scheme of Fig. 5c travel col linearly instead of antiparallel a particular situation arises. The simultaneous interaction of both waves with the molecule may be regarded as a resonant stimulated Raman scattering process. It can be shown39 that in such a case the width yqr of the double resonance signal is given by... 

Up to now/ the dimer laser system has been described alone in terms of population inversion between suitable energy levels/ and for this description the condition S2 > A 2 is indeed the only necessary condition for cw laser oscillation/ as long as the thermal population density in the lower laser level remains negligibly low. However/ as this optically pumped laser system is a coherently excited three level system/ the coherent emission can also be described as stimulated Raman scattering/ which is resonantly enhanced by the common level 3 of the pump and laser transitions. This coupled two photon or Raman process does not require a population inversion between levels 3 and 2 and introduces qualitatively new aspects which appreciably influence and change the normal laser behaviour. For a detailed and deeper description of the coherently excited three level dimer... 

It is well-known that gain can be obtained by stimulated Raman scattering without population inversion between the lower, initial state a> and the higher, final state c> of the two-photon transition. Near resonance, that is when the laser photon frequency almost equals the transition frequency u, the overall transition takes place mainly ac-cording to the following scheme (1) absorption of a laser photon of frequency (2) stimulated emission of a Stokes laser photon of frequency as depicted in Fig. la. This process prevails over the reverse process shown in Fig. lb, provided. ... 

The laser combustion diagnostics techniques discussed so far utilized resonant processes, whether it be single- or multi-photon excitation, fluorescence or stimulated emission. We will now consider non-resonant processes of Raman nature. Because of its msensitivity to quenching (the lifetime of the virtual state is lO s), Raman spectroscopy is of considerable interest for quantitative measurements on combustion processes. Further, important flame species such as O2, N2 and H2 that do not exhibit IR transitions (Sect. 4.2.2) can be readily studied with the Raman technique. However, because of the inherent weakness of the Raman scattering process (Sect. 4.3) only non-luminous (non-sooting) flames can be studied. 

Inverse Raman scattering Inverse Raman scattering (IRS) is a coherent process involving stimulated loss at an anti-Stokes-shifted frequency. The term inverse Raman refers to the fact that, at resonance, the probe radiation is attenuated. In spontaneous Raman spectroscopy, on the other hand radiation at Raman-active frequencies would he generated in the course of the experiment. Inverse Raman scattering (IRS) and stimulated Raman gain (SRG) are closely related. While one involves stimulated gain at an anti-Stokes-shifted frequency, the other involves stimulated loss at a Stokes-shifted frequency. 

On the basis of the experimental data presented above, one cjin exclude on the onset several models for the "blue peak" emission. Firstly, spin-flip collision induced population inversion on the Di transition is not involved - due to both the off-resonant character of the emission and its independence on the buffer gas pressure. Similarly, pressure induced extra resonances are rejected. Stimulated electronic Raman and three photon scattering effects, both by a two or three level system, are dependent on the laser detuning and neither their frequencies are to the blue in the vicinity of the Di line (figure 2) thus, these processes are also excluded. 

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