Stokes resonance Raman process

Resonance Raman and antisymmetric scattering are involved in a novel technique involving spin-flip Raman transitions in paramagnetic molecules that can function as Raman electron paramagnetic resonance. Figure 3.2a shows a conventional vibrational Stokes resonance Raman process, while 3.2b and 3.2c show the polarization characteristics of the two distinct spin-flip Raman processes for scattering at 90°... 

The detailed studies show that the saturation of the common level, as discussed above, is not the only coupling mechanism. The interaction of the atom with a light wave generates an induced dipole moment which is at small intensities proportional to the field amplitude. At higher intensities the nonlinear terms in the induced polarization become important. If two waves with frequencies and simultaneously act in resonance with the atom, these nonlinear terms produce sum and difference frequencies For two transitions which share a common level the difference frequency - W2 is in resonance with the atomic transition b c (2 2 - 3 2) and will therefore modulate the atomic polarization at the frequency - 0)2 The phenomenon can be regarded as a resonant Raman process where the 0.63 ym transition generates the Stokes line at X = 1.15 ym and both waves force the electronic polarization to oscillate at the difference frequency (see Sect.9.4). 

Marks, D. L., Vinegoni, C., Bredfeldt, J. S., and Boppart, S. A. 2004. Interferometric differentiation between resonant coherent anti-Stokes Raman scattering and nonresonant four-wave-mixing processes. AppZ. Phys. Lett. 85(23) 5787-89. 

Figure Bl.2.5. Comparison of several light seattering processes, (a) Rayleigh scattering, (b) Stokes and anti-Stokes Raman scattering, (c) pre-resonance Raman scattering, (d) resonance Raman scattering and (e) fluorescence where, unlike resonance Raman scattering, vibrational relaxation in the excited state takes place. From [3], used with permission.

Figure 8.3 Schematic representation of two eletronic states (ground and excited) and their respective vibrational levels (the eletronic and vibrational levels are not represented on the same scale). The arrows Indicate the types of transitions that can occur among the different levels. It Is Important to say that in the case of Raman scattering, if the laser line (XJ used has energy similar to one electronic transition of the molecule, the signal can be intensified by a resonance process, know as the resonance Raman effect. In the figure, and laser line and scattering frequencies, respectively (just the Stokes, Vs < Vg, component is shown in the diagram)...

Fig. 1 Representation of the processes that occur (from left to right) in non-resonant Rayleigh scattering, Stokes and anti-Stokes Raman and resonance Raman spectroscopy.

Figure 5 Ladder graphs for four-wave mixing effects containing Raman processes. In all cases there is assumed an intermediate Raman-type resonance at the frequency Q. (A) The coherent anti-Stokes Raman (CARS) process. (B) The process responsible for stimulated Raman spectroscopy (SRS) as well as the Raman-induced Kerr effect (TRIKE). (C) The coherent Stokes Raman spectroscopy (CSRS). Adapted with permission from Levenson MD (1982), Introduction to Nonlinear Laser Spectroscopy. New York Academic Press.

Coherent excitation of quantum systems by external fields is a versatile and powerful tool for application in quantum control. In particular, adiabatic evolution has been widely used to produce population transfer between discrete quantum states. Eor two states the control is by means of a varying detuning (a chirp), while for three states the change is induced, for example, by a pair of pulses, offset in time, that implement stimulated Raman adiabatic passage (STIRAP) [1-3]. STIRAP produces complete population transfer between the two end states 11) and 3) of a chain linked by two fields. In the adiabatic limit, the process places no temporary population in the middle state 2), even though the two driving fields - pump and Stokes-may be on exact resonance with their respective transitions, 1) 2)and... 

X lT + E r Xij ki, which refer in their order of appearance to the Cartesian polarization components of the CRS, pump, probe, and Stokes fields in the four-wave mixing process [31]. In transparent and optically inactive media, where the input frequencies are away from any electronic transition frequencies, and only the molecular ground state is populated, the selection rules of both resonant coherent and spontaneous Raman scattering are identical... 

Fig. 6.2. Illustration of the frequency-time dependences of pump and Stokes pulses in three different CRS excitation pulse schemes and their corresponding spectral resolution of Raman shifts. A Using a pair of transform-limited femtosecond pulses of broad spectral and narrow temporal widths results in a broad bandwidth of Raman shifts that exceeds the line width of a single Raman resonance. B Using transform-limited picosecond pulses of broad temporal and narrow spectral width readily provides high spectral resolution matching the Raman resonance line width to be probed. Selection of a Raman resonance shifted by AQr is achieved by tuning the frequency of one of the laser beams by the same amount. C Spectral focusing of a pair of identically linear chirped pump and Stokes femtosecond pulses results in a narrow instantaneous frequency difference in the CRS process, thus also providing narrow-bandwidth CRS excitation. Selection of a Raman resonance shifted by AQr is achieved by adjusting the time delay At between the pulses. Shifted pulses in (B) and (C) are depicted hatched...

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