Active spectroscopies, nonlinear light

Akhmanov, S. A., and Koroteev, N. I. 1981. Nonlinear optical methods in light scattering Active spectroscopy of light scattering. Moscow Nanka. 

Lee D and Albrecht A C 1993 On global energy conservation in nonlinear light matter interaction the nonlinear spectroscopies, active and passive Adv. Phys. Chem. 83 43-87... 

ON GLOBAL ENERGY CONSERVATION IN NONLINEAR LIGHT-MATTER INTERACTION THE NONLINEAR SPECTROSCOPIES, ACTIVE... 

Several other forms of nonlinear spectroscopy have been developed that are not strictly based on Raman-active vibrations or rotations. Degenerate four-wave mixing (DFWM) is a technique where all input and output frequencies are identical. Because it does not involve the generation of light at new frequencies, it can rely on non-local mechanisms other than the local electronic polarizability (e.g. electrostric-tion). The selection rules for DFWM are closely related to those of one-photon techniques (e.g. absorption). DFWM using infrared beams is therefore used to probe infrared absorbing transitions instead of Raman-active transitions. 

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