Double resonance spectra have been obtained for nitrogen and benzene molecules in the low density region of a molecular beam by use of stimulated Raman pumping followed by ionization probing. For N 2 the photoionization step was via a 2ϩ2 resonantly enhanced multiphoton ionization process using the a 1 ⌸ g (vϭ2)←X 1 ⌺ g (vϭ1) two-photon resonance. The detection limit is 10 3 -10 4 times lower than for the direct optical measurement of a stimulated Raman signal. For benzene, a more efficient 1ϩ1 ionization step permitted measurements at state densities less than 10 10 cm Ϫ3 and the Raman step yielded linewidths of 30 MHz for the Q-branch lines of the weak Raman active 16 fundamental.
An experimental CARS set-up for resonant solid-state coherent spectroscopy is described. Resonance CARS spectra of one unsymmetrically (TSFBS) and two symmetrically (TS6, FBS) substituted diacetylene monomer single crystals, with a low content of polymer, were obtained with this apparatus. Particularly, spectra of the C-C and the C=C stretching vibrations of the polymer chains, recorded for different pump laser frequencies, were investigated. The occurrence of broad spectral CARS features which are non-resonant and Stokes shifted relative to other observed CARS lines is discussed.
We have performed polarized resonance coherent anti-Stokes Raman experiments on the permanganate ion doped in potassium perchlorate single crystals at temperature T=15 K. At this temperature the m (Cs) site splitting of the excited degenerate 1T2-electronic level of the permanganate ion and the vibronic structure are well resolved. We report on the A′-ν1 coherent anti-Stokes Raman excitation profile which shows a strong dependence on the frequency of the pump laser. The simulation of the experimental results is performed by using the transform theory, which enables one to calculate the resonance Raman excitation profile solely from the measured absorption spectrum. According to the well known relation between the third-order nonlinear susceptibility and the Raman polarizability the coherent anti-Stokes excitation profile is given by a simple product of two Raman excitation profiles which are shifted relative to each other. The linear and the quadratic electron-phonon coupling as well as the influence of non-Condon terms were taken into account. Since the transform theory is mode selective in case there are no mode mixing effects the results derived from the description of the Raman excitation profiles and the coherent anti-Stokes excitation profiles are more definite. For this purpose one only needs the model parameters of the mode of interest. The simulation of the A′-ν1 coherent anti-Stokes profile shows that a small amount of non-Condon coupling yields to a better agreement with the experiment.
Solid-state absorption, resonance Raman and resonance CARS spectroscopy were applied to investigate the vibronic properties of the permanganate ion doped in KCIO, single crystals. Experiments were performed for all three types of spectroscopy at 15 K. At this temperature the vibronic structure in the absorption spectrum is highly resolved, allowing detailed studies of resonance Raman and resonance CARS spectroscopy. Selected experimental results are presented.
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