I. Tehver

Femtosecond resonance enhanced CARS for background-free detection of organic molecules

We study the coherent excitation profile (CEP) of resonance enhanced femtosecond CARS in a model system zinc phthalocyanine in a polymer film host as a prospective technique for detection and identification of molecular species in ambient environments. A new method of suppressing the non-resonant FWM background is demonstrated. Transform theory is applied to calculate CEP based on the absorption spectrum, and good agreement between theory and experiment is obtained.

Optical transitions in the centres with soft dynamics in the final state

Anomalous optical spectra of the centres with strongly-reduced local elastic springs in the final electronic states are studied. In this case the transitions take place to the vicinity of a flat potential curve in the configurational coordinate space which corresponds to a very strong mode mixing at the electronic transition. To take this mixing into account we apply the operator transform method. We have found that the mixing results in a strong enhancement of the effect of low-frequency vibrations, causing an intensive low-energy phonon sideband. In this case the spectrum (or its envelope) has the shape of an asymmetrical lambda-letter. Suchtype spectra have been observed experimentally. The spectra may also reveal the Airy-type oscillations if the potential of the final state has a finite slope. This-type oscillations have been observed in the hot luminescence; the oscillating structure of zero-point lines in the absorption spectra of super-fluid 4He droplets probably has an analogous origin.

Raman Scattering for Weakened Bonds in the Intermediate States of Impurity Centres

A theory of the Raman scattering in resonance with an electronic transition causing a strong softening of vibrations is proposed. In this case the potential surface of the excited state has a flat minimum or maximum in the configurational coordinate space. Two cases of the vibronic coupling are considered: (1) the coupling with a single coordinate and (2) the coupling with the phonon continuum. To describe the Raman scattering the Fourier-amplitude method is applied. In the first case the calculations are performed for the pseudo-Jahn-Teller effect in the excited state. In the second case, despite a strong mixing of phonons, the equations for the Raman Fourier amplitudes can be factorized and solved analytically. It is predicted that the second-order Raman scattering will be strongly enhanced. Moreover, the second-order Raman scattering is also enhanced as compared to the first-order scattering. The Raman excitation profiles show a structure caused by the Airy oscillations. The theory is applied to the triplet-triplet optical transition in Na2 molecule confined at the surface of a 4He droplet.