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We present the results of experimental and theoretical studies of fastanisotropic relaxation and rotational diffusion in the first electron excitedstate of biological coenzyme NADH in water-ethanol solutions. The experimentshave been carried out by means of a novel polarization-modulation transientmethod and fluorescence polarization spectroscopy. For interpretation of theexperimental results a model of the anisotropic relaxation in terms of scalarand vector properties of transition dipole moments and based on theBorn-Oppenheimer approximation has been developed. The results obtained suggestthat the dynamics of anisotropic rovibronic relaxation in NADH under excitationwith 100~fs pump laser pulses can be characterised by a single vibrationrelaxation time $\tau_v$ laying in the range 2--15~ps and a single rotationdiffusion time $\tau_r$ laying in the range 100--450~ps a subject of ethanolconcentration. The dependence of the times $\tau_v$ and $\tau_r$ on thesolution polarity (static permittivity) and viscosity has been determined andanalyzed. Limiting values of an important parameter $\langleP_2(\cos\theta(t))\rangle$ describing the rotation of the transition dipolemoment in the course of vibrational relaxation has been determined fromexperiment as function of the ethanol concentration and analyzed.

Anisotropic relaxation in NADH excited states studied by polarization-modulation pump-probe transient spectroscopy