We present the results of experimental and theoretical studies of fast
anisotropic relaxation and rotational diffusion in the first electron excited
state of biological coenzyme NADH in water-ethanol solutions. The experiments
have been carried out by means of a novel polarization-modulation transient
method and fluorescence polarization spectroscopy. For interpretation of the
experimental results a model of the anisotropic relaxation in terms of scalar
and vector properties of transition dipole moments and based on the
Born-Oppenheimer approximation has been developed. The results obtained suggest
that the dynamics of anisotropic rovibronic relaxation in NADH under excitation
with 100~fs pump laser pulses can be characterised by a single vibration
relaxation time τv laying in the range 2--15~ps and a single rotation
diffusion time τr laying in the range 100--450~ps a subject of ethanol
concentration. The dependence of the times τv and τr on the
solution polarity (static permittivity) and viscosity has been determined and
analyzed. Limiting values of an important parameter ⟨P2(cosθ(t))⟩ describing the rotation of the transition dipole
moment in the course of vibrational relaxation has been determined from
experiment as function of the ethanol concentration and analyzed.Comment: 14 pages, 13 figure