Intramolecular dephasing. Picosecond evolution of wavepacket states in a molecule with intermediate-case level structure

Abstract

We present preliminary picosecond optical decay measurements of pyrazine seeded in He supersonic expansions. Pyrazine is a molecule which, in the vicinity of the first excited singlet, is classified as having “intermediate-case” level structure. From the effects of (i) a static homogeneous magnetic field, (ii) variation of the bandwidth of transform-limited excitation pulses and (iii) the rotational-state dependence of the spontaneous emission decay, we conclude that the fast-component decay contains very little triplet character. It follows that the fast decay represents evolution of a primarily singlet state into mixed singlet and triplet states. The quasi-stationary state that is prepared by photon absorption, and the molecular decay, are very sensitive to the coherence width of the laser pulse, which effect is characteristic of phase and not population relaxation in the molecule. We also find that the density of rotational states plays an important role in determining the evolution of the initially excited wave-packet state into a compound state.