Vibrational coherence in the composition-selected wavepacket of photoexcited pyrimidine.

Coherent wavepacket motion in photoexcited pyrimidine has been initiated and visualized in real time using femtosecond time-resolved ion-yield spectroscopy. A coherent superposition of at least four low-frequency Frank-Condon (FC) active modes is created in the first excited electronic state (S1), leading to a vibrational wavepacket. Its composition is manipulated experimentally by tuning the excitation wavelength in the range 309–313 nm to populate the selected vibrational levels. Interference among these vibrational levels is directly characterized by a clear quantum beat superimposed on a single-exponential decay. Fourier transform analysis of the wavelength-dependent transients shows modulation at different frequencies, providing a direct signature of multi-mode vibrational coherence resulting from the coherent excitation process. The sensitivity of the parent-ion transient to the vibrational wavepacket dynamics probably arises because different modes are connected by variable FC factors to the 3s and 3p Rydberg states. [ABSTRACT FROM AUTHOR]