Ultrafast Wave Packet Propagation Phenomena in Excited Alkali Trimers

2 Ultrafast Wave Packet Propagation Phenomena in Excited Alkali Trimers 

Vibronically coupled internal degrees of freedom are the main reason for the very distinct properties of small metal clusters. In diatomic systems, with their single vibrational mode, these phenomena do not yet occur. Therefore, an isolated oscillating dimer vibrates until eventually it radiates or predissociates. With an increasing number of participating atoms in a molecule, however the number of coupled vibrations drastically increases. This increase makes the identification of individual energetic redistribution channels extremely difficult, unless simple systems are chosen. 

In Sects. 3.2.1-3.2.4 a detailed analysis of the sodium trimer excited to its electronic B state is presented. A short review of Naa, with special interest in the spectroscopy of its electronic B state, is given in Sect. 3.2.1. Employing laser pulses of moderate intensities with durations of either 1.4 ps or llOfs to excite the Naa B state, different vibrational modes of the excited trimer are detected selectively in the real-time spectra. While the picosecond laser 

These complementary experimental results can be explained to a great extent by quantum dynamical simulations of the real-time experiments. In Sect. 3.2.2, first the results obtained by means of two-dimensional (2d) ab initio potential-energy surfaces are briefly summarized. Even more sophisticated calculations are performed on three-dimensional (3d) ab initio potential-energy and transition dipole surfaces (Sect. 3.2.3). There, all three vibrational degrees of freedom of the Nas molecule are included in the theoretical treatment. The time-dependent wave packet dynamics elucidate the effect of ultrafast state preparation on the molecular dynamics. Extensive theoretical calculations indicate the possibility of initiating the molecular dynamics predominantly in selected modes during a certain time span by variation of the pump pulse duration (Sect. 3.2.4). 

Another interesting triatomic candidate for femtosecond real-time spectroscopy is the potassium trimer. Although it has often been tried, conventional spectroscopy does not reveal any theoretically predicted electronic state of K3. Ultrafast processes such as intermolecular vibrational redistribution (IVR) or even photodissociation on a picosecond timescale were considered responsible for this fact. With femtosecond real-time spectroscopy, however, the predicted electronic state (comparable to the Nas B state) should be detectable (Sect. 3.2.5). 

---