Pulse-Width-Controlled Molecular Dynamics

Most of the pump control experiments carried out so far have used diatomic molecules, because in such simple systems, with only one vibrational degree of freedom, the dynamics can be controlled relatively easily. In larger molecular systems with three or more vibrational degrees of freedom, the situation becomes much more complicated and it is an interesting question whether the concept of controlled molecular dynamics can still be realized. Here, it is shown that different vibrational modes of the sodium trimer can be selectively excited during an electronic excitation with ultrashort laser pulses. For this reason, it should in future be possible to control subsequent reactions. The relevant control parameter in these investigations is the duration of the pump pulse. 

For this, various 3d quantum ab initio simulations of the wave packet dynamics in Naa B are presented here and compared to ultrashort laser pump probe experiments. In addition to exact QD calculations, an a proximate QD method is suggested to simulate the main features of a pump probe spectrum. The simulations provide satisfactory results in comparison to exact QD calculations. By means of these two methods it is possible to reproduce and to explain the different experimental pump probe spectra. The 310 fs oscillation in the femtosecond pump probe experiment [62, 81] can clearly be assigned to the Qs vibration, while the 3ps oscillation of the picosecond pump probe experiment [306, 379] is caused by a slow pseudorotational wave packet motion. 

Real-Time Spectrum Obtained with 120 fs Pump Pulses. In order to simulate the femtosecond experimental ion signal, the following pulse parameters were used duration fpWHM = 120fs, intensity I = 520MWcm  

The approximate QD pump probe signal (Fig. 3.47) still yields a good agreement with the experimental and the exact QD theoretical results. The 320 fs oscillation structure dominates in all cases. Moreover, one of the slower pseudorotational vibrations with a period of about 1 ps can also be detected by our approximate QD method. A detailed analysis by means of the Fourier transform of the corresponding autocorrelation function [378] and of the induced wave packet dynamics reveals that this oscillation is caused by a slow pseudorotational vibration in the coordinate (f. As the Fourier spectrum shows the wave packet prepared in the B state is centered around 621 nm, i.e. between the vibrational states = 5 and 6 of the (f mode. The energy distance between these two eigenstates corresponds to a vibrational period of about 1 ps. It is one of the fastest pseudorotational vibrations which can be observed in the absorption spectrum and is the next slower vibration after the Qs mode. This interpretation of the 1 ps oscillation is confirmed by analyzing the induced wave packet dynamics. Here, an accumulation of the 

Summarizing the strengths of the approximate QD method, it reflects the dominant wave packet motions quite well, while the intensities cannot be reproduced exactly. Nevertheless, the comparison between the exact QD and the approximate QD method demonstrates that the latter method enables an efficient study of the dynamics that are induced with different laser pulse parameters even for larger molecules. 

---

S. Rutz, E. Schreiber, and L. Woste, Pulse Width Controlled Molecular Dynamics Symmetric Stretch Versus Pseudorotations in Nas(B) in Femto-chemistry Ultrafast Chemical and Physical Processes in Molecular Systems, M. Chergui (ed.) (World Scientific, Singapore, 1996), p 319. 

---