Control systems selective mode excitation

For the coherent control of reactions at surfaces, the manipulation of adsorbate motion is essential. Cs/Pt(lll) is a suitable system which provides us with a good opportunity to test whether or not we can excite preferentially one of the two modes whose frequencies are very close to each other by using tailored laser pulses. We have demonstrated the mode-selective excitation of coherent surface phonon modes on Cs/Pt(lll) by synthesized femtosecond pulse trains. 

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. 

Many exciting questions remain in gas phase reaction dynamics. The goal of bond and state selective chemistry is still not fully realized, although this type of control has been observed in the local mode H-I-H20 system [18, 19]. An interesting new approach for chemical control involves preparing reactants in a coherent superposition of states [24], for which some theory already exists [25]. In applying these techniques to complex reactive systems, one hopes that the hard won coherence is not lost before the reaction proceeds. 

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