Control of chemical reactions with ultrashort pulses

3 Control of chemical reactions with ultrashort pulses 

Ultrashort pulses can help us place a wave-packet in a well-localized region on an excited state potential. Moreover, the details of the optical excitation, specifically the momentum in the different vibrations, can have a decisive influence on the outcome. In Section 7.4 we discussed control in the frecpiency domain and emphasized interference between different pathways as an essential element in making the process go one way or the other. Here we consider how we might control chemical reactions in the time domain. 

To generalize this approach consider a pulse that is tailor-made to have prescribed phase relations between its frequency components. Such a shaped pulse is what was used in the example mentioned aheady in Section 1.1. A learning feedback approach is to start from an arbitrary pulse shape and feed the output to a (computer) controller that then seeks to change the shape of the 

Upper trace no phase difference between the two laser pulses. The wave-packet promoted by the first pulse and the newly promoted wave-packet can interfere constructively whenever the time delay between the pulses is a multiple of 300 fs so that the first wave-packet is back in the Franck-Condon window. Lower trace a jc phase difference between the two laser pulses. At intervals of 300 fs the returning wave-packet interferes destructively with the new wave-packet (adapted from N. F. Scherer eta/., J. Chem. Phys. 95, 1487 (1991). For other experiments showing interference between wave-packets excited by two pulses see Baumert eta . (1997). Even an electron can be so controlled during electron transfer (Barthel eta/., 2001 Martini eta ., 2001 Bardeen, 2001)]. 

The ns (n + 2)s transition is two-photon allowed. Detection of the npper level is through its one-photon fluorescence to the np state. The object of the control is either to populate or not to populate the upper state using the interference between different two-photon transitions. The principle is very much the same as in Section 7.4 bnt the implementation is different. Here only one pulse is used but this pulse is ultrashort so it is a superposition of many pulses each with its own well-defined ffeqnency. It is the interference between these components that allows us to direct the outcome. The shape of the pulse, meaning the amplitude 

---