Ultrashort laser pulses, photoexcitation

A suitable method for a detailed investigation of stimulated emission and competing excited state absorption processes is the technique of transient absorption spectroscopy. Figure 10-2 shows a scheme of this technique. A strong femtosecond laser pulse (pump) is focused onto the sample. A second ultrashort laser pulse (probe) then interrogates the transmission changes due to the photoexcitations created by the pump pulse. The signal is recorded as a function of time delay between the two pulses. Therefore the dynamics of excited state absorption... 

The direct photoexcitation of water molecules by ultrashort laser pulses is used for the investigation of primary events occurring from 10 s (thermal orientation of water molecules and ultrafast proton transfer) to 10" s (primary reactions of a solvated electron with protic species) (57,58,61-65). The nonlinear interaction of ultrashort UV pulses (typically less than 100 fs in duration and having a power of 10 W cm" ) with water molecules triggers multiple electron photodetachment channels within a hydrogen bond network (see equations 4-7). An initial energy deposition via a two-photon absorption process (2 X 4 eV) leads to the formation of nonequilibrium states of an excess electron... 

We mimic the photoexcitation of the system by an ultrashort laser pulse by instantly switching from the ground-state N=N torsional potential yo(9) to the excited-state potential 14(9) (see Figure 7.1). Following this nonequilibrium preparation at time t = 0, the system isomerizes along excited-state N=N potential within 250 fs. 

Two main approaches to the control of molecules using wave interference in quantum systems have been proposed and developed in different languages . The first approach (Tannor and Rice 1985 Tannor et al. 1986) uses pairs of ultrashort coherent pulses to manipulate quantum mechanical wave packets in excited electronic states of molecules. These laser pulses are shorter than the coherence lifetime and the inverse rate of the vibrational-energy redistribution in molecules. An ultrashort pulse excites vibrational wave packets, which evolve freely until the desired spacing of the excited molecular bond is reached at some specified instant of time on a subpicosecond timescale. The second approach is based on the wave properties of molecules as quantum systems and uses quantum interference between various photoexcitation pathways (Brumer and Shapiro 1986). Shaped laser pulses can be used to control this interference with a view to achieving the necessary final quantum state of the molecule. The probability of production of the necessary excited quantum state and the required final product depends, for example, on the phase difference between two CW lasers. Both these methods are based on the existence of multiple interfering pathways from the initial... 

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