Excited state Femtochemistry

Initial femtochemistry theoretical models were simply generalizations of those for the single-photon DIET processes, i.e., as dynamics induced by multiple electronic transitions (DIMET) [100]. The idea is simply that even if the excited state residence is too short to cause excitation to a ground state continuum after resonant scattering (tR < tc), it can still cause some vibrational excitation in the ground state. If resonant... 

The main experimental methods in femtochemistry are based on the pump— probe methods. The pump pulse at the frequency ni creates a wave packet in the electronically excited state and determines the zero time moment at which die inter-nuclear distance in the transition state is Ro = / t(t=0). The dynamics of the wave packet, which can be considered as its motion over the PES, represents the dynamics of transition state, that is, the time evolution of the intemuclear distance in [B...Cf. After some delay time t, the second femtosecond pulse is produced at the frequency TI2. This pulse is called the probe pulse because it determines the existence of the wave packet on the PES, i.e., the intemuclear distances R(t) in the time moment t = t. 

One of the most interesting applications of Femtochemistry is the stroboscopic measuring of observables related to molecular motion, for instance the vibrational periods or the breaking of a bond [1], Because femtosecond laser fields are broadband, a wave packet is created by the coherent excitation of many vibrational states, which subsequently evolves in the electronic potential following mostly a classical trajectory. This behavior is to be contrasted to narrow band selective excitation, where perhaps only two (the initial and the final) states participate in the superposition, following typically a very non-classical evolution. In this case one usually is not interested in the evolution of other observables than the populations. 

The year 2003 is the tenth anniversary of the first Femtochemistry Conference and the fiftieth anniversary of Watson and Crick s celebrated discovery of the DNA double helix [1], Remarkable progress has been made in both fields femtosecond spectroscopy has made decisive contributions to Chemistry and Biology, and advances in the elucidation of static nucleic acid structures have profoundly transformed the biosciences. However, much less is known about the dynamical properties of these complex macromolecules. This is especially true of the dynamics of the excited electronic states, including their evolution toward the photoproducts that are the end result of DNA photodamage [2],... 

There are now direct experimental confirmations [41-43], to be discussed in Section 13.3, of the effect of the pulse shaping. In addition, the role of the phase j-1 between two pulses, predicted in both OCT [104,119] and in CC studies [94,96,1R>J. has been confirmed experimentally by Fleming et al. [137,138], Girard et al. [JO- j 142], Kinrot et al, [143], and Warmuth et al. [144] in the so-called wave paiket interferometry experiments [145], For example, Fleming et al, [137,138] [and Warmuth et al. [144] describe experiments where the fluorescence from I2 in the F t state is influenced by constructive or destructive interference between two wave pad.-, j ets induced by two-phase related excitation pulses. This study relates to a large voliuiiejy of work on wave packet interferometry in atoms [146-148], as well as to various." femtochemistry experiments, where similar effects were seen in absorption [106] . jj... 

One of the most exciting possibilities of ultrafast laser techniques is to follow the course of fundamental chemical reactions on the relevant timescale at which they occur. Previously, it was only possible to know the individual states of molecules A and B before reacting and the final state of the compound molecule AB. In contrast, the details of the chemical reaction can now be followed on a femtosecond scale with information on how chemical bonds are formed and broken. In particular, the existence of transition states has been demonstrated. This new field of science is frequently referred to as femtochemistry [9.191-9.204], for which A. Zewail was awarded a Nobel prize in chemistry (1999). 

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