Excitation pathways, phase control

Two lines of inquiry will be important in future work in photochemistry. First, both the traditional and the new methods for studying photochemical processes will continue to be used to obtain information about the subtle ways in which the character of the excited state and the molecular dynamics defines the course of a reaction. Second, there will be extension and elaboration of recent work that has provided a first stage in the development of methods to control, at the level of the molecular dynamics, the ratio of products formed in a branching chemical reaction. These control methods are based on exploitation of quantum interference effects. One scheme achieves control over the ratio of products by manipulating the phase difference between two excitation pathways between the same initial and final states. Another scheme achieves control over the ratio of products by manipulating the time interval between two pulses that connect various states of the molecule. These schemes are special cases of a general methodology that determines the pulse duration and spectral content that maximizes the yield of a desired product. Experimental verifications of the first two schemes mentioned have been reported. Consequently, it is appropriate to state that control of quantum many-body dynamics is both in principle possible and is... 

References 29-33 introduce the notion of coherence spectroscopy in the context of two-pathway excitation coherent control. Within the energy domain, two-pathway approach to coherent control [25, 34—36], a material system is simultaneously subjected to two laser fields of equal energy and controllable relative phase, to produce a degenerate continuum state in which the relative phase of the laser fields is imprinted. The probability of the continuum state to evolve into a given product, labeled S, is readily shown (vide infra) to vary sinusoidally with the relative phase of the two laser fields < ),... 

Figure 1. Schematic illustration of two-pathway control in the (a) frequency and (b) time domains. In case (a) the ground state is excited to a coupled continuum by either one photon of frequency CO3 or three photons of frequency C >i. Control is achieved by introducing a phase lag between the two fields. In case (b) a two-pulse sequence has sufficient bandwidth to excite a superposition of two intermediate states. Control is achieved by introducing a delay, At, between the pulses, resulting in a phase difference of to At.

Although considerations such as these have led to a less active field of UV/visible spectroscopic study compared with infrared study, this wavelength region offers some unique advantages, particularly for studies oriented toward kinetics, dynamics, thermodynamics, and mechanisms of gas-phase ion chemistry. As a particular example, we can mention the ability to initiate a chemical process with the insertion of a very accurately known (single-photon) increment of several eV of internal energy at a precisely determined time (for a view of some of these possibilities, see, for instance, [132]). Control of photon polarization is possible [133]. Moreover, excited electronic reaction and dissociation pathways, often involving radical ion chemistry, can be accessed in this way, which is not possible with vibrational activation. 

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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