Coherent pulse sequences

Tannor D J and Rice S A 1988 Coherent pulse sequence control of product formation in chemical reactions Adv. Chem. Rhys. 70 441 -524... 

Tannor D J, Kosloff R and Rice S A 1986 Coherent pulse sequence induced control of selectivity of reactions exact quantum mechanical calculations J. Chem. Rhys. 85 5805-20, equations (1)-(6)... 

In 1964, the spin echo experiment was extended to the optical regime by the development of the photon echo experiment (3,4). The photon echo began the application of coherent pulse techniques in the visible and ultraviolet portions of the electromagnetic spectrum. Since its development, the photon echo and related pulse sequences have been applied to a wide variety of problems including dynamics and intermolecular interactions in crystals, glasses, proteins, and liquids (5-8). Like the spin echo, the photon echo and other optical coherent pulse sequences provide information that is not available from absorption or fluorescence spectroscopies. 

In 1993, the first ultrafast vibrational echo experiments on condensed matter systems were performed using a free electron laser as the source of temporally short, tunable infrared pulses (11). Recently, the development of Ti sapphire laser-based optical parametric amplifier (OPA) systems has made it possible to produce the necessary pulses to perform vibrational echoes using a tabletop experimental system (12,13). The development and application of ultrafast, IR vibrational echoes and other IR coherent pulse sequences are providing a new approach to the study of the mechanical states of molecules in complex molecular systems such as liquids, glasses, and proteins (14-20). While the spin echo, the photon echo, and the vibrational echo are, in many respects, the same type of experiment, the term vibrational echo is used to distinguish IR experiments on vibrations from radio frequency experiments on spins or vis/UV experiments on electronic states. In this chapter, recent vibrational echo experiments on liquids, glasses, and proteins will be described. 

Background suppression in VES is in some respects analogous to NMR background suppression techniques (56,57). In both types of spectroscopy, coherent pulses sequences are used to remove unwanted spectral features. 

Figure 1. Gradient-enhanced heteronuclear single quantum coherence pulse sequence with coherence transfer selection and artifact suppression gradients. All pulses are of phase x unless otherwise indicated.

COHERENT PULSE SEQUENCE CONTROL OF PRODUCT FORMATION IN CHEMICAL... 

V. Coherent Pulse Sequence Control of Product Formation in Chemical Reactions... 

The designs of the previously mentioned selectivity schemes ignore the possibility of control of the evolution of excitation energy via exploitation of the coherence properties of the coupled matter-electromagnetic field system. Several schemes that do exploit the coherence of the time evolution of a wavepacket excitation have recently been proposed. This chapter is concerned with one of these schemes, namely, the use of coherent pulse sequences to control product formation in chemical reactions. We shall see that this scheme follows naturally from an understanding of the characteristics of time-delayed coherent anti-Stokes Raman spectroscopy (CARS) and of photon echo spectroscopy. 

An additional result that emerges from our study concerns the extent to which wavepacket control is possible using coherent pulse sequences. In a two-level system one can exchange the phases of the two levels with a 7t pulse and, in effect, achieve time reversal of the state of the system. In a multilevel system the extent of control is much more restricted. The center of the wavepacket evolves according to the Franck-Condon principle and Hamilton s equations of motion, which in turn are dictated by nature s potential energy surfaces. What can be controlled by the experimenter is the instant at which the wavepacket changes surfaces. This concept forms the basis for a scheme for controlling the selectivity of a reaction,24,25 which we discuss in the next section. 

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