Coherent laser control of molecules

and it was the achievements in this field that led to the development of coherent laser control of molecules. 

Let us emphasize that noncoherent laser control of molecules depends on differences in the absorption spectrum between different molecular species, for example isotopic molecules. Coherent laser control is distinguished by its nontrivial manipulation of the phase coherence of excited states in molecules that can be similar in their absorption spectra but different in their phase coherence properties (Brixner et al. 20016). 

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Coherent Laser Control of the Handedness of Chiral Molecules... 

The situation with molecules is much more involved for several reasons. First, for polyatomic molecules, the intramolecular relaxation processes that occur on a subpicosecond timescale are essential. It was for exactly this reason that the first successful experiments were conducted on the noncoherent laser control of polyatomic molecules with intermolecular selectivity. Second, the phase relaxation time in a condensed medium is also on a subpicosecond scale because of the interaction between the quantum system and its surroundings. Therefore, it was only the creation of relatively simple and inexpensive femtosecond lasers that made it possible to set about realizing the ideas of the coherent laser control of unimolecular processes (Tannor and Rice 1985 Brumer and Shapiro 1986 Judson and Rabitz 1992), particularly the... 

Figure 1. Diagram of the intensity / (W/cm2) vs. duration of laser pulse tp(s) with various regimes of interaction of the laser pulse with a condensed medium being indicated very qualitatively. At high-intensity and high-energy fluence 4> = rpI optical damage of the medium occurs. Coherent interaction takes place for subpicosecond pulses with tp < Ti, tivr. For low-eneigy fluence (4> < 0.001 J/cm2) the efficiency of laser excitation of molecules is very low (linear interaction range). As a result the experimental window for coherent control occupies the restricted area of this approximate diagram with flexible border lines.

The results presented in this chapter show that the use of proper effective models, in combination with calculations based on the exact vibrational Hamiltonian, constitutes a promising approach to study the laser driven vibrational dynamics of polyatomic molecules. In this context, the MCTDH method is an invaluable tool as it allows to compute the laser driven dynamics of polyatomic molecules with a high accuracy. However, our models still contain simplifications that prevent a direct comparison of our results with potential experiments. First, the rotational motion of the molecule was not explicitly described in the present work. The inclusion of the rotation in the description of the dynamics of the molecule is expected to be important in several ways. First, even at low energies, the inclusion of the rotational structure would result in a more complicated system with different selection rules. In addition, the orientation of the molecule with respect to the laser field polarization would make the control less efficient because of the rotational averaging of the laser-molecule interaction and the possible existence of competing processes. On the other hand, the combination of the laser control of the molecular alignment/orientation with the vibrational control proposed in this work could allow for a more complete control of the dynamics of the molecule. A second simplification of our models concerns the initial state chosen for the simulations. We have considered a molecule in a localized coherent superposition of vibrational eigenstates but we have not studied the preparation of this state. We note here that a control scheme for the localiza-... 

Vala J, Duheu O, Masnou-Seeuws F, FiUet P, Kosloff R. (2000) Coher-ent control of cold-molecule formation through photoassociation using a chirped-pulsed-laser field. Phys. Rev. A 63 013412-1-12. 

One more trend in laser control is based on the use of the property of coherence of the laser light. To effect coherent laser control, it is necessary that not only the light, but also the atom (or molecule) should be in a coherent state during the interaction. For atoms in a beam or in a low-pressure gas, the phase relaxation time of their wave functions depends on spontaneous decay or on collisions and can be comparatively long (from 10 to 10 s). It was for precisely this reason that the main experiments on coherent interaction were conducted with atoms. These experiments led in the final analysis to the discovery of new effects, such as coherent population trapping (Arimondo 1996), electromagnetically induced transparency (Harris 1997), and the slow-light effect (Hau et al. 1999 Kash et al. 1999). 

Fig. 1.5 Main methods for the laser noncoherent and coherent control of molecules by lasers (Chapters 9-12).

The goal of this book is to present in a coherent way the problems of the laser control of matter at the atomic-molecular level, namely, control of the velocity distribution of atoms and molecules (saturation Doppler-free spectroscopy) control of the absolute velocity of atoms (laser cooling) control of the orientation, position, and direction of motion of atoms (laser trapping of atoms, and atom optics) control of the coherent behavior of ultracold (quantum) gases laser-induced photoassociation of cold atoms, photoselective ionization of atoms photoselective multiphoton dissociation of simple and polyatomic molecules (vibrationally or electronically excited) multiphoton photoionization and mass spectrometry of molecules and femtosecond coherent control of the photoionization of atoms and photodissociation of molecules. 

The laser control of atoms and molecules is most effective when the particles are isolated from external influences. The necessary degree of isolation differs widely between different laser control methods. For coherent control techniques, even an accidental displacement of the phase of the wave function must be excluded. For incoherent laser control techniques, it is sufficient to exclude relaxation of the quantum-state populations of the atomic particles. One should therefore consider first the interaction of an isolated simple (two-level) quantum system with a coherent light field (Fig. 2.4(a)). 

Fig. 12.1 Three waves of evolution of laser noncoherent and coherent control of molecules over a quarter of a century from nanosecond to femtosecond laser pulses. The next wave is the development of attophysics. (Modified from Letokhov 1997a.) Integrated activity (funding, number of experiments, and number of publications) in arbitary units.

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... 

Fig. 12.8 Diagram of radiation intensity I (W/cm ) versus laser pulse duration Tp (s), with the various laser-pulse-condensed-medium interaction regimes being indicated very qualitatively. At high radiation intensities I and energy fluences

A. D. Bandrauk, Y. Fujimura, and R. J. Gordon (eds.), Laser Control and Manipulation of Molecules, American Chemical Society Oxford University Press, New York 2002 W. Potz and A. Schroeder (eds.), Coherent Control in Atoms, Molecules, and Semiconductors, Kluwer Academic Publishers, Dordrecht, 1999. 

Figure 6.2 Steering of photochemical reactions by coherent control of ultrafast electron dynamics in molecules by shaped femtosecond laser pulses. Ultrafast excitation of electronic target states in molecules launches distinct nuclear dynamics, which eventually lead to specific outcomes of the photochemical reaction. The ability to switch efficiently between different electronic target channels, optimally achieved by turning only a single control knob on the control field, provides an enhanced flexibility in the triggering of photochemical events, such as fragmentation, excited state vibration, and isomerization.

Looking ahead, coherent laser pulses covering the complete spectral range of valence bond excitation from the UV to the IR spectral region are becoming available (see, e.g., [119]), and we expect SPODS to increase in importance in coherently controlled photochemistry with applications ranging from reaction control within molecules up to discrimination between different molecules in a mixture and laser-based quantum information technologies. 

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