Laser pulses, quantum dynamics techniques

The purpose of this work is to demonstrate that the techniques of quantum control, which were developed originally to study atoms and molecules, can be applied to the solid state. Previous work considered a simple example, the asymmetric double quantum well (ADQW). Results for this system showed that both the wave paeket dynamics and the THz emission can be controlled with simple, experimentally feasible laser pulses. This work extends the previous results to superlattices and chirped superlattices. These systems are considerably more complicated, because their dynamic phase space is much larger. They also have potential applications as solid-state devices, such as ultrafast switches or detectors. 

This chapter discusses the apphcation of femtosecond lasers to the study of the dynamics of molecular motion, and attempts to portray how a synergic combination of theory and experiment enables the interaction of matter with extremely short bursts of light, and the ultrafast processes that subsequently occur, to be understood in terms of fundamental quantum theory. This is illustrated through consideration of a hierarchy of laser-induced events in molecules in the gas phase and in clusters. A speculative conclusion forecasts developments in new laser techniques, highlighting how the exploitation of ever shorter laser pulses would permit the study and possible manipulation of the nuclear and electronic dynamics in molecules. 

Controlling atomic and molecular quantum states with the help of laser light has been an intensively studied field in the last years (see, e.g., [Tan-nor 1985 Judson 1992 Rice 2000 Shapiro 2003]). The ability to produce and shape femtosecond laser pulses [Zewail 1994 Manz 1995] made it possible to create specially tailored wave packets and to manipulate their dynamics in order to reach pre-assigned goals. The techniques of quantum control have been applied to the problem of steering chemical reactions [Tannor 1985 Assion 1998 Shapiro 2003] as well as intramolecular dynamics [Aver-bukh 1993 Goodson 2000],... 

This need for quantum mechanical techniques also arises in the connection between dynamics simulation and experiment when the detailed nature of light-matter interaction is important. An example is the initiation and probing of solution dynamics by ultrafast light pulses, in which the detailed time-frequency nature of the light interacts with the detailed time-frequency nature of the solution, and quantum aspects can become important. 1 At this level, the quantum dynamics of how the excitation and probe laser pulses interact with the sample must be considered in addition to all the other dynamics of the reaction process,... 

The events taking place in the RCs within the timescale of ps and sub-ps ranges usually involve vibrational relaxation, internal conversion, and photo-induced electron and energy transfers. It is important to note that in order to observe such ultrafast processes, ultrashort pulse laser spectroscopic techniques are often employed. In such cases, from the uncertainty principle AEAt Ti/2, one can see that a number of states can be coherently (or simultaneously) excited. In this case, the observed time-resolved spectra contain the information of the dynamics of both populations and coherences (or phases) of the system. Due to the dynamical contribution of coherences, the quantum beat is often observed in the fs time-resolved experiments. 

This chapter is no more than a cursory review of a few experimental methods used in chemical dynamics. It is apparent that most of these techniques have been developed only in the past decade, and that new methods continue to be invented. Most of the new methods have been made possible by the development of pulsed lasers and pulsed molecular beams. Together, they permit the state selection of molecules, the measurement of rates down to femtoseconds, and the analysis of product state distributions down to single quantum state level. Only a decade ago, the state of the art for measuring unimolecular reactions for ions and neutrals was about equal, and the amount of data for ionic systems far surpassed that for neutral reactions. However, the... 

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