Quantum Control of Gas‐Phase and Liquid‐Phase Femtochemistry

Abstract

Active control of chemical reactions on a microscopic (molecular) level, that is, the selective breaking or making of chemical bonds, is an old dream. However, conventional control agents used in chemical synthesis are macroscopic variables such as temperature, pressure or concentration, which gives no direct access to the quantum-mechanical reaction pathway. In quantum control, by contrast, molecular dynamics are guided with specifically designed light fields. Thus it is possible to efficiently and selectively reach user-defined reaction channels. In the last years, experimental techniques were developed by which many breakthroughs in this field were achieved. Femtosecond laser pulses are manipulated in so-called pulse shapers to generate electric field profiles which are specifically adapted to a given quantum system and control objective. The search for optimal fields is guided by an automated learning loop, which employs direct feedback from experimental output. Thereby quantum control over gas-phase as well as liquid-phase femtochemical processes has become possible. In this review, we first discuss the theoretical and experimental background for many of the recent experiments treated in the literature. Examples from our own research are then used to illustrate several fundamental and practical aspects in gas-phase as well as liquid-phase quantum control. Some additional technological applications and developments are also described, such as the automated optimization of the output from commercial femtosecond laser systems, or the control over the polarization state of light on an ultrashort timescale. The increasing number of successful implementations of adaptive learning techniques points at the great versatility of computer-guided optimization methods. The general approach to active control of light-matter interaction has also applications in many other areas of modern physics and related disciplines.

Electronic structure in real time: mapping valence electron rearrangements during chemical reactions

“…Since these original studies, the field of femtochemistry has rapidly evolved and it is not the aim of this perspective to review this development. Rather, the reader is referred to a number of excellent articles, reviews and books that have appeared over the years (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15).…”

Strong field quantum control by selective population of dressed states

“…An alternative approach based on the use of the combination of pulse shaping techniques [12] with adaptive feedback learning loops (closed loop) was suggested [13] for the case when the underlying potential surfaces are unknown. Implementations of this technique demonstrate the optimization of almost any conceivable physical quantity [14][15][16][17][18][19][20][21][22][23][24][25][26]. However, it is not clear whether this methodology is suitable to extract the underlying physical mechanism from the electrical fields obtained during the optimization process.…”

Adaptive Femtosecond Quantum Control

“…The main experimental tool for achieving this goal has been spectral phase (or phase-and-amplitude) shaping of femtosecond laser pulses [85,127]. This 'conventional' approach to pulse shaping addresses only the scalar properties of laser pulses, i.e., momentary frequency, phase and intensity.…”

“…Adaptive femtosecond quantum control has proven to be a very powerful tool to control a variety of chemical reactions and physical processes [15,45,49,52,53,56,59,127,159]. However, in many cases it is quite difficult to extract the control mechanism(s) utilized by the optimal pulse shape obtained in the optimal control experiment, a problem already encountered in chapter 4.…”

“…This approach has proven to be an excellent method for reaching specific control objectives [15,45,49,52,53,56,59,127,159]. However, it is usually difficult to gain insight into the reaction mechanisms at play or to extract information on the interaction of the investigated system with electromagnetic fields from the optimized field shapes.…”