Quantum molecular dynamics and control

What happens to a molecule after it has absorbed a photon? Can we influence how a molecule behaves after it has absorbed the photon? These questions have motivated a great deal of research for more than half a century and continue to do so.

During the last couple of decades, huge advances in experimental technology (e.g. femtosecond lasers, nonlinear optics, optical pulse shaping and molecular imaging) and computational methodology (e.g. multicore processors, parallelization, novel architectures and algorithms) have created the capability for studying the structure and dynamics of photoexcited molecules in unprecedented detail and have developed the potential to synthesise light fields to control the dynamics.

Understanding and controlling the dynamics of electronically excited molecules at the molecular level requires a range of tools: spectroscopy and quantum chemistry calculations to determine molecular wave functions, femtosecond pump–probe experiments and quantum dynamics calculations to monitor evolving wave functions, and control theory to determine how to influence the time-evolution. This themed issue provides an excellent illustration of the broad array of cutting-edge research methods that are available.

In his perspective article, Worth provides an excellent overview of the current capabilities of, and recent developments in quantum dynamics and control simulations (DOI: 10.1039/c0cp01740j). High-resolution spectroscopic measurements provide details of molecular structure and the wave functions of the products of photodissociation and photoionisation. For example, Drabbels and coworkers (DOI: 10.1039/c0cp00746c) employ absorption spectroscopy in helium nanodroplets to elucidate the spectroscopy and dynamics of a family of molecules based on the nucleobase adenine, Merkt (DOI: 10.1039/c0cp00191k) exploits the polarisation effects originating from double-resonance, three-photon ionisation spectroscopy to detect the Coriolis effect in the 3p Rydberg state of formaldehyde, Shapiro (DOI: 10.1039/c0cp01417f) is able to extract the phase and amplitude of evolving wave packets from fluoresecence line intensities, and Zare (DOI: 10.1039/c0cp00518e) measures the time-dependent depolarisation of aligned deuterium molecules caused by hyperfine coupling. Femtosecond pump–probe experiments and quantum chemistry calculations provide insight into the dynamics and fate of electronically excited molecules in the gas-phase and in solution. For example, Blanchet and coworkers (DOI: 10.1039/c004220j) employ time-resolved photoelectron imaging to investigate the photoionisation and photodissociation dynamics of the 6s Rydberg state of methyl iodide, Gilch and coworkers (DOI: 10.1039/c004025h) use a combination of femtosecond transient absorption spectroscopy and quantum chemistry calculations to unravel the phototautomerisation dynamics of ortho-nitrotoluene, and Marian (DOI: 10.1039/c0cp00106f) applies quantum chemical methods to an investigation of ultrafast triplet formation in 6-azauracil. Quantum dynamics calculations and control theory demonstrate the potential of controlling molecular dynamics using tailored light fields. For example, de Vivie-Riedle (DOI: 10.1039/c0cp01657h) uses optimal control theory to design shaped optical laser pulses for the chemoselective quantum control of carbonyl bonds in Grignard reactions.

In summary, this special issue exemplifies the diverse range of cutting-edge experimental and theoretical techniques that are currently available for unravelling the fine details of the dynamics of photoexcited molecules and learning how to control them. Armed with this toolkit, the stage is now set for exciting new applications to both fundamental and applied problems.

We would like to thank all the authors who were kind enough to contribute to this themed issue and RSC publishing staff for their help in putting it together.

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