This anthology, which is adapted from the Nobel Lecture, gives an overview of the field of Femtochemistry from a personal perspective, encompassing our research at Caltech and focusing on the evolution of techniques, concepts, and new discoveries. In developing femtochemistrythe study of molecular motions in the ephemeral transition states of physical, chemical, and biological changeswe have harnessed the powerful concept of molecular coherence and developed ultrafast-laser techniques for observing these motions. Femtosecond resolution (1 fs = 10-15 s) is the ultimate achievement for studies of the dynamics of the chemical bond at the atomic level. On this time scale, matter wave packets (particle-type) can be created and their coherent evolution as a single-molecule trajectory can be observed. The field began with simple systems of a few atoms and has reached the realm of the very complex in isolated, mesoscopic, and condensed phases and in biological systems such as proteins and DNA. It also offers new ...

Femtochemistry: Atomic-Scale Dynamics of the Chemical Bond†

Organic semiconductor lasers.

Electron Transfer Past and Future (R Marcus) Electron Transfer Reactions in Solution: A Historical Perspective (N Sutin) Electron Transfer--From Isolated Molecules to Biomolecules (M Bixon & J Jortner) Charge Transfer in Bichromophoric Molecules in the Gas Phase (D Levy) Long--Range Charge Separation in Solvent--Free Donor--Bridge--Acceptor Systems (B Wegewijs & J Verhoeven) Electron Transfer and Charge Separation in Clusters (C Dessent, et al) Control of Electron Transfer Kinetics: Models for Medium Reorganization and Donor--Acceptor Coupling (M Newton) Theories of Structure--Function Relationships for Bridge--Mediated Electron Transfer Reactions (S Skourtis & D Beratan) Fluctuations and Coherence in Long--Range and Multicenter Electron Transfer (G Iversen, et al) Lanczos Algorithm for Electron Transfer Rates in Solvents with Complex Spectral Densities (A Okada, et al) Spectroscopic Determination of Electron Transfer Barriers and Rate Constants (K Omberg, et al) Photoinduced Electron Transfer Within Donor--Spacer--Acceptor Molecular Assemblies Studied by Time--Resolved Microwave Conductivity (J Warman, et al) From Close Contact to Long--Range Intramolecular Electron Transfer (J Verhoeven) Photoinduced Electron Transfers Through sigma Bonds in Solution (N--C Yang, et al) Indexes

Electron transfer : from isolated molecules to biomolecules

Ultrafast vibrational dynamics of hydrogen bonds in the condensed phase.

Multicolour near-infrared femtosecond experiments of a genetically modified bacterial reaction centre show that the phase of two excited-state vibrational modes is conserved for a period of picoseconds over a large range of temperatures. The direct visualization of low-frequency nuclear vibrations in this protein, which is embedded in its natural membrane, implicates coherent nuclear motion in the primary electron transfer reaction in functional reaction centres.

Visualization of coherent nuclear motion in a membrane protein by femtosecond spectroscopy

http://authors.library.caltech.edu/43172/1/1-s2.0-S0009261413010750-main.pdf

Reprint of: Femtosecond transition-state spectroscopy of iodine: From strongly bound to repulsive surface dynamics

Ultrafast molecular vibrations and rotations are the fundamental motions that characterize chemical bonding and determine reaction dynamics at the molecular level. The timescales for these motions are typically 10^(−10) s for vibrations and 10^(−13) s for rotations. For decades, time-integrated (frequency-resolved) spectros-copy has provided a powerful tool for probing the dynamics of motion, but the motions themselves are not 'seen' directly in real-time. With femtosecond laser techniques it is now possible to follow the motions of isolated molecular systems as they occur. The requirement is that the system is excited (for vibration) and aligned (for rotation) on a timescale shorter than the vibrational and rotational periods. Here we report real-time observations of these molecular motions. The system—in this case, molecular iodine—is prepared in the particular state(s) of interest by coherent excitation with an initial femtosecond laser pulse, and the subsequent motions are probed with successive femtosecond pulses. The probe monitors changes in the interatomic distance (vibration) or molecular orientation (rotation), so that the measured signal provides direct 'snapshots' of the molecular motions.

Femtosecond laser observations of molecular vibration and rotation

The dissociation reaction of HgI2 is examined experimentally using femtosecond transition-state spectroscopy (FTS). The reaction involves symmetric and antisymmetric coordinates and the transition-state is well-defined: IHgI*-->[IHgI] [double-dagger]@B| Q[sub S[script ']]Q[sub a[script ']]q -->HgI+I. FTS is developed for this class of ABA-type reactions and recurrences are observed for the vibrating fragments (symmetric coordinate) along the reaction coordinate (antisymmetric coordinate). The translational motion is also observed as a "delay time" of the free fragments. Analysis of our FTS results indicates that the reaction wave packet proceeds through two pathways, yielding either I(2P3/2) or I*(2P1/2) as one of the final products. Dissociation into these two pathways leads to HgI fragments with different vibrational energy, resulting in distinct trajectories. Hence, oscillatory behaviors of different periods in the FTS transients are observed depending on the channel probed (~300 fs to ~1 ps). These results are analyzed using the standard FTS description, and by classical trajectory calculations performed on model potentials which include the two degrees of freedom of the reaction. Quantum calculations of the expected fluorescence of the fragment are also performed and are in excellent agreement with experiments.

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R.M. Bowman

Papers

Reprint of: Femtosecond transition-state spectroscopy of iodine: From strongly bound to repulsive surface dynamics

Femtosecond laser observations of molecular vibration and rotation

Femtosecond real-time probing of reactions. V. The reaction of IHgI

Femtosecond temporal spectroscopy and direct inversion to the potential: Application to iodine

Femtosecond selective control of wave packet population