Laser‐driven Ultrafast Hydrogen Transfer Dynamics

Abstract

The ground state tunnel splitting in a symmetric double well potential, D 0 , is associated with a tunneling time of s 0 ¼ h=2D 0 . If we take the hydrogen transfer (HT) in tropolone [1] as an example we have D 0 » 1 cm -1 and therefore s 0 » 16:7 ps. This is a rather long time as compared, e.g. with the period of an OH-stretch vibration or with the T 1 relaxation time of this vibration in solution [2]. Consequently, to study ultrafast HT in the electronic ground state the dynamics has to be initiated closer to or even above the reaction barrier what could be achieved by an interaction with an IR laser pulse. Of course, this draws on the analogy with electron-vibrational dynamics triggered after almost instantaneous optical excitation and thus switching of the electronic state. Here the driving force for the nuclear wave packet motion after excitation is due to the difference in the electron density which leads, for instance, to the rather rapid reactions observed for excited state HT [3][4][5].Building further on this analogy one could think of adiabatically separating the fast OH-stretch dynamics from the slower motions of the molecular frame. This would result in adiabatic potential energy surfaces (PES) for the slow modes in a given state of the OH-stretch mode [6][7][8]. The HT can then be viewed as a slow mode wave packet moving on the adiabatic potential whose character changes gradually from an OH-stretch excitation localized in the reactant well to an excitation localized in the product well.The first question, which is raised by this simple analogy, concerns the very possibility of exciting slow mode wave packets in a hydrogen bond at all. Taking a different perspective it touches the very issue of interpretation of the notoriously complex IR spectra [9]. In the condensed phase much of this complexity is hidden under bands broadened by the solvent interaction. Hence it was only recently that coherent wave packet motion of a 100 cm -1 hydrogen bond mode could be observed after OH-stretch excitation, although in a system which has only a single minimum potential [10]. Meanwhile coherent low-frequency dynamics has also been observed in a double minimum system (acetic acid dimer) [11]. With this Hydrogen-Transfer Reactions. Edited by

Photochemistry and Photophysics in Silica-Based Materials: Ultrafast and Single Molecule Spectroscopy Observation

“…The earliest studies of proton-transfer dyes mainly used fluorescence spectroscopy because the photoproduced K* (or Z*) generally emits with a large Stokes shift (6000–10000 cm –1 ). Advances in laser spectroscopy gave details on PES’s reactions at the femtosecond time scale in gas and condensed phases. ,,,,,,,− Coupled proton and electron motions in the molecular system, of great importance in many chemical and biological systems, have been experimentally and theoretically examined. ,− The experimental and theoretical findings not only generate new knowledge but also open new research fields with possible applications in sensing, catalysis, drug delivery, photovoltaics, and photonics (section ).…”

Infrared Laser Excitation Controlled Reaction Acceleration in the Electronic Ground State

“…IR laser control of unimolecular reaction dynamics has attracted particular attention from theory (for a recent account, see also ref ). Isomerization reactions, such as the Cope rearrangement in a semibullvalene model or various H atom transfer reactions, were studied as exemplary cases within the development of general control strategies . A common assumption has been that it is possible to excite the RC directly, i.e., without much influence of anharmonic couplings to other coordinates leading to broad absorption bands or competing VER processes.…”