The “Hydrogen-Subway”A Tunneling Approach to Intramolecular Hydrogen Transfer Reactions Controlled by Ultrashort Laser Pulses

Isomerization reactions from a reactant state to a near-degenerate product state which are typical for intramolecular hydrogen transfer may be achieved by means of ultrashort picosecond or sub-picosecond laser pulses having three consecutive time domains. Initially, the laser field is switched on such that the wave packet representing the reactant is converted into a superposition of near-degenerate delocalized states of the “dressed” molecule, with level spacing Δ E ε . In the second stage, the laser field ε is kept approximately constant until the wave packet has tunneled from the reactant to the product configuration, during the tunneling time τ ε = h /2Δ E ε . Finally the field is switched off in such a way that the wave packet is stabilized as the target product state. This approach is suggested by optimal control theory and applied to a one-dimensional model resembling substituted malonaldehydes. Here it is found that the laser-driven tunneling requires time-integrated laser field intensities fifty times smaller than the alternative pump−dump approach which drives the reactant wave packet over the potential barrier toward the product state without tunneling. Various applications and extensions of this method are discussed.