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 Delta E-epsilon. In the second stage, the laser field epsilon is kept approximately constant until the wave packet has tunneled from the reactant to the product configuration, during the tunneling time tau(epsilon) = h/2 Delta E-epsilon. 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.