Surface femtochemistry: Frustrated desorption of alkali atoms from noble metals

The electronic excitation of chemisorbed Cs atoms through the coherent phot oinduced charge transfer from the Cu(111) surface turns on repulsive forces that initiate the dissociative wave packet motion. Due to unusually slow e lectronic relaxation, with the interferometric time-resolved two-photon pho toemission technique it is possible to follow the characteristic change in the surface electronic structure due to the ensuing nuclear dynamics for si milar to 200 fs after excitation. This direct observation of the incipient bond breaking process on a metal surface provides information on a series o f complex electronic and nuclear events that previously could be gleaned on ly through analysis of the reaction products. The metal band structure, the polarizability of the Cs atom, and the mechanical properties of the surfac e strongly influence the electronic and nuclear relaxation, and therefore, the reaction efficiency. The importance of time-resolved methods in studyin g of surface reactions is underscored by significant deviations in the elec tronic and nuclear dynamics of Cs/Cu(111) from the standard models for surf ace photochemistry.