Femtosecond studies of electron tunneling at metal-dielectric interfaces Original Research Article

Femtosecond time-resolved two-photon photoemission spectrocopy (TPPE) has been used to measure the lifetimes of image potential electrons at alkane mono- and bilayers on Ag(111). The n = 1 lifetimes for zero, one, and two layers of n-heptane on Ag(111) are 32 ± 10 fs, 155 ± 20 fs, and 1580 ± 200 fs, respectively. This approximately exponential increase in lifetime is consistent with a tunneling picture in which the adlayer forms a barrier that slows the decay of an image potential electron back into the metal. The existence of the tunneling barrier is consistent with the repulsive electron affinity of the longer chain n-alkanes in the condensed phase. The lifetimes of the higher quantum states indicate that the presence of the monolayer significantly reduces coupling of the image states to the bulk band structure, so that further changes in lifetime are determined by the adlayer barrier and an attempt rate related to the classical oscillation time in the modified image potential well. These results are compared with quantitative predictions of a model by Cole which considers the tunneling barrier presented by the layer and the effect of the layer on the attempt rate. These results are considered in the context of previous TPPE studies of metal-insulator interfaces.