Transient Exciton at Ag(111) Surface

We investigate the surface states on Ag(111) by means of multi-photon photoemission using ultrashort laser pulses. The angle-resolved photoemission spectra at the non-resonant range are consistent with the well-known structures of Shockley states and image potential states. But when we tune the wavelength to the resonant range by two photon, the spectra is dominated by a non-dispersive feature, which should correspond to a localized state, and we assign it to transient exciton. Then we do time-resolved measurements and take Fourier Transformation with respect to the delay-axis. The dominant response of the Ag(111) sample is the driving frequency, which is unexpected because there is no one-photon resonant transition in the excitation scheme.