Observation of above-threshold multiphoton photoelectric emission from image-potential surface states

Abstract

Subpicosecond laser pulses have been applied to investigate multiphoton photoemission from singlecrystal Pt(1 1 1) surfaces. Image-potential surface states on the Pt(111) surface give rise to sharp resonant enhancements in the multiphoton photoemission spectrum which allow for unambiguous identification of above-threshold peaks.The phenomenon of above-threshold ionization (ATI) has been extensively studied in atomic and molecular photoionization, but how the optical physics which leads to ATI (ponderomotive effect) influences the photoelectric effect from solids has received comparably little attention.Two studies of above-threshold photoemission (ATP) from polycrystalline metal surfaces have recently been reported with moderate' or poor energy resolution. In both experiments, ATP effects were identified via the detection of electrons with energy greater than the minimum multiphoton photoeff'ect energy, E;"=nohv -+, where no is the minimum number of photons of energy hv necessary to overcome the work function N. However, the spectacular series of peaks, separated by hv, characteristic of ATI from atoms was not observed at all in Ref. 2, and only suggested in the spectra reported in Ref. 1, due to poor energy resolution.In this experiment we study the intensity dependence of the multiphoton photoemission spectrum from the clean, single-crystal Pt(111) surface. The unique feature of this sample is that it is known to possess a special class of surface electronic states known as image-potential states. These states arise due to the fact that an electron outside a metal surface can be bound to its image charge, forming a Rydberg series of states converging on the vacuum level. These states have the character of a one-dimensional hydrogen atom with a potential in the z direction of V(z) =e /4z, hence a binding energy for the first excited state of approximately -, ', Ry=0.85 eV, where Ry is the Rydberg constant. Measured image-state binding energies for a variety of materials and crystalline orientations actually range ' from 0.4 to 1.0 eV due to the departure of the crystal dielectric constant from unity as well as the details of the electron wave-function phase shift upon reflection at the metal-surface-potential discontinuity.The binding energy of the n =1 (first excited) image state on Pt(111) has been reported as 0.63 eV.When present, image states have a dramatic influence on multiphoton photoemission spectra. They provide a very strong intermediate-state resonant enhancement, giving rise to exceptionally sharp features in the photoelectron spectrum, even when there are no sharp initial-state features below the metal Fermi level. This property has made it possible for us to clearly and unambiguously identify sharp peaks in the photoemission spectrum with energy greater than E;"asarising due to ATP. Furthermore, B dN dE (ar b. unit. s) 2.0 2.4 2.8 3.2 3.6 0 ]. 2 3 4 ENERGY(eV) FIG. I. Multiphoton photoemission spectrum from a Pt (111) single-crystal surface. The laser intensity was 15 GW/cm'. due to the extreme...

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