quantised peaks is based upon the observation of such spectra in multiphoton ionisation of atomic species in the gas-phase (e.g. Agostini & Petite (1988)). In the condensed phase of matter studied here such an analogy may be too simplistic.
Electron emission from surfaces has been the subject of considerable research in the past decade. Electron scattering from monolayers of condensed gas upon a metallic substrate have shown that an additional interactive potential exists between the scattering electron and the substrate, the image potential. The presence of an image potential may have considerable influence upon excitation in the atomic/molecular monolayer {Palmer & Rous (1993)). For example, both the lifetime and excitation energy of scattering resonances are lowered by the presence of the image potential. Similarly, the greater attractive forces existing between constituent atoms/molecules in the condensed phase compared to gaseous- phase has been shown to lower the excitation energy of electronic transitions and distort the photoelectron spectra.
Thus, it would be surprising if a simple multiphoton analysis based upon gas- phased multiphoton ionisation reproduced data from a gas-covered surface. Electrons produced by multiphoton ionisation within the monolayer will be subject ^ to both the image potential and interactive potentials with neighbouring targets which will alter their kinetic energies and may lead to a widening of the observed kinetic energy distribution until all discrete structure is 'smeared out'. However, this is purely as at the present time no multiphoton io n isa tio n __ experiment within molecular/atomic monolayers have been performed.
Nevertheless, from the present data we cannot infer any discrete structure within the photoelectron spectra. This could be attributed to any analogy with an ionisation process being in error. The photoelectrons produced by a photoelectric effect arise from the fermi sea of electrons within the metals and can not be ascribed to any particular copper, gold or silver atom. Although the work function of any surface has been compared with the ionisation energy of an atom, in effect
With the different quanta of laser radiation simply penetrating to different depths of the fermi surface, the ejected photoelectron then 'scattering' from other electrons in the 'sea' prior to reaching the surface and thus losing any discrete photoelectron spectra.
Finally, photoelectric emission may be a simple tunnelling phenomena from which discrete structure is also unlikely.
Further experiments will be necessary before this problem is resolved As part of this process a preliminary experiment using 532nm radiation has been performed. Such radiation produces photons of twice the energy of the 1064nm used in the experiments described above thus doubling the separation between any structure in the photoelectron energy spectra and thence, perhaps, allowing such structure to be observed even if there is considerable broadening due to internal surface scattering and image potentials. Figures 4,14 and 4,15 show one data set collected over a long time period with alternating p- and s-polarised radiation. There is evidence of a reproducible double-peak structure with the lower peak at ^1.5eV, the higher at ^2.5eV but we ascribe the lower peak to the increased thermionic emission expected from the silver surface due to its lower reflectivity for green than infra-red radiation. This increased thermionic emission expected from all surfaces when using green radiation can only be overcome by using shorter pulsed radiation and this is currently unavailable.
190 -0 g s 0.2 0.0 6 8 ■2 0 2 4
Electron energy [eV]
{Figures 4.14 a & b} The measured electron energy distributions of ejected electrons from a silver target using the CHA. In (a) the electric field of the laser beam (hu=2.34eV) was polarised perpendicularly to the target surface, in {b}, parallel to the surface. The collection time for each spectrum was 7 hours. In both spectra, the main peak has an electron energy of 1.25eV. A second smaller peak is clearly visible at 2.75eV in {a} but is not observed in {b}.
0 2
I
. 1 0.4 0.2 0.0 6 8 2 4 ■2 0Electron energy [eV]
{Figures 4.15 a & b} The measured electron energy distributions of ejected electrons from a silver target using the CHA. The electric field of the laser beam (hu=2.34eV) was polarised perpendicularly to the target surface. The collection time for each spectrum was 10 hours. In {b} the laser pulse energy was increased by adjustment of the laser oscillator flashlamp voltage. This had the effect of extending the electron energy distribution and enhancing the high-energy peak.
0.8 g . 1 0.4
I
ts s 0.2 0.0 ■2 0 2 4 6 8192
43*1 Kinetic Energy Distribution O f Photo electrons
43*1 c Conclusions
The present experiments while clearly demonstrating the existence of the multiquantum photoelectric effect have, in contrast to the work of Luan et al
(1989), shown no evidence for discrete structure within the product kinetic energy distributions. Whether such discrete structure should be observed therefore remains uncertain and further experiments are necessary.