Two-photon photoemission

Femtosecond lasers represent the state-of-the-art in laser teclmology. These lasers can have pulse widths of the order of 100 fm s. This is the same time scale as many processes that occur on surfaces, such as desorption or diffusion. Thus, femtosecond lasers can be used to directly measure surface dynamics tlirough teclmiques such as two-photon photoemission [85]. Femtochemistry occurs when the laser imparts energy over an extremely short time period so as to directly induce a surface chemical reaction [86]. 

Petek H and Ogawa S 1997 Femtosecond time-resolved two-photon photoemission studies of electron dynamics in metals Prog. Surf. Sc/. 56 239... 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

Heterogeneous electron transfer probed with femtosecond two-photon photoemission spectroscopy... 

The electron dynamics in metals have been investigated with femtosecond time resolution in two photon photoemission experiments [3,16,17], where the idea that the signal can be described by a double convolution of an exponential decay with the laser pulse shape is commonly... 

Evers F, Rakete C, Watanabe K, Menzel D, Freund H-J. Two-photon photoemission from silver nanoparticles on thin alumina films role of plasmon excitation. Surf Sci. 2005 593 43-8. 

Andreyev O, Koroteev YM, Snchez Albaneda M et al (2006) Spin-resolved two-photon photoemission study of the surface resonance state on Co/Cu(001). Phys Rev B 74 195416... 

Time-Resolved Two-Photon Photoemission with Femtosecond Laser Pulses... 

Two-photon photoemission spectroscopy is known for its capability to reveal not only occupied but also unoccupied electronic density of states [10]. In this scheme, one photon excites an electron below the Fermi level to an intermediate state. A second photon then excites the electron from the intermediate state to a final state above the vacuum le vel. The photoelectron yields are strongly enhanced if the excitation photon energy is tuned to the resonance conditions, and the photoelectron spectrum reflects the electron lifetime in the intermediate states as well as their density of states. It is necessary to keep the employed photon energy below the work function of the sample, otherwise one photon photoemission signal becomes excessive and buries the 2PPE signals. 

Fig. 10.2 Scheme of two-photon photoemission. Hot electron dynamics are measured by inducing photoemission of hot electrons with a time-delayed probe pulse... 

Ogawa S, Petek H (1996) Two-photon photoemission spectroscopy at clean and oxidized Cu(llO) and Cu(lOO) surfaces. Surf Sci 363 313-320... 

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