Two-photon photoemission 2PPE

STS),as described in detail in Chapter 3.5 (see also Refs 25-27), provide detailed information on the decay properties of surface states. The topographical images monitor simultaneously the quality and detailed structure of the surface area under investigation. Two-photon photoemission (2PPE), as described in detail in Chapter 3.2.4 and in Refs 28, 29, in the time-resolved mode is the only technique that is able to study the decay in the time domain [30, 31], By combining this information with spectroscopic measurements, a very detailed picture of the electron dynamics emerges [1, 32],... 

The surface electronic states are typically probed with PES and I-PES (photoemission measures occupied states inverse photoemission, unoccupied states) described in Section 7.3.1, or for more accuracy with two-photon photoemission (2PPE), which is particularly suitable for analyzing the image potential states. In the 2PPE technique, a first laser hght pulse (pump) excites an electron from its initial state below the Fermi level Ep into an unoccupied intermediate state (of Shockley or image potential type). A second pulse (probe) lifts the electron to the final state above the vacuum level E c. so it can be detected as a photoelectron. One of the possible data acquisition modes is to record energy-resolved spectra at a particular delay between pump and probe pulses [51]. 

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. 

Peteket al. [148, 149] provided some indirect evidence that part of CH3OH was adsorbed at Tisc sites dissociatively. Using two-photon photoemission spectroscopy (2PPE), an empty wet electron state at about 2.3 0.2 eV above Fermi level (Ep) was detected on both reduced and stoichiometric rutile TiO2(110). However, in the case of H2O, this excited state could only be observed on reduced rutile Ti02(l 10) surfaces with simultaneous presence of monolayer water and bridging hydroxyls... 

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