Two-photon photoemission spectroscopy

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

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. 

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

Fauster, T. and Steinmann, W. (1995) Two-photon photoemission spectroscopy of image states, in Electromagnetic Waves Photonic Probes of Surfaces vol. 2, (ed. P. Halevi), North-Holland, Amsterdam, p. 347-411. 

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... 

Zhou C, MaZ, Ren Z (2012) Surface photochemistry probed by two-photon photoemission spectroscopy. Energy Environ Sci 5 6833-6844... 

Two-photon photoemission can be viewed as regular (one-photon) photoemission from a state after excitation of the surface by another photon. All the well-known concepts of photoemission can be apphed to two-photon photoemission energy, spin, and (parallel) momentum conservation for the emitted electron as well as dipole selection rales for optical transitions. The main realm of two-photon photoemission is the spectroscopy of excited intermediate states with energies above the Fermi level, which are normally unoccupied. This energy range, in particular, the part below the vacuum level, is otherwise accessible only by inverse photoemission. 

The spectroscopy or energy-resolved mode of two-photon photoemission discussed so far is performed at constant time delay between the two laser pulses, and the kinetic energy is scanned. Usually, the maximum intensity is obtained at optimum... 

In a time-resolved two-photon-photoemission experiment, the pulse duration can be comparable to the time scale of the temporal quantum-mechanical evolution of the involved states. The lifetime can be measured in the time domain, if it is long compared to the pulse duration. In the energy domain, the intrinsic Unewidth would then be narrow compared to the spectral bandwidth of the laser pulses and could not be resolved. Similarly, states with an energy difference smaller than the spectral bandwidth caimot be separated in spectroscopy, but may be resolved in the time domain. 

The photoemission techniques (PE fig. 3a,b,c,) are based on the observation of the energy and intensity distribution of the electrons emitted by a sample irradiated by a monochromatic photon source. The spectra are then interpreted in terms of electronic transitions resulting from the annihilation of these photons of known energy. The photoemission methods can be devided into two broad categories defined by the photon energy range ultraviolet photoemission (UPS) for hv < lOOeV and X-ray photoemission spectroscopy (XPS) for hv 1200-1500eV. 

Two-photon time-resolved photoemission (TPTRP) spectroscopy has been developed to directly study the dynamics of optically excited electrons at metal and semiconductor surfaces. This technique has been applied to direct measurement of hot electron relaxation in noble and transition metals [27, 28], surface-state dynamics on clean and adsorbate-covered metal surfaces [29, 30], as well as charge carrier dynamics in semiconductors, where much work has been performed. 

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