Two-photon photoelectron spectroscopy

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. 

Excitation of the surface is done by photons, and the dipole selection rales can also be apphed to the first step in two-photon photoemission. The experimental parameter space is expanded compared to regular photoemission by the option to choose different photon energies and polarizations for the two photons employed. For short photon pulses generated by femtosecond lasers, the time delay between the two photons is an additional experimental parameter that allows the time-resolved sampling of the population in the excited state. This last feature in particular is unique to two-photon photoemission and permits the detailed investigation of the electron dynamics at surfaces, which is the topic of Chapter 6. 

Note that there is no force needed to move the charge in the direction parallel to the surface. The factor 2 in the denominator of Eq. (3.2.4.1) arises because charge and image charge are separated by the distance 2z. 

Surface and Interface Science Concepts and Me0wds, First Edition. Edited by Klaus Wandelt. 

The one-dimensional potential is strictly valid only for electrons moving perpendicular to the surface, that is, with parallel momentum fe = 0 at the center of the surface Brillouin zone. It can be extended assuming a free-electron-Hke behavior for the motion parallel to the surface. The effect of the crystal potential might lead to an effective mass different from the free-electron mass. 

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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. 

Hudson BS and Kohler BE (1973) Polyene spectroscopy the lowest energy excited singlet state ofdiphenyloctatetraene and other linear polyenes. J Chem Phys 59 4984-5002 Hudson BS and Kohler BE (1974) Linear polyene electronic structure and spectroscopy. Ann Rev Phys Chem 25 437-460 Hudson BS and Kohler BE (1984) Electronic stmcture and spectra of finite linear polyenes. Synthetic Metals 9 241-253 Hudson BS, Ridyard JN and Diamond J (1976) Polyene spectroscopy. Photoelectron spectra of the diphenylpolyenes. J Am Chem Soc 98 1126-1129 Hudson BS, Kohler BE and Schulten K (1982) Linear polyene electronic structure and potential surfaces. In Lim EC (ed) Excited States, Vol 6, pp 1-95. Academic Press, New York Jones PF, Jones WJ and Davies B (1992) Direct observation of the 2 Ag electronic state of carotenoid molecules by consecutive two-photon absorption spectroscopy. Photochem Photohiol A Chem, 68 59-75... 

XPS is also often perfonned employing syncln-otron radiation as the excitation source [59]. This technique is sometimes called soft x-ray photoelectron spectroscopy (SXPS) to distinguish it from laboratory XPS. The use of syncluotron radiation has two major advantages (1) a much higher spectral resolution can be achieved and (2) the photon energy of the excitation can be adjusted which, in turn, allows for a particular electron kinetic energy to be selected. 

The photoelectronic properties of poly(dihexylgermane) were investigated by photoluminescence spectroscopy, after one- and two-photon absorption. The spectra were compared with those of the analogous poly(dihexylsilane)111. 

X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) are the two main techniques based on electron spectroscopy. In XPS, a source of photons in the X-ray energy range is used to irradiate the sample. 

The results of a preliminary study of a sample of berkelium oxide (Bk02, Bk203, or a mixture of the two) via X-ray photoelectron spectroscopy (XPS) included measured core- and valence-electron binding energies (162). The valence-band XPS spectrum, which was limited in resolution by photon broadening, was dominated by 5f-electron emission. 

Experimental data can be obtained by ultra-violet absorption spectroscopy, electron energy loss spectroscopy and photoelectron spectroscopy. UV absorption and EELs have been described briefly in Chapter 3. The former provides information only about the band-gap, while EELs gives more general information about the conduction bands. Both X-rays and UV photons can be used to generate photoelectrons these two methods are given the acronyms XPS and UPS. The energy spectrum of the emitted electrons provides information about the density of electron states in the valence bands. In principle the size of the band gap can be obtained, but care must be taken as the absolute energy... 

In addition to using X-rays to irradiate a surface, ultraviolet light may be used as the source for photoelectron spectroscopy (PES). This technique, known as ultraviolet photoelectron spectroscopy (UPS, Figure 7.38), is usually carried out using two He lines (Hel at 21.2 eV and Hell at 40.8 eV), or a synchrotron source. This technique is often referred to as soft PES, since the low photon energy is not sufficient to excite the inner-shell electrons, but rather results in photoelectron emission from valence band electrons - useful to characterize surface species based on their bonding motifs. It should be noted that both UPS and XPS are often performed in tandem with an Ar" " source, allowing for chemical analysis of the sample at depths of < 1 J,m below the surface. 

By comparing threshold and energetic electron detection in coincidence studies, one can conclude that in the former case a 50-500 times higher signals-to-noise ratio is possible, while in the latter case the photon flux of a Hel radiation source is greater by several orders of magnitude. As a final result, coincidence rates are comparable in the two types of experiment. In practice, the threshold approach seems to be preferred because a wider variety of states is available since autoionization aids to excite levels not available in conventional photoelectron spectroscopy. 

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