Photoionization Processes

The photoionization process with which we shall be concerned in both UPS and XPS is that in Equation (8.4) in which only the singly charged is produced. The selection mle for such a process is trivial - all ionizations are allowed. 

The lines of primary interest ia an xps spectmm ate those reflecting photoelectrons from cote electron energy levels of the surface atoms. These ate labeled ia Figure 8 for the Ag 3, 3p, and 3t7 electrons. The sensitivity of xps toward certain elements, and hence the surface sensitivity attainable for these elements, is dependent upon intrinsic properties of the photoelectron lines observed. The parameter governing the relative iatensities of these cote level peaks is the photoionization cross-section, (. This parameter describes the relative efficiency of the photoionization process for each cote electron as a function of element atomic number. Obviously, the photoionization efficiency is not the same for electrons from the same cote level of all elements. This difference results ia variable surface sensitivity for elements even though the same cote level electrons may be monitored. 

SNMS sensitivity depends on the efficiency of the ionization process. SNs are post-ionized (to SN" ) either hy electron impact (El) with electrons from a hroad electron (e-)heam or a high-frequency (HF-) plasma (i.e. an e-gas), or, most efficiently, hy photons from a laser. In particular, the photoionization process enables adjustment of the fragmentation rate of sputtered molecules by varying the laser intensity, pulse width, and/or wavelength. 

In Eq. (12), l,m are the photoelectron partial wave angular momentum and its projection in the molecular frame and v is the projection of the photon angular momentum on the molecular frame. The presence of an alternative primed set l, m, v signifies interference terms between the primed and unprimed partial waves. The parameter ct is the Coulomb phase shift (see Appendix A). The fi are dipole transition amplitudes to the final-state partial wave I, m and contain dynamical information on the photoionization process. In contrast, the Clebsch-Gordan coefficients (CGC) provide geometric constraints that are consequent upon angular momentum considerations. 

The cluster reactor is attached to the pulsed cluster source s condensation channel, as shown in Figure 6. (16) To it is attached a high-pressure nozzle from which a helium/hydrocarbon mixture is pulsed into the reactor at a time selected with respect to the production and arrival of the clusters. The effect of turbulent mixing with the reactant pulse perturbs the beam, but clusters and reaction products which survive the travel from the source to the photoionization regime ( 600y sec) and the photoionization process are easily detected. 

Fig. 7.15. Photophysics associated with x-ray photoelectron spectroscopy and x-ray fluorescence. As illustrated, in the XPS experiment one monitors the energy of the electron ejected from the M shell upon photoionization (process 1). In the XRF experiment, one monitors the fluorescence emitted from either the M shell after photoionization (process 2a), or from the L shell after photo ionization and radiationless decay (process 2b).

Two-photon absorption chemistry of 2AP, specifically photoionization processes, can be induced by intense nanosecond 308-nm XeCl excimer laser pulses [10]. Typical transient absorption spectra of 2AP in deoxygenated neutral aqueous solutions are shown in Fig. 1. The stronger (385 nm) and weaker (510 nm) absorption bands were assigned to 2AP radicals derived from the ionization of 2AP (bleaching near 310 nm) [10], whereas a structureless absorption band from -500 to 750 nm corresponds to the well-known spectrum of the hydrated electron (eh ) [41]. 

With increasing energy of the incident photons the photoionization process is accompanied by a rupture of valence bonds, leading to various ionized fragments, the identification of which requires complementary methods of analysis. During the last decade much progress has been achieved in the mass spectrometry of the photoionization products in various diatomic or polyatomic gases under vacuum u.v. irradiation. 

Fig. 6. Spectral efficiency of the photoionization processes in benzylamine vapor...

In ordinary spectroscopy one measures the frequency (or wavelength) and therefore the energy of photons absorbed or emitted (or scattered, as in the Raman effect) by molecules. In photoelectron spectroscopy (PES) one measures the energy of electrons emitted by molecules when they are photoionized by the absorption of high-energy (UV or x-ray) photons. If M stands for a molecule, the photoionization process can be symbolized by... 

In this experiment, no use is made of femtosecond laser pulses, although the photoionization process itself is a femtosecond event. The next natural steps are experiments using femtosecond pulses and the traditional (not projection) electron optics. This materially extends the class of possible experiments. 

B. V. Marr, Photoionization Processes in Gases, Academic Press, New York, 1967. 

If the isotopic shift of a spectral line in an atom or in a molecule is more Hi... the Doppler width, it is in principle possible to selectively excite a parti, id... isotopic species from isotopic mixtures by monochromatic light of w.u. length in coincidence with the absorption of the particular isotopic spe> < In a typical example, 2°2Hg atoms in natural Hg vapor containing 204. o 201, 200, 199, and 198 isotopes are preferentially excited by the 2b i V resonance line of 202Hg atoms. It has recently been demonstrated tlm 235U atoms are enriched in the photoionization processes of Mi. t... 

Electronic states which result from the promotion of an electron from a localized molecular orbital to a Rydberg orbital are Rydberg states . Although such states can be observed only in the gas phase, it is likely that they are involved in some photoionization processes. 

Since ionization is only a special case of excitation it is also possible to simulate the photoionization process using fast electrons. We may compare the processes... 

Several reasons have been put forward to explain the change in the angular intensity pattern of the photoelectrons. One explanation is that intermediate neutral energy levels are ac-Stark shifted into resonance and contribute new selection rules to the photoionization process [53,54], Another possibility is that the electrons of the Kr or D2 are driven into the core Kr+ or D2 in a scattering-like process that creates interference fringes in the photoelectron angular distribution due to interference between multiple scattering channels [55],... 

Recording the electrons emitted from an atom in a certain direction during a photoionization process allows one to obtain two main types of... 

B.T. Pickup, On the theory of fast photoionization processes, Chem. Phys. 19 (1977) 193. 

The interaction of a photon with an atom changes the structure of the atom, and the photon-energy-dependent change in the observables is called the dynamics in the photon-atom interaction. Therefore, in the present context of the study of photoionization processes using electron spectrometry and synchrotron radiation, the observations that can be made on the emitted electrons are all studies of dynamical properties. In the light of the foregoing discussion on the forces in the atom and the transition operator it can be concluded that photoprocesses in atoms provide a unique opportunity for fundamental investigations which explore the dynamics of many-body effects, because both the forces and the interaction... 

The properties of monochromatized synchrotron radiation have been discussed in detail in the previous section and the characteristic features of electrostatic spectrometers will be discussed in detail in Chapter 4, with examples of photoionization processes in certain atoms and specific questions of interest presented in Chapter 5. Therefore, the following discussion is restricted to basic aspects of electron spectrometry with monochromatized synchrotron radiation, in particular to some of the fundamental properties of electron spectrometers and to the special polarization properties of this radiation which require appropriate experimental set-ups for angle-resolved electron spectrometry (without spin-analysis for the determination of spin-polarization see Section 5.4). 

The combined effect of the individual energy distribution functions which are of relevance for the photoionization process and for the detection of photoelectrons, can now be discussed. As a first step, the photoionization process alone, i.e., without any detection device, is discussed with the help of Fig. 2.9. The y-axis represents an energy scale with respect to the ground state, and different states of neon are plotted along the x-direction. For photons with sufficient energy hv and with a distribution function GB( ph, Eph), one reaches the continuum above the Is... 

---