Photoelectronic spectra

In the three-stage model of electron photoemission from solids, the emitted electron intensity /( , co) ( is the energy of excited photoelectrons and o) is the photon frequency) is determined by the following factors  

Here i is the sum over all occupied states j the sum over all empty states My(k) is the matrix element of the dipole momentum operator the integration is carried out over the entire Brillouin zone. If the photon energy exceeds 30 eV, the density of virtual states is close to that of free electrons, i.e., it is proportional to y/E. Therefore, under these conditions the main features of the function P( , cu) are determined by the density of occupied states, while the density of virtual states causes only a monotonic transformation of P( ) and does not add new features to the function. The matrix element My(k) can be estimated from the photoionisation cross-section of the atomic states that contribute most to the band states from which photoelectrons are removed. However, there are few papers where the photoelectron spectrum of TiC was calculated with an allowance for the matrix element. Usually Mjj is not calculated and photoelectron spectra are compared directly with densities of band states. 

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It is advantageous if the laser system pemiits rotation of the optical polarization. Detached electrons correlated witii different final electronic states of the neutral molecule will generally be emitted with different angular distributions about the direction of polarization. Measurement of the angular distribution helps in the interpretation of complex photoelectron spectra. The angular distribution/(0) of photoelectrons is [50]... 

Photoelectron spectra of cluster ions yields cluster-bond strengdis, because each added ligand increases the bindmg energy of the extra electron in the negative ion by the amount of the ligand bond strength (provided the bond is electrostatic and does not appreciably affect the cln-omophore ion) [116]. 

Bailey C G, Dessent C E FI, Johnson M A and Bowen K FI 1996 Vibronic effects in the photon energy-dependent photoelectron spectra of the CFIjCN dipole-bound anion J. Chem. Phys. 104 6976-83... 

Figure Cl.1.4. Photoelectron spectra of V, ,(A= 17, 27, 43, and 65) at 6.42 eV photon energy, compared to tire bulk photoelectron spectmm of V(100) surface at 21.21 eV photon energy. The cluster spectra reveal tire appearance of bulk features at and how tire cluster spectral features evolve toward tire bulk. The bulk spectmm is referenced to tire Fenni level. Wu H, Desai S R and Wang L S 1996 Phys. Rev. Lett. 77 2436, figure 2.

In studying molecular orbital theory, it is difficult to avoid the question of how real orbitals are. Are they mere mathematical abstractions The question of reality in quantum mechanics has a long and contentious history that we shall not pretend to settle here but Koopmans s theorem and photoelectron spectra must certainly be taken into account by anyone who does. 

The simplest, and perhaps the most important, information derived from photoelectron spectra is the ionization energies for valence and core electrons. Before the development of photoelectron spectroscopy very few of these were known, especially for polyatomic molecules. For core electrons ionization energies were previously unobtainable and illustrate the extent to which core orbitals differ from the pure atomic orbitals pictured in simple valence theory. 

The He I ultraviolet photoelectron spectra of Kr and Xe appear similar to that of Ar but the ionization energy decreases and the spin-orbit coupling increases with increasing atomic number, as illustrated by the data in Table 8.1. 

Figure 8.13 The MgATa oxygen Is and carbon Is X-ray photoelectron spectra of a 2 1 mixture of CO and CO2 gases. (Reproduced, with permission, from Allan, C. J. and Siegbahn, K. (November 1971), Publication No. UUIP-754, p. 48, Uppsala University Institute of Physics)...

Question. For X-ray photoelectron spectra of a mixture of acetone and carbon dioxide gases, explain what you would expect to observe regarding the relative ionization energies (binding energies) and intensities in the C Is and O Is spectra. 

Figure 8.15 The carbon Is X-ray photoelectron spectra of furan, pyrrole and thiophene. The sulphur Ip spectrum of thiophene is also shown. (Reproduced with permission from Gelius, U., Allan, C. J., Johansson, G., Siegbahn, H., Allison, D. A. and Siegbahn, K., Physica Scripta, 3, 237, 1971)...

Both UPS and XPS of solids are useful techniques. So far as studies of adsorption by surfaces are concerned we would expect UPS, involving only valence orbitals, to be more sensitive. For example, if we wish to determine whether nitrogen molecules are adsorbed onto an iron surface with the axis of the molecule perpendicular or parallel to the surface it would seem that the valence orbitals would be most affected. This is generally the case but, because ultraviolet photoelectron spectra of solids are considerably broadened, it is the X-ray photoelectron spectra that are usually the most informative. 

Figure 8.20 Nitrogen Is and oxygen Is X-ray photoelectron spectra of nitric oxide (NO) adsorbed on an iron surface. 1, Fe surface at 85 K 2, exposed at 85 K to NO at 2.65 x 10 Pa for 80 s 3, as for 2 but exposed for 200 s 4, as for 2 but exposed for 480 s 5, after warming to 280 K. (Reproduced, with permission, from Kishi, K. and Roberts, M. W., Proc. R. Soc. Land., A352, 289, 1976)...

The factor limiting the resolution in ultraviolet photoelectron spectra is the inability to measure the kinetic energy of the photoelectrons with sufficient accuracy. The source of the problem points to a possible solution. If the photoelectrons could be produced with zero kinetic energy this cause of the loss of resolution would be largely removed. This is the basis of zero kinetic energy photoelectron (ZEKE-PE) spectroscopy. 

Infrared, uv, nmr spectra (66), and photoelectron spectra have been reviewed (67). Physical properties of siHcon peroxides are summarized in Reference 43. Other physical properties, eg, dipole moments, dihedral angles, and heats of combustion ate Hsted in Reference 68. The oxygen—oxygen bond strengths of various diaLkyl peroxides have been reported (69). 

The photoelectron spectra of pyridazine have been interpreted on the basis of many-body Green s function calculations both for the outer and the inner valence region. The calculations confirm that ionization of the first n-electron occurs at lower energy than of the first TT-electron (79MI21201). A large number of bands in the photoelectron spectrum of 3,6-diphenylpyridazine in stretched polymer sheets have been assigned to transitions predicted... 

The He(Ia) photoelectron spectra of the parent heterocycles have been the subject of much study. Initially the assignment of the ionization energies to appropriate occupied molecular orbitals was confused by the unexpected reversal in the sequence of the two highest occupied MOs in tellurophene relative to the other heterocycles. The reported values are compared in Table 24. The assignments are based upon comparisons with the spectra... 

In this method, photons of an energy well in excess of the ionization potential are directed onto a molecule. The photoelectron spectrum which results allows assessment of the energies of filled orbitals in the molecule, and thus provides a characterization of a molecule. Comparisons between photoelectron spectra of related compounds give structural information, for example, on the tautomeric structure of a compound by comparison of its spectrum with those of models of each of the fixed forms. 

The same group has published two articles on indazoles [indazole (36), 1-methyl- (106) and 2-methyl-indazole (107)]. The He-I and He-II spectra have been obtained and satisfactorily interpreted by means of ab initio LCGO calculations (78JST(43)33, 78JST(43)203). The PE spectra support the conclusion (Section 4.04.1.5.1) that the 1//-tautomer is by far the more stable. X-Ray photoelectron spectra of some pyrazole and pyrazoline derivatives have been reported (83MI40400). 

The vertical ionization potentials from the photoelectron spectra of some thiirane and thiirene derivatives are given in Table 3. A Walsh localized scheme of bonding is generally preferred. There is a strong hyperconjugative interaction in thiirene 1,1-dioxides between the occupied C=C tt-MO and the occupied SO2 pure sulfur d-AO. Thiirene oxides are suggested to be less aromatic than cyclopropenones and tropone. 

UV photoelectron spectra, 5, 517 Adenine, N ,9-dimethyl-UV photoelectron spectra, 5, 517 Adenine, 9-ethyl-IR spectra, 5, 518... 

Benzonaphthyridines, 2, 581-627 nomenclature, 2, 612 synthesis, 2, 612 Benzo-1,5-naphthyridines biological activity, 2, 625 Benzo[/i][ 1,5]naphthyridines photoelectron spectra, 2, 585 Benzo[/i][l, 6]naphthyridines photoelectron spectra, 2, 585 synthesis, 2, 613 Benzo[/][l, 7]naphthyridines photoelectron spectra, 2, 585 synthesis, 2, 613 Benzo-1,8-naphthyridines synthesis, 2, 608... 

Benzotriazolium tetrachlorocobaltates crystal structure, 5, 676 Benzotrifuroxans molecular dimensions, 6, 397 N NMR, 6, 398 photoelectron spectra, 6, 399 Benzotropones synthesis, 2, 308 2H-Benz[e][l,2]oxaborins benzo fused synthesis, 1, 659 synthesis, 1, 659... 

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