Photoelectron spectroscopy ionization potentials

Two especially important variants of REMPI (Johnson, et al., 1975) spectroscopy are ionization-dip (Cooper, et al., 1981) and Zero Electron Kinetic Energy (ZEKE) (Miiller-Dethlefs and Schlag, 1991 Merkt, 1997 Signorell and Merkt, 1999) photoelectron spectroscopy. Ionization-dip REMPI spectroscopy is especially useful when one wants to record free<—bound spectra from a single, selectable v, J level. Without such v, J selection, most of the oscillatory structure in a free<—bound spectrum will be washed out. One potential problem with some ionization-dip schemes is that, if the ionization transition originates from the initial level of the free<—bound transition being studied, there is a possibility that the observed linewidths will be distorted by power broadening (especially when the free final state is a weakly predissociated state with linewidth < lcm-1). 

Cederbaum L S and Domcke W 1977 Theoretical aspects of ionization potentials and photoelectron spectroscopy a Green s function approach Adv. Chem. Phys. 36 205-344 Oddershede J 1987 Propagator methods Adv. Chem. Phys. 69 201-39... 

Ultraviolet photoelectron spectroscopy allows the determination of ionization potentials. For thiazole the first experimental measurement using this technique was preformed by Salmona et al. (189) who later studied various alkyl and functional derivatives in the 2-position (190,191). Substitution of an hydrogen atom by an alkyl group destabilizes the first ionization potential, the perturbation being constant for tso-propyl and heavier substituents. Introduction in the 2-position of an amino group strongly destabilizes the first band and only slightly the second. 

Mass spectrometry can be used to determine ionization potentials by the method of Lossing (283). The values obtained can be compared with those found by photoelectron spectroscopy and those calculated by CNDO/S (134) or ab initio (131) methods using the Koopman theorem approximation. The first and second, ionization potentials concern a ir... 

Trioxanes bond angles, 3, 949 bond lengths, 3, 949 H NMR, 3, 952 ionization potential, 3, 959 IR spectra, 3, 956 photoelectron spectroscopy, 3, 959 radical cations... 

The ionization potentials of some substituted noibomadienes have been measured by photoelectron spectroscopy. The values which pertain to the n orbitals are shown ... 

Unlike the stable molecule N2O, the sulfur analogue N2S decomposes above 160 K. In the vapour phase N2S has been detected by high-resolution mass spectrometry. The IR spectrum is dominated by a very strong band at 2040 cm [v(NN)]. The first ionization potential has been determined by photoelectron spectroscopy to be 10.6 eV. " These data indicate that N2S resembles diazomethane, CH2N2, rather than N2O. It decomposes to give N2 and diatomic sulfur, S2, and, hence, elemental sulfur, rather than monoatomic sulfur. Ab initio molecular orbital calculations of bond lengths and bond energies for linear N2S indicate that the resonance structure N =N -S is dominant. 

Ghosh and Almlof published many articles discussing the XPS (X-ray photoelectron spectroscopy) and UPS (ultraviolet photoelectron spectra) and the corresponding ionization potentials of porphyrins using high-level calculations. Tliese topics are indirectly related to the tautomerism of porphyrins (for an example see 94IC6057 and 95JA4691). 

The most evident of these is the marked stability of radical cations formed in an aprotic medium by the oxidation of compounds where the first ionization potential (in the sense of photoelectron spectroscopy) is for the removal of an electron from a non-bonding orbital, e. g. thianthrene... 

Even the photoelectron spectroscopy of closed-shell molecules is valuable for the physical chemistry of radicals because a difference between the nth and the first adiabatic ionization potentials determines the excitation energy in a radical cation for a transition from the ground doublet state to the (n — 1) excited doublet state. 

Extensive discussion on the ionization potentials of 1,2,5-thiadiazole and its derivatives can be found in CHEC(1984) and CHEC-II(1996) <1984CHEC(6)513, 1996CHEC-II(4)355>. Hel photoelectron spectroscopy, inner-shell electron energy loss spectroscopy involving the S2p, S2s, Cls and Nls edges, and Sis synchrotron radiation photoabsorption spectroscopy were used to probe the occupied and unoccupied valence levels of benzothiadiazole 2 <1991MI165>. 

Before going into a detailed account of the chemistry of phanes, the author will touch on 3,4,7,8-tetrasilacycloocta-l,5-diyne briefly, since the compound illustrates the importance of a—it mixing. The ionization potential of the Si-Si bond is estimated by photoelectron spectroscopy to be 8.69 eV (9). Thus, the HOMO level of the Si-Si is comparable to most HOMOs of tt systems. Consequently, the Si-Si bond can conjugate efficiently with carbon-carbon double and triple bonds, benzene rings, and other tt systems. Most Si-Si bonds are stable enough to construct sophisticated structures by themselves and with organic molecules (10). 

The ionization potential of a molecule is the energy from the ground state of the molecule (HOMO) to the vacuum level. It is measured using UPS or XPS. The electron affinity of the molecule is the energy from the vacuum level to the LUMO. It is measured using inverse photoelectron spectroscopy (IPES) [15]. The values obtained in the gas phase are different from those obtained in the solid state, and shifts due to amorphous versus crystalline regions can be noticed. 

Energy Levels for Hole Injection. For the hole conductor TPD (6), measurements are available from different groups that allow a direct comparison of different experimental setups. The ionization potential that corresponds to the HOMO level under the assumptions mentioned above was measured by photoelectron spectroscopy to be 5.34 eV [230]. Anderson et al. [231] identified the onset of the photoelectron spectrum with the ionization potential and the first peak with the HOMO energy, and reported separate values of 5.38 and 5.73 eV, respectively. The cyclovoltammetric data reveal a first oxidation wave at 0.34 V vs. Fc/Fc+ in acetonitrile [232], and 0.48 V vs. Ag/0.01 Ag+ in dichloro-methane [102], respectively. The oxidation proceeds by two successive one-electron oxidations, the second one being located at 0.47 V vs. Fc/Fc+. 

Ionization Potentials of Methylamine (e.v.) by Electron Impact and Photoelectron Spectroscopy... 

We would therefore expect to find in monosubstituted benzenes a second ionization potential, corresponding to the unpertiu-bed orbital, at a value not very different from that of benzene ( 9-2 e.v.), as well as the higher value, corresponding to the lowest 7r-level (tti), and differing from the benzene iri-value by an amount similar to the first ionization potential difference. This has in fact been found in those compounds which have so far been studied by photoelectron spectroscopy (see Table 8). The simple monoalkylbenzenes and styrene have second ionization potentials in the range 9-0-9-1 e.v. Phenylacetylene has a second... 

Other experimental evidence leads to essentially the same conclusion regarding the n ionization of pyridine. El Sayed and Kasha (1961) have detected Rydberg series in the absorption spectrum similar to those in benzene and ascribable to n orbitals (9-266 e.v., 02 11-56 e.v., 62) and, in addition, reported a fragmentary series leading to a third ionization potential of 10-3 e.v. which they ascribed to the nitrogen lone pair. Similar values are found by photoelectron spectroscopy which also indicated the 10-3 e.v. (10-54 e.v.) level to be only weakly bonding. 

Fio. 19. Comparison of the two lowest adiabatic ionization potentials in benzene with the three lowest in pyrrole and furan. The values arranged as an energy level diagram were obtained by photoelectron spectroscopy. (T, N. Badwan and D. W. Turner, unpublished work.)... 

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