Photoionization, from

Photoelectron spectroscopy (PES) has been applied to determine the structure of 1-aza- and 1,4,7-triazatricy-clo[5.2.1.04,10]decane 37 and 40 <1997JMT(392)21>. The PES spectrum of ATQ shows four composite bands in the region 7-17 eV. A first band peaked at 7.80 eV is attributed to the NLPO (nitrogen lone-pair orbital). A second prominent broad band system, extending from 10.5 to 13.0 eV is associated with photoionizations from the cr-orbital manifold. The third composite band is produced by two photoemissions. The second band may be attributed to emissions arising from a sequence of seven near-lying MOs. 

Herzberg (Nobel prize for Chemistry, 1971) commented on the two distinct photoionizations from methane that this observation illustrates the rather drastic nature of the approximation made in the valence bond treatment of CH4, in which the 2s and 2p electrons of the carbon atom are considered as degenerate and where this degeneracy is used to form tetrahedral orbitals representing mixtures of 2s and 2p atomic orbitals. The molecular orbital treatment does not have this difficulty". 

The photoionization of a molecule to yield an electron and an unfragmented ion may be considered to be the simplest of all photodissociation reactions, and therefore also one of the simplest of the radiationless processes in an isolated molecule. In addition, because the products are charged, a combination of mass spectrometric and photometric data yields information about photoionization reactions not now available for molecular fragmentation reactions. For example, the reaction cross sections for generation of specific charged products and the total photon absorption cross section may be measured and compared, thereby yielding the residual cross section corresponding to radiationless processes other than photoionization. From this information we can deduce some of the consequences of the competition between several radiationless processes in an isolated molecule. 

Laser spectroscopy of the 1S-2S transition has been performed by Mills and coworkers at Bell Laboratories (Chu, Mills and Hall, 1984 Fee et al, 1993a, b) following the first excitation of this transition by Chu and Mills (1982). Apart from various technicalities, the main difference between the 1984 and 1993 measurements was that in the latter a pulse created from a tuned 486 nm continuous-wave laser with a Fabry-Perot power build-up cavity, was used to excite the transition by two-photon Doppler-free absorption, followed by photoionization from the 2S level using an intense pulsed YAG laser doubled to 532 nm. Chu, Mills and Hall (1984), however, employed an intense pulsed 486 nm laser to photoionize the positronium directly by three-photon absorption from the ground state in tuning through the resonance. For reasons outlined by Fee et al. (1993b), it was hoped that the use of a continuous-wave laser to excite the transition would lead to a more accurate determination of the frequency interval than the value 1233 607 218.9 10.7 MHz obtained in the pulsed 486 nm laser experiment (after correction by Danzmann, Fee and Chu, 1989, and adjustment consequent on a recalibration of the Te2 reference line by McIntyre and Hansch, 1986). 

Figure 6 Nondipole asymmetry parameter xis(e) as a function of the photoelectron energy s for the Ne Is photoionization from e Cgo calculated [36] in two different approximations 1, the 5-potential model. 2, 3 and 4, the A-potential model with the following values for Rgq, f/gQ and A (all in au) ...

Figure 7 Nondipole asymmetry parameter Y2s(s) for the Ne 2s photoionization from Ne Cgo calculated [36] in the 5-potential model (line 1) and the A-potential model (lines 2,3, and 4) with the same choice of R,n, A, and f/g0 as in Figure 6.

The main distinction between the reversible photoionization from the irreversible one is brought by the non-zero-element WB(r) arising in the rate matrix W(r) from Eq. (3.259) ... 

In the case of a deep core level, the photoelectron will leave the core region in a time short compared with the time for the localized Is hole to hop between the nuclei, and the deep core hole will in practice be localized on one of the nuclei. Diagrammatically this is shown in Fig. 41, where Figs. 41a-d describe photoionization from the symmetry adapted 1 og and 1 ou MO s and where Fig. 41 e describes the interference between the... 

Figure 18 Representation of the potential energy surfaces involved in photoionization from a nondegenerate ground state of a molecule to a Jahn-Teller active state of a molecular ion. The distortion coordinate is the Jahn-Teller active vibration. The vertical arrows represent the most probable transitions...

A relatively widely-available alternative ionization technique is atmospheric-pressure photoionization (APPI) [102]. In APPI, the ionization process is initiated by photons from a discharge lamp rather than from a corona discharge electrode. APPI is promising in the analysis of relatively non-polar analytes. Commercial systems are available (Photospray from Sciex, photoionization from Syagen Technology and other instrament manufacturers). Ionization under APPI is discussed in Ch. 6.5. 

Finally we have illustrated the power of the method by calculating the cross sections for electron scattering or photoionization from a number of diatomic and polyatomic molecules. These calculations have been among the first to include polarization and correlation in an ab initio way. The extension of our methods to inelastic electronic processes and nuclear excitation and dissociation are underway and should appear soon. 

The difference in the behavior of these four ions is easily rationalized. The ionization energies (lEs) of He and Ne fall in a region where resonant ionization preferentially removes an electron from the 3ai orbital of SiH4. Photoionization studies indicate that ionization from this orbital yields Si+ and SiH " products almost exclusively (Cooper et al., 1990). In contrast, the lEs of Kr and Xe are resonant with the lower-energy 2t2 orbital of SiH4. Photoionization from this orbital results in less dissociation, preferentially forming SiHj and SiH. Thus, the dominant products observed are consistent with the photoionization results for removal of an electron from the two distinct valence molecular orbitals of silane. 

Photoelectronic spectra of a series of six carbonylfurans (39 R = H, Me, F, Cl, Br, OH) and 2-furonitrile have been interpreted by comparison within the series and by differences in the relative effects of substituents <90CJC747>. The peaks at lower ionization energy have been assigned to photoionization from 7i-orbitals of the furan rings, orbitals localized on the carbonyl group, and orbitals of the carbonyl substituents, in agreement with MO calculations. Within the series of... 

From calculations of the electronic structure of the diphenylpolyenes, the three peaks at lowest binding energy, i.e., the peaks shown in the inset, can be identified as arising from electrons photoionized from the the highest occupied molecular n-orbitals as follows The intense... 

The photoionization from the 773 or the 773 MOs leaves the ion in the 2 or Bi states, respectively. These give rise to a Cl that manifests itself in the lack of a vibrational structure in pyrrole and thiophene (Cl near the minimum of the upper, surface) and by hue broadening in furan (Cl at... 

Europium, which has been studied also in the first on-line experiments using multistep photoionization from a thermal atomic beam, continues the large series of rare-earth elements investigated systematically at... 

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