Anion photodetachment

The e (KE) peaks in the PES (fixed hu) provide information about the relative energies of electronic states of the AB neutral molecule (for example AsO-, Lippa et al., 1998) that, owing to spin or orbital selection rules, are not easily observed by direct study of the neutral molecule spectrum. An anion photodetachment spectrum is recorded by detecting the low-energy electrons ejected as the photon energy is scanned through the successive photodetachment thresholds associated with the energies of each neutral molecule rovibronic state. 

When treating ion spectroscopy one should not forget anions. Similar spectroscopic techniques may be used as for cation spectroscopy. For instance dissociation spectroscopy is also possible for molecular anions. Since excited anionic electronic states mostly do not exist, one uses infrared multiphoton dissociation to study vibrational levels of the ground state. Another interesting technique is the photoelectron spectroscopy of anions (photodetachment photoelectron spectroscopy), which exhibit a very specific feature. This technique differs from cation <— neutral photoelectron spectroscopy in two respects (i) the final state is a neutral one thus anion photoelectron spectroscopy delivers information about neutrals rather than ionic systems, (ii) The initial state is anionic thus mass selection before spectroscopy is possible. As a result, mass selective spectroscopic information of neutral molecular systems is supplied which otherwise is not accessible. This is of particular interest for neutral systems which are only available in complex mixtures or are short-lived intermediate reaction products or radicals. 

Drzaic P S, Marks J and Brauman J I 1984 Electron photodetachment from gas phase molecular anions Gas Phase Ion Chemistry Ions and Lighted M T Bowers (New York Academic)... 

The transition-state spectroscopy experiment based on negative-ion photodetachment described above is well suited to the study of the F + FI2 reaction. The experiment is carried out tln-ough measurement of the photoelectron spectrum of the anion FH,. This species is calculated to be stable with a binding energy of... 

Gantefor G, Meiwes-Broer K H and Lutz H O 1988 Photodetachment spectroscopy of cold aluminum cluster anions Phys. Rev. A 37 2716... 

Anion photoelectron spectroscopy [37, 38] amd photodetachment techniques [39] provide accurate information on electron detachment energies of negative ions. Ten closed-shell ainions considered here exhibit sharp peaks, indicative of minor or vanishing final-state nuclear rearrangements, in their photoelectron spectra. Comparisons between theory and experiment are straiightforward, for differences between vertical and adiabatic electron detachment energies (VEDEs and AEDEs, respectively) are small. 

Fig. 4. Fast radical beam photofragment translational spectrometer. A fast free radical beam is produced from photodetachment of anions in a mass selected beam. The free radicals are then photodissociated in the downstream. (From Cyr et al.59)...

Rubeanic acid, complexes of, 3 277 Rubidium acetylide, 11 195 anion, 25 165-166 electron photodetachment, 25 166 optical spectra, 25 166 auride, 25 241 borates, 25 215 structures, 25 194 carbide, 11 195... 

Photodetachment from anions should also be mentioned [57-59]. In this case, the threshold, Uth, given by ... 

Electron photodetachment upon laser excitation of the solvent anion above 1.76 eV was observed (Fig. 2a,c) [18]. The cross section of photodetachment linearly increases between 1.76 and 3 eV (Fig. 2b). Under the same physical conditions, the photodetachment and absorption spectra of the solvent anion are identical (Fig. 2b) [20], suggesting a bound-to-CB transition the quantum yield of the photodetachment is close to unity. The photodetachment spectrum is similar to the photoelectron spectra of (C02) 9 clusters observed by Tsukuda et al. [24] in the gas phase it is distinctly different from the electron photodetachment spectra of CO2 in hydrocarbon liquids [27]. This suggests that a C-C bound, 7)2, symmetric dimer anion constitutes the core of the solvent radical anion [18,19]. 

This step generates the TMM radical anion 27, which is selected for electron photodetachment of the mass 54 (TMM) anions from the beam. Two PE band... 

Aside from the preceding argument based on the oxygenation kinetics, no quantitative experimental estimates of the singlet-triplet separation in TMB have been reported. If a suitable precursor of the TMB radical anion in the gas phase can be made, photodetachment PES of TMB would become a likely way to solve this problem. 

K. Yamashita Prof. Neumark, you talked about the experiment in which anions are photodetached to the bound states of neutral molecules, and these bound states are excited to dissociate by a second photon. However, you can also excite anions to the transition state of neutral reactions instead of bound states and can reach the resonance states, as you do for several systems. 

Electron photodetachment spectra for indenyl and fluorenyl anions have been measured and the electron affinities of the corresponding radical and bond dissociation energies of the corresponding neutrals determined. Comparison has been made with solution-phase data in an attempt to determine the dependence of gas and solution properties on ionic size and extent of charge distribution.3 Benzocyclopropenyl anion... 

The electron affinities of a number of a-silyl substituted silyl and carbon radicals were determined in photodetachment experiments and confirmed by data obtained from ab initio calculations. The authors conclude in this study that the stabilization a carbanion experiences through a-silyl substitution is approximately 14-20 kcalmol-1 per silyl group that of a silyl anion is approximately 6-14 kcal mol-1. The larger stabilization in the carbanionic systems is readily explained by stronger hyperconjugation of the anionic carbon center with the silyl groups as compared to that of the silyl anion with a silyl group. 

T. Takayanagi, A. Wada, Quantum reactive scattering calculations of photodetachment spectra of the FHD- anion, Chem. Phys. Lett. 348 (2001) 514. 

To focus the review, it is necessary to pass over some topics which are timely, relevant, and could be included were it not for space limitations. One of these is the area of electron photodetachment as applied to the study of the Franck-Condon region of the neutral PES accessed from the corresponding anion. This approach has been shown to provide information about the transition state region of several bimolecular reactions. This work was pioneered by Neumark and coworkers and an excellent review is already available (Manolopoulos et al. 1993 Neumark 1992). Another noteworthy area is state-to-state studies of vibrational predissociation in weakly bound complexes. Miller and coworkers have made impressive advances in which fully state and angle-resolved product distributions have been obtained (Bemish et al. 1994 Block et al. 1992 Bohac et al. 1986, 1992a,b Bohac and Miller 1993a,b Dayton et al. 1989), and these results have been used to bring theory and experiment into accord. The present review is limited to cases in which ultraviolet photodissociation of a complexed moiety initiates reaction. 

Accurate measurements of the electron affinity of simple Ge and Sn radicals have been obtained by threshold photodetachment experiments carried out in ion cyclotron resonance experiments183. In these experiments, measurement of the threshold frequency for removing the electron from the anion yields an upper limit for the electron affinity of the species, as shown for GeH3- in equation 28. 

Anion photoelectron spectroscopy is conducted by crossing a mass-selected beam of negative ions with a fixed-frequency photon beam and energy-analyzing the resultant photodetached electrons (Figure 21-8). There are three main regions of such an apparatus the source that generates the anions to be studied, the mass... 

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