Photodetachment techniques, anions

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. 

In order to further illustrate the scope of the NeNePo technique and the abihty of our theoretical approach to treat more complex systems, two examples, Ag4 and Au4, have been chosen for the presentation, because they exhibit qualitatively different structural properties in the anionic state and have common properties in the neutral state. In the case of the silver tetramer, the global minima of the anion and of the neutral cluster assume related rhombic structures. Therefore, after photodetachment at low temperatures (T 50 K), which ensures that only the rhombic isomer is populated, the pump step reaches the nonequilibrium rhombic configuration close to the global minimum of the neutral species, as shown on the left-hand side of Fig. 4. Notice that the well-defined initial structure is a necessary condition to observe the time scales of the processes involved in the geometric relaxation of the neutral state, and therefore the experiments should be performed at low temperatures. 

Sowada and Holroyd have used a combined A -ray-visible-dye-laser doublepulse technique to determine the photodetachment spectra of the anions of biphenyl, trans-siWhene, pyrene, perylene, benzperylene, coronene, and nitrobenzene in non-polar solvents. 

A more powerful experimental technique to probe the electronic structure of transition-metal clusters is size-selected anion photoelectron spectroscopy (PES) [70. 71. 72. 73. 74. 75 and 76]. In PES experiments, a size-selected anion cluster is photodetached by a fixed wavelength photon and the kinetic energies of the photoemitted electrons are measured. PES experiments provide direct measure of the electron affinity and electronic energy levels of neutral clusters. This technique has been used to study many types of clusters over a large cluster size range and can probe how the electronic structures of transition-metal clusters evolve from molecular to bulk [77. 78. 79, 80 and M] Research has focused on the 3d transition-metal clusters, for which there have also been many theoretical studies [82, M, M, 86, M and 89]. It is found that the electronic structure of the small transition-metal clusters is molecular in nature, with discrete electronic states. However, the electronic structure of the transition-metal clusters approaches that of the bulk rapidly. Figure Cl. 14 shows that the electronic structure of vanadium clusters with 65 atoms is already very similar to that of bulk vanadium [90]. Other 3d transition-metal clusters also show bulk-like electronic structures in similar size range [78]. 

Experimental evidence for the vibrational structure of XHX transition states has been provided by photoelectron spectroscopy of XHX- anions with X = Cl, Br, and I (134,160-163). This technique, by inducing photodetachment of an electron from the XHX" anions, probes the Franck-Condon region, which is believed for these systems to include geometries in the vicinity of the transition state region for the neutral systems. Spectral bands have been interpreted as evidence for trapped-state resonances associated with asymmetric stretch-excited levels of the transition state (160-163), and they are in general agreement with synthetic photoelectron spectra calculated from the scattering computations of Schatz (17-19). In recent experimental spectra (158,162), more closely spaced oscillations have been observed these are apparently related to rotational thresholds as described by Schatz. 

Photoelectron spectroscopy can also be used to study negative ions. In this case, the technique is termed negative ion (anion) PES or photodetachment PES. 

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. 

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