Photodetachment technique

Kitsopoulos T N, Chick C J, Zhao Y and Neumark D M 1991 Study of the low-lying electronic states of Sij and Si. using negative ion photodetachment techniques J. Chem. Phys. 95 1441... 

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. 

Table 1 presents a list of the electron affinities for several selected atomic and molecular species of interest to the present discussions results determined by the photodetachment technique have been selected from the literature. No experimental measurements of the electron affinity of 0 or S are available. However, lattice energy calculations and a variety of isoelectronic extrapolation tech-... 

The photodetachment threshold values for Ga, As, and Pb (0.30(15), 0.81(3), and 1.10(5) eV, respectively) were determined by an older photodetachment technique [30]. The PES values for Ga, 0.43(3), and As, 0.814(8), are more precise, but are supported by the PD values [31, 32]. The PES value for Pb, 0.364(8), is significantly lower than the PD value, 1.10(5) eV. The photodetachment values for the majority of the other main group elements have also been confirmed by photoelectron spectroscopy. The values for the halogens have been determined by most methods photodetachment thresholds, photoabsorption, ion pair photodissociation, relative and absolute surface ionization methods, and the Born Haber cycle. Values for H, Li, C, F, Cl, Cu, Ge, Br, Nb, Ag, Sn, I, W, Re, Au, and Pb have also been determined by relative and absolute surface ionization methods, as shown in Table 8.2. 

The availability of laser photodetachment techniques has permitted more accurate experimental determinations of electron affinities. Even so, tables of electron affinities list some calculated values, in particular for the formation of multiply charged ions. One method of estimation uses the Born-Haber cycle, with a value for the lattice energy derived using an electrostatic model. Compounds for which this is valid are limited (see Section 5.15). 

Recently, Zewail and co-workers have combined the approaches of photodetachment and ultrafast spectroscopy to investigate the reaction dynamics of planar COT.iii They used a femtosecond photon pulse to carry out ionization of the COT ring-inversion transition state, generated by photodetachment as shown in Figure 5.4. From the photoionization efficiency, they were able to investigate the time-resolved dynamics of the transition state reaction, and observe the ring-inversion reaction of the planar COT to the tub-like D2d geometry on the femtosecond time scale. Thus, with the advent of new mass spectrometric techniques, it is now possible to examine detailed reaction dynamics in addition to traditional state properties." ... 

While one might expect that the techniques developed for photodissociation studies of closed-shell molecules would be readily adaptable to free radicals, this is not the case. A successful photodissociation experiment often requires a very clean source for the radical of interest in order to minimize background problems associated with photodissociating other species in the experiment. Thus, molecular beam photofragment translation spectroscopy, which has been applied to innumerable closed-shell species, has been used successfully on only a handful of free radicals. With this problem in mind, we have developed a conceptually different experiment [4] in which a fast beam of radicals is generated by laser photodetachment of mass-selected negative ions. The radicals are photodissociated with a second laser, and the fragments are detected in coinci-... 

Although theoretical techniques for the characterization of resonance states advanced, the experimental search for reactive resonances has proven to be a much more difficult task [32-34], The extremely short lifetime of reactive resonances makes the direct observation of these species very challenging. In some reactions, transition state spectroscopy can be employed to study resonances through "half-collision experiments," where even very short-lived resonances may be detected as peaks in a Franck-Condon spectrum [35-38]. Neumark and coworkers [39] were able to assign peaks in the [IHI] photodetachment spectrum to resonance states for the neutral I+HI reaction. Unfortunately, transition state spectroscopy is not always feasible due to the absence of an appropriate Franck-Condon transition or due to practical limitations in the required level of energetic resolution. The direct study of reactive resonances in a full collision experiment, such as with a molecular beam apparatus, is the traditional and more usual environment to work. Unfortunately, observing resonance behavior in such experiments has proven to be exceedingly difficult. The heart of the problem is not a... 

Until the fairly recent development of photodetachment and other techniques, the electron affinities of atoms were usually only poorly determined from electron impact studies. Consequently, numerous attempts were made to employ a variety of extrapolation procedures with data from within a given isoelectronic sequence in an attempt to obtain more reliable values or estimates where no experimental data existed. 

The ground state electronic configuration for P (g) is given by Hotop and Lineberger (2, 5), Rosenstock (6), and Massey (7). The fine sturcture seperatton has been measured experimentally via a tunetable laser photodetachment threshold technique (3, 4) and is that recommended by Hotop and Lineberger (2). 

By 1967 the kinetic model for nondissociative thermal electron attachment and revised values for the electron affinities of 16 aromatic hydrocarbons and 7 aromatic carbonyl compounds were reported [24-26]. The ECD Ea values were correlated to theoretical calculations, electronegativities, spectroscopic data, and reduction potentials. The majority of these remain the most precise electron affinities for such compounds. Some values are assigned to excited states based on the multistate model of the ECD postulated in the 1990s [27, 28]. The electron affinities of atoms, molecules, and radicals were reviewed in 1966 [24]. The relative Ea of nitrobenzene, CS2, and SO2 were measured by the thermal charge transfer techniques and the Ea of O2 by photodetachment [30-32]. 

As new values were obtained, atomic electron affinities were reviewed periodically beginning in 1953 [1-13]. All the available experimental, extrapolated, and theoretical values were tabulated in 1984 [7]. Presently, experimental values are available at the NIST website [12]. Prior to 1970 the majority of the values for the main group elements were determined by the Born Haber cycle, electron impact, or relative and absolute equilibrium surface ionization techniques. However, values for C, O, and S had been measured by photodetachment [1-3]. By the mid-1970s virtually all the Ea of the main group elements in the first three rows had been measured by photon methods [4-7]. By the early 1980s values were obtained for the transition elements by photon techniques [7, 8]. In the 1990s the values of Ca, Sr, and Ba were measured [9-13]. Recently, experimental values have been reported for Ce, Pr, Tm, and Lu [14-17],... 

TABLE 8.2 Atomic Electron Affinities (in eV) Determined by Photodetachment, Photoelectron Spectroscopy and Surface Ionization Techniques [4, 7 12]... 

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