Laser photodetachment

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-... 

Fig. 2. Laser photodetachment-reionization of CH2N02 plotted as a difference of laser-on and laser-off measurements. The upward peaks are due to dissociations upon photodetachment followed by reionization with 02 and cation detection. The downward peaks are due to dissociations upon collisional detachment due to residual 02 followed by reionization with 02...

Electronic energy levels are from the laser photodetachment study of Feldmann (3). He found that the C ( D) state has a... 

This value, recommended by Hotop and Lineberger (2), is based on a laser photodetachment electron... 

The ground state configuration for Ni"(g) is given by Hotop and Lineberger (2, 4) and Rosenstock et al. (5) and Masey (6). The fine structure separation has been measured experimentally be laser photodetachment electron spectrometry (3) and is that recommended by Hotop and Lineberger (2). 

The laser photodetachment study of Celotta et al. (j.) which yielded a precise value for EA(02) also yielded precise values for r and as well as values for and We adopt these values and calculate assuming a Morse function for the ground... 

Electronic energy levels are from the laser photodetachment electron spectrometry study of Kasdan et al. (3). These workers found that the Si"( D) and Si"( P) states have binding energies of 0.523 0.005 eV and 0.029 0.005 eV, respectively, relative... 

As a further example of accurate calculations for a transition metal system, consider the difficulty of determining the ground state of FeH. The laser photodetachment spectroscopic studies of FeH by Stevens et al. ° have been interpreted in terms of a A ground state and a low-lying A state only... 

We present the results of experimental studies of photon-negative ion interactions involving the dynamics of two electrons. Resonances associated with doubly excited states of Li and He" have been observed using laser photodetachment spectroscopy. Total and partial photodetachment cross sections have been investigated. In the former case, the residual atoms are detected irrespective of their excitation state, while in the latter case only those atoms in specific states are detected. This was achieved by the use of a state selective detection scheme based on the resonant ionization of the residual atoms. In addition, in the case of Li-photodetachment, the threshold behavior of the Li(2 P)+e-(ks) partial cross section has been used to accurately measure the electron affinity of Li. 

Dipole-bound anions are characterized both by their small electron affinities and diffuse nature. Laser photodetachment of dipole-bound anions leave the neutral molecule in the same (or approximately so) vibrational state as that of the anion. The diffuse nature of the dipole-bound anions... 

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). 

In photodetachment studies we need not be concerned with the coulombic interaction that dominates the neutralization channel in MPI at times <10 ° s, but since the threshold for ionization of an anion is higher in the liquid than in the gas phase, we obtain once again information on the polarization energy of the ion in the liquid prior to photo-detachment. The laser photodetachment mechanism has been used with inorganic ions, such as Fe(CN)g or organic ions such as phenolate, supported in a wide range of aqueous or organic liquids. 

SOa is discussed in the last section of this chapter. The dye-laser photodetachment of electrons from SH has a threshold at 538.7 nm, indicating an electron affinity for SH of 2.301 eV.110 The reaction rate between CS and O (61)... 

Abbreviations used in the tables calc = calculated value PT = photodetachment threshold using a lamp as a light source LPT = laser photodetachment threshold LPES = laser photoelectron spectroscopy DA = dissociative attachment attach = electron at-tachment/detachment equilibrium e-scat = electron scattering kinetic = dissociation kinetics Knud=Knudsen cell CT = charge transfer CD = collisional detachment and ZEKE = zero electron kinetic energy spectroscopy. 

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