Photoionization and Photodetachment

TABLE 4.4 Threshold Wavelength and Cross Section for Photoionization 

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The HSCC equations have been solved for various Coulomb three-body processes, such as photoionization and photodetachment of two-electron systems and positronium negative ions [51, 105-111], electron or positron collisions [52, 112-115], ion-atom collisions [116-119], and muon-involving collision systems [103, 114, 120-125]. Figures 4.6, 4.7, 4.8, 4.9, and 4.10 are all due to HSCC calculations. Figure 4.12 illustrates the good agreement between the results of HSCC calculations [51] and the high-resolution photoionization experiment on helium [126]. See Ref. [127] for further detailed account of the comparison between the theory and experiment on QBSs of helium up to the threshold of He+(n = 9). 

M. Faubel, in Photoionization and Photodetachment edited by Cheuk-You Ng, Advanced Series in Physical Chemistry, lOA, Part. 1, (World Scientific River Edge,... 

Wang L S 2000 Photodetachment photoelectron spectroscopy of transition metal oxide species Photoionization and Photodetachment Advanced Series in Physical Chemistry 10, ed C Y Ng (Singapore World Scientific)... 

The continuous spectrum is also present, both in physical processes and in the quantum mechanical formalism, when an atomic (molecular) state is made to interact with an external electromagnetic field of appropriate frequency and strength. In conjunction with energy shifts, the normal processes involve ionization, or electron detachment, or molecular dissociation by absorption of one or more photons, or electron tunneling. Treated as stationary systems with time-independent atom - - field Hamiltonians, these problems are equivalent to the CESE scheme of a decaying state with a complex eigenvalue. For the treatment of the related MEPs, the implementation of the CESE approach has led to the state-specific, nonperturbative many-electron, many-photon (MEMP) theory [179-190] which was presented in Section 11. Its various applications include the ab initio calculation of properties from the interaction with electric and magnetic fields, of multiphoton above threshold ionization and detachment, of analysis of path interference in the ionization by di- and tri-chromatic ac-fields, of cross-sections for double electron photoionization and photodetachment, etc. 

The above fomuilae for the absorption spectrum can be applied, with minor modifications, to other one-photon spectroscopies, for example, emission spectroscopy, photoionization spectroscopy and photodetachment spectroscopy (photoionization of a negative ion). For stimulated emission spectroscopy, the factor of fflj is simply replaced by cOg, the stimulated light frequency however, for spontaneous emission... 

Photoionization of neutral atoms and molecules and electron-ion collisions, for example, are rich in infinite Rydberg series of Feshbach resonances. On the other hand, only a finite number of Feshbach (and possibly shape) resonances occur in electron-neutral collisions and photodetachment of an electron attached to a neutral species, with an exception of the following cases. 

The reaction AB —) I hv AB e is the basis of photoelectron spectroscopy and photodetachment methods. Many precise and accurate ionization potentials of molecules have been obtained by studying the photoionization of neutral molecules. The same principles apply to the photon methods for determining electron affinities, except that negative ions are studied. The electron affinities of over 1,000 atoms, radicals, clusters, and small molecules have been determined using... 

Photodetachment/Photoionization Photodetachment and photoionization refer to the processes of removing an electron from either a negative ion or a neutral molecule. 

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." ... 

By comparing the chemical shifts and peak heights of an unknown with standards or known reference materials, some predictions of the unknown structure can be made. In some exceptional but important cases, such as the photodetachment of negative ions, the wavelength of light causing photoionization is in the visible or even the near infrared, allowing extremely precise structure determinations. 

The first NeNePo experiments dealt with silver clusters, Ag3, Ags, Ag7, and Ag9, particularly with the first of these. The photodetachment and photoionization were done with a single titanium-sapphire laser producing pulses of approximately 60 fs duration. Doubled in frequency, these could be tuned over a wavelength span from above 420 to below 390 nm. As with the dimer, photodetachment was a one-photon process and photoionization a two-pho-ton process. (The clusters of odd numbers of atoms could be studied this way the even-numbered clusters require at least three photons in the available energy range for photoionization). The interval between pulses could be varied from zero (simultaneous pulses) to 100 ps the two pulses were made to differ in intensity by about a factor of 2, and either could be the leading pulse. 

An ejected photoion can have nonzero transversal velocity, Autr, if a part of the excitation energy is converted to kinetic energy during the photodetachment from the surface. It is assumed that the average kinetic energy of the transverse motion of a photoion equals kin- Then, in a way similar to the above procedure, it is possible to estimate the spatial resolution due to this effect. To achieve the spatial resolution Ax of 3-5 A at Fmax = 0.2 eV/A and r - 103 A, it is necessary to realize a very precise MPI of the chro-mophore on the cooled tip with excess of transversal kinetic energy of only = 10-3 eV. 

Sofar the imaging results of Fig. 3.1 were discussed in very classical terms, using the notion of a set of trajectories that take the electron from the atom to the detector. However, this description does not do justice to the fact that atomic photoionization is a quantum mechanical proces. Similar to the interference between light beams that is observed in Young s double slit experiment, we may expect to see the effects of interference if many different quantum paths exist that connect the atom to a particular point on the detector. Indeed this interference was previously observed in photodetachment experiments by Blondel and co-workers, which revealed the interference between two trajectories by means of which a photo-detached electron can be transported between the atom and the detector [33]. The current case of atomic photoionization is more complicated, since classical theory predicts that there are an infinite number of trajectories along which the electron can move from the atom to a particular point on the detector [32,34], Nevertheless, as Fig. 3.2 shows, the interference between trajectories is observable [35] when the resolution of the experiment is improved [36], The number of interference fringes smoothly increases with the photoelectron energy. 

The molecular electron affinity of N02 has been found92 to have a lower limit of 2.5 0.1eV, from collisional ionization data, and a value of 2.36 0.1eV has been obtained by photodetachment from NO2.107 The ionization potential of N02 has been found to be 9.62 eV by photoionization mass spectrometry.108... 

Note that solvated electrons can also be produced by photodetachment of electrons from certain anions or photoionization of molecules using UV excitation.Then the mechanism of metal ion reduction is expected to be quite similar to the radiolytic processes of Figures 2a, 2b and 2c. However, the matrices used as cluster hosts are generally not transparent to light, and the reduction is often restricted to the surface. 

Finally, electrons have been generated with nanosecond pulses from the KrF exciplex laser at 248 nm via photodetachment of OH and Cl in aqueous and methanol solutions, and could be via one-photon photoionization in liquids with the ArF laser at 193 nm, and via one and two-photon processes in aqueous naphthol and naphtholate with nanosecond pulsed nitrogen lasers at 337 nm. With the improved fluxes and short pulses now available from gain modulation techniques applied to the TEA exciplex lasers, semiconductor lasers and F-center lasers, they may prove to be convenient sources for future studies of electron relaxation and transfer processes from the ultraviolet to infrared region. 

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