Electron photodetachment spectroscopy

Similar experiments, involving electron transfer between an anion and a neutral molecule, yield relative or absolute EAs. The method has been used to determine relative free energies for electron attachment for a variety of metallocenes and /3-diketonate molecules. Electron photodetachment spectroscopy of negatively charged ions is another source for obtaining electron affinities of molecules. These data provide an important component of thermochemical cycles involving oxidation/reduction of metal complexes, and serve as a basis for obtaining other thermochemical values. 

Wetzel, D. M., 8c Brauman, J. I. (1987). Electron photodetachment spectroscopy of trapped negative ions. Chemical Reviews, 87, 607. 

A second role for mass spectrometry in the investigation of reactive intermediates involves the nse of spectroscopy. Althongh an important nse of ion spectroscopy is the determination of thermochemical properties, including ionization energies (addition or removal of an electron), as in photoelectron or photodetachment spectroscopy, and bond dissociation energies in ions, as in photodissociation methods, additional spectroscopic data can also often be obtained, inclnding structural parameters such as frequencies and geometries. 

Determination of the Singlet-Triplet Gap in TMM by Electron Photodetachment Photoelectron Spectroscopy ... 

Photodetachment spectroscopy of negative ions like IHI- and similar systems, studied by Neumark and coworkers (Metz, Kitsopoulos, Weaver, and Neumark 1988 Weaver, Metz, Bradforth, and Neumark 1988 Metz et al. 1990 Bradforth et al. 1990 Neumark 1990 Weaver and Neumark 1991) has provided the first conclusive manifestation of reactive resonances for a purely repulsive PES. The idea of the experiment goes as follows a photon with frequency u detaches the electron from the negative ion producing e- and IHI. If the PES for the neutral molecule is dissociative, the IHI complex subsequently breaks apart into I and HI. 

The electron affinity (31.9 0.7 kcalmoH ) and gas-phase acidity (361.5 2.8 kcalmoH ) for MesGeH have been determined using photodetachment spectroscopy. The bond dissociation energy (Ge-H) derived from these values was determined to be 79.8 3.5 kcalmoH, in excellent agreement with other determinations. ... 

Photoelectron spectroscopy, the measurement of the intensity and energy of electrons photodetached from an ion by a fixed-energy photon beam. 

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. 

The next paragraphs focus on the most recent advances in electron photodetachment processes in aqueous ionic solutions. Interesting results on ultrafast UV-IR spectroscopy of photoexcited aqueous chloride ions are presented in Figure 5-8. A complex photokinetic model of time-resolved data has been considered and explained in detail in recent pubhcations (85, 86). The primary photophysical and photochemical events triggered by one- or two-photon processes can be summarized with the following equations ... 

The second electron photodetachment channel occurring from excited electronic states of an ionic solute has been discriminated by femtosecond near-infrared spectroscopy (1.24-1.41 eV). This channel is characterized by the presence of two transient subbands peaking around 1.41 eV and wholly... 

Yourshaw I, Zhao Y and Neumark D M 1996 Many-body effects in weakly bound anion and neutral clusters zero electron kinetic energy spectroscopy and threshold photodetachment spectroscopy of Ar Br" n = 2-9) and Ar r (n =... 

The electron affinities listed in the table below agree within the error limits given. They were obtained by laser photoelectron spectroscopy (LPES) on the PHg ion [19] and PH2 photodetachment using a tunable laser (LPD) [20, 21] or an Xe arc lamp (with a grating monochromator) [21] and ion cyclotron resonance (ICR) spectrometry [20, 21]. All values are reported in two reviews on electron affinities [22] and electron photodetachment [23], and may be considered as adiabatic (see the remarks below the table) ... 

Visible and UV light sources, which excite electronic transitions, can be used also for PD spectroscopy. By scanning the frequencies of the radiation emitted from the UV/vis light source and measuring PD or electron photodetachment as a function of excitation wavelength, an electronic action spectrum can be constructed in the same way as a vibrational action spectrum is constructed using an IR source. 

Photo-excitation of gas-phase ions may result in the photodetachment of an electron rather than photo-fragmentation. Coulombic considerations dictate that this process is more prevalent for anions than for cations. Electron photodetachment action spectroscopy of trapped anions has proved also to be a valuable source of molecular information. In some systems, electron photodetachment and PD compete. The mechanisms for these two processes in large molecules are yet to be understood fully consequently, their branching ratios in specific experimental conditions cannot be predicted as yet. One exciting possibility is the idea of using frequency and phase-shaped pulses to promote selected photochemical pathways. 

Part 3. Ion Spectroscopy. In Chapter 9, we return to the theme of ion photodissociation, which was included also in Volume IV, Part 6, in an exploration of trapped-ion photodissociation, electron photodetachment, and fluorescence. Trapped-ion fluorescence may offer an alternative approach for the elucidation of ion conformation. Whereas these spectroscopic experiments require high ion densities, much attention is directed to the spectroscopic study of single ions confined in an ion trap. Chapters 10 and 11 are illustrative of such studies, with the former devoted to the study of a single molecular ion in a linear ion trap and the latter to a single atomic ion in Paul-type ion traps. While both types of studies require extensive cooling of the subject ion, once such cooling has been achieved, the ions can remain confined for many hours. 

Since the extra electron generally has a low binding energy, most negative ions can be ionized (photodetachment) by visible or infrared lasers. The remaining ions are separated from the neutral molecules formed in the photodetachment process by a deflecting electric field. An example of sub-Doppler photodetachment spectroscopy of can be found in [9.85]. 

---