Direct photodetachment, electron from

In the late 1960s several major advances were made in the study of thermal electron reactions. These were based on the ECD, the extension of the magnetron method of studying electron molecule reactions to determine equilibrium constants for electron molecule reactions, and the invention of high-pressure thermal electron negative-ion sources for mass spectrometry [5-7], Electron swarms were also used to determine rate constants for thermal electron reactions [8, 9]. The electron affinities of molecules were measured using electron and alkali metal beams [10, 11]. Relative electron affinities were obtained from the direction of the reaction of a negative ion with a molecule [12, 13], Other major advances were photodetachment and photoelectron spectroscopy [14—17],... 

The total photodetachment cross section describes the probability that an electron is detached from a negative ion following the absorption of a photon, regardless the excitation state of the residual atom or the energy or direction of the emitted electron. A total cross section is the sum of partial cross sections for detachment into each of the energetically allowed continua. This is illustrated in Fig. 1. Here we show the three possible channels accessible to a doubly excited state of Li" that lies just below the Li(32P) detachment threshold. The total cross section may be determined by... 

The direct photoexcitation of water molecules by ultrashort laser pulses is used for the investigation of primary events occurring from 10 s (thermal orientation of water molecules and ultrafast proton transfer) to 10" s (primary reactions of a solvated electron with protic species) (57,58,61-65). The nonlinear interaction of ultrashort UV pulses (typically less than 100 fs in duration and having a power of 10 W cm" ) with water molecules triggers multiple electron photodetachment channels within a hydrogen bond network (see equations 4-7). An initial energy deposition via a two-photon absorption process (2 X 4 eV) leads to the formation of nonequilibrium states of an excess electron... 

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]. 

The photoelectron spectra of the mass-selected j-triazine (1,3,5-triazme, denoted Tz) cluster anions, Tz ( = 1-6), were obtained at various photon energies to investigate the electronic character of the clusters. This study provides the first direct observation of an isolated molecular anion of azabenzene, Tz. From the photoelectron spectrum taken at 1064 nm, the electron affinity (EA) of Tz was determined to be 0.03 eV. By examining the effect of the Jahn-Teller distortion in Tz, active vibrations associated with photodetachment were identified. A series of Ar-solvated clusters of Tz were also studied, which provided indirect evidence of asymmetric charge distribution in Tz caused by the Jahn-Teller distortion <2003JCP4320>. 

The equilibrium concentration of the ions A and B participating in the equlibrium can be directly observed by mass spectrometry. Thus, the ffee-energy change can be derived from the equilibrium constant, since the concentrations of the neutral species are known in advance. Similarly, by measuring the temperature dependence of the equilibrium constants, the associated enthalpy and entropy can be obtained from van t Hoff plots. By measuring a series of interconnecting equlibria, an appropriate scale can be established. The primary standard in such work has frequently been SO2 whose electron affinity is well established by electron photodetachment. ... 

Negatively charged, mass-selected silver trimers are caught in a linear quadrupole trap. There, their excess electrons are photodetached with a pulse of radiation in the range of 400 nm, with a duration of nearly 100 fs. The neutrals prepared in this way would remain in the trap for nanoseconds or even milliseconds. However, a second ultrashort pulse of the same radiation, intense enough to induce TPI of the clusters, is directed into the trap after a preselected delay, allowing the neutral to carry out internal motion, from a small fraction of a vibration to many vibrations. The positive ions generated in this way, are then mass-analyzed and collected. Thus, the yield of... 

The femtosecond spectroscopic data summarized in the table 1 demonstrate that the reactivity of electron trapping in concentrated ionic aqueous solutions is identical to what has been observed in pure liquid water. These experiments, which involved electron photodetachement from the ferrocyanide ion demonstrate that electron solvation does not proceed through a direct electron capture by pre-existing deep traps suggested as previously (Wiesenfeld and Ippen, 1980). 

---