Electron emission from liquids

Two cases can be distinguished in the electron emission from liquids (1) emission of thermalized electrons and (2) emission of hot electrons. The emission of thermal electrons occurs when the electron source is located in the liquid at a distance from the surface which is greater than the attenuation length of hot electrons. The electrons become thermalized in the liquid before they approach the liquid/vapor interface. The second case is usually realized in the process of photoemission where VUV-light is impinging on the liquid surface. 

If Vq 0, transfer of delocalized electrons from the liquid into the vapor is a thermally activated process. Delocalized electrons in the vapor phase can transfer easily into liquids with Vq 0. If Vq 0, delocalized electrons can in principle transfer with no activation energy from the liquid into the vapor, while delocalized electrons in the vapor phase encounter the barrier I Vq I. In most cases, in liquids with Vq 0 the electron transport proceeds via localized states. When a localized electron comes close to the liquid surface, temporary trapping occurs due to the image force. Emission into the vapor phase is then a thermally activated process, too. 

The barrier Vois superimposed on this potential. Several cases can be distinguished. (1) An electron in the vapor space is approaching a liquid with Vq 0. The electron is attracted by the image potential but it cannot enter the liquid due to the barrier presented by Vq. Surface states are formed. An example of this is liquid helium (see Section 6.8). (2) An electron is approaching a liquid with Vq 0 in the vapor space. In this case, the electron transfer is readily effected. (3) An electron is in the liquid approaching the liquid/vapor interface. It is repelled by the image potential. If in addition, Vq 0, then a barrier for electron transfer exists which the electrons have to overcome by thermal activation. If Vq 0, then in principle electron transfer should 

The image barrier described by Equations 1 and 2 also has an effect in the transfer of electron bubbles at the boundary between liquid He and liquid He. The dielectric constant ( He) is slightly smaller than ( He). An electrostatic barrier of about 0.5 meV is produced (Kuchnir et al., 1970 Schoepe and Rayfield, 1973). 

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Figure 14 Diode cell for electron emission from liquids.

The attenuation length of VUV light in organic liquids is of the order of 10 nm. Electrons are emitted from ionization events extending into the bulk of the liquid layer. Electron emission, only from surface molecules, is observed when excited metastable He-atoms impinge on the liquid surface (Morgner et al., 1992). Photon-induced electron emission from liquid surfaces and analysis (ESCA) has been developed into a powerful tool for the study of polar liquids and solutions (Siegbahn, 1985). 

Vq values are of theoretical and practical importance. In the language of physics, Vq is identified as the bottom of the conduction band, while in chemical terms, it is called the electron affinity of the liquid. As discussed in Section 7.6, Vq reflects the subtle balance of repulsive and attractive forces acting on the electron. The more negative the value, the greater the influence of attraction, while positive Vq values point toward the decisive influence of repulsion. Vq data are compiled in Table 3. The measured data were obtained either by the photoelectric method (see Section 6.2), by electron emission from liquids, or by comparison of photoconductivity and photoemission data (see Section 6.5). Data measured by the photoelectric effect may suffer from the effect of a charged double layer at the metal liquid interface. In principle, the values obtained by the electron emission method or from the comparison of photoconductivity and photoemission thresholds should be free from this ambiguity. 

Liquids have their energy levels broadened due to the interaction of the closely packed atoms of the liquid. Such dense packing leads to an overlap of the electronic emission from various closely spaced rotational and vibrational levels, thereby producing emission spectrum widths of the order of to Hz. 

Electron emission from a liquid into the adjacent vapor phase represents another interface phenomenon. Two cases can be distinguished (1) excess electrons are already present in the bulk of the liquid and are drawn to the interface by an electric field and (2) electrons are emitted from the valence levels of the molecules or atoms comprising the liquid or a solute by absorption of ultraviolet photons or by multiphoton absorption. 

Electron emission from nonpolar organic liquids... 

Boriev, I. A., Balakin, A. A., and Yakovlev, B. S., Electron emission from nonpolar organic liquids (in Russian), Khim. Vysokikh Energii, 12, 20,1978. 

With the advent of picosecond-pulse radiolysis and laser technologies, it has been possible to study geminate-ion recombination (Jonah et al, 1979 Sauer and Jonah, 1980 Tagawa et al 1982a, b) and subsequently electron-ion recombination (Katsumura et al, 1982 Tagawa et al, 1983 Jonah, 1983) in hydrocarbon liquids. Using cyclohexane solutions of 9,10-diphenylanthracene (DPA) and p-terphenyl (PT), Jonah et al. (1979) observed light emission from the first excited state of the solutes, interpreted in terms of solute cation-anion recombination. In the early work of Sauer and Jonah (1980), the kinetics of solute excited state formation was studied in cyclohexane solutions of DPA and PT, and some inconsistency with respect to the solution of the diffusion equation was noted.1... 

Emission spectroscopy utilizes the characteristic line emission from atoms as their electrons drop from the excited to the ground state. The earliest version of emission spectroscopy as applied to chemistry was the flame test, where samples of elements placed in a Bunsen burner will change the flame to different colors (sodium turns the flame yellow calcium turns it red, copper turns it green). The modem version of emission spectroscopy for the chemistry laboratory is ICP-AES. In this technique rocks are dissolved in acid or vaporized with a laser, and the sample liquid or gas is mixed with argon gas and turned into a plasma (ionized gas) by a radio frequency generator. The excited atoms in the plasma emit characteristic energies that are measured either sequentially with a monochromator and photomultiplier tube, or simultaneously with a polychrometer. The technique can analyze 60 elements in minutes. 

Ideally, we would like to study the structure and composition of supported, dispersed catalyst particles in the same configuration used in the chemical technology. However, the determination of the atomic surface structure of the catalyst particle that is situated inside the pores of the high-surface-area support by LEED, for example, is not possible. This technique requires the presence of ordered domains 200 A or larger to obtain the sharp diffraction features necessary to define the surface structure. Even Auger electron emission, which is the property of individual atoms and can even be obtained from liquid surfaces, can only be employed for studies of supported catalyst surfaces with difficulty. Identification of the active sites does require the determination of the structure and composition of the catalyst surface, however. To avoid the difficulties of carrying out these experiments on supported... 

In very pure nonpolar dielectric liquids, electron injection currents at very sharp tips follow the Fowler-Nordheim voltage dependence (Halpem and Gomer, 1969), just as is the case in solid insulators, and in a gas, as described before. In a study of the electrochemical behavior of CNT cathodes (Krivenko et al., 2007) direct experimental proof was found of electron emission into the liquid hexamethylphosphortriamide, which was chosen because it is a convenient solvent for the visualization of solvated electrons at room temperature the solution will show an intense blue coloration upon the presence of solvated electrons. Electron spin resonance showed prove of a free electron. Electrogenerated (as opposed to photogenerated) solvated electrons have been used in the synthesis of L-histidinol (Beltra et al., 2005), albeit that in that work the electrons were generated electrochemically from a solution of LiCl in EtNH2, which is a solvent that is easier to handle than liquid ammonia (boiling points at atmospheric pressure are 17 °C and -33.34 °C, respectively). 

The plasma ionic liquid interface is interesting from both the fundamental and the practical point of view. From the more fundamental point of view, this interface allows direct reactions between free electrons from the gas phase without side reactions - once inert gases are used for the plasma generation. From the practical point of view, ionic liquids are vacuum-stable electrolytes that can favorably be used as solvents for compounds to be reduced or oxidised by plasmas. Plasma cathodic reduction may be used as a novel method for the generation of metal or semiconductor particles, if degradation reactions of the ionic liquid can be suppressed sufficiently. Plasma anodic oxidation with ionic liquids has yet to be explored. In this case the ionic liquid is cathodically polarized causing an enhanced plasma ion bombardment, that leads to secondary electron emission and fast decomposition of the ionic liquid. 

A liquid scintillation counter is actually two photon counters connected in coincidence for measuring the shower or pulse of electrons resulting from the relaxation of fluorescent molecules excited by b-particle emission. In the out-of-coincidence mode, the instrument is a single photon counter, i.e., it counts single photon events. 

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