Electron quantum tunneling

The distortion caused by the field allows an electron to pass from the molecule to the tip if the applied potential is positive or from the tip to the molecule if the potential is negative. This is called field ionization (FI), and the electron transfer occurs through quantum tunneling. Little or no vibrational excitation occurs, and the ionization is described as mild or soft. 

Classically there should not be a current between the sample and the tip, but as the distance becomes below 0.5 nm quantum mechanics takes over and electrons may tunnel through the gap, giving rise to a tunnel current on the order of 1 nA, which can then be measured. The experimental set-up is shown schematically in Fig. 4.27. 

Effusive beam technique, 157-158 Electron bombardment flow radiolysis, 238 Electrospray ionization and ionic clusters, 168 Enantiomers, separation techniques, 154-155 Enantioselectivity of enzymes, 148 Enthalpy-entropy compensation plots, 261 Enthalpy of activation, and quantum tunneling, 67, 70-71... 

The plane of closest approach of hydrated ions, the outer Helmholtz plane (OHP), is located 0.3 to 0.5 run away from the electrode interface hence, the thickness of the interfacial compact layer across which electrons transfer is in the range of 0.3 to 0.5 nm. Electron transfer across the interfacial energy barrier occurs through a quantum tunneling mechanism at the identical electron energy level between the metal electrode and the hydrated redox particles as shown in Fig. 8-1. 

Electron-transfer proteins have a mechanism that is quite different from the conduction of electrons through a metal electrode or wire. Whereas the metal uses a continuous conduction band for transferring electrons to the centre of catalysis, proteins employ a series of discrete electron-transferring centres, separated by distances of I.0-I.5nm. It has been shown that electrons can transfer rapidly over such distances from one centre to another, within proteins (Page et al. 1999). This is sometimes described as quantum-mechanical tunnelling, a process that depends on the overlap of wave functions for the two centres. Because electrons can tunnel out of proteins over these distances, a fairly thick insulating layer of protein is required, to prevent unwanted reduction of other cellular components. This is apparently the reason that the active sites of the hydrogenases are hidden away from the surface. 

Fig. 9. The effect [26] of the concentration of CC14 on the order, n, of the electron phototransfer reactions from (a) naphthalene and (b) diphenylamine to CC14 with respect to light intensity and (c) schematic representation of the two-quantum over barrier and the one-quantum tunnel electron phototransfer from a donor to an acceptor.

In Newtonian mechanics, the particle cannot be found below V = 0, therefore Newtonian mechanics always corresponds to V = 0 [e.g., Eq. (398)], and this represents, classically, an insurmountable barrier to a particle such as an electron attempting to enter the Higgs region below V = 0. In quantum mechanics, however, an electron may enter the Higgs region by quantum tunneling, which occurs when E < V = 0. The wave function for this process is well known to be [68]... 

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