Tunneling efficiency

Engelkes VB, Beebe JM, Frisbie CD (2004) Length-dependent transport in molecular junctions based on SAMs of alkanethiols and alkanedithiols effect of metal work function and applied bias on tunneling efficiency and contact resistance. J Am Chem Soc 126 (43) 14287-14296... 

The origin of the tunneling efficiency can be traced to a specific feature of the PES, specifically, the presence of an extremely narrow barrier between the initial state (physisorbed N2 molecule) and the final state (chemisorbed N atoms). The model PES proposed by Haase et al. [1989] is characterized by two coordinates an N-N stretching coordinate (r) directed along the surface, and the distance (z) from the surface to the N2 center of mass. In the initial state, r0 = 1.10A, and z0 = 2.48A. The corresponding vibrational frequencies are 2180 and 355 cm-1. At the transition state r = 1.14 A, and z = 2.25 A (Figure 9.5). Comparing... 

With the recent surge in surface analysis techniques, the characterization of surface-adsorbed SPMs has blossomed. This is particularly tme for the use of scanning tunneling microscopy (STM), where the conjugated core of the SPM is expected to show a higher tunneling efficiency than nonconjugated portions (e.g.,... 

Apart from these nCB examples, there have also been reports of STM images of ferro-electric liquid crystals on graphite surfaces [19-21]. The interesting conclusion from these results, in particular from those of [21], is that the enchanced tunneling efficiency of the benzene rings cannot simply be explained via the highest occupied or lowest unoccupied molecular orbitals. 

From Figure 49.2, it can be seen that the quadrupole assembly provides a potential well to contain the ions in their journey along the main quadrupole axis. The potential well of the quadinpole has not very steep sides and, compared with steep-sided hexapoles or higher -poles or ion tunnels, the quadrupole is not as efficient as the others in containing ions inside the rod assembly. 

As the number of rods is increased, the rate of increase in ion efficiency falls off, and the devices become more difficult to construct. Consequently, quadrupoles and hexapoles are used most frequently in commercial instruments. More recently, the concept has been extended to a set of ring electrodes, which are the most efficient ion guides (Figure 49.8). Flowever, although the ring set (ion tunnel) uses RF fields similar to the ones outlined here, there are sufficient differences that are not discussed here. 

Ion tunnels are even more efficient in their guiding motion on ions than are the multipoles. 

Figure 9-25. Field-induced tunneling relative t]uanlunt efficiencies of organic LEDs calculated for various carrier mobilities and barrier heights (I KT , ,=,=0.2 2...//,= If) /( =I0"8,...

This is a method which is very attractive in principle and which has been applied to yield approximate barriers for a number of molecules. There are, however, difficulties in its use. In the first place, it is not easy to measure the intensities of microwave lines with accuracy. There are unsolved problems of saturation, reflections in the wave guide, and variation of detector efficiency with frequency which are presumably reponsible for the fact that measurements made with ordinary wave guide spectrometers are not very reproducible. In addition, both the spectral lines may be split into components by tunnelling from one potential minimum to another and this splitting, even though it is not resolved, can alter the apparent intensity. Furthermore, it is often difficult to find pairs of lines such that neither is obscured by Stark lobes from the other. 

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