Volume 85, Issue 9, 01 May 1999
Index of content:
- GENERAL PHYSICS: NUCLEAR, ATOMIC, AND MOLECULAR (PACS 01-39)
85(1999); http://dx.doi.org/10.1063/1.370129View Description Hide Description
A method of performing scanning tunneling spectroscopy (STS) at a constant height while maintaining feedback control is described. By using sinusoidal voltage oscillations the ac tunneling current can be separated from the displacement current present in current–voltage measurements. The method allows statistically well averaged spectra spanning both occupied and unoccupied states of the sample to be obtained from a continuous set of curves. The unoccupied state on graphite is observed using the technique described, and the utility of the technique is further demonstrated by the detection of several bulk states on and two transition metal dichalcogenides. A weighting towards k perpendicular states is seen in the STS spectra. These results show that the fast alternating current form of STS is suitable for probing low amplitude electron states on metal surfaces within 2 eV of the Fermi level.
85(1999); http://dx.doi.org/10.1063/1.370130View Description Hide Description
This article presents a new phase jump phenomenon in a differential heterodyneinterferometer. When the intensity of one arm of an interferometer changes from greater to less (or less to greater) than that of another, a phase jump of will take place if the phase difference between two arms is prefixed at π. We call this phenomenon phase jump. To demonstrate phase jump, a modified differential heterodyneinterferometer has been setup. The intensity of one arm is modulated as the arm is scanning across an edge. Both theoretical and experimental investigations are conducted in detail. The theoretical and experimental results indicate that the phase slope of the phase jump is infinite and the amplitude of output signal is zero when a phase jump occurs. Meanwhile the position of phase jump is very sensitive to the displacement. Therefore, both phase and amplitude signals of phase jump are suitable for precise position index and may find wide potential applications in the fields of edge detection, alignment, optical storage, and so on.
85(1999); http://dx.doi.org/10.1063/1.370131View Description Hide Description
When gas proportional scintillation counters (GPSC) are used to detect very low energy x rays, the addition of the light noble gas neon to the usual xenon filling improves the collection of primary electrons that originate near the detector window. However, xenon–neon mixtures have lower electroluminescence yields than pure xenon. Increasing the scintillation electric field jeopardizes the energy resolution because of the additional fluctuations introduced by electron multiplication. In this work we investigate the effect of a limited amount of charge multiplication on the electroluminescence yield and the energy resolution R of a xenon–neon GPSC using both Monte Carlo simulation and experimental measurements. We consider xenon–neon mixtures with 5%, 10%, 20%, 30%, 40%, 50%, 70%, 90%, and 100% Xe at a total pressure of 800 Torr. Comparing the experimental and Monte Carlo data for 5.9 keV x rays, we conclude that optimum value of R is reached in a region of weak ionization with a charge gain of less than 2. By extrapolating the experimental results for R to infinite light yield we obtain the intrinsic energy resolution for 5.9 keV x rays in all mixtures. From these results we can predict Fw values, where F is the relative variance in the number of primary electrons (the Fano factor) and w is the mean energy required to produce a primary electron. From a comparison between Monte Carlo and experimental electroluminescence yields, F and w values are estimated for 5.9 keV x rays in the various mixtures.