Frequency standards contribution

Table 1. Most significant contributions to the estimated frequency standard uncertainty of the iodine stabilized Nd YAG laser of PTB...

This paper describes the progress of a laser resonance experiment which aims to measure the Lamb shift in hydrogenic silicon with an accuracy that will allow it to test the two-loop binding corrections mentioned above. This in turn should allow the viability of calculable frequency standards, based on transitions in lower-Z one-electron systems such as hydrogen and He+, to be assessed. Following a review of some theoretical contributions to hydrogenic energy levels, the details of the laser resonance experiment are outlined. 

As has already been pointed out, two different methods of calibration have been used in the work so far reported. One can measure in terms of H/3, and in this way determine the ground state Lamb shift. If instead one uses a frequency standard, one can determine the IS Lamb shift by allowing for the other contributions to the measured IS - 2S interval this requires a value of the Rydberg constant from another source. Alternatively, if one assumes the IS Lamb shift known from theory at the level of accuracy of the experiment, one can regard the measurement as a determination of the Rydberg constant. All the most recent work has been in terms of the external standards provided by the tellurium transitions, but these introduce an uncertainty which in the case of the cw experiments dominates the final error. This situation must be... 

In this contribution we speculate on some new techniques of optical frequency comparison by use of modulated lasers, and on a possible new optical frequency standard based on the methane stabilised HeNe laser. He start by reviewing our... 

Lyman-a X-rays is about 100 keV. If the measurements are improved and the QED contributions can be understood with sufficient accuracy, such Lyman-a transitions could one day become calculable frequency standards in the X-ray region. 

The calculated extinction spectrum of a polydispersion of small aluminum spheres (mean radius 0.01 jam, fractional standard deviation 0.15) is shown in Fig. 11.4 both scales are logarithmic. In some ways spectral extinction by metallic particles is less interesting than that by insulating particles, such as those discussed in the preceding two sections. The free-electron contribution to absorption in metals, which dominates other absorption bands, extends from radio to far-ultraviolet frequencies. Hence, extinction features in the transparent region of insulating particles, such as ripple and interference structure, are suppressed in metallic particles because of their inherent opacity. But extinction by metallic particles is not without its interesting aspects. 

The standard heat of sorption, Afl°, can then be interpreted in terms of appropriate energy contributions and the corresponding entropy, AS°, in terms of various degrees of freedom of the guest molecules relative to the host lattice (Table II) (10). An example of the use of the above method is given elsewhere in this volume (11). In comparing observed standard entropies with those based on the models of Table II, only empirical methods (12) are available for a priori estimates of the frequencies v. Nevertheless, comparisons have been of considerable interest (10). 

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