Abstract
Hydrogen is the simplest atom, comprising only a proton and an electron. This simplicity means its properties can be calculated theoretically with impressive accuracy, currently of order 10-11. At the same time, rapid advances in the techniques of laser spectroscopy have paved the way for experimental measurements at a comparable level of precision. This article begins with an outline of the problems encountered in the theoretical calculations and explains where the current uncertainties lie. Next, methods of Doppler-free laser spectroscopy are described and recent key experiments based on them reviewed. In the latest measurements, the Rydberg constant Rinfinity , the scaling factor for all transition frequencies in hydrogen, has been determined with a precision approaching 10-11. This means that all other transitions in hydrogen, ranging from the microwave to the ultraviolet, can be considered as metrological standards to this same level of precision.
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