THE HYDROGEN-ATOM, A TOOL FOR METROLOGY

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 R(infinity), 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.