D. Aumiler, T. Ban, R. Beuc and G. Pichler
Institute of Physics, Zagreb

A new method for the atom number density determination in alkali vapour is proposed. The method is based on the comparison of numerical simulation of the alkali self-broadened resonance lines absorption profiles with measured ones. The numerical simulation calculates absorption profiles starting from the relevant set of alkali ab initio potential curves, using semiclassical stationary point approximation. As a result, the reduced absorption coefficient (absorption coefficient divided by the square of the atom number density) is obtained. By adjusting the temperature parameter of the model, it is shown that in the spectral region around resonance lines excellent agreement between simulated and experimental absorption profiles can be achieved. The least squares fitting of the simulated profiles to the measured profiles determines the vapour temperature with the uncertainty of less than ±1°C. The measurements were made on rubidium vapour generated in a homemade crossed heat-pipe oven. In this way we were able to measure the Rb atom number densities with accuracy better than ±3%.
The advantage of this method is that the measurement of the sole physical quantity - absorption coefficient is sufficient for the accurate determination of the atom number density. In particular, one does not need to know the temperature of the vapour in order to determine the atom number density. This is especially important in experiments in which exact vapour temperature measurements are not possible, as is the case when a heat-pipe oven is used. Similar application of the method could also be found in plasma diagnostics of pulsed high-pressure alkali light sources, which we are currently investigating in our laboratory.