Changes in mean-square nuclear charge radii in Er from optical isotope shifts by laser-atomic-beam spectroscopy

High-resolution laser-atomic-beam spectroscopy has been applied to investigate transitions 4f126s2-4f115d6s2 and transitions starting from metastable states of the configuration 4f115d6s2 or ErI. Precise values for the transition isotope shifts and for the hyperfine constants of 10 levels belonging to the configuration 4f115d6s2 and 9 levels belonging to high lying even-parity levels have been determined. From the isotope shift data relative changes of mean-square nuclear charge radii delta for all the stable Er isotopes have been deduced: vertical stroke168,170vertical stroke1,vertical stroke166,168vertical stroke0.984(15), vertical stroke164,166vertical stroke0.984(15), vertical stroke162,164vertical stroke1.198(18),vertical stroke166,167vertical stroke0.3357(50). Absolute delta values have been obtained by calibrating the optical data with a value from muonic X-ray spectroscopy: vertical stroke168,170vertical stroke0.121(10) fm2. The odd-even staggering parameter for 167Er has been determined: 0.683(10). Our results are compared with experimental data from electronic X-ray spectroscopy and data from calculations with the extended liquid-drop and the droplet model. (orig.)