Window resonances below the strontium 6s threshold

Absolute photoabsorption cross sections of strontium have been measured in the energy range extending from the 5p(3/2) threshold up to above the 6s th reshold. The synchrotron radiation source of the Synchrotron Radiation Rese arch Center at Hsin-Chu was used as the background continuum. Strontium vap our was confined in a heatpipe oven. The absolute column density was determ ined by simultaneously measuring the temperature distribution profiles and the total pressure in the heatpipe. Absolute cross sections were determined using the Beer-Lambert law. The observed spectrum was analysed using a com bination of a recent extension of the jj-coupled eigenchannel R-matrix adap ted to handle highly correlated levels and the phase-shifted multichannel q uantum defect theory. The main features of the observed spectrum, dominated by the presence of many window-type resonances, are well reproduced and th e structures are identified. The observed absorption windows result from th e fact that only continua are directly excited from the ground state.