beta-parameter measurements of state-selected rotational transitions near the v(+)=0 threshold of para-H2

Photoelectron angular distributions in H2 and their dependence on photon energy have been measured for the first time with full rotational selection. In particular, the transitions X1Sigmag (? = 0,N = 0) -> X2Sigma+ g (?+ = 0,N+ = 0, 2) have been studied in the energy region from 15.47 to 15.75 eV. This energy region is dominated by autoionizing structure, largely Rydberg states converging on the ?+ = 1 threshold of the H2 ion. VUV radiation from the GASPHASE beamline at the Elettra Synchrotron Radiation Source has been used, in conjunction with a hemispherical photoelectron spectrometer featuring a magnetic angle-changing device, to measure the photoelectron angular distributions of the rotationally selected states. Variations of the asymmetry parameter, ?, with photon energy have been extracted from the angular distributions. The value of the asymmetry parameter for the 0-2+ transition is essentially constant, despite the presence of a large number of resonances, and has a value close to 0.2, consistent with p-wave electron emission during the autoionization process. Although variations in ? are observed for the 0-0+ transition, these are attributed to 2-2+ contributions and the underlying constant value is also then consistent with p-wave emission.