Angle dependence of the spin-orbit branching ratio

As 3d spin-orbit branching ratios have been measured as functions of azimuthal angle for the 1 monolayer As/Si(111)-(1 X 1) and As/Si(001)-(2 X 1) systems using angle resolved photoemission. At a photon energy of hv = 72 eV, the branching ratio is larger than the statistical value for both surfaces. Furthermore, strong changes in the branching ratios at this energy are observed as functions of azimuthal angle and these changes reflect the symmetries of the respective surfaces. Constant final state measurements confirm for the As/Si(111) surface that the deviation from the statistical value is primarily due to the rapidly increasing As 3d photoionization cross section at this energy and show that the observed angle dependence is mostly due to photoelectron diffraction effects. At hv = 120 eV, which is at the maximum of the As 3d cross section, the branching ratio from As/Si(111) is statistical and independent of angle further confirming that the nonstatistical values for this surface are due to the photoionization cross section dependence on energy. For the As/Si(001) surface, however, an azimuthal angle dependence is still observed in the constant final state measurements suggesting that the nonstatistical values as due in part to some other mechanism.