Orientation of the $\mathrm{O} 2p$ holes in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_1$ ${\mathrm{Cu}}_2$${\mathrm{O}}_8$

Unoccupied $\mathrm{O} 2p$ states are observed in ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_1$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_8$ near the Fermi level. They show up as a spike in the $\mathrm{O} 1s$ absorption edge at threshold ($h\nu =528.2\mathrm{}$ eV). The orientation of these $\mathrm{O} 2p$ states is determined via optical selection rules from the polarization dependence of the absorption using single crystals. The $\mathrm{O} 2p$ holes are found to have at least 95% $p_{x,y}$ character with $x$, $y$ in the superconducting $a-b$ plane.