Spectral momentum density of graphite from (e,2e) spectroscopy: Comparison with first-principles calculation.

We have measured the spectral momentum density p(E,q} of graphite by (e,2e) spectroscopy for momentum parallel and perpendicular to the crystal e axis. In the independent-electron approximation, p(E, q}=go ~ U„(G) ~ 25(q —lt —G)5(E —E(t}}where the one-electron wave function is %„(rl=e'"' go U„(G)e'o' and G is a reciprocal-lattice vector. The measurements covered a range of momentum parallel to the c axis equal to 0& ~ q ~ & 1.84 A and a range of momentum perpeno ] dicular to the c axis equal to 0( ~ q ~ (2.35 A . The energy range spanned the valence band of graphite from 4.4 eV above the Fermi energy to 27.6 eV below the Fermi energy. The momentum resolution was OA7 and 0.73 A (full width at half maximum) for momentum parallel and perpendicular to the c axis, respectively. The energy resolution was 8.6 eV. The maximum coincidence rate was -0.02 counts/sec, The band structure Eik} and spectral density