  We introduce a new method for determining accurate values of the valence-band
maximum in x-ray photoemission spectra. Specifically, we align the sharpest
peak in the valence-band region of the experimental spectrum with the
corresponding feature of a theoretical valence-band density of states curve
from ab initio GW theory calculations. This method is particularly useful for
soft and hard x-ray photoemission studies of materials with a mixture of
valence-band characters, where strong matrix element effects can render
standard methods for extracting the valence-band maximum unreliable. We apply
our method to hydrogen-terminated boron-doped diamond, which is a promising
substrate material for novel solar cell devices. By carrying out photoemission
experiments with variable light polarizations, we verify the accuracy of our
analysis and the general validity of the method.
