  Resonant optical excitation of apical oxygen vibrational modes in the normal
state of underdoped YBa2Cu3O6+x induces a transient state with optical
properties similar to those of the equilibrium superconducting state. Amongst
these, a divergent imaginary conductivity and a plasma edge are transiently
observed in the photo-stimulated state. Femtosecond hard x-ray diffraction
experiments have been used in the past to identify the transient crystal
structure in this non-equilibrium state. Here, we start from these
crystallographic features and theoretically predict the corresponding
electronic rearrangements that accompany these structural deformations. Using
density functional theory, we predict enhanced hole-doping of the CuO2 planes.
The empty chain Cu dy2-z2 orbital is calculated to strongly reduce in energy,
which would increase c-axis transport and potentially enhance the interlayer
Josephson coupling as observed in the THz-frequency response. From these
calculations, we predict changes in the soft x-ray absorption spectra at the Cu
L-edge. Femtosecond x-ray pulses from a free electron laser are used to probe
these changes in absorption at two photon energies along this spectrum, and
provide data consistent with these predictions.
