ion with its five oxygen neighbors. The optical gap of undoped
Li2B4O7 is large with transparency extending beyond 165 nm [24].
The samples were irradiated at room temperature using an
X-ray tube operating at 50 kV. Irradiation times were 60 min. The
initially clear samples were green to the eye after the exposure to
the X-rays. Optical emission and excitation data were acquired at
room temperature using a Horiba Fluorolog-3 spectrofluorometer.
The excitation source was a xenon lamp and the detector was a
Hamamatsu R928 photomultiplier tube. The double-grating
monochromators used to obtain the excitation and emission
spectra were wavelength-calibrated using a low-intensity Hg
lamp. Emission intensities were corrected for system response
using a calibrated white light source. Because the emission bands
were broad, a λ2 correction was included during the conversion
from wavelength to energy [25]. This provides correct line shapes
and peak positions when plotting PL intensities vs. energy. The PLE
spectra were corrected for variations in the intensity of the
exciting light that reached the sample (i.e., these spectra are
adjusted for wavelength-dependent changes in incident
photon flux).