  Defect centers in hexagonal boron nitride represent room-temperature
single-photon sources in a layered van der Waals material. These light emitters
appear with a wide range of transition energies ranging over the entire visible
spectrum, which renders the identification of the underlying atomic structure
challenging. In addition to their eminent properties as quantum light emitters,
the coupling to phonons is remarkable. Their photoluminescence exhibits
significant side band emission well separated from the zero phonon line (ZPL)
and an asymmetric broadening of the ZPL itself. In this combined theoretical
and experimental study we show that the phonon side bands can be well described
in terms of the coupling to bulk longitudinal optical (LO) phonons. To describe
the ZPL asymmetry we show that in addition to the coupling to longitudinal
acoustic (LA) phonons also the coupling to local mode oscillations of the
defect center with respect to the entire host crystal has to be considered. By
studying the influence of the emitter's wave function dimensions on the phonon
side bands we find reasonable values for size of the wave function and the
deformation potentials. We perform photoluminescence excitation measurements to
demonstrate that the excitation of the emitters is most efficient by LO-phonon
assisted absorption.
