  Vibrational levels of the electronic ground states in dye molecules have not
been previously explored at high resolution in solid matrices. We present new
spectroscopic measurements on single polycyclic aromatic molecules of
dibenzoterrylene embedded in an organic crystal made of para-dichlorobenzene.
To do this, we use narrow-band continuous-wave lasers and combine spectroscopy
methods based on fluorescence excitation and stimulated emission depletion
(STED) to assess individual vibrational linewidths in the electronic ground
state at a resolution of ~30 MHz dictated by the linewidth of the electronic
excited state. In this fashion, we identify several exceptionally narrow
vibronic levels with linewidths down to values around 2GHz. Additionally, we
sample the distribution of vibronic wavenumbers, relaxation rates, and
Franck-Condon factors, both in the electronic ground and excited states for a
handful of individual molecules. We discuss various noteworthy experimental
findings and compare them with the outcome of DFT calculations. The highly
detailed vibronic spectra obtained in our work pave the way for studying the
nanoscopic local environment of single molecules. The approach also provides an
improved understanding of the vibrational relaxation mechanisms in the
electronic ground state, which may help to create long-lived vibrational states
for applications in quantum technology.
