  Single-molecule tip-enhanced Raman spectroscopy (TERS) under ultra-high
vacuum (UHV) and cryogenic conditions enables exploration of the relations
between the adsorption geometry, electronic state, and vibrational fingerprints
of individual molecules. TERS capability of reflecting spin states in
open-shell molecular configurations is yet unexplored. Here we use the tip of a
scanning probe microscope to lift a perylene-3,4,9,10-tetracarboxylic
dianhydride (PTCDA) molecule from a metal surface to bring it into an
open-shell spin one-half anionic state. We reveal a correlation between the
appearance of a Kondo resonance in the differential conductance spectroscopy
and concurrent characteristic changes captured by the TERS measurements.
Through a detailed investigation of various adsorbed and tip-contacted PTCDA
scenarios, we infer that the Raman scattering on the suspended PTCDA is
resonant with a higher excited state. Theoretical simulation of the vibrational
spectra enables a precise assignment of the individual TERS peaks to
high-symmetry Ag modes, including the fingerprints of the observed spin state.
These findings highlight the potential of TERS in capturing complex
interactions between charge, spin, and photophysical properties in nanoscale
molecular systems, and suggest a pathway for designing spin-optical devices
using organic molecules.
