\section*{Methods}
$\mathrm{TbPc_2}$\, molecules were synthesized by the procedure reported in Refs.~\onlinecite{DeCian1985,Gonidec2012} followed by purification in a column chromatograph with silicagel and toluene as an eluent. 
The single-crystal Au(111) substrate was prepared by repeated cycles of Ar$^+$ ion bombardement and annealing at 700~K. 
$\mathrm{TbPc_2}$\, was thermally evaporated at ultra-high vacuum conditions from a quartz crucible at 700~K after thorough degassing for $>12$~h at 363~K and $<1$~h at 473~K. 
The static magnetic field perpendicular to the sample was obtained by mounting a SmCo permanent magnet close to the sample and measured \textit{ex situ}. 
STM experiments were performed after \textit{in situ} transfer of the sample, employing electrochemically etched W tips deoxidized by annealing in vacuum. 
Impurity- and tip effects were minimized by multiple tip formings between the experiments, resulting in Au-coated tips. 
The $\mathrm{d}I/\mathrm{d}V$\, signal was obtained from the first-harmonic tunneling current signal detected by lock-in technique with sinusoidal modulation of the sample voltage at 704~Hz and 10--20~mV peak-to-peak. 
Reliable tip performance was established by accurately reproducing the characteristic conductance signature of the Au(111) surface state well-known in the literature \cite{Chen1998}. 
For the rf resonance experiments, a sinusoidal ac tunneling voltage from an rf signal generator is coupled in via a bias tee to the sample (further details see Ref.~\cite{Mullegger2014b}). 
The modulation frequency was swept between 150~MHz and 4~GHz, respectively, at a rate of 10~MHz/s. 
The resulting rf current does not affect the tunnel distance, because the radio frequency is several orders of magnitude larger than, both, the cutoff frequency of the STM feedback loop and the bandwidth of the high-gain current amplifier of the STM. 
The maximum rf power level at the sample was $-20$~dBm.  

