  It has been recently proposed that single molecule spectroscopy could be
employed to detect the motion of nano-mechanical resonators. Estimates of the
coupling constant (g) between the molecular two-level system and the oscillator
indicate that it can reach values much larger than the mechanical resonating
pulsation (omega_m) and the two-level system linewidth (Gamma). Other
experimental realization of the same system are also approching this strong
coupling regim. In this paper we investigate the behavior of the system in the
limit for slow mechanical oscillator omega_m << Gamma}. We find that, for
sufficiently large coupling, the system undergoes a bistability reminiscent of
that observed in optical cavities coupled to mechanical resonators.
