Zero-Phonon Line as the Corner Stone of Single Impurity Center Spectroscopy

1. In conventional optical addressing the diffraction limit λ3 selects a body about 1010 molecules to be under illumination. Out of them 104 are impurities, if their relative concentration is 10-6. Single impurity molecule spectroscopy (SMS) has to deal with one molecule at the back-ground of 1010molecules, whose frequencies are out of resonance with the excitation by a few thousands of cm-1 (the host molecules) and 104 molecules in the inhomogeneous impurity band distributed over about 1-1000 cm-1 around the resonance. The single molecule subject to SMS (which is really a spectroscopy, not only detection) must have a sharp and intense absorption line towering well above the spectral background created by the other 1010 molecules under illumination [1,2]. The purely electronic zero-phonon line (ZPL), “the optical analog of the Mossbauer γ-resonance line” ([3] and references therein) is a proper candidate to that role.