SINGLE-MOLECULE SPECTROSCOPY - MAXIMUM EMISSION RATE AND SATURATION INTENSITY

We consider the collection efficiency for photons emitted by a single molecule in a solid excited by monochromatic light at low temperature, which depends upon the dipole radiation pattern; the orientation of the emission dipole, total internal reflection, and the numerical aperture of the collecting optics. By using the complete expression for the collection efficiency and varying the collection angle, we have determined the orientation of the dipole of a single terrylene molecule in hexadecane to be almost perpendicular to the substrate. By using saturation data we find that the angle between the optical axis of the collection paraboloid and the emission dipole moment is 16 +/- 6 degrees. We estimate that the true saturation intensity of terrylene (for dipole moment parallel to the electric field of the light) is about (80+/-30) mW/cm(2) and that the ratio of the intersystem crossing rate to the total decay rate of the triplet state is about 3.3 +/- 1.2.