OPTICAL-DETECTION OF MAGNETIC-RESONANCE IN A SINGLE MOLECULE

MAGNETIC resonance spectroscopy1 is a powerful tool for molecular characterization and structure determination. The sensitivity of conventional approaches is limited to about 10(10) electron spins or 10(16) nuclear spins; this sensitivity can be improved to about 10(5) spins by polarizing the spins via optical pumping and detecting optical rather than microwave photons2. Recently, fluorescence from single molecules was detected by tuning a single-frequency laser in the inhomogeneously broadened fluorescence excitation band of a dilute dispersion of pentacene in a host crystal of p-terphenyl3,4. Here we report that, by combining single-molecule fluorescence spectroscopy with optically detected magnetic resonance for the pentacene-doped p-terphenyl system, we can detect magnetic resonance in a single pentacene molecule. We observe two of the three possible transitions between sublevels of the metastable triplet state. The spectral lineshapes indicate that the proton nuclear spin states change during the measurement, leading to spectral diffusion within the magnetic resonance line.