Isotope effect in the linewidth distribution of single-molecule spectra in doped toluene at 2Â K

The low-temperature dynamics of two different varieties of a low-molecular glass-protonated toluene (C6H5CH3) and perdeuterated toluene (C6D5CD3)-weakly doped with tetra-tert-butylterrylene (TBT) molecules was investigated with single-molecule spectroscopy. In both glassy matrices the distributions of the spectral widths were measured for a large number of single TBT molecules at T=2 K. The marked shift of the distribution upon deuteration of matrix was observed and attributed to an isotope effect which is qualitatively analyzed. It was found that at this temperature the tunneling dynamics of the matrix dominates the broadening behavior and hydrogen/deuterium atoms are involved in the tunneling motions.