Does the standard model of low-temperature glass dynamics describe a real glass?

We present the first experimental single-molecule (SM) spectroscopy study of the local dynamics in molecular glasses (frozen toluene and deuterated toluene weakly doped with a substituted terrylene) at T=2-30 K. Surprisingly, the dynamics does not follow the standard model of low-temperature glasses. An extra contribution to the dynamics was detected causing drifts and irreproducible jumps of the SM spectra. The isotope effects in the SM linewidth distribution and the density of states (boson peak) show that the fast dynamics at T=2 K is due to excitations of tunneling nature, whereas at T>7 K it is related to vibrations; H/D atoms are directly involved in both types of excitations.