Two-pulse photon echoes from zinc-meso-tetraphenylporphine/polymethylmethacrylate are not consistent with the tunneling two-level system model

We argue that the results of a recent two-pulse photon echo study below 2 K on the system zinc-meso-tetraphenyIporphine/polymethyl [S. J. Zilker and D. Haarer, J. Chem. Phys. 105, 10648 (1996)] are inconsistent with the tunneling two-level system model of low temperature glasses. More specifically, the experimental echo decay is exponential in time, and the decay rates are linear in temperature, while the theoretical predictions from the model are nonexponential in time and nonlinear in temperature, for the parameters that appear to be appropriate for this system. (C) 1998 American Institute of Physics. [S0021-9606(98)03120-1]