The low-temperature dynamics of rhodamine 640 in polyvinylalcohol

Abstract

The low temperature (1.2–2.1 K) dynamics of rhodamine 640 in polyvinyl alcohol have been investigated by 2-pulse photon echo measurements. The low temperature dephasing results of this study cannot be fit utilizing only the tunneling two-level-system (TLS) model used to describe the higher temperature (10–30 K) data of Nakanishi et al. [J. Chem. Phys. 100, 3442 (1994)] on the same system. In the present study, both the low temperature (1.2–2.1 K) data and the high temperature (10–30 K) data of Nakanishi et al. are fit by a model which includes both a TLS and an electron–phonon contribution to the optical dephasing, yielding a homogeneous dephasing rate (T2*)−1=aTα+b(exp(ΔE/kT)−1)−1, where α=4/3, a=(2915 ps)−1, b=(2.9 ps)−1 and ΔE=16.7 cm−1. The present results indicate that TLS models alone are not sufficient to describe the optical dynamics of Rh640 in PVA at temperatures above 5 K. This is particularly important for studies which attempt to use chemical or physical modification of the matrix to investigate the nature of TLS.