Temperature dependence of optical dephasing in an organic polymer glass (PMMA) from 300 K to 30 K

The temperature dependence of the optical dephasing mechanism in an organic polymer glass, polymethylmethacrylate ͑PMMA͒, was studied from 300 K to 30 K using the dye IR144 as a probe. Transient grating and three pulse photon echo measurements were made, and the three pulse photon echo peak shift ͑3PEPS͒ was recorded as a function of temperature. The peak shift data reveal time constants of ϳ6 fs and ϳ60 fs, along with vibrational beats and a long-time constant value for the peak shift. The 6 fs component is attributed to intramolecular vibrations and the 60 fs component to librational degrees of freedom of the PMMA itself. This contribution appears slightly underdamped and the fitted spectral density matches well with the Raman spectrum of PMMA. The two ultrafast decays are insensitive to temperature. For temperatures above 80 K the long-time peak shift increases linearly as temperature decreases but at 80 K the shift levels off and decreases for temperatures between 80 and 30 K. Fit values for the inhomogeneous width ͑500 cm Ϫ1 ͒ and the reorganization energy ͑378 cm Ϫ1 ͒ describe the initial value of the peak shift, its decay, the absorption spectrum, and the three-pulse photon echo signal quite well at both high and low temperature. We were not very successful in describing the temperature dependence of the long-time peak shift, although the insensitivity of the dynamics to temperature could be qualitatively accounted for. At low temperature the imaginary portion of the line shape function, which is temperature independent, contributes significantly to the response, while at high temperature the dephasing is dominated by the real part of the line shape function. A more sophisticated model is required to quantitatively describe the data.