Investigation of electronic excitation dynamics in molecular crystals via picosecond laser spectroscopy

Although the spectroscopy of aromatic crystals such as naphthalene has been known for decades, recent advances in laser spectroscopy have made it possible to study the dynamic nature of the molecular exciton in these systems. Electronic excitation dynamics studied by picosecond time-resolved laser spectroscopy in the molecular crystal naphthalene yields information about energy transfer which can not be obtained otherwise. The nature of these dynamics are affected greatly by the exciton type and exciton band structure of this system. Study of steady state spectroscopy gives information about the equilibrium population of the exciton states while time-resolved spectroscopy can show the effects of electronic energy transfer. Results showing the effects of impurity induced states on exciton trapping in $\beta$-methylnaphthalene doped naphthalene crystals are presented. Time-resolved and steady state spectroscopy are used to show the presence of secondary traps in this system. The data is interpreted using an energy funnel approach which is consistent with thermal assisted de-trapping of the secondary trap back into the exciton band even at low temperatures. Determination of intra-band exciton scattering is optically detected using picosecond time-resolved laser spectroscopy in the quasi-two-dimensional first singlet exciton of naphthalene. Thermalization of the exciton band is complete within a few hundred picoseconds for pure material. Comparing inelastic exciton scattering results of nominally pure crystal to those with substitutional and isotopic impuries can test impurity scattering effects on the inelastic scattering rate. Comparing results to one-dimensional exciton scattering mark the diverse method of approaching equilibrium with thermalization times differing by orders of magnitude