Distinguishing the glass, crystal, and quasi-liquid layer in 1-methylnaphthalene by using fluorescence signatures

The fluorescence of crystalline 1-methylnaphthalene is monomer-like, while liquid and glass exhibit excimeric emissions. We detail the temperature dependence of the fluorescence emission and excitation spectra in the range of between 77 K and 295 K. These spectra provide exhaustive information about the state and temperature of 1-methylnaphthalene. The glass, formed by abrupt quenching in liquid nitrogen or methane, devitrifies at (155 +/- 5) K, and the liquid then undergoes cold crystallization at around 170 K. In 1-methylnaphthalene crystals, an excimeric emission appears at approximately 40 K below the melting point, a process we ascribe to the formation of dimers due to surface premelting; such a quasi-liquid layer exists at the surface well below the freezing point, remaining uncrystallized. The premelted layer is clearly distinguishable from the bulk glass via fluorescence spectroscopy, which facilitates state identification.