Fluorescence excitation spectra of 1-methyl-, 2-methyl-, and 2-ethyl-naphthalenes in a supersonic free jet.

Fluorescence excitation spectra of 1-methyl-, 2-methyl- and 2-ethyl-naphthalenes have been observed with the excitation of a xenon lamp in a supersonic free jet. The excitation spectrum of 1-methylnaphthalene was much the same with that of naphthalene, while 2-methyl- and 2-ethyl-naphthalene molecules exhibited entirely different spectral profile from 1-methylnaphthalene. Methylation or ethylation on the position 2 of naphthalene ring caused an increase of the origin intensity and changes of vibronic band frequency, which can be explained by symmetry reduction and mode mixing.