Study of intersystem crossing in naphthalene and 1-methylnaphthalene in collision-free conditions and pressure effects

Intersystem crossing processes were studied for gaseous naphthalene and 1-methylnaphthalene excited at 265 nm by the fourth harmonic of a Nd glass laser, in collision-free conditions and in the presence of inert gas (SF6). Using a method based on wavelength-selective quenching of a dye laser, we obtained the absorption spectra of the thermalized triplet as well as of triplet molecules in the initially populated levels (isoenergetic with the excited singtet ones). The pressure effects on the triplet growth and decay were determined by kinetic spectrophotometry; fluorescence decay measurements were also made. It was shown that the triplet is populated efficiently in the absence of collisions and that the S-T transition is practically irreversible; the initially populated triplet levels are depopulated mainly by vibrational relaxation even at the lowest pressures (similar to 0.1 torr).