Picosecond excitation and selective intramolecular rates in supersonic molecular beams. IV. Alkylanthracenes

Abstract

To assess the role of alkylation on IVR, the dynamics of jet cooled 9-methyl and 9-hexylanthracene excited to single vibronic levels (SVL) in S1 are investigated and compared with the parent molecule, anthracene, whose picosecond IVR dynamics are now well characterized. Vibrations in S1 and S0 are analyzed. Decay rates and SVL fluorescence spectra are also presented. The decay rates as a function of excess vibrational energy increase rapidly at low energy but become relatively constant at high energy. The approximate energy threshold at which the decay rate ‘‘saturates’’ is dependent on the substitutent; anthracene (≊1800 cm−1), 9-methylanthracene (≊1000 cm−1), 9-hexylanthracene (≊400 cm−1), and A–(CH2)3–φ (≤400 cm−1). These identified thresholds are discussed and related to IVR processes. Finally, some comments on the importance of low frequency modes to IVR are given.