When a sample molecule is excited to high vibrational levels in the excited state, rapid intramolecular relaxation occurs by an interaction with a dense manifold of vibrational levels in the ground state. Due to vibronic mixing, a combination of vibrational quanta is redistributed and the state is no longer identical with the initial state excited. Therefore, a complicated and broad-band structure appears in a conventional excitation spectrum, due to breakdown of selection rules. This common feature in supersonic jet spectrometry makes It difficult to assign a specific component even for a sample containing only a single interference compound. For example, selective detection of β-naphthol is difficult for a sample containing anthracene, and this situation is similar to m-toluidine and 2-methylnaphthalene. However, this unwanted photoemission is completely suppressed by a combination of supersonic Jet spectrometry and synchronous scan luminescence spectrometry monitoring resonance fluorescence (SSJ/R-SSL). Furthermore, the number of spectral lines is reduced, giving a simpler spectrum. Thus SSJ/R-SSL spectrometry provides a useful analytical means for selective analysis of organic molecules such as polycyclic aromatic hydrocarbons.
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