Identification of separate isoenergetic routes for vibrational energy flow in p-fluorotoluene

A deceptively simple feature in the S₁ ← S₀ spectrum of p-fluorotoluene (pFT), 1013 cm^-1 above the origin, is studied using both zero-electron-kinetic-energy (ZEKE) and two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy. It is found to consist of a cornucopia of overlapped transitions to eigenstates that arise from numerous interacting levels. A significant variation in the activity is seen employing both the ZEKE and 2D-LIF techniques. Detailed insight into the complicated spectra can be achieved, owing to the large number of vibrational wavenumbers that have been previously determined for the S₀, S₁, and D₀ ⁺ states, summarized herein. It is found that the activity is dominated by two overtones, which are individually interacting with other levels, so providing largely independent routes for vibrational energy flow at the same internal energy. Additionally, other weak features located 900-1050 cm^-1 above the origin are examined.