Unravelling overlaps and torsion-facilitated coupling using two-dimensional laser-induced fluorescence

• Published in: Molecular physics 2019-11, Vol.117 (21), p.3011-3026
• Main Authors: Kemp, David J., Gardner, Adrian M., Tuttle, William D., Wright, Timothy G.
• Format: Article
• Language: English
• Subjects: 2D-LIF; Coupling; Coupling (molecular); dispersed fluorescence; Excitation spectra; fluorescence excitation; Laser induced fluorescence; LIF; Spectrum analysis; Symmetry; Torsion; torsions; Wavelengths
• ISSN: 0026-8976; 1362-3028
• DOI: 10.1080/00268976.2018.1554865

Two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy is employed to identify contributions to fluorescence excitation spectra that arise from both overlapping bands and coupling between zero-order states (ZOSs). Evidence is found for the role of torsional motion in facilitating the coupling between vibrations that particularly involves the lowest-wavenumber out-of-plane vibrational modes. The experiments are carried out on jet-cooled p-fluorotoluene, where the molecules are initially in the lowest two torsional levels. Here we concentrate on the 390-420 cm −1 features in the S 1  ← S 0 excitation spectrum, assigning the features seen in the 2D-LIF spectrum, aided by separate dispersed fluorescence spectra. The 2D-LIF spectra allow the overlapping contributions to be cleanly separated, including some that arise from vibrational-torsional coupling. Various coupling routes open up because of the different symmetries of the lowest two torsional modes; these combine with the vibrational symmetry to provide new symmetry-allowed vibration-torsion ('vibtor') interactions, and the role of the excited m = 1 torsional level is found to be significant.