Two-dimensional laser induced fluorescence spectroscopy of van der Waals complexes: fluorobenzene-Arn (n=1,2)
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The technique of two-dimensional laser induced fluorescence (2D-LIF) spectroscopy has been used to observe the van der Waals complexes fluorobenzene-Ar and fluorobenzene-Ar2 in the region of their S1-S0 electronic origins. The 2D-LIF spectral images reveal a number of features assigned to the van der Waals vibrations in S0 and S1. An advantage of 2D-LIF spectroscopy is that the LIF spectrum associated with a particular species may be extracted from an image. This is illustrated for fluorobenzene-Ar. The S1 van der Waals modes observed in this spectrum are consistent with previous observations using mass resolved resonance enhanced multiphoton ionisation techniques. For S0, the two bending modes previously observed using a Raman technique were observed along with three new levels. These agree exceptionally well with ab initio calculations. The Fermi resonance between the stretch and bend overtone has been analysed in both the S0 and S1 states, revealing that the coupling is stronger in S0 than in S1. For fluorobenzene-Ar2 the 2D-LIF spectral image reveals the S0 symmetric stretch van der Waals vibration to be 35.0 cm−1, closely matching the value predicted based on the fluorobenzene-Ar van der Waals stretch frequency. Rotational band contour analysis has been performed on the fluorobenzene-Ar math transition to yield a set of S1 rotational constants A′ = 0.05871 ± 0.00014 cm−1, B′ = 0.03803 ± 0.00010 cm−1, and C′ = 0.03103 ± 0.00003 cm−1. The rotational constants imply that in the S1 00 level the Ar is on average 3.488 Å from the fluorobenzene centre of mass and displaced from it towards the centre of the ring at an angle of ∼6° to the normal. The rotational contour for fluorobenzene-Ar2 was predicted using rotational constants calculated on the basis of the fluorobenzene-Ar geometry and compared with the experimental contour. The comparison is poor which, while due in part to expected saturation effects, suggests the presence of another band lying beneath the contour.