Two-dimensional laser induced fluorescence spectroscopy of van der Waals complexes: Fluorobenzene-Arn (n = 1,2)

Abstract

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 $\overline {0_0^0 }$000¯ 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.

Methyl rotor dependent vibrational interactions in toluene

“…This is analogous to the previously reported situation for observing absorption transitions associated with van der Waals modes. 17 The rotor energies in both S 1 and S 0 can be determined from the positions of these features in the 2D-LIF spectral image. The S 0 rotor energies can also be observed via weak m = 0 emission features.…”

“…Our investigation is based on the technique of two-dimensional laser induced fluorescence (2D-LIF). [16][17][18][19][20][21][22][23][24][25] This technique involves measuring a region of the dispersed fluorescence spectrum as the laser is scanned over absorption features. The dispersed fluorescence effectively provides a filter that allows different species to be separated.…”

Intermolecular vibrations of fluorobenzene-Ar up to 130 cm−1 in the ground electronic state

“…11 (FB levels and transitions are shown with no bar, FB-Ar transitions with a single bar, and FB-Ar 2 transitions with a double bar.) The spectrum shown is a composite of three separately acquired segments.…”

“…11,13 The salient features for the present experiments are as follows. The frequency doubled output of a Nd:YAG pumped dye laser (coumarin 503 laser dye; 0.3 cm −1 doubled line width; 10 Hz repetition rate) is passed vertically through a stainless steel chamber containing the pulsed nozzle source, the vertical propagation matching the vertical entrance slit of the spectrometer.…”

“…This is significantly above both our previous work and the limits of reported ab initio calculations. 8,9,11 FB-Ar was the subject of this study for several reasons. First, as noted above, the results of high level ab initio calculations have been reported for this complex [8][9][10] and the experimental data provide a test of their accuracy.…”