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Methyl rotor dependent vibrational interactions in toluene
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10.1063/1.4795439
/content/aip/journal/jcp/138/13/10.1063/1.4795439
http://aip.metastore.ingenta.com/content/aip/journal/jcp/138/13/10.1063/1.4795439

Figures

Image of FIG. 1.
FIG. 1.

(a) A 2D-LIF spectral image near the ( , ) band region of toluene. The series of horizontal features at the top of the image are associated with absorption from m = 0 and 1 to various methyl rotor states in S1. The series of vertical features on the left of the image are associated with emission from m′ = 0 and 1 to various methyl rotor states in S0. The weak sequence of horizontal features near 37 450 cm−1 is due to transitions of the toluene-Ar van der Waals complex (Ref. 49 ). Assignments for the toluene features are given in the figure. The horizontal axis at the top of the figure shows the displacement relative to the band at 37 476.6 cm−1 (Ref. 26 ). (b) The LIF spectrum extracted from (a) by vertical integration of a horizontal slice encompassing the Δm = 0 emission features. The horizontal slice covered the emission region 37 467 to 37 485 cm−1. The small peak at +4.3 cm−1 is due to 13C isotopomers in natural abundance (Ref. 18 ).

Image of FIG. 2.
FIG. 2.

LIF spectra of the and components associated with (a) , (b) , (c) , and (d) absorption transitions. These spectra have been extracted from 2D-LIF spectral images by integrating vertically over the and emission bands, respectively. The spectra in (a) were extracted from Fig. 1 , the spectra in (b) and (c) were extracted from Fig. 4 , and the spectra in (d) were extracted from Fig. 3 . This illustrates that the m′ = 0 and m′ = 1 levels associated with a S1 vibration can be monitored separately through an appropriate choice of emission wavelengths. The horizontal axis shows the displacement relative to the band at 37 476.6 cm−1 (Ref. 26 ).

Image of FIG. 3.
FIG. 3.

(a) 2D-LIF spectral image for the ( , ) band and associated m transitions. The image is analogous to that shown in Fig. 1 for the ( , ) region. The 142−111−191201 Fermi resonance is revealed through the two intense features in the top left hand corner of the image where the single ( , ) feature would otherwise be seen. The assignments for selected features are displayed in the figure. The Δm = 0 transitions are omitted from the labels while the Δm ≠ 0 transitions are only labeled for the ( , ) features. The horizontal axis at the top of the figure shows the displacement relative to the band at 37 476.6 cm−1 (Ref. 26 ). (b) The LIF spectrum is obtained by vertically integrating the 2D-LIF spectral image.

Image of FIG. 4.
FIG. 4.

The 2D-LIF spectral image obtained by scanning the laser over the absorption features , , , , , while monitoring emission from the excited states to 111 m 0, 111 m 1, 111 m 2, and 111 m 3(+). This image reveals the 111 component of the coupled states at m = 0 and 1. The horizontal axis at the top of the figure shows the displacement relative to the band at 37 476.6 cm−1 (Ref. 26 ).

Image of FIG. 5.
FIG. 5.

The 2D-LIF spectral image obtained by scanning the laser over the same absorption features shown in Fig. 4 but with emission monitored to 191201 m 0 and 191201 m 1, revealing the 191201 component in the coupled states. The horizontal axis at the top of the figure shows the displacement relative to the band at 37 476.6 cm−1 (Ref. 26 ).

Image of FIG. 6.
FIG. 6.

The 2D-LIF spectral image obtained by scanning the laser over the same absorption features shown in Fig. 4 but emission monitored to 142 m 0 and 142 m 1 (the transitions to 142 m 2 and 142m3(+) are too weak to be observed here), revealing the 142 component in the coupled states (this is the lower set of features in the figure). The horizontal axis at the top of the figure shows the displacement relative to the band at 37 476.6 cm−1 (Ref. 26 ).

Image of FIG. 7.
FIG. 7.

The 2D-LIF spectral image obtained by scanning the laser over the region where the absorption features , , and are expected while monitoring emission from the excited states to 111m3(+). This reveals the 111 component of the Femi resonance at m′ = 3a 1. Only one band is seen, indicating that the resonance is so weak that it is unobserved here. The feature at the top left hand corner of the figure is associated with excitation of the band of toluene-Ar (Ref. 48 ), which dissociates prior to emission, populating rotational states of the toluene 00 level (see also Fig. 3(b) ). The vertical series of features on the right hand side of the figure is associated with emission following excitation of . The horizontal axis at the top of the figure shows the displacement relative to the band at 37 476.6 cm−1 (Ref. 26 ).

Image of FIG. 8.
FIG. 8.

An energy level diagram showing the relative energies of the 142−111−191201 Fermi resonance coupled states at m = 0 and m = 1 and the energies of the “zero-order” states obtained by deperturbing the local Fermi resonance as discussed in the text. The obtained coupling matrix elements, V, coupling the “zero-order” states are given in the diagram. The energy of the perturbed 142 m 1 level is calculated to be 5.31 cm−1 above the perturbed 142 m 0 level.

Image of FIG. 9.
FIG. 9.

An energy level diagram showing the relative energies of the 111−191 torsion-vibration coupled states and the energies of the “zero-order” states obtained by deperturbing the coupling as discussed in the text. The obtained coupling matrix elements, V, are given in the diagram.

Image of FIG. 10.
FIG. 10.

REMPI spectrum in the region of the 142−111−191201 Fermi triad provided by Gardner et al. (Ref. 27 ).

Image of FIG. 11.
FIG. 11.

An energy level diagram showing the coupling scheme deduced for the anharmonic and torsion-vibration couplings influencing the 142−111−191201 Fermi triad and responsible for its m dependence.

Tables

Generic image for table
Table I.

A comparison of the different nomenclatures and mode numbering schemes used to label the vibrational modes of toluene.

Generic image for table
Table II.

The calculated methyl rotor levels in 00 and 00 toluene based on the constants determined in high resolution studies.

Generic image for table
Table III.

The relative intensities extracted from the 2D-LIF images shown in Figs. 4–6 . The values determined from the calculated eigenvectors (Table VI ) are shown for comparison.

Generic image for table
Table IV.

The energy separations of the molecular eigenstates observed for the m = 0 and m = 1 levels of the 142−111−191201 resonance. The values determined from quantum beat experiments are also shown.

Generic image for table
Table V.

The results of the deconvolution for m = 0 and m = 1 levels of the 142−111−191201 resonance. a

Generic image for table
Table VI.

The eigenvector matrices determined in this study and that determined by Davies et al. (Ref. 7 ). The eigenvector matrix provides the coefficients for the molecular eigenstates expressed as linear combinations of the zero-order states, assuming the local Fermi resonance to be the only perturbation.

Generic image for table
Table VII.

The rotor-vibration levels calculated to lie within ±100 cm−1 of 111 m 0 and 111 m 1 and possessing the correct symmetry to couple via a torsion-vibration mechanism.

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/content/aip/journal/jcp/138/13/10.1063/1.4795439
2013-04-02
2014-04-25
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Methyl rotor dependent vibrational interactions in toluene
http://aip.metastore.ingenta.com/content/aip/journal/jcp/138/13/10.1063/1.4795439
10.1063/1.4795439
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