Wave Packet Movements near the Conical Intersection between Two Excited Potential Surfaces May Create Observable Molecular Oscillations

A femtosecond pump-probe experiment is performed on tetrakis(dimethylamino)ethylene. The evolution resulting from the $\mathrm{\pi }\mathrm{-}{\mathrm{\pi }}^{\mathrm{*}}$ excitation of the CC double bond corresponds to movement along a single adiabatic potential surface with deformation along several coordinates and passage near a conical intersection. Surprisingly, this movement excites the umbrella mode of the amino groups, resulting in a measurable oscillation regime.

Figure 1

Sketch showing the conical intersection between the $V$ and $Z$ molecular states of TDMAE. The white arrow pictures the wave packet movement out of the Franck-Condon as explained in the text.

Figure 2

Signals measured with the TOF-MS set at masses 185 amu [curves (a)] and 200 amu [curves (b)] as a function of the pump(266 nm)-probe(400 nm) delay. Curve (c) recalls a 0 eV photoelectron signal recorded using a different probe wavelength, 800 nm [4]. The solid lines passing through the experimental points are fits, as explained in the text. The right inset is a Fourier transform of curves (a).