Gazing Down a Funnel

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Science  25 Apr 2008:
Vol. 320, Issue 5875, pp. 426
DOI: 10.1126/science.320.5875.426c

Because electrons generally move about much more rapidly than nuclei, most chemical reactions are modeled using a framework of potential energy surfaces in which effectively instantaneous electronic transitions between surfaces precede vibrational rearrangements confined to a single surface. However, this framework can break down in certain polyatomic reactions that couple vibrational and electronic motion through a feature linking two surfaces in a cone-shaped, or conical, intersection. Farrow et al. use ultrafast spectroscopy to extract the precise timing and details of vibrational coupling as electrons rush down through such a funnel in the energetic landscape after excitation of a square planar naphthalocyanine molecule coordinated to a central silicon moiety. Specifically, they monitor the polarization anisotropy decay of the electronic absorption signal, upon which periodic intensity fluctuations are superimposed that correspond to coherent vibrational motion. Modeling of the data supports a transition time of <200 fs for the relatively modest relaxation energy pertaining in this molecule; the electrons still outpace the nuclear vibrations, though only by a small margin. The data suggest that in chemical reactions with much higher driving forces, transitions through conical intersections could occur within several femtoseconds. — JSY

J. Chem. Phys. 128, 144510 (2008).

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