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Direct mapping of curve-crossing dynamics in IBr by attosecond transient absorption spectroscopy

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Science  05 Jul 2019:
Vol. 365, Issue 6448, pp. 79-83
DOI: 10.1126/science.aax0076

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A panoramic view of photodissociation

As light pulses get shorter in time, they correspondingly get broader in frequency. Kobayashi et al. take advantage of both properties of attosecond pulses to elucidate iodine monobromide (IBr) photodissociation by detecting ultrafast bromine and iodine spectral shifts simultaneously. A preliminary burst of light weakens the I–Br bond. Then, as the atoms fly apart, they reach a configuration where the bond vibration can couple ground and excited electronic states. The broadband probe pulse reveals rapid changes in each atom's electronic structure at this juncture.

Science, this issue p. 79

Abstract

The electronic character of photoexcited molecules can abruptly change at avoided crossings and conical intersections. Here, we report direct mapping of the coupled interplay between electrons and nuclei in a prototype molecule, iodine monobromide (IBr), by using attosecond transient absorption spectroscopy. A few-femtosecond visible pulse resonantly excites the B(Π30+), Y(0+), and Z(0+) states of IBr, and the photodissociation dynamics are tracked with an attosecond extreme-ultraviolet pulse that simultaneously probes the I-4d and Br-3d core-level absorption edges. Direct comparison with quantum mechanical simulations unambiguously identifies the absorption features associated with adiabatic and diabatic channels at the B/Y avoided crossing and concurrent two-photon dissociation processes that involve the Y/Z avoided crossing. The results show clear evidence for rapid switching of valence-electronic character at the avoided crossing.

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