Exposure to UV light induces damage to the human DNA. This process, including many others, is mediated by so-called Conical Intersections (CIs). Their understanding is important in, for example, medical developments, highly efficient solar cells, and photochemical processes.

CIs are formed when two or more electronic states become degenerate. The so-called Born-Oppenheimer approximation, which allows for the formal separation of nuclear and electronic degrees of freedom, breaks down near a CI. The molecules can funnel back to the ground state via the strongly coupled electronic states in the vicinity of a CI, thus creating a non-radiative decay channel.
 

Optical Nano Cavities

When a molecule passes through a CI, a superposition of electronic states is created due to the branching of the nuclear wave packets.  This superposition creates an electronic coherence, which can be considered a unique signature of the CI. The electronic states populated by the branching of nuclear wave packets can, in principle, be observed using femtosecond pulses in the energy domain.

Whereas, probing the generated electronic coherence via ultra-fast oscillation in the time domain requires a spectral bandwidth of several electron volts, which can only be provided by attosecond pulses. However, the uncertainty principle prohibits most spectroscopic techniques from resolving phenomena in the time and energy domain simultaneously.

Researchers at Fysikum have proposed a technique, which uses attosecond pulse trains  to observe CIs rather than isolated attosecond pulses. The theoretical study suggests that it is possible to simultaneously observe the branching of nuclear wave packets into the states involved in CI and the generated electronic coherence via photoelectron spectroscopy.  The special properties of such attosecond pulse trains allow for partially bypassing the restriction imposed by the time-energy-uncertainty principle. The major advantage of this idea is that attosecond pulse trains are readily available in state-of-the-art laboratories by means of the high-harmonic generation process.

More information
Journal reference:
Deependra Jadoun, Markus Kowalewski, “Time-Resolved Photoelectron Spectroscopy of Conical Intersections with Attosecond Pulse Trains”, J. Phys. Chem. Lett., 12, 8103 (2021).
https://doi.org/10.1021/acs.jpclett.1c01843

Scientific Contact:
Markus Kowalewski, markus.kowalewski@fysik.su.se