Attosecond Core-level Photoionization Delays of Aromatic Molecules

We conducted attosecond angular streaking on the K-shell photoionization of a series of azabenzenes (pyridine, pyrimidine, and 1,3,5-triazine). The core electrons are ionized by the attosecond soft-X-ray pulses from the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory [1], and the photoelectron momentum is modulated by a co-propagating femtosecond IR dressing field (1300 nm) which encodes the ionization time to the angular distribution into the coaxial velocity-map-imaging (c-VMI) detector perpendicular to the light-propagation direction [2]. The relative photoionization delays between N-1s and C-1s have been measured at photon energies ranging from 400-450 eV, where the C-1s delays are negligibly small because of the high kinetic energy of the electrons. The measured ionization time delay shows two maxima in this range for all of the three azabenzenes, which are assigned to shape resonances [3] and are reproduced by core-level calculations using the Schwinger variational principle with a correlation-polarization potential. The observed increasing trend of time delay in the sequence pyridine-pyrimidine-1,3,5-triazine at both resonances is theoretically interpreted by the trapping of the photoelectron by the molecular potential, which is modulated by the number of nitrogen atoms in the aromatic rings and the symmetry-restricted coupling between the high- and low-angular-momentum states.



References

[1] Duris J et al 2020 Nat. Photonics 14 30-36

[2] Li S et al 2022 Science 375 285-290

[3] Huppert M et al 2016 Phys. Rev. Lett. 117 093001