Wigner time delay in the $4d$ giant-resonance region of iodoalkanes

Abstract

The iodine $4d$ giant resonance provides an unusually strong and predominantly iodine-centered photoionization channel in iodoalkanes. Recent attosecond measurements by Schröder et al. [Phys. Rev. A 112, L051104 (2025)] reported pronounced, nonmonotonic, and isomer-dependent variations of the streaking-derived $4d$ photoemission delay across the molecular series. We calculate field-free molecular Eisenbud--Wigner--Smith (EWS) delays from photoionization amplitudes represented in a multicenter B-spline basis of the time-dependent density-functional theory (TDDFT). A tensor formulation is developed in which the delay for randomly oriented molecules is evaluated from the same rank-0 and rank-2 contractions that determine the cross section and anisotropy parameter. The resulting TDDFT delays of CH$_3$I, C$_2$H$_5$I, 2-iodopropane, and both calculated conformers of 1-iodopropane form a narrow bundle throughout the 90--118~eV region and closely follow atomic-I and I$^-$ reference calculations. This robustness is consistent with the similarly weak molecular dependence of the giant resonance cross section and angular distribution, but it is in marked contrast with the much larger molecular variation found by streaking experiments. The asymptotic Coulomb--laser-coupling/continuum--continuum correction is controlled mainly by the photoelectron energy and residual charge and is therefore expected to be nearly common to the iodoalkane series. This does not exclude target-specific molecular IR effects beyond the universal Coulomb term.