Spin-orbit delays and relativistic effects in methyl iodide

The measurement of attosecond time delays can be used to study the timing of electronic dynamics related to photoionization in atoms and molecules. While previous studies have measured the impact of relativistic corrections on atomic time delays, similar work has not been performed for molecular delays. It is possible to measure molecular time delays via a halogen atom which acts as a probe of the molecular environment. However, relativistic effects become increasingly relevant as the mass of the probe increases. In this work, we extend the study of molecular time delays by comparing the relative delays between the photoelectron wave packets of the E(1/2) and E(3/2) spin-orbit states of methyl iodide. We observe a maximum spin-orbit delay of 98 ± 46 as near threshold, followed by a sharp decrease. We compare the measured values to an ab initio relativistic atomic theory for I- and a non-relativistic molecular theory with an artificial spin-orbit splitting. By comparing the two theories, the behavior of the spin-orbit delay can be explained as being the combined result of spin-orbit activated interchannel coupling (SOIAIC) and spin-flip transitions. When the number density of two-electron transitions is large, we also observe a significant increase in the relative delay between spin-orbit split states. We are the first to measure a molecular spin-orbit delay, thereby highlighting the significant role of relativistic corrections on molecular time delay measurements.