We analyze a time delay of one- and twoelectron photoemission from an atom or a molecule after absorption of an attosecond XUV pulse. We establish this delay by solving the time dependent Schrödinger equation (TDSE) and by subsequent tracing of the fieldfree evolution of the photoelectron wave packet. This delay can also be related to the energy derivative of the phase of the complex photoionization amplitude which makes the time delay a sensitive probe of many-electron correlation and the means to perform a complete photoionization experiment.

This technique was first used to analyze the relative time delay between photoemission from the 2s and 2p shells in Ne. The experimental time delay difference of τ2p-τ2s = 21±5 as was reported while theoretical modeling could only account for less than a half of this value. This disparity of results opened up a wide discussion but subsequent theoretical papers broadly confirmed our initial prediction. Our next application of the time delay analysis was in a joint experimental and theoretical study of the 3s photoionization of argon near the Copper’s minimum which is affected strongly by correlation with the 3p shell. Because of this inter-shell correlation, the relative time delay τ3p-τ3s is large and negative in Ar while the corresponding delay τ2p-τ2s is small and positive in Ne, where correlation plays no role.

In theoretical study of single-photon double ionization (DPI) of He, we demonstrated that an attosecond time delay measurement can distinguish between the two leading correlational mechanisms of this process. The shakeoff mechanism is driven by a fast rearrangement of the atomic core after departure of the primary photoelectron. In contrast, the knockout mechanism involves a slow interaction of the primary photoelectron with the remaining electron bound to the singly charged ion. In molecules, unlike in atoms, the photoemission time delay becomes sensitive to molecular orientation relative to the polarization axis of light. When the molecular orientation is known, like in the case of DPI of H2, the time delay can be used as a delicate tool of the phase analysis. The time-delay is shown to have poles at the point corresponding to the kinematic nodes of the reaction. Study of the delay in the vicinity of a pole can provide information about phases of the DPI amplitudes. In single electron photoionization, when the molecule does not break up, the time delay measurement can be used to determine the molecular orientation.


Seminar, September 12, 2012, 16:00. Seminar Room

Hosted by Prof. Jens Biegert