Recent advances in laser technology have led to the generation of attosecond laser pulses, whose time duration is in the range of the natural time scale of electron dynamics, and thus make the observation and even control of electron dynamics in atoms and molecules possible. While the single-electron dynamics is more thoroughly studied, the correlated dynamics of two electrons is less understood, especially in the context of resolving the ultrafast temporal information in double photoionization.
In this thesis, we first study the energy exchange via electron correlation upon photon absorption over large distances in the two-site double photoionization of the helium dimer, which is found to be a two-step process. In the first step, one electron in one atom absorbs the photon and gets ionized. In the second step, this electron propagates towards the neighboring atom and knocks out the other electron. We then introduce the Hamiltonian reduction method to further study the effects of different interactions in the single and double photoionization of the helium dimer. In addition, two-center interferences in the double ionization of the dimer are studied.
Next, we analyze the selection rules for the emission of two electrons from the helium atom, the helium dimer, and general molecules following the absorption of a few photons in an intense laser field. In particular, the back-to-back emission of the two electrons with equal energy sharing is either suppressed or not depending on the number of photons absorbed from the field.
Finally, we study the time delay between the single and double photoionization processes. We first propose a self-consistent-time method to account for the Coulomb-laser coupling effect and obtain the intrinsic photoabsorption time delay measured by the attosecond streak camera. We then proceed to time resolve the correlated emission of two electrons in the knockout process of the helium dimer with respect to the first step of single ionization.