Bicircular Attoclock with Molecules

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In this theory project, we investigate the dynamics of electrons in diatomic molecules on the attosecond time scale. We use a specially tailored form of intense light, known as a bicircular field, which is a superposition of two counter-rotating circularly polarized laser fields of different colors. The time-dependent Schrödinger equation is solved to calculate the photoelectron momentum distributions resulting from ionization of molecules by bicircular fields. From these results we infer information about the attosecond-scale temporal evolution of strong-field ionization (hence the name attoclock) and its dependence on the molecular orientation. The location of the maximum in the momentum distribution provides information about the trajectory of the outgoing electron. In this way, the attoclock serves as a nano-ruler to retrieve the birth position of the electron and the asymptotic shape of the potential. The ionization time for a specified momentum is obtained by analyzing the response to an additional probe field with variable delay. In contrast to previous attoclock schemes using light with near-circular polarization, the bicircular field has the desirable property that it can be tailored such that the ionizing-field direction is nearly constant during the escape of the electron. Moreover, in contrast to conventional linear polarization, this field avoids complications such as rescattering or interference of electron trajectories.To analyze and interpret the numerically calculated momentum distributions, we use models based on Newtonian electron trajectories. In particular, we extend the classical backpropagation method from atoms to molecules. Furthermore, we use a trajectory-free, purely quantum-mechanical method to find the ionization time from an integral representation of the time-dependent Schrödinger equation.