Photoelectron circular dichroism refers to the forward/backward asymmetry in the photoelectron angular distribution with respect to the propagation axis of circularly polarized light. It has recently been demonstrated in femtosecond multi-photon photoionization experiments with randomly oriented camphor and fenchone molecules [C. Lux et al., Angew. Chem. Int. Ed. 51, 5001 (2012); C. S. Lehmann et al., J. Chem. Phys. 139, 234307 (2013)]. A theoretical framework describing this process as (2+1) resonantly enhanced multi-photon ionization is constructed, which consists of twophoton photoselection from randomly oriented molecules and successive one-photon ionisation of the photoselected molecules. It combines perturbation theory for the light-matter interaction with ab initio calculations for the two-photon absorption and a single-center expansion of the photoelectron wavefunction in terms of hydrogenic continuum functions. It is verified that the model correctly reproduces the basic symmetry behavior expected under exchange of handedness and light helicity. When applied it to fenchone and camphor, semi-quantitative agreement with the experimental data is found, for which a sufficient d wave character of the electronically excited intermediate state is crucial.

We demonstrate coherent control over the photoelectron circular dichroism in randomly oriented chiral molecules, based on quantum interference between multiple photoionization pathways. To significantly enhance the chiral signature, we use a finite manifold of indistinguishable (1+1') REMPI pathways interfering at a common photoelectron energy but probing different intermediate states.We show that this coherent control mechanism maximizes the number of molecular states that constructively contribute to the dichroism at an optimal photoelectron energy and thus outperforms other schemes, including interference between opposite-parity pathways driven by bichromatic (ω, 2ω) fields as well as sequential pump-probe ionization.Chiral molecules are non-superimposable mirror images of each other, referred to as enantiomers. Recent advances in measuring enantiomer-sensitive observables in gas phase table-top experiments [1-4] have brought chiral molecules into the spotlight of current AMO research. One of these observables is photoelectron circular dichroism (PECD), i.e., the differential photoelectron angular distribution obtained by ionizing randomly oriented molecules with left circularly and right circularly polarized light [1,[5][6][7][8][9]. PECD is a purely electric dipole effect, yielding much stronger signals than traditional absorption circular dichroism (CD), which involves also the magnetic dipole of the probed transition. It can be quantified by the odd-moment coefficients in the expansion of the photoelectron angular distribution into Legendre polynomials. The simplest explanation for PECD is provided by perturbation theory for one-photon ionization [10]: It is the small difference in magnitude between dipole matrix elements with opposite sign m quantum number, occurring only for chiral molecules, that results in a net effect when averaging over all molecular orientations. More intuitively, two non-parallel vectors are needed to provide an orientation with which to probe the handedness of the molecular scaffold and create a pseudo-scalar observable. While in traditional CD these are the electric and magnetic dipole moment, the photoelectron momentum provides the second vector in PECD. This picture connects PECD with the general framework for electric-dipole-based chiral observables [11]. Perturbation theory can also explain the PECD observed in resonantly enhanced multi-photon ionization (REMPI) [1], in terms of the electronically excited intermediate state of the REMPI process [12]. Dependence of the chiral signal on excitation wavelength is then understood in terms of probing different intermediate states [13]. Whether PECD is amenable to coherent control by suitably shaping the ionizing pulses is an open question [14].Here, we address this question by making use of opti- * lgreenman@phys.ksu.edu mal control theory and show that, for a chiral methane derivative, CHBrClF, quantum interference between distinct two-photon ionization pathways significantly enhances PECD. To this end, we combine a many-body desc...

Photoelectron spectra and photoelectron angular distributions obtained in photoionization reveal important information on e.g. charge transfer or hole coherence in the parent ion. Here we show that optimal control of the underlying quantum dynamics can be used to enhance desired features in the photoelectron spectra and angular distributions. To this end, we combine Krotov's method for optimal control theory with the time-dependent configuration interaction singles formalism and a splitting approach to calculate photoelectron spectra and angular distributions. The optimization target can account for specific desired properties in the photoelectron angular distribution alone, in the photoelectron spectrum, or in both. We demonstrate the method for hydrogen and then apply it to argon under strong XUV radiation, maximizing the difference of emission into the upper and lower hemispheres, in order to realize directed electron emission in the XUV regime.

Photoionization with attosecond pulses populates hole states in the photoion. Superpositions of hole states represent ideal candidates for time-dependent spectroscopy, for example via pump-probe studies. The challenge consists in identifying pulses that create coherent superpositions of hole states while satisfying practical constraints. Here, we employ quantum optimal control to maximize the degree of coherence between these hole states. To this end, we introduce a derivative-free optimization method with sequential parametrization update (SPA optimization). We demonstrate the versatility and computational efficiency of SPA optimization for photoionization in argon by maximizing the coherence between the 3s and 3p 0 hole states using shaped attosecond pulses. We show that it is possible to maximize the hole coherence while simultaneously prescribing the ratio of the final hole state populations.

We report two schemes to generate perfect anisotropy in the photoelectron angular distribution of a randomly oriented ensemble of polyatomic molecules. In order to exert full control over the anisotropy of photoelectron emission, we exploit interferences between single-photon pathways and a manifold of resonantly-enhanced two-photon pathways. These are shown to outperform nonsequential (ω, 2ω) bichromatic phase control for the example of CHFClBr molecules. We are able to optimize pulses that yield anisotropic photoelectron emission thanks to a very efficient calculation of photoelectron momentum distributions. This is accomplished by combining elements of quantum chemistry, variational scattering theory, and time-dependent perturbation theory.