SFB-1319: Subproject C1

Photoelectron circular dichroism (PECD) is a fascinating phenomenon which manifests itself as a forward-backward asymmetry in the laboratory-frame photoelectron angular emission distribution from chiral molecules. Almost all previous studies of the one-photon and multiphoton PECD in the gas phase were performed for randomly oriented chiral molecules. Here, the asymmetry effect, which survives after averaging over all molecular orientations, is on the order of a few percent. At present, it is anticipated that the magnitude of PECD is governed by the ability of the outgoing photoelectron to probe the asymmetry of molecular potential by multiple scattering effects while escaping the ion. One of the successful long-term projects of our group consists in the development of advanced nonstandard approaches for the theoretical description of electron continuum waves in molecules. Recently, we have demonstrated that chiral asymmetry can be significantly enhanced already by fixing one molecular orientation axis. Our preliminary work directly relates PECD with the molecular frame photoelectron angular distribution (MFPAD), which is unambiguously determined by the partial photoionization amplitudes. Therefore, a proper theoretical accompaniment of experimental MFPADs should allow for reconstruction of the absolute configuration of a target. Moreover, fixing three-dimensional orientation of a target in space may in principle result in a 100% effect, which makes PECD of spatially-oriented chiral molecules a more sensitive tool for the enantiomeric excess determination.

The main long-term aim of this CRC is to be able to manipulate and exploit chirality on a single-molecule level. This aim, in turn, can be achieved only after a successful qualitative understanding, quantitative characterization, and systematic quantification of the molecular chirality, which are the main goals of the present project. In order to achieve these goals, we propose a comprehensive theoretical investigation of PECD in the one-photon inner-shell ionization, fragmentation, excitation and decay spectra, as well as in the multiphoton and tunnel outer-shell ionization of the partially and fully spatially-oriented chiral molecules. The suggested theoretical study requires developments of the theoretical and computational approaches for the quantitative description of the electron continuum spectrum in molecules, which are available in our group. Based on well-justified approximations, these theoretical methods must be as efficient as possible in order to enable a prompt and reliable support for the angle-resolved experiments with chiral molecules proposed in this CRC. The main long-term research questions, which we would like to address with these theoretical methods, are:
1.  At which orientations of a chiral molecule one can expect larger circular dichroism effects and why?
Are the Auger decay spectra chiral sensitive and which additional information could they provide?
What can be learned on chirality by strong-field tunnel ionization spectroscopy?
What kind of information on original chirality is deposited in the fragments?
For how long do original chiral signatures preserve in the full set of fragments during dissociation?
How to reconstruct the absolute configuration by 3D tomographic momentum distributions?
What new can be learned about molecular chirality by spin-resolved spectroscopy?
What are the individual roles of the initial and final electronic states involved in the process?
Are there any qualitative correlations between different observables?
Is there a quantitative relation between observables and the asymmetry of the electronic potential?
How to quantify, optimize/maximize, control, and exploit information on different observables?
During the first four years, we would like to address the issues 1–6 from the full list of the long-term research questions. To this end, we suggest to investigate alaninol, (deuterated) methyloxirane, trifluoromethyloxirane, and fenchone. Restricting our interest to these samples, we propose theoretical studies of:

  • One-photon inner-shell ionization, excitation and Auger decay of spatially-oriented targets. Thereby, we provide a comprehensive theoretical support for synchrotron radiation experiments on static chirality suggested in the project A1.

  • Multiphoton PECD by coherent broadband excitation, studied experimentally in A3, and passage from the multiphoton ionization regime by optical laser pulses to the tunnel ionization regime by intense IR pulses, which will be experimentally addressed in the project A2.

  • Ionization of fragments during dissociation of inner-shell ionized chiral molecules. Here, we offer theoretical support for one-photon pump – one-photon (or multiphoton) probe experiments on a transient chirality, which will be conducted in the project A4.

In addition, we will theoretically study a possibility to access absolute configuration and electronic structure reconstruction via the one-photon slow-electron 3D momentum tomography by elliptically polarized light, which is of potential interest to experimental projects A4.