The application of a matrix-based reconstruction protocol for obtaining Molecular Frame (MF) photoelectron angular distributions (MFPADs) from laboratory frame (LF) measurements (LFPADs) is explored. Similarly to other recent works on the topic of MF reconstruction, this protocol makes use of time-resolved LF measurements, in which a rotational wavepacket is prepared and probed via photoionization, followed by a numerical reconstruction routine; however, in contrast to other methodologies, the protocol developed herein does not require determination of photoionization matrix elements, and consequently takes a relatively simple numerical form (matrix transform making use of the Moore-Penrose inverse). Significantly, the simplicity allows application of the method to the successful reconstruction of MFPADs for polyatomic molecules. The scheme is demonstrated numerically for two realistic cases, N2 and C2H4. The new technique is expected to be generally applicable for a range of MF reconstruction problems involving photoionization of polyatomic molecules.
Photoelectron images have been measured following the ionisation of aligned distributions of gas phase naphthalene and aniline molecules. Alignment in the adiabatic regime was achieved by interaction with a 100 ps infrared laser pulse, with ionisation achieved in a two-photon resonant scheme using a low intensity UV pulse of ∼6 ps duration. The resulting images are found to exhibit anisotropy that increases when the alignment pulse is present, with the aniline PADs peaking along the polarisation vector of the ionising light and the naphthalene PADs developing a characteristic four-lobed structure. Photoelectron angular distributions (PADs) that result from the ionisation of unaligned and fully aligned distributions of molecules are calculated using the ePolyScat ab initio suite and converted into two-dimensional photoelectron images. In the case of naphthalene excellent agreement is observed between experiment and the simulation for the fully aligned distribution, showing that the alignment step allows us to probe the molecular frame, but in the case of aniline it is clear that additional processes occur following the one-photon resonant step.
Photoionization of molecular species is, essentially, a multi-path interferometer with both experimentally controllable and intrinsic molecular characteristics. In this work, XUV photoionization of impulsively aligned molecular targets (N2) is used to provide a time-domain route to “complete” photoionization experiments, in which the rotational wavepacket controls the geometric part of the photoionization interferometer. The data obtained is sufficient to determine the magnitudes and phases of the ionization matrix elements for all observed channels, and to reconstruct molecular frame interferograms from lab frame measurements. In principle this methodology provides a time-domain route to complete photoionization experiments, and the molecular frame, which is generally applicable to any molecule (no prerequisites), for all energies and ionization channels.
Photoionization is a complex quantum mechanical process, with a range of interfering channels playing a role in even the simplest case. For problems in quantum metrology and sensing, a detailed understanding of the process is desirable for accurate measurements; quantum control is also a possible outcome of such understanding. New research in this area will build on recent cutting-edge work at NRC (see below), which probed the fundamental quantum physics of photoionization in atoms and molecules, and metrology work which demonstrated the retrieval of electron wavefunctions via interferometric time-domain measurements.
Four areas of photoionization interferometry are the target of current research:
Metrology and control with rotational wavepackets.
Metrology and control with shaped laser pulses.
Quantum dynamics probed via photoionization interferometry.
Fundamental properties of photoion and photoelectron coherence.
Depending on interests and experience, project work will be in one (or more) of these areas.
An introduction to this topic, and recent work, can be found in Paul’s DAMOP 2017 talk Phase-sensitive Photoelectron Metrology (below), and via our blog.
Originally presented at ITAMP’s (Institute for Theoretical, Atomic and Molecular and Optical Physics, part of the Harvard-Smithsonian Center for Astrophysics) workshop on Ultrafast atomic and molecular physics with cutting-edge light sources: New opportunities and challenges, back in Nov. 2013.
Single and multi-photon ionization of aligned molecular ensembles is examined, with a particular focus on the link between the molecular axis distribution and observable in various angle-integrated and angle-resolved measurements. To maintain generality the problem is treated geometrically, with the aligned ensemble cast in terms of axis distribution moments, and the response of observables in terms of couplings to these moments. Within this formalism the angular momentum coupling is treated analytically, allowing for general characteristics—independent of the details of the ionization dynamics of a specific molecule—to be determined. Limiting cases are explored in order to provide a phenomenology which should be readily applicable to a range of experimental measurements, and illustrate how observables can be sensitive to fine details of the alignment, i.e. higher-order moments of the axis distribution, which are often neglected in experimental studies. We hope that this detailed and comprehensive treatment will bridge the gap between existing theoretical and experimental works, and provide both quantitative physical insights and a useful general phenomenology for researchers working with aligned molecular ensembles.