Probing Correlated Ionization Dynamics
Final Report Abstract
Correlated quantum dynamics is one of the most intriguing phenomena in the microscopic world. When intense laser radiation interacts with multi-electron and molecular systems, it appears natural that many electrons are affected such that correlations should play a role. However, disregarding obvious correlation effects induced by photoelectrons driven back into the parent ion’s electron cloud, there is, surprisingly, hardly any clear evidence of correlation effects in strong-field laser-matter interaction. The idea of the project, thus, has been to develop experimental techniques and schemes that are sensitive to correlation effects and, hopefully, pin down one of them. Characteristic of our experimental approach is the use of few-cycle laser pulses and an ion-beam apparatus. The former allows one to confine ionization to a very short period of time, ideally to a single half-cycle or less. As a consequence, the correlated dynamics should stand out when several electrons are ionized within a very short time. Moreover, when the carrier-envelope phase is measured concurrently, even the dependence of ionization on the shape of the waveform can be investigated. The ion beam, on the other hand, allows for the use of exceptional targets, including several of fundamental significance, like He+ , H2+ , and HeH+ . In addition, our ion-beam setup measures the complete momenta of the ionic fragments, which reflect the sum-momenta of the photoelectrons. The challenge of the technique comes from the low ion beam density and the correspondingly low event rates – besides the complexity of the setup. The first measurement was on laser-induced multiple-ionization of Ne+ . With a detailed analysis of the ion momentum distributions, we were able to reconstruct the time of ionization for each individual ionization step in the chain. Evidence for small variations of the ionization times depending of the final charge state was obtained. These results induced the idea to turn to the investigation of photoionization where the final product is He2+ , but where He and He+ are the precursors. Correlations should be readily identifiable in the difference of the He2+ momentum spectra since correlated photoionization dynamics can only occur for He. We built a charge exchange unit for our ion beam and successfully performed experiments. However, as actually suspected by a referee, the results, while promising, were difficult to interpret because of predominant population of the n=2 exited states. This gave rise to an entirely new idea, namely the investigation of spin effects: The electrons in para-helium have opposite spin, those in ortho-helium the same spin. An electron ionized and subsequently driven back to the parent ion, thus, scatters at an electron with equal or opposing spin. Theoretical investigations have shown that significant effects can be expected. In a collaboration with a group in Vienna, double ionization of He by few-cycle laser pulses was investigated. This measurement revealed a new experimental effect suggestive of correlated ionization. However, detailed theoretical modeling revealed that small pulse aberrations could also induce similar effects. Such aberrations are difficult to rule out because of the broad bandwidth of few-cycle pulses. The broad bandwidth of few-cycle pulses also has also consequences in the spatial variation of the phase in the focus. The monochromatic analogue is the Gouy phase, which has been known for more than 100 years. In collaboration with colleagues from Erlangen, we measured the phase with nano-tip. Our results show marked differences to the monochromatic case, in agreement with earlier theoretical investigations. The implications for many, if not most, experiments using few-cycle pulses are profound. Nevertheless, so far, the phase evolution in broadband laser foci has largely been modeled using the Gouy phase. Our experiment was published in Nature Physics. In conclusion, the project has been very successful. As so often is the case in science, the work on the original idea has raised a number of new questions and induced several new ideas. For example, we performed detailed experiments on He+ and H2^+. The project has also been the starting point of a project on HeH+ within the DFG priority program SPP 1840.
Publications
- Determination of the absolute carrier-envelope phase by angle-resolved photoelectron spectra of Ar by intense circularly polarized few-cycle pulses, Physical Review A 95, 053410 (2017)
S. Fukahori, T. Ando, S. Miura, R. Kanya, K. Yamanouchi, T. Rathje, Gerhard G. Paulus
(See online at https://doi.org/10.1103/PhysRevA.95.053410) - Numerical investigation of the sequential double-ionization dynamics of helium in fewcycle laser field shapes, Physical Review A 95, 023411 (2017)
P. Wustelt, M. Möller, M. S. Schöffler, X. Xie, V. Hanus, A. M. Sayler, A. Baltuska, G. G. Paulus, M. Kitzler
(See online at https://doi.org/10.1103/PhysRevA.95.023411) - Tracing the phase of focused broadband laser pulses, Nature Physics 13, 947 (2017)
D. Hoff, M. Krüger, L. Maisenbacher, A. M. Sayler, G. G. Paulus, P. Hommelhoff
(See online at https://doi.org/10.1038/NPHYS4185) - Using the focal phase to control attosecond processes, Journal of Optics 19, 124007 (2017)
D. Hoff, M. Krüger, L. Maisenbacher, G. G. Paulus, P. Hommelhoff, A. M. Sayler
(See online at https://doi.org/10.1088/2040-8986/aa9247)
