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Zhao J.,Space Time Research | Zhao J.,National University of Defense Technology | Lein M.,Space Time Research
Physical Review Letters | Year: 2013

From the numerical solution of the time-dependent Schrödinger equation, we obtain the times of ionization and return of the laser-driven electron in high-order harmonic generation by probing the dynamics with a second harmonic field polarized orthogonal to the fundamental field and observing the harmonic emission in dependence on the two-color delay. Our retrieval method using complex-time evolution gives ionization and return times in excellent agreement with the quantum-orbit model, while a retrieval based on real-time classical dynamics can introduce substantial errors. Because of the imaginary parts, the harmonic signal polarized along the probe field is nonzero for any two-color delay. The tunneling time can be retrieved under an assumption for the return time. © 2013 American Physical Society.

Lein M.,Space Time Research
Journal of Modern Optics | Year: 2011

Motivated by recent experimental progress in precision investigation of strong-field ionisation by angular streaking 1, we employ streaking by a linearly polarised half-cycle pulse as a theoretical tool to analyse the strong-field ionisation of atoms. For moderate field strengths, the electron momentum distribution is consistent with the interpretation that ionisation occurs most probably at the peak of the field followed by classical electron motion under the combined force of the laser field and the atomic potential. At high field strengths, the momentum distributions together with the classical model indicate that ionisation occurs preferentially before the peak of the field. Surprisingly, the momentum at the peak of the distribution scales almost linearly with the intensity of the half-cycle pulse. © 2011 Taylor & Francis.

Tudorovskaya M.,Space Time Research | Tudorovskaya M.,University of Kassel | Lein M.,Space Time Research
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

We investigate high-order harmonic generation from laser-irradiated systems that support a shape resonance. From the numerical solution of the time-dependent Schrödinger equation, we calculate the harmonic spectra and the time-frequency analysis of the harmonic intensity and phase. The analysis reveals the separate contributions of the short and long trajectories as well as the resonance. A range of harmonics is strongly enhanced by the presence of the resonance irrespective of the pulse length. The signature of the resonance remains significant after coherent summation over intensities as a simple method to simulate macroscopic effects. The time-frequency analysis supports the recently proposed four-step mechanism of the enhanced harmonic generation process. © 2011 American Physical Society.

Dreissigacker I.,Space Time Research | Lein M.,Space Time Research
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2014

Motivated by recent experiments on circular dichroism in the photoelectron momentum distributions from strong-field ionization of chiral molecules [C. Lux, Angew. Chem. Int. Ed. 51, 5001 (2012)1433-785110.1002/anie.201109035; C. S. Lehmann, J. Chem. Phys. 139, 234307 (2013)JCPSA60021-960610.1063/1.4844295], we investigate the origin of this effect theoretically. We show that it is not possible to describe photoelectron circular dichroism with the commonly used strong-field approximation due to its plane-wave nature. We therefore apply the Born approximation to the scattering state and use this as a continuum-state correction in the strong-field approximation. We obtain electron distributions for the molecules camphor and fenchone. In order to gain physical insight into the process, we study the contributions of individual molecular orientations. © 2014 American Physical Society.

Zhao J.,Space Time Research | Zhao J.,National University of Defense Technology | Lein M.,Space Time Research
New Journal of Physics | Year: 2012

In this paper autoionizing states in the one-dimensional helium atom are investigated by numerical solution of the time-dependent twoelectron Schrodinger equation. The atom is irradiated by an extreme ultraviolet (XUV) attosecond pulse and a time-delayed infrared few-cycle laser pulse. The XUV pulse populates a superposition of doubly excited states, leading to Fano resonances in the photoelectron spectrum. It is demonstrated that the Fano line profile is strongly modified by the presence of the laser field. Laser-induced coupling between the different doubly excited states causes the population of autoionizing states that cannot be reached by absorbing a single XUV photon from the ground state. The resulting additional peaks in the photoelectron spectrum are modulated as a function of time delay. Furthermore, the photoelectron spectrum exhibits a fringe pattern that is determined by the time delay but is independent of the details of the laser pulse. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Jhala C.,Space Time Research | Lein M.,Space Time Research
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2010

The multiconfiguration time-dependent Hartree approach is applied to study the electron-nuclear correlation in the dynamics of molecules subject to strong external laser fields, using the example of a model hydrogen molecular ion. The ground state of the system is well described by as few as two single-particle functions per degree of freedom. A significantly larger but moderate number of configurations is required to predict laser-induced fragmentation probabilities and high-order harmonic generation spectra accurately, showing that the correlation between electronic and nuclear degree of freedom is strongly increased by the presence of the laser field. © 2010 The American Physical Society.

Petersen I.,Space Time Research | Henkel J.,Space Time Research | Lein M.,Space Time Research
Physical Review Letters | Year: 2015

Strong-field ionization of aligned diatomic and polyatomic molecules such as O2, N2, C2H4, and others in circularly polarized laser fields is investigated theoretically. By calculating the emission-angle-resolved lateral width of the momentum distribution perpendicular to the polarization plane, we show that nodal planes in molecular orbitals are directly imprinted on the angular dependence of the width. We demonstrate that orbital symmetries can be distinguished with the information obtained by observing the lateral width in addition to the angular distributions. © 2015 American Physical Society.

Dreissigacker I.,Space Time Research | Lein M.,Space Time Research
Chemical Physics | Year: 2013

We investigate theoretical models for the lateral width of the electron momentum distribution after recollision-free strong-field ionization of atoms. We review the derivation of the tunneling formula and demonstrate that the pre-exponential factor in the saddle-point approximation cannot be neglected if quantitative results are desired. We calculate the widths for hydrogen as well as argon and neon atoms. We compare to results from the time-dependent Schrödinger equation, and to the experimental results from [L. Arissian, C. Smeenk, F. Turner, C. Trallero, A.V. Sokolov, D.M. Villeneuve, A. Staudte, P.B. Corkum, Phys. Rev. Lett. 105 (2010) 133002]. © 2012 Elsevier B.V. All rights reserved.

Van Der Zwan E.V.,Space Time Research | Van Der Zwan E.V.,University of Kassel | Lein M.,Space Time Research
Physical Review Letters | Year: 2012

Accurate molecular imaging via high-order harmonic generation relies on comparing harmonic emission from a laser-irradiated molecule and an adequate reference system. However, an ideal reference atom with the same ionization properties as the molecule is not always available. We show that for suitably designed, very short laser pulses, a one-to-one mapping from high-order harmonic frequencies to electron momenta in above-threshold ionization exists. Comparing molecular and atomic momentum distributions then provides the electron recollision amplitude in the molecule for enhanced molecular imaging. The method retrieves the molecular recombination transition moments highly accurately, even with suboptimal reference atoms. © 2012 American Physical Society.

News Article | August 12, 2015

The immense amount of data governments collect every day is enormously useful and valuable, but it is also mostly siloed and inaccessible even to those that own and manage it. If the government owners of data can't use it effectively, what hope have citizens who may want to use it to inform their own life choices? Kelvin Watson, deputy chief executive organisation capability and services at Statistics NZ, says the current government is very focused on data-driven decision making. In 2013 it commissioned a piece of work called "Analysis for outcomes" and the IDI was one result. Statistics was well positioned. Apart from its role as the lead agency for the Census and other studies, it had already been integrating government data for over a decade, effectively running an integration service for the public sector. Now, in secure data labs dotted around the country, researchers are using anonymised data from across government to deliver data-driven policy and to build tools to help citizens make better decisions. And while data in the IDI needs to be constantly refreshed to maintain its currency, Statistics is already trialing a tool that could enable analysis in real time. Watson says all individual identifiers are stripped out after the various IDI data sets are matched. The only people who can access the data have to prove bona fide research requirements. There is stringent vetting both of researchers and of projects, he says. The data labs in which research takes place are not connected to internet and at the end of the research, all outputs are checked to ensure they adhere to confidentiality rules. "Individuals can never be identified and the information is never used for case management decisions for individuals," Watson says. Data comes into the IDI from the likes of the Inland Revenue Department, Ministry of Social Development and the Ministry of Education in various formats. It is then processed and mapped into tables for use. The strength of the IDI is the data is already linked through a name or a common number for individuals or by a probabilistic measure - such as a record having the same name and date of birth - before it is anonymised. "Data integration is where a lot of the power is," Watson says. "We don't have researchers having to work that out. It's already done." Despite that, it's not a case of coming in and ticking a few boxes, but Statistics makes a set of standard research tools available as well. "You still need to have quite a few smarts around how you manage your data," Watson says. "The whole point of the IDI is about the life pathways," says Watson. "What happens to someone in education and what does it mean for them in employment? What does low education achievement mean in terms of potentially going into a life of crime or whatever.?" The social policy aspects are the easiest to describe but Watson says the spectrum of research is broad. One piece of work looked at study options, what individuals studied and then tracking that through to what they are doing in employment. From that came a careers tool to help young people make their own informed study decisions. But already Statistics is trying to go one better, trialing a tool from Melbourne based Space Time Research that could enable data analysis in real time. "It's potentially the next generation but it's still a twinkle in the eye at the moment," Watson says. Where IDI data dates and needs to be refreshed. the Space Time tool uses a federated data model. That could allow a researcher to access a front end and say they want to look at this cohort from the Ministry of Social Development connected to that cohort from Education and do some analysis. The tool would then draw up-to-date data down, already linked and confidentialised. "It means data would be only ever held at the home agency," Watson says. All up, it's pretty advanced stuff. "When we talk to national statistics offices around the world they look at the IDI and say they need to get there too," says Watson.

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