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Strong-field dynamics

 

Our research focuses on the investigation and strong-field control of ultrafast electron dynamics in atoms and molecules, restructuring and dissociation dynamics in molecules, as well as on the invention of new methods for measurements on the attosecond time-scale and the generation of XUV and X-ray pulses. We apply intense few-cycle laser pulses with a fully characterized electric field and intense multi-color synthesized waveforms. Typical observables are energies, angles and momenta of photo-ions and photo-electrons or XUV/X-ray photon spectra. Here we present some exemplary results.

 

1. Ionization of polyatomic molecules

 

We investigated the ejection protons from a series of polyatomic hydrocarbon molecules (methane, ethylene, 1,3-butadiene, hexane) exposed to 27 fs laser pulses from our Titanium-Sapphire laser system. We found that the energies of the protons are surprisingly high – too high to be consistent with Coulomb explosions from typical molecular ionic charge states, see Figure 1.

Using multi-particle coincidence imaging we were able to decompose the observed proton energy spectra into the contributions of individual fragmentation channels, see Figure 2. We could show that the molecules can completely fragment already at relatively low peak intensities of a few 1014 W/cm2, and that the protons are ejected in a concerted Coulomb explosion from unexpectedly high charge states. Our observations can be explained by enhanced ionization taking place at many C-H bonds in parallel – a thus far unreported highly efficient type of ionization [Roither2011].

 

High-energy proton spectra

Figure 1: Measured proton energy spectra (left) and cutoff energies (right) for 1,3-butadiene and hexane recorded with linearly polarized laser pulses of different peak intensities from below 1014 [spectrum (1)] to slightly above 1015 W/cm2 [spectrum (6)]. The red squares and blue circles correspond to linearly and circularly polarized light, respectively.

 

We showed the mechanism for hydrocarbon molecules using coincidence momentum imaging, but we believe that such a molecular decomposition process should occur during the interaction of strong laser pulses with any polyatomic molecule, when the time scale of the intramolecular nuclear motion matches the laser pulse duration. In the particular case of hydrocarbon molecules, the very fast motion of hydrogen atoms in C-H bonds on the order of 10 fs can make this process very efficient even for the interaction with quite short pulses.

 

Decomposed proton spectra

Figure 2: Decomposition of the total proton energy spectra (gray lines) for ethylene (a,b) and 1,3-butadiene (c,d) into the proton spectra of separate fragmentation channels (colored lines). The black line shows the sum of all individual spectra.

 

 

2. Attosecond electron wavepacket interferometry


Interferometry is a powerful technique providing access to a relative phase of interfering optical or matter waves. We experimentally and theoretically demonstrated [Xie2011] a self-referenced wavefunction retrieval of a valence electron wavepacket during its creation by strong-field ionization, based on a distinct separation of interferences arising at different time scales, see Figure 3. Our work showed that the measurement of sub-cycle electron wavepacket interference patterns can serve as a tool to assess structure and dynamics of the valence electron cloud in atoms and molecules on a sub-10-attosecond time scale.

Interference structures schematics

Figure 3: Interferences of electron wavepackets created and driven by sculpted ω-2ω laser pulses with a relative phase φ between the two colors. A free electron born at time tb reaches a final momentum along the light polarization direction given by the negative vector potential at birth time, p=-A(tb). Electron wavepackets that reach the same final momentum interfere. This is possible either for wavepackets created during different cycles [gray dots and lines] or for wavepackets created within the same cycle [blue dots and lines]. The former are separated in time by multiples of the fundamental cycle period T giving rise to interference fringes separated by the photon energy ℏω=2πℏ/T, referred to as ATI-peaks or intercycle interferences. The latter result from sub-cycle time delays and lead to a modulation of the ATI peaks.

 

Interference patterns extracted from measured electron momentum spectra, shown in Figure 4, sensitively depend on the shape of the laser field cycle which we control by varying the relative phase φ between the two colors that the sculpted laser field is composed of. The experimental spectra have been recorded by coincidence momentum imaging. The simulated spectra, also shown in Figure 4, result from a numerical solution of the time-dependent Schrödinger equation (TDSE) in three spatial dimensions. All spectra feature a strong ionization signal in the central stripe (|py|<0.2 au) and weaker finger-like structures for |py|>0.2 au.

 

Measured 2D interference structures

Figure 4: Two-dimensional electron wavepacket interference patterns. (a)-(c) Measured interferograms extracted from electron momentum spectra for single ionization of helium atoms for various relative phases φ of a two-color laser field with its laser polarization direction along pz. The gray bars blank out regions where our detector has no resolution for electrons. (d)-(f) Solutions of the time-dependent Schrödinger equation for a single-cycle pulse for the same values of φ as in (a)-(c). In the lower half of (d) and (f) we also show the TDSE results for a multi-cycle pulse for which ATI fringes appear.

 

The position and shape of the sub-cycle interference peaks depend sensitively on the shape of the laser field cycle, i.e. on φ. By contrast, the ATI peaks are created by interference of wavepackets released during different laser cycles and their positions thus reflect the periodicity T of the field giving equispaced peaks in energy independent of φ. Sub-cycle and ATI fringes can thus be clearly separated from each other by studying the variation of the longitudinal electron spectrum with φ, see Figure 5. The sub-cycle fringes appear as bow-like structures whose positions vary strongly with φ. The strong asymmetry of the spectra about pz=0, which is a further consequence of the two-color field, allows to detect them well apart from low-energy resonances and leads to a much broader spectral detection range and therewith to a strongly enhanced useful temporal probe window. Maximum fringe spacing and highest momenta are reached for φ=(0.5+n)π. In contrast, the position of the ATI peaks is independent of φ only determined by T (or, equivalently, ω).

 

Field-control of sub-cycle interference structures

Figure 5: Control of sub-cycle interference patterns with a sculpted laser field. Interference fringes extracted from the measured ion momentum spectra as a function of the longitudinal momentum pz and the relative phase φ. ATI peaks are independent of φ and form straight lines at fixed pz. By contrast, sub-cycle interferences are strongly dependent on φ and form bow-like structures.

 

 

3. Molecular restructuring and proton migration dynamics

 

Polyatomic molecules subject to strong laser pulses are exposed to electric field strengths that are comparable to or may even exceed the intra-molecular Coulomb binding fields. As a consequence, the molecules can become singly or multiply ionized during their interaction with the laser field. The details of the accompanying field driven internal electronic dynamics are still far from being understood and strongly depend on the parameters of the laser field as well as on the electronic structure of the molecules. After the removal of electrons not only the charge density will redistribute very quickly within the molecule, also the molecule itself may undergo partly severe structural deformation.

A very interesting restructuring process is the migration of hydrogen atoms or protons. Protons are known to take a special role in polyatomic molecules since their dynamics take place on a timescale that is in between the one of the sub-femtosecond motion of electrons and the one of the other moieties, that due to their much bigger mass is by at least an order of magnitude slower. Eventually, after or during the geometric restructuring and migration processes, the multiply charged complex may break into two or several charged fragments that are driven apart by Coulomb repulsion, and the excessive molecular potential energy is released into kinetic energy of the resulting set of final fragment ions.

In this project we investigate the dynamics of laser-induced intra-molecular proton migration and large-scale molecular restructuring prior to fragmentation [Xu2010a, Zhang2011]. The goal is to move from observation to control by steadily improving our understanding of the molecular response to the laser field. Using coincidence momentum imaging it is possible to selectively investigate the momentum correlation between certain moieties in a given fragmentation reaction and therewith to reveal the break-up dynamics, see Figure 6.

Path-selective investigation of molecular fragmentation

Figure 6: Experimentally obtained momentum correlation map of two ionic fragments in a three-body break-up reaction of 1,3-Butadiene induced by a 25 fs laser pulse with an intensity of 1.5×1014 W/cm2. For the here shown fragmentation channel the molecule breaks along the center bond and additionally a proton is ejected from one of the two fragments. By using momentum correlation it is possible to show that events in region A are created by a fragmentation dynamics where first the proton is ejected and then the center bond is broken, and events in region B are created when first the center bond breaks and then a proton is ejected [Zhang2011].

 

From the measured momentum correlation maps we can numerically reconstruct the position of the proton prior to Coulomb explosion, see Figure 7. These proton maps revealed that not only one but also two protons can migrate to the same molecular site prior to molecular fragmentation [Xu2010b], see the explanation in the figure caption.

 

Proton mapProton mapProton map

Figure 7: Intra-molecular spatial distributions of the protons as numerically reconstructed from the measured momentum values of the three recorded fragment moieties by using an algorithm that assumes concerted fragmentation dynamics. (a)-(c) show the results of the numerical reconstruction for 3 different fragmentation channels as indicated in the figure. The position of the two heavy moieties is depicted below each panel. The two dominant regions, where the protons are situated left and right of the molecule's center of mass are labeled by Ai and Bi, where i=1..3 indicates the 3 identified fragmentation channels. The region denoted with A2 is special, since it indicates the ejection of a proton from CH3+. This is remarkable as the appearance of this moiety with its 3 hydrogen atoms already indicates the “capture“ of an additional hydrogen atom or proton. In turn, this means that 2 protons (or hydrogen atoms) must have migrated to this molecular site prior to Coulomb explosion [Xu2010b, Zhang2011] – a process that our experiments revealed for the first time.

 

 

References

[Roither2011] S. Roither, X. Xie, D. Kartashov, L. Zhang, M. Schöffler, H. Xu, A. Iwasaki, T. Okino, K. Yamanouchi, A. Baltuska, and M. Kitzler,
High Energy Proton Ejection from Hydrocarbon Molecules Driven by Highly Efficient Field Ionization
Physical Review Letters 106, 163001 (2011).

[Xie2011] X. Xie, S. Roither, D. Kartashov, E. Persson, D.G. Arbó, Li Zhang, S. Gräfe, M. Schöffler, J. Burgdörfer, A. Baltuska, and M. Kitzler
Attosecond probe of valence electron wavepackets by sub-cycle sculpted laser fields
submitted (2011)

[Xu2010a] H. Xu, T. Okino, K. Nakai, K. Yamanouchi, S. Roither, X. Xie, D. Kartashov, M. Schöffler, A. Baltuska, and M. Kitzler
Hydrogen migration and C–C bond breaking in 1,3-butadiene in intense laser fields studied by coincidence momentum imaging
Chemical Physics Letters 484, 119-123 (2010).

[Xu2010b] H. Xu, T. Okino, K. Nakai, K. Yamanouchi, S. Roither, X. Xie, D. Kartashov, L. Zhang, A. Baltuska, and M. Kitzler
Two-proton migration in 1,3-butadiene in intense laser fields
Physical Chemistry Chemical Physics  12, 12939-12942 (2010).

[Zhang2011] Li Zhang, S. Roither, X. Xie, D. Kartashov, M. Schöffler, H. Xu, A. Iwasaki, S. Gräfe, T. Okino, K. Yamanouchi, A. Baltuska, and M. Kitzler
Path-selective investigation of intense laser pulse-induced fragmentation dynamics in triply charged 1,3-butadiene
submitted (2011)

 

Last update Dec. 5, 2011