Coincidence imaging of multi-electron dynamics in atoms and molecules

We investigate ultrafast electron dynamics in atoms and the dynamics of electronic, vibrational and rotational wave functions in molecules using (synthesized) intense (few-cycle) laser pulses and pump-probe techniques. Our typical observables are energies, angles and momenta of photoions and photoelectrons, as well as molecular fragmentation branching ratios.

Our primary imaging method is coincidence momentum imaging on cold beams of single atoms and molecules, also called COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) [Ullrich97, Dörner00]. A spectrometer based on this technique is often referred to as a momentum microscope. This type of spectrometer is capable of gathering the final electron and ion momenta of all emerging charged particles. A schematic of our apparatus can be seen in Figure 1.

Fig. 1: Schematics of the cold target recoil ion momentum spectroscopy (COLTRIMS) apparatus.

Recently conducted experiments are the investigation of the fragmentation dynamics of polyatomic carbon-hydrogen molecules, the ionization behavior of atoms under the influence of tailored attosecond half-cycle pulses, and recollision-free correlated multi-electron ionization dynamics of atoms using circularly polarized ultra-strong, ultra-short laser pulses.

1. Molecular fragmentation experiments

Using our COLTRIMS device we studied the fragmentation behaviour of a 10-atomic carbon-hydrogen molecule, 1,3-Butadiene, in response to a strong laser pulse of ~25fs in duration from our chirped pulse Titanium-Sapphire amplification system. Butadiene was chose because it is one of the smallest molecules that posses delocalized π electrons which give rise to a high polarizability. Thus the laser driven electronic dynamics may strongly influence the fragmentation behaviour. In a first experiment the break-up of the doubly ionized molecule into two singly charged particles was studied [Xu10]. This work revealed the existence of proton migration channels, which have been extensively studied in the literature. In a follow-up experiment, triggered by the findings of the first study and by reports in the literature of unexpectedly high energies of protons being ejected from carbon-hydrogen molecules, the fragmentation of the triply ionized molecule into 3 particles, where one of them is a proton, was investigated [Roither10a]. Such experiments necessitate the use of coincidence momentum imaging techniques. The following channels were considered: (i) C₄H₆³⁺ → H⁺ + C₂H₂⁺ + C₂H₃⁺, C₄H₆³⁺ → H⁺ + CH₂⁺ + C₃H₃⁺, and (ii) C₄H₆³⁺ → H⁺ + CH₃⁺ + C₃H₂⁺.

Fig. 2: Momentum correlation map between 2 measured ionic fragments of 1,3-Butadiene. 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 (1) that the two fragmentation events happen sequentially, and (2) whether first the proton is ejected and then the center bond is broken (scenario A), or whether first the center bond breaks and then a proton is ejected (scenario B).

A three-particle break-up can take place in a one-step, concerted manner or sequentially, where one bond breaks after the other. If the fragmentation proceeds sequentially, the proton can be emitted during the first or the second fragmentation event. Because we measured all 3 fragments in coincidence, we could use the correlation between their momenta and it was possible to show, that for all three channels the fragmentation proceeds sequentially, see Figure 2, albeit with different probabilities of proton ejection during the first and the second fragmentation for the different fragmentation channels [Roither10a]. In this experiment we also investigated the dependence of the fragmentation behaviour on the pulses’ polarization state and intensity. It was found that the fragmentation behaviour is neither sensitive to the intensity nor the polarization state, which shows that (1) the molecular ion is not promoted to a highly excited state by the recolliding electrons, and (2) that the polarizability of the bound electrons, that can be excited by strong, linearly polarized laser pulses and which may lead to non-adiabatic electron dynamics with sub-sequent enhanced fragmentation, as was reported in the literature, does not play a prominent role for this molecule.

2. Attosecond half-cycle pulses – kicked electrons

This experiment was dedicated to the investigation of the ionization behaviour of atoms that are strongly kicked by attosecond half-cycle pulses. The motivation for this investigation came from the well-known subject of kicked Rydberg atoms, where picosecond half-cycle pulses can be used to manipulate highly excited bound-state wave packets that orbit on a picosecond time scale. In order to drive, shape, manipulate and probe the dynamics of ground state electronic wave packets in atoms and/or molecules by impulsive kicking, it is necessary to apply attosecond half-cycle pulses. We used ω/2ω laser fields to generate the simplest incarnation of attosecond half-cycle pulses and applied them to a series of atoms [Xie10]. We demonstrated that we can indeed drive electrons after tunnel ionization directly in momentum space and we used a simple semi-classical model to explain the results qualitatively. Detailed comparison of experimental results with those of the simulations showed, however, that the interaction with the ω/2ω laser field is more complicated and cannot be described quantitatively by static tunneling theory and Newtonian mechanics.

Fig. 3: Measured momentum distributions of electrons correlated with singly charged Helium as a function of the relative phase of ω and 2ω, Δφ.

Figure 3 shows measured momentum spectra of electrons that are correlated with singly charged Helium ions as a function of the relative phase of the ω and the 2ω laser field. Within a classical model both the variation of the mean value and the intensity of the spectra with the relative phase can be easily understood as an effect of the changing laser field waveform. The measurement, however, shows strong deviations from the classically expected spectra in that it exhibits more complicated fringe structures and maxima of the spectra at different relative phase values. The discrepancies are explained by using numerical solutions of the time-dependent Schrödinger equation.

3. Recollision-free correlated multi-electron ionization

Fig. 4: Ion momentum map of singly (left), doubly (center) and triply (right) ionized Neon atoms created by circularly polarized 5-6 fs pulses at intensities above the barrier suppression regime.

Another recently conducted experiment [Roither10b] was dedicated to the investigation of possibly correlated multi-electron dynamics during strong-field ionization of atoms by very intense few-cycle laser pulses. Circularly (elliptically) polarized laser pulses with pulse durations from 5 fs to ~12 fs were focused to intensities in excess of the barrier suppression ionization regime of doubly charged Helium and multiply charged Neon atoms, and the momentum spectra of the resulting ions were recorded by COLTRIMS. Due to momentum conservation the momentum spectrum of an ion is the vectorial sum of all electron momenta. Any correlation that is present in the emitted electrons will be manifest in the ion spectrum as well. Figure 4 shows typical ion momentum spectra as recorded during this experiment.

References:

[Dörner00] R. Dörner, V. Mergel, O. Jagutzki, L. Spielberger, J. Ullrich, R. Moshammer, and H. Schmidt-Böcking.
Cold Target Recoil Ion Momentum Spectroscopy: A 'Momentum Microscope' to View Atomic Collision Dynamics.
Physics Reports 330, 95-192 (2000).

[Ullrich97] J. Ullrich, R. Moshammer, R. Dörner, O. Jagutzki, V. Mergel, H. Schmidt-Böcking, and L. Spielberger.
Recoil Ion Momentum Spectroscopy.
J. Phys. B 30, 2917 (1997).

[Xu10] H.L. 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.
Chem. Phys. Lett. 484, 119-123 (2010).

[Roither10a] L. Zhang, S. Roither, X. Xie, D. Kartashov, M. Schöffler, H.L. 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.
manuscript submitted (2011).

[Xie10] X. Xie, S. Roither, D. Kartashov, E. Persson, L. Zhang, M. Schöffler, M. Lezius, R. Dörner, J. Burgdörfer, and A. Baltuska, and M. Kitzler.
Driving electronic wave packets by attosecond half-cycle pulses.
manuscript in preparation (2010).

[Roither10b] S. Roither, X. Xie, D. Kartashov, M. Schöffler, L. Zhang, F. Sturm, B. Ulrich, M. Lezius, R. Dörner, A. Baltuska, and M. Kitzler.
Correlated strong field multi-electron ionization.
manuscript in preparation (2010).

Last update March 8, 2010