Probing molecular dynamics by laser-induced backscattering holography
In electron microscopy, 3-dimensional atomic-scale resolution can be achieved by exploiting the hologram created from a scattered signal wave and an unscattered reference wave. If this kind of electron holography could reach femtosecond or attosecond temporal resolution, then we could access the space and time dynamics of both electrons and ions in molecules.
A coherent electron wave can also be created by strong-field ionization. A molecule subjected to an intense infrared laser pulse emits an electron wave packet each half optical cycle. In the oscillating laser field this electron wave can return to its origin. Upon return, the electron wave is partly scattered, forming the signal wave, and interferes with the unscattered part, producing a photoelectron hologram in momentum space.
Up to now, no direct molecular signal was measured in strong-field photoelectron holography due to the forward scattering problem.
We overcome this problem by using differential holography and show how an interference structure can be extracted that contains a backscattered electron. Such a backscattering hologram is particularly sensitive to the nuclear and electron dynamics.
Above: (a), Sketch of the experiment: H2 is tunnel-ionized in a strong laser field. Depending on the time of ionization, different electron trajectories are possible. In the oscillating laser field it may come to backscattering at return (case A). These electrons interfere with indirect electrons started half a laser cycle later (case B). Simultaneously, a wave packet is started in the 1ssg state of H2+ (see subfigure (b)). In the presence of the field, this state couples to the dissociative 2psu state, which may lead to dissociation. Depending on the observed proton momentum these are associated to bond-softening (BS) and above-threshold dissociation (ATD) (subfigure (c)). The direction of the proton in the laboratory frame is used for post-alignment. By measuring the 3D momentum of the electron and ion in coincidence, it is possible to measure photoelectron spectra for different molecular alignments.
Above: Forward scattering is the dominant process. In our experiment, we exploit the molecular alignment dependence of forward and backward scattering. Whereas forward scattering does not show an alignment dependence, backward scattering does. In (a), we show the measured 3D electron momentum distribution for aligned D2 molecules leading to ATD. The intensity is plotted on a logarithmic scale. Above-threshold-ionization (A) and forward scattering photoelectron holography (B) are clearly visible. (b), Differential holograms of the measured photoelectron momentum distributions between aligned and anti-aligned molecules correlated with BS or ATD in H2 or D2. The normalized difference for the ATD channel of deuterium was mirrored. The fishbone structure from backward scattering holography is indicated as feature C. A fringe shift, highlighted by the green dotted line, is due to combined nuclear and electron dynamics in the time during ionization and recollision (backscattering).
Haertelt, M.; Bian, X.; Spanner, M.; Staudte, A.; and Corkum, P. B. Physical review letters, 116: 133001. 2016.