Strong field dissociative ionization of the D2+ : Nuclear wave packet analysis

Attosecond Probing of Nuclear Vibrations in the D2+ and HeH+ Molecular Ions

We study the ultrafast photodissociation of small diatomic molecules using attosecond laser pulses of moderate intensity in the (extreme) ultraviolet regime. The simultaneous application of subfemtosecond laser pulses with different photon energies─resonant in the region of the molecular motion─allows one to monitor the vibrational dynamics of simple diatomics, like the D2+ and HeH+ molecular ions. In our real-time wave packet simulations, the nuclear dynamics is initiated either by sudden ionization (D2+) or by explicit pump pulses (HeH+) via distortion of the potential energy of the molecule. The application of time-delayed attosecond pulses leads to the breakup of the molecules, and the information on the underlying bound-state dynamics is imprinted in the kinetic energy release (KER) spectra of the outgoing fragments. We show that the KER-delay spectrograms generated in our ultrafast pump-probe schemes are able to reconstruct the most important features of the molecular motion within a given electronic state, such as the time period or amplitude of oscillations, interference patterns, or the revival and splitting of the nuclear wave packet. The impact of probe pulse duration, which is key to the applicability of the presented mapping scheme, is investigated in detail.

Tracing the vibrational dynamics of sodium iodide via the spectrum of emitted photofragments

We study by real-time wave packet simulations the ultrafast photodissociation dynamics of the sodium iodide molecule with the aim to trace molecular vibrational motion in a bound electronic state. Applying a few-cycle infrared pump laser pulse, a nuclear wave packet is created in the ground electronic state via the dynamic Stark shift of the potential energy curves of the molecule. To probe this coherent motion in the ground state, we propose to use a series of ultrashort laser pulses with different photon energies that resonantly promote the spread-out wave packet to the repulsive excited state. As the kinetic energy release (KER) spectrum of the dissociating photofragments is sensitive to the shape of the vibrational wave packet, in our pump-probe scheme, KER-delay spectrograms generated for different probe photon energies are used to monitor the molecular motion in the bound state. In our numerical analysis supported by a simple analytical model, we show that for sufficiently long probe pulses the proposed mapping scheme reaches its limits as nuclear wave packet interferences wash out the observed images. The appearance of these interferences is attributed to nuclear wave packet amplitudes that are generated at the first and second half of the probe pulse with the same energy but with a certain time delay. In our detailed numerical survey on the laser parameter dependence of the presented scheme, we find that resonant probe pulses with a few femtosecond duration are suitable for a qualitative mapping of the bound-state molecular motion.

Selecting two-photon sequential ionization pathways in H2 through harmonic filtering

Recent experiments in gas-phase molecules have shown the versatility of using attosecond pulse trains combined with IR femtosecond pulses to track and control excitation and ionization yields on the attosecond timescale. The interplay between electron and nuclear motions drives the light-induced transitions favoring specific reaction paths, so that the time delay between the pulses can be used as the tracking parameter or as a control knob to manipulate the molecular dynamics. Here, we present ab initio simulations on the hydrogen molecule to demonstrate that by filtering the high harmonics in an attosecond pulse train one can quench or enhance specific quantum paths thus dictating the outcome of the reaction. It is then possible to discriminate the dominant sequential processes in two-photon ionization, as for example molecular excitation followed by ionization or the other way around. More interestingly, frequency filters can be employed to steer the one- and two-photon yields to favor electron emission in a specific direction.