Strong laser-pulse-driven ionization and Coulomb explosion of hydrocarbon molecules

Field ionization and Coulomb explosion of small hydrocarbon molecules driven by intense laser pulses are studied in a combined theoretical and experimental framework. The spectra of ejected protons calculated by the time-dependent density functional approach are in good agreement with the experimental data. The results of the simulations give detailed insight into the correlated electron and nuclear dynamics and complement the experiment with a time-dependent physical picture. It is demonstrated that the Coulomb explosion in the studied molecular systems is a sudden, all-at-once fragmentation where the ionization step is followed by a simultaneous ejection of the charged fragments.

Figure 1

Snapshots of the 3D electron density and ionic positions of a C${}_4$H${}_6$ molecule subject to a laser pulse. For an animated version, see the supplemental material [28].

Figure 2

Number of valence electrons left in the system as a function of time for the entire ensemble of spatial configurations used in the calculations.

Figure 3

(a,b) Number of valence electrons remaining in the molecular system (blue lines) during ionization by a laser pulse (gray lines) for methane and 1,3-butadiene. (c,d) Displacements of individual protons (green lines) and carbon ions (dark red lines) from their initial positions. (e,f) Kinetic energies of individual protons (green lines) and carbon ions (dark red lines). While the data shown for the displacements and kinetic energies are for some randomly picked molecular spatial orientation, the same qualitative behavior could be observed for the majority of other orientations. (g,h) Measured carbon energy spectrum (red line) decomposed into the contributions of singly and doubly charged carbons (gray lines).

Figure 4

Histograms of computed proton energies for methane (a,b) and 1,3-butadiene (c), shown by the red bars, compared to experimentally obtained spectra (lines). The blue lines are the measured total proton energy spectra. For methane, these spectra are decomposed into the contribution coming from complete molecular fragmentations (black lines) resulting in a singly (red line) and doubly (green line) charged carbon ion. Laser peak intensities are indicated in the panels.