Electronic Predetermination of Ethylene Fragmentation Dynamics

We experimentally investigate the dependence of the fragmentation behavior of the ethylene dication on the intensity and duration of the laser pulses that initiate the fragmentation dynamics by strong-field double ionization. Using coincidence momentum imaging for the detection of ionic fragments, we disentangle the different contributions of ionization from lower-valence orbitals and field-driven excitation dynamics to the population of certain dissociative excited ionic states that are connected to one of several possible fragmentation pathways towards a given set of fragment ions. We find that the excitation probability to a particular excited state and therewith the outcome of the fragmentation reaction strongly depend on the parameters of the laser pulse. This, in turn, opens up new possibilities for controlling the outcome of fragmentation reactions of polyatomic molecules in that it may allow one to selectively enhance or suppress individual fragmentation channels, which was not possible in previous attempts of controlling fragmentation processes of polyatomic molecules with strong laser fields.

Popular Summary

In molecules, the constituent atoms are bound together by their electrons, whose motions can be very fast. Rearranging the atoms, or splitting them apart, is ultimately determined by the electrons. A fundamentally important and fascinating question is then to what extent, and by which mechanisms, electronic processes can influence the slower molecular restructuring and splitting processes, or even determine their outcome. The answer to this question may also guide the design of novel methods for controlling chemical reactions. In this experimental paper, we demonstrate that selective fragmentation of ethylene molecules can be achieved by controlled distortion of the electron cloud induced by quickly ripping off two electrons with strong and very short laser pulses.

Depending on the quantum states (molecular orbitals) the electrons are removed from, the ionization of the molecule triggers the motion of the heavy atoms and may eventually lead to a split of the molecule into two ionic fragments. But, knowing what the two fragments are does not immediately tell us which molecular orbitals the electrons are removed from, as different pathways can lead to the same ionic fragments. By analyzing the energies and directions of the two fragments against possible fragmentation pathways and quantum-chemical simulations, we have identified, for a number of fragmentation pathways, their corresponding molecular orbitals. Furthermore, our results show that the parameters of the laser pulses, specifically intensity and duration, can be used to select the fragmentation pathway.

Our work may open up new possibilities for controlling the outcome of fragmentation reactions of polyatomic molecules such as the selective enhancement or suppression of specific combinations of fragments.

Figure 1

(a) Molecular orbitals of ethylene calculated by GAMESS [37] as described in the text. (b) Calculated PESs of ground ($1^1{A}_g$, black line) and selected excited ionic states of the ethylene dication. The ${}^3{B}_{3u}$ state is formed by removing one HOMO electron and one HOMO-2 electron, the ${}^1{B}_{3g}$ by removing one HOMO-1 electron and one HOMO-2 electron, and the $2^1{A}_g$ and $3^1{A}_g$ states are formed by removing two HOMO-1 electrons and two HOMO-2 electrons, respectively. The vertical black dashed line indicates the C-C equilibrium distance of 2.53 a.u. The colored arrows (labeled by I, II, and III) refer to the three fragmentation pathways of channel (1), as discussed in the text.

Figure 2

(a)–(d) Slices through the 3 D momentum distributions in the $yz$ plane ($|p_x|<5\mathrm{a}\mathrm{.}\mathrm{u}\mathrm{.}$) for the indicated fragments from channels (2) (a),(b) and (1) (c),(d). Pulse duration 4.5 fs (left) and 25 fs (right), intensity $5\times 10^{14}\mathrm{W}/{\mathrm{cm}}^2$ for each panel, polarization direction horizontal ($z$). (e) Normalized angular momentum distributions for channels (1) and (2). Pulse parameters as indicated. Polarization direction horizontal. (f) $⟨{\cos }^2\theta ⟩$ over pulse duration for channels (1) and (2). Peak intensity as indicated.

Figure 3

(a) Normalized KER spectra of fragmentation channel (1) for three different laser intensities as indicated. (b)–(e) Angular distributions over KER of ${\mathrm{CH}}_2^{+}$ fragments from channel (1) measured at the indicated laser intensities. (d),(e) Selections of the angular distribution shown in (c) for $|p_x|<15\mathrm{a}\mathrm{.}\mathrm{u}\mathrm{.}$ (d) and $|p_x|>40\mathrm{a}\mathrm{.}\mathrm{u}\mathrm{.}$ (e) in a plane parallel to the $yz$ plane, with ${\theta }_{yz}$ the angle to the $z$ axis in this plane. See text for details.

Figure 4

(a) Ratio of event numbers for channel (1) with respect to those for channel (2) measured for a pulse duration of 25 fs as a function of laser peak intensity. (b) Sum of event numbers in channels (1) and (2) normalized to the detected counts of the parent ion ${\mathrm{C}}_2{\mathrm{H}}_4^{2+}$ over laser pulse duration for the two peak intensities $5\times 10^{14}\mathrm{W}/{\mathrm{cm}}^2$ (red) and $8\times 10^{14}\mathrm{W}/{\mathrm{cm}}^2$ (blue).