Ultrafast Dissociation of Metastable ${\mathrm{CO}}^{2+}$ in a Dimer

We triply ionize the van der Waals bound carbon monoxide dimer with intense ultrashort pulses and study the breakup channel $(\mathrm{CO}{)}_2^{3+}\to {\mathrm{C}}^{+}+{\mathrm{O}}^{+}+{\mathrm{CO}}^{+}$. The fragments are recorded in a cold target recoil ion momentum spectrometer. We observe a fast ${\mathrm{CO}}^{2+}$ dissociation channel in the dimer, which does not exist for the monomer. We found that a nearby charge breaks the symmetry of a ${\mathrm{X}}^3\mathrm{\Pi }$ state of ${\mathrm{CO}}^{2+}$ and induces an avoided crossing that allows a fast dissociation. Calculation on the full dimer complex shows the coupling of different charge states, as predicted from excimer theory, gives rise to electronic state components not present in the monomer, thereby enabling fast dissociation with higher kinetic energy release. These results demonstrate that the electronic structure of molecular cluster complexes can give rise to dynamics that is qualitatively different from that observed in the component monomers.

Figure 1

(a) Momentum distribution of ${\mathrm{CO}}^{+}$. Black dots show experimental data; red and blue curves show the fitting with two Gaussian distributions. (b) Newton diagram for all events in the channel $(\mathrm{CO}{)}_2^{3+}\to {\mathrm{C}}^{+}+{\mathrm{O}}^{+}+{\mathrm{CO}}^{+}$. (c)–(d) are Newton diagrams for events in different subsets. Corresponding ${\mathrm{CO}}^{+}$ momentum is shown in the top right insets. (c) $|{\mathbf{p}}_{{\mathrm{CO}}^{+}}|<100\mathrm{a}.\mathrm{u}.$ or $|{\mathbf{p}}_{{\mathrm{CO}}^{+}}|>120\mathrm{a}.\mathrm{u}.$; (d) 100 a.u. $<|{\mathbf{p}}_{{\mathrm{CO}}^{+}}|<120\mathrm{a}.\mathrm{u}.$. (e) and (f) Sketches of direct and sequential dissociation processes. Black arrows, the final momentum of each fragment; red arrow, momentum of ${\mathrm{CO}}^{2+}$ in the first dissociation step; gray arrows, momentum of ${\mathrm{C}}^{+}$ or ${\mathrm{O}}^{+}$ from the second dissociation step. Geometry with minimum energy is shown as an example.

Figure 2

(a) Kinetic energy distribution of ${\mathrm{CO}}^{+}$, solid red curve for the sequential dimer breakup $(\mathrm{CO}{)}_2^{3+}\to {\mathrm{C}}^{+}+{\mathrm{O}}^{+}+{\mathrm{CO}}^{+}$ (subset II) and dashed green curve for the two-particle dimer breakup $(\mathrm{CO}{)}_2^{2+}\to {\mathrm{CO}}^{+}+{\mathrm{CO}}^{+}$. Kinetic energy for the latter channel is multiplied by 2 to compensate for the charge difference in the two channels. (b) Kinetic energy of ${\mathrm{CO}}^{+}$ in the breakup channel $(\mathrm{CO}{)}_2^{3+}\to {\mathrm{CO}}^{2+}+{\mathrm{CO}}^{+}$ as a function of propagation time. (c) Kinetic energy of ${\mathrm{C}}^{+}$ and ${\mathrm{O}}^{+}$, solid red curve for the second step of dimer sequential breakup, dashed black curve for monomer breakup ${\mathrm{CO}}^{2+}\to {\mathrm{C}}^{+}+{\mathrm{O}}^{+}$. (d) Total kinetic energy release in the three-particle dimer breakup process. Solid blue curve is for subset I, direct breakup channel; solid red curve for subset II, sequential channel. The dotted curves are for specific geometries in the subset I: orange curve for $\theta <20°$ and purple curve for $80<\theta <100°$. (e) Bond angle distribution of the dimer in subset I.

Figure 3

(a) Potential energy curves of ${\mathrm{CO}}^{2+}$ with a point charge. The Coulomb field of the point charge perturbs the ${\pi }_x$ and ${\pi }_y$ orbitals differently, which breaks the symmetry of one ${\mathrm{X}}^3\mathrm{\Pi }$ electronic state. An avoided crossing arises between one component of the ${\mathrm{X}}^3\mathrm{\Pi }$ states and the ${{}^3\mathrm{\Sigma }}^{-}$ state. (b) Ab initio potential energy curves of the ${\mathrm{CO}}^{+}\text{−}{\mathrm{CO}}^{2+}$ complex. Arrows show possible dissociation pathways. Kinetic energy release from pathway (1) is 1.2 eV higher than from pathway (2).