Multiple Photofragmentation Pathways with Different Recoil Anisotropy from a Metal-Ion–Ligand Complex

Spatial recoil anisotropy that is dependent upon the fragment-ion species is reported for the first time for a metal-ion–ligand complex after a single photoexcitation process by linearly polarized light. Upon excitation to the lowest three excited states of ${\mathrm{M}\mathrm{g}}^{+}$-${\mathrm{I}\mathrm{C}\mathrm{H}}_3$, originating from the ${\mathrm{M}\mathrm{g}}^{+}{}^2\mathrm{P}$ states, fragment ions of ${\mathrm{M}\mathrm{g}\mathrm{I}}^{+}$ and ${\mathrm{I}\mathrm{C}\mathrm{H}}_3^{+}$ are found to have clear and different angular dependences, which are also characteristic of the excited states. These are explained from the results of theoretical work in that the calculated ground-state complex has a bent structure and further in that each transition dipole moment vector of the complex almost coincides with the ${\mathrm{M}\mathrm{g}}^{+}$ $3p$ orbital lobe direction in each case. The fragment ions are concluded to be formed along dissociative potential surfaces which are crossed by the initially excited states, in a much faster process than the rotational period of the complex.

Figure 1

(a) Photodissociation spectrum of ${\mathrm{M}\mathrm{g}}^{+}$-${\mathrm{I}\mathrm{C}\mathrm{H}}_3$ obtained experimentally (left), potential energy curves of ground and excited states along the Mg-I bond length by CIS/DFT calculation (middle), and energy levels of the fragments (right). Potential curves are drawn so as to correlate with the molecular orbital picture at long range, although curve crossings are avoided adiabatically between the states with the same symmetry. Dotted curves are the excited states with CT nature correlated with $\mathrm{M}\mathrm{g}+{\mathrm{I}\mathrm{C}\mathrm{H}}_3^{+}$. (b) The most stable structure of the parent complex in its ground state. Natural charges on Mg and I are indicated in parentheses.

Figure 2

Time-of-flight profiles $F(t)$ of the fragment ions from ${\mathrm{M}\mathrm{g}}^{+}$-${\mathrm{I}\mathrm{C}\mathrm{H}}_3$ with $\mathit{E}\parallel \mathit{Z}$ and $\mathit{E}\perp \mathit{Z}$. (a) and (c) Photolysis at 266 nm, (b) and (d) photolysis at 355 nm. The peaks at $84.5\mu \mathrm{s}$ in (a) and (b) are due to the parent ${\mathrm{M}\mathrm{g}}^{+}$-${\mathrm{I}\mathrm{C}\mathrm{H}}_3$ complex remaining after photodissociation. (c) and (d) are the expanded figures of (a) and (b) in the region of 78 to $82.5\mu \mathrm{s}$ (${\mathrm{I}\mathrm{C}\mathrm{H}}_3^{+}$ and ${\mathrm{M}\mathrm{g}\mathrm{I}}^{+}$ fragment region), respectively.

Figure 3

Velocity distribution along the $\mathit{Z}$ direction $F^{\prime }(v_Z)$ for (a) ${\mathrm{I}\mathrm{C}\mathrm{H}}_3^{+}$ and (b) ${\mathrm{M}\mathrm{g}\mathrm{I}}^{+}$ fragments at photolysis wavelength of 266 nm. ○: Obtained with $\mathit{E}\parallel \mathit{Z}$, gray ●: with $\mathit{E}\perp \mathit{Z}$. Solid curves: Fitted results.

Figure 4

Schematic drawing of the correlation with highest probability among vectors $\mathit{v}$, $\mathbf{\mu }$, $\mathit{E}$, and $\mathit{Z}$ for 266 nm excitation (Band III, ${\mathrm{M}\mathrm{g}}^{+}$ $3s$ to $3p_z$). (a) and (b) The ${\mathrm{I}\mathrm{C}\mathrm{H}}_3^{+}$ fragment formation, (c) and (d) ${\mathrm{M}\mathrm{g}\mathrm{I}}^{+}$ formation. (a) and (c) the condition under $\mathit{E}\parallel \mathit{Z}$, (b) and (d) under $\mathit{E}\perp \mathit{Z}$.