Photodissociation dynamics of weakly bound $\mathrm{He}{{\mathrm{H}}_2}^{+}$ in intense light fields

Photoinduced dynamics of a weakly bound triatomic molecule $\mathrm{He}{{\mathrm{H}}_2}^{+}$ exposed to electromagnetic radiation is investigated by time-dependent quantum wave-packet propagation. Adopting a two-dimensional linear H-H-He model, the three lowest-lying potential energy surfaces (PESs) and corresponding dipole moment surfaces are constructed. One of the two characteristic excited PESs of $\mathrm{He}{{\mathrm{H}}_2}^{+}$ leads to the charge-transfer reaction ${{\mathrm{H}}_2}^{+}+\mathrm{He}\to {\mathrm{H}}_2+\mathrm{H}{\mathrm{e}}^{+}$ and the other corresponds to the first excited state of ${{\mathrm{H}}_2}^{+}$ perturbed by the presence of He. When $\mathrm{He}{{\mathrm{H}}_2}^{+}$ is exposed to a femtosecond intense ultraviolet light pulse ($I=4\times {10}^{14}\mathrm{W}\mathrm{c}{\mathrm{m}}^{-2}, \lambda =400\mathrm{nm}$), both of the two excited PESs are found to be coupled with the light field and a variety of reaction pathways become opened so that HeH, $\mathrm{He}{\mathrm{H}}^{+}, {\mathrm{H}}_2,{{\mathrm{H}}_2}^{+},\mathrm{H},{\mathrm{H}}^{+}$, He, and $\mathrm{H}{\mathrm{e}}^{+}$ are produced. Simulations also show that the anharmonic coupling between the two stretching vibrational modes in $\mathrm{He}{{\mathrm{H}}_2}^{+}$ leads to the stabilization of the ${{\mathrm{H}}_2}^{+}$ moiety against the decomposition into $\mathrm{H}+{\mathrm{H}}^{+}$ compared with bare ${{\mathrm{H}}_2}^{+}$. The theoretical findings of the formation of $\mathrm{He}{\mathrm{H}}^{+}$ composed of the most abundant elements in the universe are also discussed in view of the theoretical modeling of the chemical reactions proceeding in the primordial gas and in the interstellar medium.

Figure 1

Visual presentation of the Jacobi coordinates (left panel) and the $R_1=r\left(\mathrm{H}–\mathrm{H}\right)$ and $R_2=r\left(\mathrm{H}–\mathrm{He}\right)$ stretching coordinates (right panel).

Figure 2

Three lowest-lying adiabatic potential energy surfaces of the linear ${(\mathrm{H}–\mathrm{H}–\mathrm{He})}^{+}$ molecule as a function of the $\mathrm{H}–\mathrm{H}$ and $\mathrm{H}–\mathrm{He}$ distances. Asymptotic atom-diatom dissociation products are also indicated.

Figure 3

Different $\mathrm{He}{{\mathrm{H}}_2}^{+}$ photodissociation channels and product yields, which are defined as the probability density absorbed at the asymptotic regions of the coordinate space, as a function of time, when the molecule is exposed to a Gaussian-shaped light pulse ($\lambda =400\mathrm{nm}, I=4\times {10}^{14}\mathrm{W}\mathrm{c}{\mathrm{m}}^{-2}, \mathrm{\Gamma }=11.4\mathrm{fs}$ FWHM of the electric field).

Figure 4

Nuclear wave-packet components on the three potential energy surfaces at different times. Contour lines of the respective potential energy surfaces are indicated in red.

Figure 5

Temporal evolution of the populations on the (blue) ground electronic state, (red) first excited $[{\mathrm{H}}_2{}^{+}\left({}^2{\mathrm{\Sigma }}_{\mathrm{u}}^{+}\right)—\mathrm{He}]$ electronic state, and (green) second excited $[{\mathrm{H}}_2\left({}^1{\mathrm{\Sigma }}_{\mathrm{g}}^{+}\right)—{\mathrm{He}}^{+}]$ electronic state. The external field amplitude reaches its peak at 0 fs.

Figure 6

(a) Temporal evolution of the norm of the wave packet of ${{\mathrm{H}}_2}^{+}$ (red) and that of $\mathrm{He}{{\mathrm{H}}_2}^{+}$ (blue), and (b) distributions of the norms of wave-packet projections onto the vibrational eigenstates of ${{\mathrm{H}}_2}^{+}$ as a function of time, as computed by Eq. (2).