Polar di-halogen molecules solvated in bosonic helium clusters: The paradigm of $\mathrm{I}\mathrm{Cl}\left(X\right)$

Energies and structures of ${}^4\mathrm{He}_N\text{−}\mathrm{I}\mathrm{Cl}\left(X\right)$ complexes, $N⩽30$, are determined within the framework of a recently developed Hartree-like approach [Phys. Rev. A 71, 033203 (2005)], in which the He atoms play the role of the electrons and the I and Cl atoms play the role of the nuclei. The potential energy of the system is represented as a sum of the He-ICl triatomic and He-He pair interactions fitted to results of ab initio calculations. The validity of the approach is evaluated through comparisons with the results of “exact” variational calculations performed for the cases of $N=1$ and 2. The procedure, which furnishes also the wavefunctions, allows for simulation of the infrared spectra of the ICl molecule embedded in the bath of ${\mathrm{He}}_N$ clusters. Similarly to the case of the “bare” ICl, when the clusters are formed from bosonic He atoms the absorption selection rules lead only to the $P$ and $R$ branches of the spectra (the $Q$ branches are missing). The spectra exhibit a monotonically increasing blueshift, albeit with a decreasing rate, as the cluster size increases.

Figure 1

Minimum energy $W_m$ in ${\mathrm{cm}}^{-1}$, and distance of minimum energy $R_m$ in Å, as functions of $\theta$ and $r$. Solid, dotted, and dashed lines correspond to $r=2.271$, 2.321, and $2.386\mathrm{\AA }$, respectively.

Figure 2

Total energies $E_N$ (open circles) and single-helium evaporation energies, $\left(E_N\text{−}{E}_{N-2}\right)∕2$ (solid circles), as a function of the cluster size $N$.

Figure 3

(a) Computed angular density distributions for the He atoms surrounding the ICl molecule for the different cluster sizes. Inset: Comparison of the angular distributions for two helium atoms from variational (full lines) and Hartree calculations (dashed lines). The distributions are normalized to the number of helium atoms, $N$. (b) Angular distributions of the five lowest independent-particle picture orbitals.

Figure 4

Radial densities for the helium distributions surrounding the ICl dopant molecule. The distributions are normalized to 1. Inset: Radial distributions of the five lowest independent-particle picture orbitals.

Figure 5

Computed ground stretching energy and half width for the VP process $\mathrm{He}\text{−}\mathrm{I}\mathrm{Cl}(X,v_f=1)\to \mathrm{He}+\mathrm{I}\mathrm{Cl}(X,v_f^{\prime }=1)$, as functions of the orientation $\theta$.

Figure 6

IR absorption cross sections, for the size selected clusters, showing the first four more intense branches. Cross sections corresponding to $N=2$ are referred to the right vertical axis, while the rest ($\sim 10$ times weaker) are represented on the left one.