Structure determination of alkali trimers on helium nanodroplets through laser-induced Coulomb explosion

Abstract

Alkali trimers, Ak${}_3$, located on the surface of He nanodroplets are triply ionized following multiphoton absorption from an intense femtosecond laser pulse leading to fragmentation into three correlated Ak${}^{+}$ ions. Combining the information from three-fold covariance analysis of the emission direction of the fragment ions and from their kinetic energy distributions $P(E_{\text{kin}})$, we find that Na${}_3$, K${}_3$, and Rb${}_3$ have an equilateral triangular structure, corresponding to that of the lowest-lying quartet state ${}^4{\mathrm{A}}_2^{\prime }$, and determine the equilibrium bond distance $R_{\text{eq}}$(Na${}_3$) = 4.65 $\pm$ 0.15 Å, $R_{\text{eq}}$(K${}_3$) = 5.03 $\pm$ 0.18 Å, and $R_{\text{eq}}$(Rb${}_3$) = 5.45 $\pm$ 0.22 Å. For K${}_3$ and Rb${}_3$ these values agree well with existing theoretical calculations, while for Na${}_3$ the value is 0.2-0.3 Å larger than the existing theoretical results. The discrepancy is ascribed to a minor internuclear motion of Na${}_3$ during the ionization process. Also, we determine the distribution of internuclear distances $P(R)$ under the assumption of fixed bond angles. The results are compared to the square of the internuclear wave function $|\Psi (R){|}^2$.

7 pages, 5 figures