Alkali-metal dimers, Ak2, residing on the surface of He nanodroplets, are doubly ionized due to multiphoton absorption from an intense 50-fs laser pulse leading to fragmentation into a pair of alkali-metal cations. Based on the measured kinetic energy distributions P(Ekin) of the Ak+ fragment ions, we retrieve the distribution of internuclear distances P(R) via the Ak22+ potential curve. Results are obtained for Na2, K2, Rb2, and Cs2 in both the 11ςg+ ground state and the lowest-lying triplet state 13ςu+ and for Li2 in the 13ςu+ state. For Li2, K2, and Rb2, the center of the measured P(R) is close to the center of the wave function ψ(R) of the vibrational ground state in the 11ςg+ and 13ςu+ states, whereas for Na2 and Cs2 small shifts are observed. For all the Ak2, the width of the measured P(R) is broader than |ψ(R)|2 by a factor of 2-4. We discuss that resonance effects in the multiphoton ionization and interaction of the Ak+ ion with the He droplet give rise to the observed deviations of P(R) from |ψ(R)|2. Despite these deviations, we deem that timed Coulomb explosion will allow imaging of vibrational wave packets in alkali-metal dimers on He droplets surfaces.
