Time-resolved Coulomb explosion imaging of vibrational wave packets in alkali dimers on helium nanodroplets

J. Chem. Phys. 161, 224301 (2024) Vibrational wave packets are created in the lowest triplet state \triplet of $\mathrm{K_2}$ and $\mathrm{Rb_2}$ residing on the surface of helium nanodroplets, through non-resonant stimulated impulsive Raman scattering induced by a moderately intense near-infrared laser pulse. A delayed, intense 50-fs laser pulse doubly ionizes the alkali dimers via multiphoton absorption and thereby causes them to Coulomb explode into a pair of alkali ions $\mathrm{Ak^+}$. From the kinetic energy distribution $P(E_\mathrm{kin})$ of the $\mathrm{Ak^+}$ fragment ions, measured at a large number of delays, we determine the time-dependent internuclear distribution $P(R,t)$, which represents the modulus square of the wave packet within the accuracy of the experiment. For both $\mathrm{K_2}$ and $\mathrm{Rb_2}$, $P(R,t)$ exhibits a periodic oscillatory structure throughout the respective 300 ps and 100 ps observation times. The oscillatory structure is reflected in the time-dependent mean value of $R$, $\langle R \rangle(t)$. Fourier transformation of $\langle R \rangle(t)$ shows that the wave packets are composed mainly of the vibrational ground state and the first excited vibrational state, in agreement with numerical simulations. In the case of $\mathrm{K_2}$, the oscillations are observed for 300 ps corresponding to more than 180 vibrational periods with an amplitude that decreases gradually from 0.035 {\AA} to 0.020 {\AA}. Using time-resolved spectral analysis, we find that the decay time of the amplitude is $\sim$ 260 ps. The decrease is ascribed to the weak coupling between the vibrating dimers and the droplet.