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

Abstract

Vibrational wave packets are created in the lowest triplet state \triplet of ${\mathrm{K}}_2$ and $\mathrm{R}{\mathrm{b}}_2$ 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 $\mathrm{A}{\mathrm{k}}^{+}$. From the kinetic energy distribution $P(E_{\mathrm{kin}})$ of the $\mathrm{A}{\mathrm{k}}^{+}$ 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 $\mathrm{R}{\mathrm{b}}_2$, $P(R,t)$ 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 $R$, $⟨R⟩(t)$. Fourier transformation of $⟨R⟩(t)$ 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 Å to 0.020 Å. 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.

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