Spectroscopy of nS, nP, and nD Rydberg series of Cs atoms on helium nanodroplets

Abstract

The preparation of an artificial superatom consisting of a positive charge inside a superfluid helium nanodroplet and an electron in an orbital surrounding the droplet is of fundamental interest and represents an experimental challenge. In this work, nanodroplets of several thousand helium atoms are doped with single caesium (Cs) atoms. While on the droplet, the Cs valence electron is excited in two steps through an intermediate state into nS, nP, and nD states. The excitation is monitored by laser induced fluorescence or, for high principal quantum numbers, by resonant three-photon-ionization. On-droplet Rydberg excitations are resolved up to about n = 20. The energies are compared with those of free Cs atom Rydberg states and quantum defects as well as the on-droplet ionization threshold are derived.

Picosecond solvation dynamics of alkali cations in superfluidHe4nanodroplets

“…1 and 2) and the deeper dimple structure for Rb compared to Cs [56]. The calculated shifts are significantly larger than the ionization thresholds that have been determined by extrapolation of the Rydberg series for these systems [67,68], or measured directly via two-step ionization [31]. This difference can be attributed to the fact that experiment and theory do not probe the same ionization threshold.…”

Photoionisaton of pure and doped helium nanodroplets

“…In contrast, the solvation and desolvation dynamics of metal atoms attached to He droplets is now well characterized even for excited levels approaching the ionization threshold and above [58,65,75,128,143]. Surprisingly, the dopantdroplet complex can be quite accurately represented by a pseudo-diatomic molecule in many cases [72,129].…”

“…However, in a more recent theoretical study, Ancilotto et al questioned the stability of such a system arguing that the electron orbiting outside the droplet may pull the positive ionic core close enough to the surface that fast electron-ion recombination occurs [79]. REMPI spectra, photoelectron and ZEKE measurements revealed clearly resolved Rydberg series of broadened peaks up to principal quantum numbers n ∼ 20 in the absorption spectra of Na, Rb and Cs attached to He droplets [58,65,80]. While lifetimes shorter than the laser pulse (∼ 10 ns) were inferred for Na Rydberg states with n 20, lifetimes τ ≈ 1 µs were measured for states with n 100.…”

“…Upon electronic excitation, Ak atoms and small molecules tend to desorb off the surface of He droplets due to repulsive forces acting between the excited Ak atom or molecule and the He droplet as a whole. A simplified representation of the state-dependent interaction of Ak atoms with He nanodroplets is given by the "pseudo-diatomic model", in which the Ak dopant constitutes one atom and the entire He droplet the other [63][64][65][66][67]. This model provides a simple interpretation of the absorption spectra measured by laser-induced fluorescence emission or by REMPI and of momentum distributions of desorbed Ak atoms.…”

“…, where E i,dr denotes the ionization energy of the CsHe N complex and d dr (n) = d at (n) + ∆d dr is the quantum defect consisting of the atomic term d at (n) and an -state dependent constant addend ∆d dr ∼ −0.1 which accounts for the perturbation by the He droplet [58,65]. R ∞ = 13.606 eV is the Rydberg constant.…”