Dynamics of solvation and desolvation of rubidium attached to He nanodroplets

Abstract

The real-time dynamics of photoexcited and photoionized rubidium (Rb) atoms attached to helium (He) nanodroplets is studied by femtosecond pump-probe mass spectrometry. While excited Rb atoms in the perturbed 6p-state (Rb * ) desorb off the He droplets, Rb + photoions tend to sink into the droplet interior when created near the droplet surface. The transition from Rb + solvation to full Rb * desorption is found to occur at a delay time τ ∼ 600 fs for Rb * in the 6pΣ-state and τ ∼ 1200 fs for the 6pΠ-state. Rb + He ions are found to be created by directly exciting bound Rb * He exciplex states as well as by populating bound Rb + He-states in an photoassociative ionization process.

Quantum dynamics of Rb atoms desorbing off the surface of He nanodroplets

“…In particular, for λ = 415 nm, which corresponds to excitation of mainly the 6pΠ 3/2 -state, the simulated rising edges of both the mean speed and the [RbHe] + ion yield match the experimental results very well [panels e) and h)]. The better agreement of experiment and simulation as compared to our previous model calculations [28] is mainly due to the different initialization procedure of the excited wave packet, which supports the model assumptions made in the present approach. Furthermore, the simulated widths σ and skewnesses γ match the experimental values quite well for long delays ∆t.…”

“…Due to the conservation of energy in the wave packet propagation, this final speed distribution relates to an initial spatial distribution |ψ * (R)| 2 of the wave packet on the repulsive PEC at short distances. This approach differs from the one we used previously for a mixed quantum-classical simulation [28]. There, the wave packet propagation was initialized based on the spectral profile of the pump laser pulse.…”

“…Nevertheless, for Rb atoms attached to He nanodroplets, we were able to reproduce the general trends of the experimental velocity and timeresolved photoionization signals using the PDM [7,28]. In particular, the interplay between desorption of the excited Rb dopant atom and the falling back of the Rb + and [RbHe] + ions into the He droplet when ionization occurs at short pump-probe delay times was unambiguously established by comparing to classical trajectory simulations based on the PDM [28]. However, these simulations yielded systematically shorter time constants for the desorption dynamics.…”

Density functional theory of doped superfluid liquid helium and nanodroplets

“…(77) yields the results summarised in Photoexcitation and photoionisation of Rb atoms attached to helium droplets has been studied in real-time dynamics experiments. 267 It was shown that excitation of Rb atoms from the 5s to the 6p states leads to their detachment. Upon subsequent ionisation of the excited Rb atom (Rb * ), the interaction with helium becomes attractive.…”

“…The critical time τ c separating these processes is called the fall-back time. 267 Recently, this study has been extended to the Rb 5p ← 5s transition. 31 Both transitions have been simulated with TDDFT as described in Sec.…”