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arXiv:1809.04846 · Physical Review A

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

Nikolay V. Dozmorov, Alexey V. Baklanov, Johannes von Vangerow, Frank Stienkemeier, Jordan A. M. Fordyce, Marcel Carto Constantin Mudrich

Published Oct 2, 2018• 11 citations
arXivHTMLPDF

Abstract

The desorption of excited rubidium atoms off the surface of helium nanodroplets is studied in detail using femtosecond time-resolved photoion and photoelectron imaging spectroscopy in combination with quantum wave-packet simulations. The good agreement of the measured time-dependent velocity distributions with the simulation when exciting the Rb dopant atoms into the $6p$ state supports the pseudodiatomic model for the Rb-He droplet interaction, even on the level of quantum wave-packet dynamics. Time-resolved photoelectron spectra reveal the partitioning of excitation energy into the dopant and the droplet degrees of freedom.

DOI
10.1103/physreva.98.043403

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The desorption of excited rubidium atoms off the surface of helium nanodroplets is studied in detail using femtosecond time-resolved photoion and photoelectron imaging spectroscopy in combination with quantum wave-packet simulations. The good agreement of the measured time-dependent velocity distributions with the simulation when exciting the Rb dopant atoms into the 6p6p6p state supports the pseudodiatomic model for the Rb-He droplet interaction, even on the level of quantum wave-packet dynamics. Time-resolved photoelectron spectra reveal the partitioning of excitation energy into the dopant and the droplet degrees of freedom.
Rubidium dimers residing on the surface of He nanodroplets are doubly ionized by an intense femtosecond laser pulse leading to fragmentation into a pair of Rb+{\mathrm{Rb}}^{+}Rb+ ions. We show that the kinetic energy of the Rb+{\mathrm{Rb}}^{+}Rb+ fragment ions can be used to identify dimers formed in either the XXX $^{1}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$ ground state or in the lowest-lying triplet state, $a\text{ }^{3}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$. From the experiment, we estimate the abundance ratio of dimers in the aaa and XXX states as a function of the mean droplet size and find values between $4\ensuremath{\mathbin:}1$ and $5\ensuremath{\mathbin:}1$. Our technique applies generally to dimers and trimers of alkali atoms, here also demonstrated for Li2{\mathrm{Li}}_{2}Li2​, Na2{\mathrm{Na}}_{2}Na2​, and K2{\mathrm{K}}_{2}K2​, and will enable femtosecond time-resolved measurements of their rotational and vibrational dynamics, possibly with atomic structural resolution.
We present a combined experimental and theoretical study of the RbSr molecule. The experimental approach is based on the formation of RbSr molecules on helium nanodroplets. Utilizing two-photon ionization spectroscopy, an excitation spectrum ranging from 11,600 up to 23,000 cm(-1) was recorded. High level ab initio calculations of potential energy curves and transition dipole moments accompany the experiment and facilitate an assignment of transitions. We show that RbSr molecules desorb from the helium droplets upon excitation, which enables dispersed fluorescence spectroscopy of free RbSr. These spectra elucidate X(2)Σ(+) ground and excited state properties. Emission spectra originating from states corresponding to the Rb(5s(2)S) + Sr(5s5p(3)P) asymptote were recorded; spin-orbit coupling was included for the simulation. The results should provide a good basis for achieving the formation of this molecule in cold collisions, thus offering intriguing prospects for ultracold molecular physics.
Abstract Alkali atoms and small clusters are known to reside on the surface of a helium droplet rather than its inside as most other dopant species. A theoretical investigation suggested that alkali clusters (Li–Rb) exceeding a certain critical size can become submerged in the droplet, which was experimentally confirmed for sodium and potassium. Here, we report an analogous experimental study of rubidium cluster submersion by means of electron impact mass spectrometry. We recorded size distributions of Rb cluster ions at various electron energies between 8 and 160 eV. Our data suggest that Rb clusters attached to helium droplets undergo a gradual submersion transition similar to potassium, ultimately leading to the full submersion of clusters larger than ∼100 Rb\sim 100~\hbox {Rb}∼100 Rb ∼ 100 Rb atoms. Our findings are consistent with previous theoretical and experimental studies. Graphic abstract
We measured laser-induced-fluorescence (LIF) and beam-depletion (BD) spectra of rubidium atoms (5S-5P transition) on the surface of superfluid helium nanodroplets (M-He_{N} with M=Rb). It is known that when M is a lighter alkali atom electronic excitation always leads to detachment of the excited atom (M;{*}). The dissociation energy, few tens cm;{-1}, comes either as photon excess energy or from the barrierless formation of a M;{*}-He exciplex. We observe that this picture does not hold when M=Rb and the photon excess energy is small: we are able to excite atoms without detaching them from the droplet, thanks to a barrier preventing formation of the exciplex. This system is ideally suited for optical spin pumping in a He nanodroplet, whose achievement we explicitly demonstrate in a pump-probe magnetic circular dichroism experiment.
The capture dynamics of a Ne atom by a superfluid helium nanodroplet (((4)He)N=1000; T = 0.37 K), Ne + ((4)He)N→ Ne@((4)He)N', was investigated using a quantum approach (TDDFT (helium) + quantum wave packet (Ne)) at zero angular momentum and a rather wide range of Ne atom initial mean velocities (〈v0〉: 90-1300 m s(-1)). This is probably the first quantum dynamics study focusing on the pick up process and the evolution of the dopant inside the nanodroplet and the second more detailed investigation on this topic. For 〈v0〉 = 210 m s(-1) and above the atom is always captured, but for lower velocities the probability of capture is somewhat below the unity and decreases as 〈v0〉 diminishes. The main energy exchange begins with the collision of the atom with the nanodroplet surface, and the excess of energy placed in the doped nanodroplet is progressively released by the evaporation of a small amount of (4)He atoms. Once the atom has entered into the nanodroplet its mean position follows an oscillatory trajectory, due to multiple sequential collisions with the inner surface of the nanodroplet, and its mean velocity reaches values which are below Landau's critical velocity. This probably corresponds to the general behavior of nanodroplets with a bulk-like region when moderate collision energies (i.e., similar to the ones considered here) are involved. In the future we hope to investigate the influence of angular momentum on the mechanism of the pick up process, using the same quantum dynamics method.
Silver atoms, initially embedded in nanodroplets of superfluid helium (roughly 104{10}^{4}104 atoms in size), are found to move to the surface and desorb when excited to the broadened 5p5p5p level. The absorption of a second photon within less than 10 ns leads to the population of previously unobserved Rydberg states (20<n<60)(20<n<60)(20<n<60) of free silver atoms. This is the first result showing laser induced movement of a foreign atom from the inside to the surface of a nanodroplet of superfluid helium.