Cs atoms on helium nanodroplets and the immersion of Cs+ into the nanodroplet

Abstract

We report the non-desorption of cesium (Cs) atoms on the surface of helium nanodroplets (He(N)) in their 6(2)P(1/2) ((2)Π(1/2)) state upon photo-excitation as well as the immersion of Cs(+) into the He(N) upon photo-ionization via the 6(2)P(1/2) ((2)Π(1/2)) state. Cesium atoms on the surface of helium nanodroplets are excited with a laser to the 6(2)P states. We compare laser-induced fluorescence (LIF) spectra with a desorption-sensitive method (Langmuir-Taylor detection) for different excitation energies. Dispersed fluorescence spectra show a broadening of the emission spectrum only when Cs-He(N) is excited with photon energies close to the atomic D(1)-line, which implies an attractive character of the excited state system (Cs∗-He(N)) potential energy curve. The experimental data are compared with a calculation of the potential energy curves of the Cs atom as a function of its distance R from the center of the He(N) in a pseudo-diatomic model. Calculated Franck-Condon factors for emission from the 6(2)P(1/2) ((2)Π(1/2)) to the 6(2)S(1/2) ((2)Σ(1/2)) state help to explain the experimental data. The stability of the Cs∗-He(N) system allows to form Cs(+) snowballs in the He(N), where we use the non-desorbing 6(2)P(1/2) ((2)Π(1/2)) state as a springboard for ionization in a two-step ionization scheme. Subsequent immersion of positively charged Cs ions is observed in time-of-flight mass spectra, where masses up to several thousand amu were monitored. Only ionization via the 6(2)P(1/2) ((2)Π(1/2)) state gives rise to a very high yield of immersed Cs(+) in contrast to an ionization scheme via the 6(2)P(3/2) ((2)Π(3/2)) state. When resonant two-photon ionization is applied to cesium dimers on He droplets, Cs(2) (+)-He(N) aggregates are observed in time-of-flight mass spectra.

Picosecond solvation dynamics of alkali cations in superfluidHe4nanodroplets

“…In particular, there is a clear droplet-size dependence especially apparent in the case of Cs, as simulations of Cs + on a flat helium surface have revealed. This reconciles the outcome of our calculations, carried out for a fairly small droplet, with the well established experimental fact that upon in situ ionization of the neutral species, Ba, Rb, and Cs cations remain attached to helium droplets made of several thousand atoms [29][30][31][32]. It should be noted, however, that in these experiments the alkalis have been ionized via an intermediate state.…”

“…Using this procedure the first solvation shell is found to host 19.2 (21.4) atoms for Rb + (Cs + ). These values are somewhat larger than those found experimentally [32,34] and by QMC calculations [3]. Figure 3 shows the equilibrium configuration of Ba + @ 4 He 1000 .…”

“…For an impurity residing at the surface of a helium droplet, such as an alkali atom or heavy alkaline-earth atom, the cation is expected to become fully solvated by the helium, i.e., sink into the helium droplet. Several studies indeed indicate that following the ionization of alkali atoms the created cations remain attached to the droplet [29][30][31][32]. These experiments, however, provide no details on the location of the cation.…”

Photoionisaton of pure and doped helium nanodroplets

“…The energy shift increases when going from heavy to light Ak atoms and from small to large He droplets due to the difference in polarization energies associated with the submerged Ak metal cations [81]. Furthermore, is was shown that large RbHe n and CsHe n snowballs are formed by resonant two-photon ionization of Rb and Cs via the lowest p 1/2 -states due to the creation of Rb + and Cs + cations at the droplet surface which subsequently submerge into the droplet interior [82,83]. Recently, the alkaline-earth metal atom barium (Ba), which is thought to reside in a dimple at the droplet surface somewhat deeper than Ak atoms [84], was studied using various spectroscopic techniques [59].…”

Dynamics of solvation and desolvation of rubidium attached to He nanodroplets

“…Various recent experimental and theoretical studies have addressed the dynamics of solvation and desolvation of ionized or excited metal atoms off the surface of He nanodroplets [7,[12][13][14][15][16][17][18][19][20][21]. So far, these studies have concentrated on measuring the total yield and the final velocity of the ejected atoms as a function of the atomic species and the electronic state of excitation.…”