LETTER pubs.acs.org/JPCL
Ionization Thresholds of Alkali Metal Atoms on Helium Droplets Moritz Theisen, Florian Lackner, G€unter Krois, and Wolfgang E. Ernst* Institute of Experimental Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria/EU ABSTRACT: Superfluid helium droplets (HeN) have attracted strong interest as cold hosts for the investigation of weakly bound molecules. Whereas the hostdopant interaction is weak for neutral molecules, ion impurities may be surrounded by frozen shells of polarized helium atoms. An extreme example of the different behavior is given by alkali metal impurities that stay at the surface of the droplet but immerse into the droplet as cations releasing a considerable amount of binding energy. We report measurements of the photoionization efficiency for the rubidium-HeN and cesium-HeN systems and find that the ionization threshold is lowered compared with the free atoms. The corresponding energy shift increases when going from heavy to light alkali metals and from small to large helium droplets. Both effects can be explained by the difference in polarization energies associated with submerged alkali metal cations. The findings agree qualitatively well with recent calculations of helium snowball formation around alkali metal cations. SECTION: Dynamics, Clusters, Excited States
H
elium droplets (HeN) serve as unique spectroscopic matrix for the isolation of atoms, molecules, and clusters in a nanoscale superfluid environment of ∼0.4 K temperature1 while allowing the application of basically all molecular beam techniques.2 The superfluid property permits unhindered nuclear motion of the dopant and sharp lines in microwave and infrared spectra.1,2 Electronic excitation spectra may be strongly broadened because of the Pauli repulsion by the surrounding helium.2 So far, mainly neutral impurities have been studied in and on helium droplets. Because mass selection of dopants is sometimes an important issue, the interaction of ionic species with helium is a subject of increasing interest. On the basis of the electrostatic potential produced by the dielectric response of the helium to an ionic impurity, Lehmann and Northby3 presented a potential for the motion of an ion inside a helium droplet. While they derive an almost harmonic trapping potential for any ionic impurity with the minimum in the center of the droplet and a positive anharmonicity that lets the potential energy rise steeply toward the droplet surface, the absolute depth depends of course on the species. Because of the significantly different interaction energy of neutral and ionized impurities with helium, a deviation of the ionization threshold (IT) from the free atom or molecule value is to be expected. As neutrals, alkali metal (Ak) atoms and molecules play a special role in helium droplet isolation spectroscopy because of the extremely weak alkali metalhelium interaction. As already observed in the first experiments at Princeton,4 alkali metal dopants reside on the droplet surface and do not migrate into the helium. Ak ions, however, will strongly polarize nearby helium atoms and surround themselves by a helium snowball.5 In fact, the immersion of rubidium6 (Rb) and cesium7 (Cs) ions into helium droplets was recently reported. Galli, Ceperley, and Reatto5 present theoretical results for the interaction energy of alkali metal ions with surrounding helium in small droplets that allow an estimate for the trend in the shift of ITs for Na, K, and Cs. r 2011 American Chemical Society
Experimentally, the determination of the on-droplet IT requires ionization sources of the appropriate energy and the detection of the well-defined ion product. Simply detecting ionized alkali metals with attached helium may result from ionization of larger Ak aggregates on helium droplets and subsequent fragmentation in the same cluster beam. Rubidium and cesium offer the unique opportunity to avoid this ambiguity through a two-step ionization taking advantage of an intermediate resonance. Whereas the lighter Ak atoms detach from the surface of the HeN after electronic excitation, the excitation of RbHeN8 into the 52P1/2 (2Π1/2) state and CsHeN7 into the 62P1/2 (2Π1/2) state does not lead to detachment but leaves the excited atoms residing on the surface. Corresponding excitation and emission spectra of CsHeN and RbHeN are shown in refs 7 and 8, respectively. These findings have allowed a number of investigations that make use of this bound n2P1/2 (2Π1/2) state (Rb: n = 5, Cs: n = 6) as springboard for further excitation or ionization6,7,9 of Ak-HeN complexes while excluding a simultaneous detection of fragmentation products. The recent assignment of nS, nP, and nD Rydberg series of Cs atoms on helium droplets10 is an example. The notation of electronically excited states of Ak-HeN with moderate principal quantum number n is based on a pseudodiatomic model, in which the HeN acts as one large atom and the “internuclear axis” from the droplet center to the surface located Ak atom serves as quantization axis for orbital and spin momenta in a Hund’s coupling case a. As described in detail in refs 9 and 11, a state is then characterized by its atomic designation in the LS coupling scheme (e.g., n2P1/2) and the diatomic molecule case a notation (e.g., 2Π1/2). In the detailed study of ref 10, this pseudodiatomic picture was judged to be valid up to n ≈ 10 Received: August 12, 2011 Accepted: October 17, 2011 Published: October 17, 2011 2778
dx.doi.org/10.1021/jz201091v | J. Phys. Chem. Lett. 2011, 2, 2778–2782
The Journal of Physical Chemistry Letters
LETTER
for CsHeN. For higher excitations, the role of the “internuclear axis” vanishes, and we are faced with atomic-like Rydberg states with an indication that the Ak ion core starts to sink into the droplet.10 In this work, helium droplets are produced in a supersonic beam expansion of 60 bar of helium gas through a 5 μm diameter nozzle at temperatures between 14 and 20 K, yielding average sizes of helium droplets between 5000 and 15 000 atoms/droplet according to the scaling laws of Harms, Toennies, and Dalfovo.12 Because details of the experimental setup, the doping with Ak atoms, and the applied laser systems have been provided in refs 7, 9, and 10, we will restrict ourselves to explaining the applied laser excitation/ionization scheme in the following. Rb (Cs) monomers on HeN are ionized in a resonant two-photon ionization (R2PI) scheme with two pulsed lasers. First, they are excited into their nondesorbing n2P1/2 (2Π1/2) state (Rb: n = 5, Cs: n = 6) with laser 1 (L1, Ti:sapphire laser Coherent Indigo S, pulse duration 30 ns, pulse energy 90180 μJ, line width
