Radiative lifetimes of highly excited states in rubidium
2014 Measurements have been made of the natural radiative lifetimes of highly excited rubidium (9 ~ n ~ 18) S and D states. These results, as well as those already published concerning P and F states, agree well with the theoretical lifetimes calculated by the Bates-Damgaard method. J. Physique 41 (1980) 119-121 1 FTVRIER 1980, Classification Physics Abstracts 32.70 In the past few years, measurements of the radiative lifetimes of highly excited states of alkali atoms have been reported [1-3]. These results would clearly be of interest for physicists working in atomic physics, plasma physics, astrophysics. Moreover they allow interesting comparisons with theoretical calculations. We have already published measurements of the radiative lifetimes for some Rydberg P and F states in rubidium [4, 5]. In order to obtain a consistent set of lifetime values for a given alkali atom, we present here measurements concerning the nS and nD states of rubidium (9 n 18). Comparison is given between the obtained experimental values and the theoretical ones calculated by using a modified Bates-Damgaard Method [6, 7] (here after refered as B.D.). We use the classical method of time resolved laser induced fluorescence [4]. Rubidium atoms contained in a cell are excited in a given high-lying nS or nD state by stepwise pulsed excitation via the first resonant 5P state. For convenience the 5p3,2 state is chosen as the intermediate state for the excitation of the nS levels, the observation of the population decay being made on the nS -+ 5P 1/2 line. The nD states are produced by using light pulses whose wavelengths correspond either, to the 5PI/2 --+ nD3/2 transitions for n 13, or to the 5P3/2 -+ nD3/2,5/2 transitions for n &#x3E; 13; the population decay is observed on these same lines. Such a choice is determined by the fact that we are also interested in the study of some collisional properties of the nD states (results concerning the collisional properties of both the S and D levels will be published elsewhere [8]). Thus our reported lifetimes refer to the nD3/2 states for n 13 and to the nD states, without allowance for fine structure, for n &#x3E; 13. This is of no importance since the lifetimes corresponding to the two fine structure levels are expected to be very close, for a given n value. A high-speed photon counting system allows the determination of the decay of the excited.state population as a function of time for a given rubidium pressure. The extrapolation to zero rubidium pressure yields the natural radiative lifetime. The experimental set-up, shown in figure 1, has been already widely Fig. 1. Experimental set-up. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01980004102011900
Ultralong range Rydberg molecules : investigation of a novel binding
In this work the existence of a novel type of molecule is shown whose binding is based solely on electron-atom scattering. These molecules are composed of a Rydberg atom and a ground state atom - here Rb(ns) and Rb(5s) - where the binding arises from scattering of the Rydberg electron from the ground state atom. The ground state atom is bound to the Rydberg wavefunction, which has radii on the order of 100nm, and thus these molecules are of long-range. Although the theoretical description of this interaction goes back to the famous work of Fermi in the year 1934 which was expanded in the following decades by Omont and Greene, the experimental proof of their existence was still missing. Up to now, experimental evidence for the ultralong-range Rydberg molecules was found only in spectral line broadenings. 
The present experiments have demonstrated the direct photoassociation of these ultralong-range Rydberg molecules for Rb(ns)-Rb(5s) triplet states from a cold and dense sample of rubidium 87 atoms for a range of principal quantum numbers, n, between 34 and 40. The high-resolution photoassociation spectra allowed for the observation of several vibrational states separated by only a few Megahertz. Based on the model by Greene the vibrational ground states v=0 are assigned and the dependence of their binding energy on the principal quantum number, n, is used to extract the triplet s-wave scattering length for electron-Rb(5s) collisions from the data. The remarkably good agreement between experiment and theory for the vibrational ground state v=0 over a range of principal quantum numbers attests to the accuracy of Fermi's original pseudopotential approach in describing interactions in excited multi-atom systems. 
Aside from this good agreement for the state v=0, the measurements also show that contributions from p-wave scattering cannot be neglected, as a number of excited vibrational states is observed in the experiment that are not predicted from the theory assuming only s-wave scattering. Calculations of Li, Pohl and coworkers have shown that a full solution of the electron-atom scattering is necessary for the assignment of all vibrational states. This work finds the molecular bound states to be very sensitive to both s-wave as well as p-wave electron-atom scattering and the Rydberg molecules turn out to be an accurate experimental platform to study electron-atom collisions in a previously inaccessible ultralow-energy regime. 
In general, the ultralong-range Rydberg molecules investigated here for rubidium are expected for all species with negative scattering length for electron-atom interaction, e. g. the other alkali atoms. Rydberg atom and ground state atom do not even have to be of the same chemical element, the only requirement is a negative scattering length of the ground state atom for electron scattering. Moreover, the binding force of the Rydberg electron is not restricted to a single ground state atom and molecules with more than one ground state atom can be created. The fact that these exotic molecules have huge bond lengths but nevertheless a vibrational spectrum with several states make a new type of ultracold Rydberg chemistry feasible. In the future, larger polymers or even heteronuclear molecules can be realized, where the number of atoms, the constituent atomic elements and the addressed Rydberg state allow to precisely select the properties of the long-range molecule. 
Diese Arbeit beschaftigt sich mit langreichweitigen Rydberg-Molekulen. Sie sind nur sehr schwach gebunden und deshalb nur unter speziellen Bedingungen erzeugbar und nachweisbar. Darin liegt auch der Grund, weshalb ihre Existenz zwar schon lange von Theoretikern angenommen wurde, bisher aber nicht im Experiment gezeigt werden konnte. Es gab schon fruh Hinweise auf diese Art von Molekulen, beginnend mit den Arbeiten der Italiener Edoardo Amaldi, Emilio Segre und Enrico Fermi im Jahr 1934, bis hin zu neueren Daten von Niemax und Kollegen von 2006. Aber bisher fehlte der "Beweis" ihrer Existenz in der Form, das sie gezielt erzeugt und isoliert studiert werden konnten. 
In dieser Arbeit werden die langreichweitigen Rydberg-Molekule fur Rb(ns) Zustande durch Photoassoziation erzeugt und ihre grundlegenden Eigenschaften untersucht. Die Messungen umfassen Photoassoziationsspektren fur Hauptquantenzahlen n zwischen 34 und 40, in denen die Vibrationsstruktur der Rydberg-Molekule erstmals aufgelost werden kann. Die Abhangigkeit der Bindungsenergie des Schwingungsgrundzustands v=0 von der Hauptquantenzahl wird systematisch studiert und mit theoretischen Vorhersagen verglichen. Dabei zeigt sich, das das theoretische Modell fur eine Streulange von A=-18.0 Bohr-Radien fur die Elektron-Atom-Streuung in Rubidium hervorragend mit den experimentellen Ergebnissen ubereinstimmt. Neben dem Vibrationsspektrum werden der Stark-Effekt und die Lebensdauern der langreichweitigen Rydberg-Molekule untersucht. Wahrend die Polarisierbarkeiten der Rydberg-Molekule gut mit denen der Rydberg-Atome ubereinstimmen und damit das theoretische Modell bestatigen, zeigen die Rydberg-Molekule deutlich verkurzte Lebensdauern. Diese Beobachtung steht im Gegensatz zu der Annahme des Modells, das die Wellenfunktion des Rydberg-Atoms durch die Prasenz des Grundzustandsatoms nicht beeinflust wird. Ferner werden molekulare Ionen im Experiment identifiziert, die eindeutig auf einen Zerfallskanal unter Mitwirkung des Grundzustandsatoms hinweisen. 
Neben den Untersuchungen von zweiatomigen Rydberg-Molekulen wird in dieser Arbeit die Photoassoziation von dreiatomigen Molekulen demonstriert, sogenannter Trimere, bei denen ein Rydberg-Atom zwei Grundzustandsatome an sich bindet. Dabei kann die Wechselwirkung zwischen den Grundzustandsatomen vernachlassigt werden, wie hier anhand der Bindungsenergien der Trimere fur Hauptquantenzahlen von n=35-37 gezeigt werden kann. 
Damit weisen diese Messungen den Weg zu komplexeren Systemen, denn die bindende Kraft des Rydberg-Elektrons ist weder auf ein einzelnes Grundzustandsatom beschrankt, noch mussen Rydberg- und Grundzustandsatom grundsatzlich von derselben Spezies sein. Grosere polyatomare Molekule oder sogar heteronukleare Molekule konnten zukunftig realisiert werden. Einzige Anforderung an die beteiligten chemischen Elemente ist eine negative Streulange der Grundzustandsatome, damit ein attraktives Molekulpotential entsteht. Das heist das chemische Element des Rydberg-Atoms spielt keine Rolle: Sogar gebundene Zustande zwischen einem chemisch gebundenen Molekul im Rydberg-Zustand und einem oder mehreren Grundzustandsatomen sind denkbar. Die Realisierung solch exotischer Molekule liegt aber wohl noch in weiter Ferne, denn die Kuhlung von Molekulen in den Bereich von wenigen Mikrokelvin ist heute noch auf wenige chemische Verbindungen beschrankt und erfordert sehr grosen experimentellen Aufwand.
On the accuracy of machine programs for calculating oscillator strengths by Coulomb approximation
Abstract A comparison of the results of three different programs for the calculation of Coulomb approximation oscillator strengths is given.