Calculations of static dipole polarizabilities of alkali dimers. Prospects for alignment of ultracold molecules

The rapid development of experimental techniques to produce ultracold alkali molecules opens the ways to manipulate them and to control their dynamics using external electric fields. A prerequisite quantity for such studies is the knowledge of their static dipole polarizabilities. In this paper, we computed the variations with internuclear distance and with vibrational index of the static dipole polarizability components of all homonuclear alkali dimers including Fr$_2$, and of all heteronuclear alkali dimers involving Li to Cs, in their electronic ground state and in their lowest triplet state. We use the same quantum chemistry approach than in our work on dipole moments (M. Aymar and O. Dulieu, J. Chem. Phys. 122, 204302 (2005)), based on pseudopotentials for atomic core representation, Gaussian basis sets, and effective potentials for core polarization. Polarizabilities are extracted from electronic energies using the finite-field method. For the heaviest species Rb$_2$, Cs$_2$ and Fr$_2$ and for all heteronuclear alkali dimers, such results are presented for the first time. The accuracy of our results on atomic and molecular static dipole polarizabilities is discussed by comparing our values with the few available experimental data and elaborate calculations. We found that for all alkali pairs, the parallel and perpendicular components of the ground state polarizabilities at the equilibrium distance $R_e$ scale as $(R_e)^3$, which can be related to a simple electrostatic model of an ellipsoidal charge distribution. Prospects for possible alignment and orientation effects with these molecules in forthcoming experiments are discussed.Comment: Accepted for publication in J Chem Phy

Triplet-singlet conversion in ultracold Cs2_2 and production of ground state molecules

We propose a process to convert ultracold metastable Cs$_2$ molecules in their lowest triplet state into (singlet) ground state molecules in their lowest vibrational levels. Molecules are first pumped into an excited triplet state, and the triplet-singlet conversion is facilitated by a two-step spontaneous decay through the coupled $A^{1}\Sigma_{u}^{+} \sim b ^{3}\Pi_{u}$ states. Using spectroscopic data and accurate quantum chemistry calculations for Cs$_2$ potential curves and transition dipole moments, we show that this process has a high rate and competes favorably with the single-photon decay back to the lowest triplet state. In addition, we demonstrate that this conversion process represents a loss channel for vibrational cooling of metastable triplet molecules, preventing an efficient optical pumping cycle down to low vibrational levels

Photoionization spectroscopy of excited states of cold cesium dimers

Photoionization spectroscopy of cold cesium dimers obtained by photoassociation of cold atoms in a magneto-optical trap is reported here. In particular, we report on the observation and on the spectroscopic analysis of all the excited states that have actually been used for efficient detection of cold molecules stabilized in the triplet a^3Sigma_u^+ ground state. They are: the (1)^3Sigma_g^+ state connected to the 6s+6p asymptote, the (2)^3Sigma_g^+ and (2)^3Pi_g states connected to the 6s+5d asymptote and finally the (3)^3Sigma_g^+ state connected to the 6s + 7s asymptote. The detection through these states spans a wide range of laser energies, from 8000 to 16500 cm-1, obtained with different laser dyes and techniques. Information on the initial distribution of cold molecules among the different vibrational levels of the a^3Sigma_u^+ ground state is also provided. This spectroscopic knowledge is important when conceiving schemes for quantum manipulation, population transfer and optical detection of cold cesium molecules.Comment: 24 pages, 11 figures. Note: tables are available separately. Accepted in Molecular Physic

Light-assisted ion-neutral reactive processes in the cold regime: radiative molecule formation vs. charge exchange

We present a combined experimental and theoretical study of cold reactive collisions between laser-cooled Ca+ ions and Rb atoms in an ion-atom hybrid trap. We observe rich chemical dynamics which are interpreted in terms of non-adiabatic and radiative charge exchange as well as radiative molecule formation using high-level electronic structure calculations. We study the role of light-assisted processes and show that the efficiency of the dominant chemical pathways is considerably enhanced in excited reaction channels. Our results illustrate the importance of radiative and non-radiative processes for the cold chemistry occurring in ion-atom hybrid traps.Comment: 5 pages, 4 figure

Etude théorique de processus cohérents dans les alcalinos-terreux

Cette thèse présente l'étude théorique de processus cohérents d'excitation et d'ionisation dans les alcalino-terreux. Les paramètres atomiques indispensables pour décrire la dynamique sont calculés en combinant la méthode de matrice R et la théorie du défaut quantique à plusieurs voies (MQDT). Nous analysons les résultats expérimentaux obtenus par l'équipe d'Elliott concernant le contrôle des taux de production de trois états ioniques du baryum obtenus par interférence entre deux chemins d'ionisation quasi-résonnants avec un état intermédiaire dans un processus à deux photons de couleurs différentes. Introduisant un hamiltonien effectif, nous montrons que la dynamique d'évolution suit l'état adiabatique formé par l'excitation cohérente des deux états intermédiaires. Nous utilisons un formalisme MQDT dépendant du temps, valable en champ faible, pour analyser l'expérience de Ramsey optique, réalisée par van Leeuween et al, portant sur le contrôle des énergies et des distributions angulaires de photoélectrons émis par un paquet d'onde autoionisant créé dans le calcium par excitation du sœur isolé d'un état de Rydberg. Le flux radial instantané d'électrons créé par une ou deux impulsions pouvant présenter une dérive de fréquence est aussi étudié. Nous présentons un formalisme nouveau adapté à l'étude de la dynamique de paquets d'onde autoionisants créés par des impulsions brèves et intenses excitant des résonances étroites. Les résonances sont introduites explicitement dans les équations d'évolution. Leur énergie complexe (position et largeur), leurs couplages avec les voies ouvertes induits par le potentiel atomique et ceux avec les états discrets dûs aux laser sont déduits des expressions MQDT par le calcul numérique des pôles et des résidus de la matrice de diffusion physique S et de l'opérateur déplacement lumineux. Le lien entre ces pôles et les états de Siegert permet d'associer une fonction d'onde aux résonances et donc de les identifier.Theoretical investigations on the dynamics of coherent excitation and ionization processes in alkaline-earth atoms are reported. Atomic parameters required to the dynamical treatment are calculated with the R-matrix method in combination with the multichannel quantum defect theory (MQDT). We investigate the photoionization yields into three continuum channels of barium observed by Elliott et al in the study of coherent control through two-photon two-color interfering paths resonantly enhanced by an intermediate state. The dynamics studied by introducing an effective hamiltonian is described as an adiabatic process governed by the coherent excitation of the intermediate states. By using a time-dependent MQDT formalism valid in the weak field limit, we analyze the experiment on coherent control of energy and angular distribution of autoionized wave packets performed in calcium by van Leeuwen et al using isolated core excitation from a Rydberg state and optical Ramsey-like pump-probe technique. The dynamics of autoionization processes is investigated by studying the time dependence of radial electron flux created by one or two short pulses with a possible chirp and observed at fixed macroscopic distances from the atom. We develop a new formalism suitable to investigate autoionizing wave packets of narrow resonances created by short and intenses pulses. Resonances are explicitly introduced in the evolution equations. Their energies (positions and widths) in the complex plane, their couplings with continuum channels due to atomic interactions and their couplings with low-lying discrete states induced by the laser field are deduced from the MQDT formulation and are numerically calculated from the poles and the residues of the physical scattering matrix S and of the light shift operator. The relation between these poles and the Siegert states is presented and identification of the resonances is deduced from the corresponding wave functions.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF