Optical control of ground-state atomic orbital alignment: Cl(P-2(3/2)) atoms from HCl(v=2,J=1) photodissociation

H(35)Cl(v=0,J=0) molecules in a supersonic expansion were excited to the H(35)Cl(v=2,J=1,M=0) state with linearly polarized laser pulses at about 1.7 microm. These rotationally aligned J=1 molecules were then selectively photodissociated with a linearly polarized laser pulse at 220 nm after a time delay, and the velocity-dependent alignment of the (35)Cl((2)P(32)) photofragments was measured using 2+1 REMPI and time-of-flight mass spectrometry. The (35)Cl((2)P(32)) atoms are aligned by two mechanisms: (1) the time-dependent transfer of rotational polarization of the H(35)Cl(v=2,J=1,M=0) molecule to the (35)Cl((2)P(32)) nuclear spin which is conserved during the photodissociation and thus contributes to the total (35)Cl((2)P(32)) photofragment atomic polarization] and (2) the alignment of the (35)Cl((2)P(32)) electronic polarization resulting from the photoexcitation and dissociation process. The total alignment of the (35)Cl((2)P(32)) photofragments from these two mechanisms was found to vary as a function of time delay between the excitation and the photolysis laser pulses, in agreement with theoretical predictions. We show that the alignment of the ground-state (35)Cl((2)P(32)) atoms, with respect to the photodissociation recoil direction, can be controlled optically. Potential applications include the study of alignment-dependent collision effects

PREPARATION ET MANIPULATION DES ATOMES ET DES MOLECULES DE CESIUM

At the time of my arrival in Laboratoire Aimé Cotton (LAC) in November 2006, the activity of the experimental group of cold molecules in LAC was, on one hand on the formation and manipulation of cold molecules in a Magneto-Optical Trap (MOT), and on the other, in the realization of a dipole trap for cesium atoms. The preparation of a dipole trap of Cs atoms aimed both in the preparation of Bose-Einstein Condensation (BEC) of atomic Cs, and in the study of preparation and manipulation of Cs dimers in ultra-low temperatures. My enrollment in the activity of the experimental group of cold molecules in LAC was both in the preparation of the dipole trap, and in the study of cold molecule's creation and manipulation. In the first part of my thesis, I describe the studies conducted in the period November 2006 to October 2008. In this period, I focused my efforts on the study of different techniques for the loading of a dipole trap with Cs atoms, using a pre-existing set-up. The target was the creation of a cold and dense trapped atomic sample, in which the evaporation technique could be applied, to further cool the sample down to the critical temperature for the creation of a Cs BEC. At the beginning of the experiment, a Cs BEC had been reported only once [Web03], after years of unsuccessful efforts by many groups [Sod98, Boir98, Thom04]. Despite the difficulty of the subject, the interest in preparing samples of ultra-cold Cs atoms remained high, especially due to experiments related to ultra-cold molecules, as the creation of a molecular BEC [Herb03], or the formation of Cs trimers and the observation of Efimov states [Lee07, Knoo08]. The strategy upon which the dipole trap experiment was based, is theoretically studied in a previous publication of the group [Comp06], which considers rapid evaporation of a dense atomic sample in a crossed, deep dipole trap. This dense dipole trap, is provided by superimposing the dipole laser to a much larger trapped atomic sample, the so-called atomic reservoir. Collisions in this atomic reservoir can thermalize the sample and lead to the transfer of atoms in the dipole trap. Since this process can last for relatively large time intervals, it can result to higher loading efficiencies with comparison to alternative, instantaneous loading methods. This approach is very different to the one used in the only successful BEC experiment at that time [Web03], in which a shallow, very cold, but not so dense dipole trap, is prepared with the use of Raman Sideband Cooling. In the theoretical proposal reported in [Comp06], a magnetic trap was considered for the realization of the atomic reservoir, while the initial experimental study of this approach is the subject of a previous thesis in our group [Stern08]. However, the general ideas considered in this approach, allowed for the substitution of this magnetic reservoir, by several atomic traps. Thus, the experimental studies discussed in the first part of my thesis, are the continuation of the work made during the thesis of G. Stern [Stern08], with whom I collaborated in the beginning of my thesis. In particular, I studied the loading a dipole trap from a magnetic trap reservoir, and compared it to the loading obtained when the magnetic reservoir is replaced by a Dark-SPOT and a Compressed MOT (C-MOT). Furthermore, all these reservoir-loading methods are compared to a simpler, instantaneous-loading method which involves optical molasses. By June 2008, it was made clear that our experimental approach to the dipole trap loading, could not lead to the preparation of a sufficiently cold and dense atomic sample, in which evaporating cooling could be applied for the preparation of cesium atoms in ultra-low temperatures. In the same time, the approach considered in the first successful realization of a Cs BEC [Web03] gained ground, since the experiments reported in [Hung08] showed that it could provide with a Cs BEC with a fast and relatively simple experimental sequence. Thus, we also attempted the preparation of an ultra-cold Cs sample with the use of a shallow, not very dense, but very cold dipole trap provided by Raman Sideband Cooling. Unfortunately, a series of experimental problems related to the old vacuum system used in our experiment, prevented us from creating an ultra-cold atomic cesium sample with such an approach, despite the encouraging preliminary results. On the same period, the studies of the manipulation of cold molecules created in a MOT, conducted by members of the experimental group of cold molecules in LAC, advanced considerably, leading to the demonstration of the vibrational cooling technique reported in [Vit08]. The operating principle of the vibrational cooling technique is similar to the one of optical pumping in atoms [Kast66]. In this process, molecules that initially lie in different vibrational levels, are simultaneously excited by shaped broadband light and are accumulated to a single vibrational level via spontaneous emission. The accumulation to a single vibrational level, is accomplished by choosing to remove from the shaped pulse, all frequencies resonant to transitions from this level and thus turn it to a dark state. The technique enjoys simplicity and generality, and its demonstration opened the way for many interesting extensions, some of which are the subject of the second part of my thesis. More particularly, my activity in the cold molecule experiment which is discussed in the second part of my thesis, considered several extensions and generalizations of the vibrational cooling technique. The first extension to be considered, was the transfer of the molecular population to any pre-selected vibrational level, via optical pumping induced by more sophisticated, shaped femptosecond pulses, and is also discussed in [Sof09]. Another extension, considered the realization of vibrational cooling and molecular population transfer with the use of a broadband, non-coherent, diode light source, instead of a femptosecond laser and is reported in [Sof09b]. Another extension was considered to be the vibrational cooling of Cs molecules in their ground triplet electronic state, in addition to the ground singlet state, that was so far manipulated. Despite the optimistic initial predictions, the experimental study did not led to considerable results. However, this 'failed' experimental study, provides with an opportunity to revisit the various key elements of the vibrational cooling technique, and to consider the possible reasons that can lead to its failure. Such a discussion is particularly useful for the following study of the extension of the vibrational cooling technique to heteronuclear molecules through the example of NaCs. Finally, the generalization of the vibrational cooling technique to include rotation, which is theoretically considered in various publications of the group in which I participated [Vit09, Sof09, Sof09c], is discussed. In addition to these theoretical considerations, I discuss the preliminary experiments considered for rotational cooling, which involve the preparation of rotationally resolved depletion spectroscopy, and which are also discussed in [Fio09].Au début de mon travail de thèse dans l'équipe Atomes et Molécules Froides du Laboratoire Aimé Cotton, l'intérêt du groupe était, d'un côté, la préparation et la manipulation des molécules de Césium créées via photoassociation, et de l'autre, à la préparation d'un échantillon de Césium à basse température et de grande densité dans un piége dipolaire. Initialement, j'ai participé aux études de realization du piége dipolaire atomique, et ensuite aux études de préparation et de manipulation des molécules. Dans la période entre Novembre 2006 jusqu'a Octobre 2008 j'ai développé une série de techniques différentes pour le chargement d'un piége dipolaire à partir d'un réservoir atomique, réalisé soit par un piége magnétique, soit par un piége du type 'Dark-SPOT', soit par C-MOT (piége magnéto-optique comprimé) et un mêlasse optique. Au commencement du mon travail, un BEC de Césium a été rapporté un seule fois [Web03], après des années des efforts de plusieurs équipes [Sod98, Boir98, Thom04]. De plus, l'intérêt sur l'atome de Césium a augmenté à cause des expériences liées à la formation de trimères de Césium et des résonances du type 'Efimof' [Lee07, Knoo08]. La stratégie sur laquelle notre approche pour le chargement du piége dipolaire était basée, est discutée dans une publication rédigée avant mon intégration au sein d'équipe Atomes et Molécules Froides du Laboratoire Aimé Cotton [Comp06]. Il s'agit d'un chargement à partir d'un réservoir obtenu par un piége magnétique. L'objectif de cette proposition était la préparation d'un échantillon ultra froid avec un dispositif expérimental beaucoup plus simple que celui de la référence [Web03]. De plus, la proposition [Comp06] prédit la préparation d'un condensat de Césium en un temps beaucoup plus court que les temps de préparation rapportée dans la référence [Web03]. L'étude de la réalisation expérimentale de cette proposition théorique a déjà commencé, dans le cadre de la thèse de G.Stern [Stern08]. J'ai continué cette étude et j'ai réalisé plusieurs études avec différents types de réservoir (Dark-SPOT, C-MOT) et aussi de un type de chargement différent qui est basé sur le refroidissement par bandes latérales (Raman-Sideband Cooling). Toutes les piéges préparés par ces méthodes, avait une densité inférieure a celle nécessaire pour la réalisation d'un processus de refroidissement évaporatif, qui est nécessaire pour la réalisation d'un condensat. Les problèmes techniques que nous avons rencontrés vers Mai 2008 (destruction de vide), nous ont conduit d'arrêter les études de refroidissement des atomes, et de s'orienter vers les études de manipulation des molécules de Césium. Pendant cette période, l'équipe avait beaucoup progressé dans manipulation des molécules de Césium, et plus spécifiquement sur la réalisation du refroidissement du degré de liberté qui correspond à la vibration des molécules. La nouvelle technique que l'équipe avait introduit et qui est rapporté à la référence [Vit08], permet le refroidissement de la vibration des molécules de Césium par un laser femtoseconde faconné, avec un processus de pompage optique [Kast66]. Mon objectif dans la thématique de la manipulation des molécules froides, était d'étudier la généralisation de cette technique. Par example le transfert de la population moléculaire dans un seul état vibrationnel pré – sélectionné a été observé. Ce résultat est discuté dans la référence [Sof09]. Une autre généralisation est la réalisation du refroidissement vibrationnel et du transfert de la population moléculaire avec une source de lumière non cohérente, résultat qui est discuté dans la référence [Sof09b]. Une autre généralisation importante est le refroidissement de la rotation moléculaire. Cette étude théorique est discutée dans les articles de l'équipe auxquels j'ai participé [Vit09, Sof09, Sof09c], et les études expérimentales préliminaires dans la référence [Fio09]. Finalement, la généralisation de la technique pour le refroidissement vibrationnel des molécules heteronucleaires est discuté dans le dernière chapitre de ma thèse. Le document de ma thèse est donc divisé en deux parties, dans chacune d'elles je discute en détails ma contribution scientifique dans l'équipe Atomes et Molécules Froides du Laboratoire Aimé Cotton

Kinetic energy distribution of the rescattering electrons from asymmetric

When high intensity pulses are used to ionize an atom or molecule, the electrons produced can be driven back to the ionic core by the laser’s electric field, where they can collide with the ion, resulting to a plethora of phenomena such as high harmonic generation, non-sequential double ionization and more. Here, we consider ionization using an asymmetric $$\omega$$/2$$\omega$$ pulse, and we study the dependence of the kinetic energy distribution of the returning electrons on the relative phase $$\phi$$ and electric field amplitude ratio $$\gamma$$ between the two components of the asymmetric pulse. We find that for a specific combination of $$\gamma$$ and $$\phi$$, the kinetic energy of the vast majority of the returning electrons which return to the ion, follows a sharp, nearly monochromatic distribution. We examine the effect of small variations of the asymmetric pulse parameters $$\gamma$$ and $$\phi$$, as well as the effect of pulse duration and multiple returns of the electron to the ionic core. We find that the kinetic energy distribution remains narrow for a variety of such conditions, demonstrating the experimental feasibility of the process. This way, $$\omega$$/2$$\omega$$ asymmetric pulses can offer control over a variety of rescattering-related processes, such as high-harmonic generation, for which we give an example

Broadband lasers to detect and cool the vibration of cold molecules

By using broadband lasers, we demonstrate the possibilities to control the cold molecules formed via photoassociation. We present first a detection REMPI scheme [1] to systematically investigate the mechanisms of formation of ultracold Cs2 molecules in deeply bound levels of their electronic ground state X1+ g . This broadband detection scheme could be generalized to other molecular species. Then we report a vibrational cooling technique trough optical pumping obtained by using a shaped mode locked femtosecond laser [2]. The broadband femtosecond laser excites the molecules electronically, leading to a redistribution of the vibrational population in the ground state via a few absorption - spontaneous emission cycles. By removing the laser frequencies corresponding to the excitation of the v = 0 level, we realize a dark state for the so-shaped femtosecond laser, yielding with the successive laser pulses to an accumulation of the molecules in the v = 0 level, i.e. a laser cooling of the vibration. The simulation of the vibrational laser cooling allows us to characterize the criteria to extend the mechanism to other molecular species. We finally discuss the generalization of the technique to the laser cooling for the rotation of the molecule