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arXiv:1005.3283 · Molecular Physics

Photoionization spectroscopy of excited states of cold caesium dimers

Nadia Bouloufa-Maafa, Elena Favilla, Matthieu Viteau, Amodsen Chotia, A. Fioretti, C. Gabbanini, M. Allegrini, M Aymar, D. Comparat, Olivier Dulieu, P. Pillet

Published Aug 4, 2010• 13 citations
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Abstract

Photoionization spectroscopy of cold caesium 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 ground state. They are: the state connected to the 6s + 6p asymptote, the and (2)3Π g states connected to the 6s + 5d asymptote and finally the state connected to the 6s + 7s asymptote. The detection through these states spans a wide range of laser energies, from 8000 to 16,500 cm−1, obtained with different laser dyes and techniques. Information on the initial distribution of cold molecules among the different vibrational levels of the ground state is also provided. This spectroscopic knowledge is important when conceiving schemes for quantum manipulation, population transfer and optical detection of cold caesium molecules.

DOI
10.1080/00268976.2010.495732

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Photoionization spectroscopy of cold caesium 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 ground state. They are: the state connected to the 6s + 6p asymptote, the and (2)3Π g states connected to the 6s + 5d asymptote and finally the state connected to the 6s + 7s asymptote. The detection through these states spans a wide range of laser energies, from 8000 to 16,500 cm−1, obtained with different laser dyes and techniques. Information on the initial distribution of cold molecules among the different vibrational levels of the ground state is also provided. This spectroscopic knowledge is important when conceiving schemes for quantum manipulation, population transfer and optical detection of cold caesium molecules.
In the molecular photoassociation process, two free cold atoms absorb one photon by a resonant way, to create an excited molecule in a well defined ro-vibrational state. The photoassociative spectroscopy of alkaline dimers yields accurate determinations for the long-range parts of molecular potential curves correlated to the first s + p asymptote. In contrast to atoms, laser cooling of molecules is very difficult because of the lack of a closed two-level scheme for recycling the population. The molecular photoassociation of cold atoms opens also a promising alternative for the formation of cold ground state molecules. Photoassociation of cold cesium atoms has permitted the first observation of translationally cold ground state molecules. The case of the cesium atom is very favorable because the 0 g - and 1 u purely long-range states below the dissociation limit 6s 1/2 +6p 3/2 present Condon points at intermediate distances. Such configurations offer rather efficient channels for the creation of singlet or triplet ground state Cs 2 molecules after spontaneous decay.
We have observed translationally cold electronic ground-state cesium dimers produced directly in a magneto-optical trap and also by photoassociation of cold atoms in this trap. These neutral molecules were detected using photoionization and time-of-flight spectroscopy. Initial experiments indicated a cold molecular formation rate of about $600{\mathrm{s}}^{\mathrm{\ensuremath{-}}1},$ with no photoassociation laser present. The measured translational temperature of the neutral cesium dimers was approximately 100 \ensuremath{\mu}K.
We have performed high precision photoassociation spectroscopy of ultracold cesium gas. Using trap-loss fluorescence detection and controlling the background cesium pressure we were able to photoassociate atoms into excited states of ultracold molecules with large detunings up to 56 cm(-1) below the Cs(6S(1/2)) + Cs(6P(1/2)) atomic asymptote. Vibrational progressions are assigned to 0(g)(-), 0(u)(+), and 1(g) long-range states. By fitting the spectral data to the LeRoy-Bernstein expression, the effective coefficients of the leading long-range interactions and the vibrational quantum number at dissociation are obtained. In addition we have observed spectral perturbations between states of the same symmetry belonging to different asymptotes (6P(1/2) and 6P(3/2)). The perturbations are manifested through irregular vibrational level spacings and are especially pronounced in the 0(u)(+) symmetry. Many observed rotational levels indicate d- and higher partial wave contributions to the photoassociation cross section in the presence of trapping laser light, while spectral regions with only weak features suggest nodes in the lower state wave functions corresponding to the two ground state atoms asymptote.
An ultrahigh resolution photoassociation spectrum of caesium atoms in a magneto-optical trap is presented. Hyperfine structure of the excited state molecule is obtained by using the lock-in method based on modulated cold atoms in this spectrum. Amplitude of resonant lines related to the rotational levels increases with photoassociation laser intensity, and saturation effect of photoassociation of cold atoms is observed in our experiment. The saturation intensity of photoassociation is deduced by fitting the experimental data to a saturation model based on scattering theory. Differences among saturation intensities of different rotational progressions in the ν = 55 vibrational state of the caesium molecular long-range 0 g − state have been found.
Translationally cold caesium molecules are created by photoassociation below the 6s + 6p(1/2) excited state and selectively detected by resonance enhanced two photon ionization (RE2PI). A series of excited vibrational levels belonging to the 0(-)(g) symmetry is identified. The regular progression of the vibrational spacings and of the rotational constants of the 0(-)(g) (6s + 6p(1/2)) levels is strongly altered in two energy domains. These deviations are interpreted in terms of resonant coupling with deeply bound energy levels of two upper 0(-)(g) states dissociating into the 6s + 6p(3/2) and 6s + 5d(3/2) asymptotes. A theoretical model is proposed to explain the coupling and a quantum defect analysis of the perturbed level position is performed. Moreover, the resonant coupling changes dramatically the spontaneous decay products of the photoexcited molecules, strongly enhancing the decay into deeply bound levels of the a(3)Σ(+)(u) triplet state and of the X(1)Σ(+)(g) ground state. These results may be relevant when conceiving population transferring schemes in cold molecule systems.
We have performed photoassociative spectroscopy of cold Cs atoms in a vapor-cell magneto-optical trap, within $10{\mathrm{cm}}^{\ensuremath{-}1}$ below the dissociation limit 6s2S1/2+6p2P3/26s^{2}S_{1/2}+6p^{2}P_{3/2}6s2S1/2​+6p2P3/2​. Our detection method is based on pulsed-laser photoionization of Cs2{\mathrm{Cs}}_{2}Cs2​ molecules, selectively detected through a time-of-flight mass spectrometer. Temporal and spatial analysis of Cs2+{\mathrm{Cs}}_{2}^{+}Cs2+​ ions show for the first time the formation of translationally cold Cs2{\mathrm{Cs}}_{2}Cs2​ triplet ground state molecules, at a temperature $T\ensuremath{\sim}300\ensuremath{\mu}\mathrm{K}$.