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Journal of Physics B Atomic Molecular and Optical Physics

Ro-vibrational cooling of molecules and prospects

M. Hamamda, P. Pillet, Hans Lignier, D. Comparat

Published Aug 11, 2015• 26 citations
Publisher

Abstract

By using a simple spectral shaping technique, optical pumping with laser fields has allowed us to manipulate, on demand, the rotational and vibrational population of di-atomic molecules. We review this method developed first on cold Cs 2 then on ions and discuss its extension to other molecular species as well as its feasibility on a molecular beam. This paves the way to the production of brighter molecular beams and thus a new method of producing cold molecular cloud. We finally present some ideas based on Sisyphus cooling with expected results on neutral as well as ionic molecules.

DOI
10.1088/0953-4075/48/18/182001

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By using a simple spectral shaping technique, optical pumping with laser fields has allowed us to manipulate, on demand, the rotational and vibrational population of di-atomic molecules. We review this method developed first on cold Cs 2 then on ions and discuss its extension to other molecular species as well as its feasibility on a molecular beam. This paves the way to the production of brighter molecular beams and thus a new method of producing cold molecular cloud. We finally present some ideas based on Sisyphus cooling with expected results on neutral as well as ionic molecules.
Optical pumping techniques using laser fields combined with photo-association of ultracold atoms leads to control of the vibrational and/or rotational population of molecules. In this study, we review the basic concepts and main steps that should be followed, including the excitation schemes and detection techniques used to achieve ro-vibrational cooling of Cs 2 molecules. We also discuss the extension of this technique to other molecules. In addition, we present a theoretical model used to support the experiment. These simulations can be widely used for the preparation of various experiments because they allow the optimization of several important experimental parameters.
A review of our recent experiments on broadband vibrational cooling of cold cesium molecules and of the related theory is presented. Our method is based on repetitive optical pumping cycles driven by laser light which is broad enough to excite all populated vibrational levels. Originally, the accumulation of molecular population in a particular, pre-selected vibrational level was achieved by removing from the broadband light all frequencies that could excite that vibrational level and thus making it a ‘dark state’ of the system. Here, we focus onto an additional, more sophisticated shaping method, which consists of selecting only specific frequency components that excite molecules into vibrational levels that favourably decay into the pre-selected level. The population transfer to any desired state can thus be optimised, i.e. the total population transfer to the desired vibrational level is maximised while the number of absorption–emission cycles required for the vibrational cooling is minimised. Finally, we apply this optimised technique to some more complex and still experimentally open cases: the pumping into the ground state for the case of Cs2 homonuclear molecules, the rotational pumping into a pre-selected ro-vibrational level and the NaCs as an example for heteronuclear molecules.
Some of us have recently reported (Viteau et al 2008 Science 321 232–4) vibrational cooling of translationally cold Cs 2 molecules into the lowest vibrational level v =0 of the singlet X 1 Σ g ground electronic state. Starting from a sample of cold molecules produced in a collection of vibrational levels of the ground state, our method was based on repeated optical pumping by laser light with a spectrum broad enough to excite all populated vibrational levels but frequency-limited in such a way to eliminate transitions from v =0 level, in which molecules accumulate. In this paper, this method is generalized to accumulate molecules into an arbitrary selected 'target' vibrational level. It is implemented by using ultrashort pulse shaping techniques based on liquid crystal spatial light modulator. In particular, a large fraction of the initially present molecules is transferred into a selected vibrational level such as v =1, 2 and 7. Limitations of the method as well as the possible extension to rotational cooling are also discussed.
Abstract. Some of us have recently reported in Science [1] vibrational cooling of translationally cold Cs2 molecules into the lowest vibrational level v = 0 of the singlet X1Σg ground electronic state. Starting from a sample of cold molecules produced in a collection of vibrational levels of the ground state, our method was based on repeated optical pumping by laser light with a spectrum broad enough to excite all populated vibrational levels but frequency-limited in such a way to eliminate transitions from v = 0 level, in which molecules accumulate. In this paper this method is generalized to accumulate molecules into an arbitrary selected ”target ” vibrational level. It is implemented by using ultrashort pulse shaping techniques based on Liquid Crystal spatial light modulator. In particular a large fraction of the initially present molecule is transferred into a selected vibrational level such as v = 1, 2 and 7. Limitations of the method as well as the possible extension to rotational cooling are also discussed. PACS numbers: 06.60.Jn; 32.80.Qk; 33.20.Tp; 37.10.Mn; 42.65.Re; 42.79.Kr 1.
The possibility of controlling all the motion as well as the internal quantum state of a sample of molecules is a long term goal in the cold molecules field. Although many different techniques have been used to produce ultra-cold molecules, in this paper, we will concentrate on the optical pumping technique successfully used to achieve rotational and vibrational cooling of Cs2molecules. We will review the different photo-association schemes for molecule formation, the detection schemes through photoionization, the ro-vibrational cooling into a single level and finally the electronic conversion. In addition, we will present a theoretical model for both ro-vibrational cooling and electronic conversion that can be used for the preparation of different experiments.
The methods producing cold molecules from cold atoms tend to leave molecular ensembles with substantial residual internal energy. For instance, cesium molecules initially formed via photoassociation of cold cesium atoms are in several vibrational levels nu of the electronic ground state. We applied a broadband femtosecond laser that redistributes the vibrational population in the ground state via a few electronic excitation/spontaneous emission cycles. The laser pulses are shaped to remove the excitation frequency band of the nu = 0 level, preventing re-excitation from that state. We observed a fast and efficient accumulation ( approximately 70% of the initially detected molecules) in the lowest vibrational level, nu = 0, of the singlet electronic state. The validity of this incoherent depopulation pumping method is very general and opens exciting prospects for laser cooling and manipulation of molecules.