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Molecular Physics

Ro-vibrational cooling of diatomic molecules Cd2 and Yb2: rotational energy structure included

Tomasz Urbańczyk, J. Koperski

Published Dec 1, 2019• 1 citation
Publisher

Abstract

Method of ro-vibrational cooling of Cd2 and Yb2 molecules occupying initially their excited ro-vibrational levels in the ground electronic state is proposed. The method employs the c31u(53P2)↔X10g+(51S0) and 30u+(63P1)↔X10g+(61S0) electronic transitions in Cd2 and Yb2, respectively. The cooling relies on a successive decrease of population of the ground-state vibrational υ″ and rotational J″ levels using a series of laser-induced excitations from a selected ground-state (υ″, J″) to the excited-state (υ′=υ″−1, J′=J″−1) ro-vibrational levels, followed by spontaneous de-excitations to the ground state. The proposal is an extension of the vibrational cooling method recently proposed by Urbańczyk et al. [Mol. Phys. 116, 3475–3486 (2018)].

DOI
10.1080/00268976.2019.1694712

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Method of ro-vibrational cooling of Cd2 and Yb2 molecules occupying initially their excited ro-vibrational levels in the ground electronic state is proposed. The method employs the c31u(53P2)↔X10g+(51S0) and 30u+(63P1)↔X10g+(61S0) electronic transitions in Cd2 and Yb2, respectively. The cooling relies on a successive decrease of population of the ground-state vibrational υ″ and rotational J″ levels using a series of laser-induced excitations from a selected ground-state (υ″, J″) to the excited-state (υ′=υ″−1, J′=J″−1) ro-vibrational levels, followed by spontaneous de-excitations to the ground state. The proposal is an extension of the vibrational cooling method recently proposed by Urbańczyk et al. [Mol. Phys. 116, 3475–3486 (2018)].
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.
The use of a broadband, frequency shaped femtosecond laser on translationally cold cesium molecules has recently demonstrated to be a very efficient method of cooling also the vibrational degree of freedom. A sample of cold molecules, initially distributed over several vibrational levels, has thus been transfered into a single selected vibrational level of the singlet X1g ground electronic state. Our method is based on repeated optical pumping by laser light with a spectrum broad enough to excite all populated vibrational levels but limited in its frequency bandwidth with a spatial light modulator. In such a way we are able to eliminate transitions from the selected level, in which molecules accumulate. In this paper we briefly report the main experimental results and then address, in a detailed way by computer simulations, the perspectives for a complete" cooling of the molecules, including also the rotational degree of freedom. Since the pumping process strongly depends on the relative shape of the ground and excited potential curves, ro-vibrational cooling through different excited states is theoretically compared.
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.