Production of ultracold diatomic and triatomic molecular ions of spectroscopic and astrophysical interest

B Roth, P Blythe, H Daerr, L Patacchini, S Schiller
2006 Journal of Physics B: Atomic, Molecular and Optical Physics  
We have produced large samples of ultracold (<20 mK) ArH molecular ions, by sympathetic cooling and crystallization via laser-cooled Be + ions in a linear radio-frequency trap. As technique, we used chemical reactions with sympathetically cooled noble gas atomic ions or N + 2 and O + 2 molecular ions. These ultracold molecules are interesting targets for high-precision measurements in fundamental physics and may open new routes for the study of state-selective chemical reactions relevant to
more » ... ons relevant to interstellar chemistry. S1242 B Roth et al was demonstrated with an accuracy at the few per cent level, by measuring the Zeeman effect in low-lying rotational transitions of ArH + and ArD + , produced in a discharge source [4] . The use of ultracold ArH + ions may allow us to improve precision of such measurements. Precision measurements of one or several ro-vibrational transition frequencies over time could serve to test the constancy of the nuclear-to-electron mass ratio. A technique to perform highprecision spectroscopy on non-fluorescing ions has recently been demonstrated [5] . Another interesting perspective is to use certain ultracold heteronuclear diatomics, in 1,2,3 or 2 states which are among the most frequent electronic ground states in molecules, as model systems for the implementation of schemes for internal state manipulation [3, 2] . For this purpose, molecules with a relatively simple hyperfine structure of the ro-vibrational transitions are more favourable, in order to limit the number of laser sources required for internal cooling schemes. Being among the most abundant molecules in interstellar clouds, the chemistry of hydrogen molecular ions is relevant to astronomy. At present, the interstellar gas-phase chemistry of H + 3 and its deuterated isotopomers is not completely understood [6, 7] . Measurements of state-specific reactions of H + 3 via high-resolution infrared spectroscopy can provide valuable input for theories of ion-molecule gas-phase chemistry and precise calculations of molecular transition frequencies. Such measurements could so far only be performed on warm samples [8] . Ultracold ensembles of triatomic hydrogen molecular ions, possibly cooled to their ro-vibrational ground state using cryogenic techniques, could lead to improved studies. As an example, translationally ultracold molecular ions in lowlying ro-vibrational levels (populated at room temperature) could be excited to higher rovibrational levels (not populated at room temperature), using standard IR laser sources. Chemical reactions between state-prepared ultracold molecular ions and (state-prepared) ultracold neutral molecules, which are endothermic when the ions are in low-lying vibrational levels, but exothermic for the excited vibrational levels, could then be studied. A powerful method for cooling molecular ions to translational temperatures in the millikelvin regime is sympathetic cooling. The translational energy of molecular ions can be reduced by interaction with directly cooled (laser-cooled) atomic ions. Under strong cooling, i.e., when the translational temperature drops to about 50 mK, the ions undergo a phase transition to an ordered state, a Coulomb crystal, characterized by well-defined sites [9] [10] [11] . Typical interparticle distances are in the range of a few tens of micrometres. This method has been applied to an increasing number of molecules (and atoms), since it overcomes the lack of closed optical transitions required for direct cooling and is independent of electric or magnetic dipole moments or the internal level structure [12, 13] . Recently, we have reported the production of ultracold molecular hydrogen ions, e.g., H + 2 , H + 3 , and their deuterated isotopomers HD + , D 2 H + , H 2 D + and D + 2 , via sympathetic cooling using laser-cooled Be + ions in a linear rf trap [13] . The molecular ions were produced by in situ ionization of neutral molecular gases (H 2 , D 2 or HD) in the ion trap using low-energy electrons. Ion crystals of various sizes and ion species ratios were formed with the fraction of molecules exceeding 70%. Chemical reactions were involved in some of the molecule species production. Since the method used also leads to the formation of unwanted impurity ions purification was applied in order to remove those ions from the crystal [14, 13] . However, crystal purification usually also leads to partial loss of the molecular species of interest. In this work, we demonstrate a novel, more efficient production method for the molecular hydrogen ions H + 3 , D + 3 , D + 2 , and for D + . We use heavier atomic or molecular ions, e.g. Ar + , Ne + , Kr + , N + 2 or O + 2 , as reactants in order to produce ultracold and pure ensembles of H + 3 . The fraction of atomic to molecular ions can be controlled. The method was also applied to produce various ultracold deuterated hydrogen isotopomers, D + 3 , D + 2 and D + atomic ions. Furthermore, multi-step chemical reactions were used to produce and reliably detect
doi:10.1088/0953-4075/39/19/s30 fatcat:lnujrs6dnjhunpbmc3uj5oynqi