J. Chem. Sci. Vol. 124, No. 1, January 2012, pp. 93–97. c  Indian Academy of Sciences. Quenching vibrations by collisions in cold traps: A quantum study for MgH + (X 1  + ) with 4 He( 1 S) # DOMENICO CARUSO a , MARIO TACCONI a , FRANCO A GIANTURCO a, ∗ and ERSIN YURTSEVER b a Department of Chemistry, The University of Rome ‘Sapienza’ and P.le AldoMoro 5, 00185 Rome, Italy b Department of Chemistry, Koc University, Rumelifeneriyalu and Sariyer, 34450 Istanbul, Turkey e-mail: fa.gianturco@caspur.it Abstract. Quantum dynamics of superelastic collisions involving vibrational levels of MgH + (X 1  + ) ions in cold traps, interacting with 4 He( 1 S) as a buffer gas at relative temperatures down to millikelvins, is discussed using an ab initio computed potential energy surface. The relative efficiency of collisional cooling with respect to collisional quenching of the internal vibrations is examined from the results of the relative sizes of the relevant cross sections in relation to predicting actual behaviour in cold traps. The present study indicates the feasibility of cooling vibrationally ‘hot’, trapped ions with the buffer gas. Keywords. Scattering theory; ion-molecule collisions vibrational quenching. 1. Introduction The ability to trap and cool dilute atomic gases has certainly revolutionized physics over the last twenty years. Hence, considerable attention has now turned to study and create trapped cold and ultracold molecular gases. 1–3 Although molecules are more complex than atoms and thus more difficult to cool below mK tempe- ratures, they offer the possibility of investigating new effects associated with their specific properties like their internal state populations, their ability to orient themselves in traps using electromagnetic fields and their ability to undergo reactions, the latter being now studied in dilute gases and at temperatures below 1 K, thus holding promises to access chemical process domi- nated by quantum effects. Dissipative cooling scheme needs to be applied after the molecules are slowed and trapped, 4,5 to produce molecular species down to 1 mK, without significant loss of the relevant molecules, because of the existence of the competition between translational energy and the internal energy of each molecule. An important number of such cooling schemes have become available for cooling molecules: they include stochastic, cavity, evaporative and sympathetic cool- ing. 6 In the case of sympathetic cooling, the molecular # Dedicated to Prof. N Sathyamurthy on his 60th birthday ∗ For correspondence gas to be cooled is co-trapped within a colder gas at higher concentration and therefore heat flows from the hotter to the colder gas. This method has been experi- mentally demonstrated using collisions with cold helium to cool and trap molecules in the 100 mK range. 6 Of critical importance to successful sympa- thetic cooling is the dominance of elastic over inelastic processes across a broad range of collisional energies. 7 Inelastic losses that are due to state-changing colli- sions to untrappable states and reactions are particularly worrying since such processes lead to leakage from the trap and prevent thermalization. 8,9 In the case of ionic molecules, they are usually stored in multipolar traps 10 and the need to bring and main- tain them to a particular, trappable, quantum state is no longer a concern as long as the particles charge state is conserved. Nevertheless, multipolar traps can have somewhat strict requirements on the trajectories of the ionic particles within the trap. Only a limited portion of the phase-space accessible to the ions in the trap corres- ponds to stable trajectories, i.e., those that prevent the escaping of the ions from the trapping potential. Fur- thermore, the mixture of helium with MgH + is basi- cally a non-reactive system, so charge exchange pro- cesses and subsequent neutralization of MgH + is to be considered completely suppressed. What we should try to understand, therefore, is if the collisions between the ions and the buffer gas atoms could disturb the trajecto- ries of the trapped ions in such a way that they become unstable within the trap. 93