Sympathetic Cooling of Molecular Ions in Selected Rotational and Vibrational States Produced by Threshold Photoionization

Abstract

We present a new method for the generation of rotationally and vibrationally state-selected, translationally cold molecular ions in ion traps. Our technique is based on the state-selective threshold photoionization of neutral molecules followed by sympathetic cooling of the resulting ions with lasercooled calcium ions. Using N þ 2 ions as a test system, we achieve >90% selectivity in the preparation of the ground rovibrational level and state lifetimes on the order of 15 minutes limited by collisions with background-gas molecules. The technique can be employed to produce a wide range of apolar and polar molecular ions in the ground and excited rovibrational states. Our approach opens up new perspectives for cold quantum-controlled ion-molecule-collision studies, frequency-metrology experiments with stateselected molecular ions and molecular-ion qubits. [5], and novel schemes for quantum-information processing [6]. Such experiments not only require precise control over the translational motion of the molecules, but also over their internal, in particular, rotational-vibrational, quantum state. The preparation of fully quantum-state-selected ultracold molecules and molecular ions constituted one of the major challenges in the field over the past decade and has only very recently been achieved for neutral diatomics synthesized from ultracold atoms (see, e.g., Refs. [1,3,4] and references therein).Translationally cold molecular ions, on the other hand, are conventionally produced from ''hot'' samples by sympathetic cooling using the Coulomb interaction with lasercooled atomic ions [7]. Because low-energy collisions between ions are dominated by the Coulomb interaction which does not couple to the internal degrees of freedom, sympathetically cooled ions exhibit broad distributions of rotational-state populations [8,9]. In such translationally cold, but internally warm samples population can be accumulated in the rotational ground state using optical pumping schemes as demonstrated in two recent studies by Staanum et al. [10] and Schneider et al. [11]. In their experiments, continuous excitation of selected rovibrational transitions in combination with population redistribution aided by blackbody radiation (BBR) enabled them to increase the ground-state population to 37% in MgH þ [10] and 78% in HD þ [11]. Although these values are about an order of magnitude higher than the corresponding thermal populations at room temperature, the state preparation is not complete which reflects the challenges associated with optical pumping in systems with a large number of simultaneously populated levels. Moreover, because the schemes used thus far rely on population transfer via dipole-allowed transitions, they cannot be applied to fundamental apolar ions such as H þ 2 , N þ 2 , and O þ 2 . In the present Letter, we demonstrate a complementary method for the production of rovibrationally state-selected, translationally cold molecular ions which circumvents the problems associated with optical pumping applied to a broad distribu...

Forbidden Vibrational Transitions in Cold Molecular Ions: Experimental Observation and Potential Applications

“…For photoionization, a two-color [2+1'] resonance-enhanced multiphoton ionization (REMPI) scheme was employed. [12,13] By setting the energy of the third photon just above the lowest ionization threshold of N 2 , the N 2 + ions were only produced in the X 2 nuclear spin isomers. As seen in Fig.…”

A Dynamic Ion–Atom Hybrid Trap for High‐Resolution Cold‐Collision Studies

“…In combination with techniques for the state preparation of the molecular ions, see, e.g., Ref. [37], energy and state controlled cold-chemistry experiments are now within reach paving the way for the characterisation of cold ion-neutral processes at an unprecedented level of detail.…”

Observation of electric-dipole-forbidden infrared transitions in cold molecular ions

“…Our approach capitalizes on the very long interrogation times (up to several minutes) enabled by the sympathetic cooling and subsequent spatial localization of individual N 2 + ions in a Coulomb crystal 10 . The sensitivity of the measurement is enhanced by confining the population of the molecules to a single rotational quantum state and using a highly sensitive charge-transfer (CT) reaction scheme for the detection of absorption events 19,20 .…”

“…Our approach capitalizes on the very long interrogation times (up to several minutes) enabled by the sympathetic cooling and subsequent spatial localization of individual N 2 + ions in a Coulomb crystal 10 . The sensitivity of the measurement is enhanced by confining the population of the molecules to a single rotational quantum state and using a highly sensitive charge-transfer (CT) reaction scheme for the detection of absorption events 19,20 .In our experiments, we focused on the observation of specific hyperfine components of the S(0) line of the IR fundamental excitation of N 2 + -that is, the transition from the vibrational and rotational ground state (v + = 0, N + = 0) to the second rotationally excited level (N + = 2) of the first vibrationally excited state (v + = 1) within the electronic ground state. Here, v + and N + denote the vibrational and rotational quantum numbers of the ion, and and refer to the lower and upper level of the transition, respectively.…”

“…2b and Methods). By setting the energy of the third photon just above the lowest rovibrational ionization threshold of N 2 , it was ensured that the N 2 + ions were produced only in the X + 2 + g , v + = 0, N + = 0 rovibronic ground state 19 . Typically 20-25 state-selected N 2 + ions were generated every experimental cycle and sympathetically cooled by the interaction with simultaneously trapped laser-cooled Ca + ions.…”