Whereas atom-molecule collisions have been studied with complete quantum-state resolution, interactions between two state-selected molecules have proven much harder to probe. Here, we report the measurement of state-resolved inelastic scattering cross sections for collisions between two open-shell molecules that are both prepared in a single quantum state. Stark-decelerated hydroxyl (OH) radicals were scattered with hexapole-focused nitric oxide (NO) radicals in a crossed-beam configuration. Rotationally and spin-orbit inelastic scattering cross sections were measured on an absolute scale for collision energies between 70 and 300 cm(-1). These cross sections show fair agreement with quantum coupled-channels calculations using a set of coupled model potential energy surfaces based on ab initio calculations for the long-range nonadiabatic interactions and a simplistic short-range interaction. This comparison reveals the crucial role of electrostatic forces in complex molecular collision processes.

We present an experimental study on the rotational inelastic scattering of OH (X 2 3/2 ,J = 3/2,f ) radicals with He and D 2 at collision energies between 100 and 500 cm −1 in a crossed beam experiment. The OH radicals are state selected and velocity tuned using a Stark decelerator. Relative parity-resolved state-to-state inelastic scattering cross sections are accurately determined. These experiments complement recent low-energy collision studies between trapped OH radicals and beams of He and D 2 that are sensitive to the total (elastic and inelastic) cross sections [Sawyer et al., Phys. Rev. Lett. 101, 203203 (2008)], but for which the measured cross sections could not be reproduced by theoretical calculations [Pavlovic et al., J. Phys. Chem. A 113, 14670 (2009)]. For the OH-He system, our experiments validate the inelastic cross sections determined from rigorous quantum calculations.

We present a combined experimental and theoretical study on the rotationally inelastic scattering of OH (X 2 Π 3/2 , J = 3/2, f ) radicals with the collision partners He, Ne, Ar, Kr, Xe, and D2 as a function of the collision energy between ∼ 70 cm −1 and 400 cm −1 . The OH radicals are state selected and velocity tuned prior to the collision using a Stark decelerator, and field-free parity-resolved state-tostate inelastic relative scattering cross sections are measured in a crossed molecular beam configuration. For all OH-rare gas atom systems excellent agreement is obtained with the cross sections predicted by close-coupling scattering calculations based on accurate ab initio potential energy surfaces. This series of experiments complements recent studies on the scattering of OH radicals with Xe [Gilijamse et al., Science 313, 1617 (2006)], Ar [Scharfenberg et al., Phys. Chem. Chem. Phys. 12, 10660 (2010)], He, and D2 [Kirste et al., Phys. Rev. A 82, 042717 (2010)]. A comparison of the relative scattering cross sections for this set of collision partners reveals interesting trends in the scattering behavior.PACS. PACS-key discribing text of that key -PACS-key discribing text of that key

Nonadiabatic transitions are known to be major loss channels for atoms in magnetic traps but have thus far not been experimentally reported upon for trapped molecules. We have observed and quantified losses due to nonadiabatic transitions for three isotopologues of ammonia in electrostatic traps by comparing the trapping times in traps with a zero and a nonzero electric field at the center. Nonadiabatic transitions are seen to dominate the overall loss rate even for the present samples that are at relatively high temperatures of 30 mK. It is anticipated that losses due to nonadiabatic transitions in electric fields are omnipresent in ongoing experiments on cold molecules. The recent development of a large variety of methods and devices for the manipulation and trapping of neutral polar molecules offers new opportunities for molecular physics experiments ͓1͔. Decelerated beams and trapped samples of polar molecules can be used to study intrinsic molecular properties, such as energy level splittings ͓2,3͔ and lifetimes of metastable states ͓4,5͔, with unprecedented precision. Once the densities of the trapped molecules become high enough and their temperatures become low enough, the intermolecular interactions are anticipated to enable interesting new studies and applications ͓6,7͔. For all these studies it is not only of importance to increase the phase-space density of the trapped molecules but also to increase the time during which the molecules can stay confined in the trap, i.e., to reduce the trap loss processes. Neutral polar molecules in low-field seeking states are routinely trapped in magnetostatic or electrostatic traps by exploiting the Zeeman or Stark effect, respectively ͓8,9͔. These traps typically exhibit zero field at the trap center. Within a certain area around the trap center, the trapped molecules can undergo nonadiabatic transitions, widely also referred to as spin flip or Majorana transitions, from a trapped state into a state in which the molecules are no longer trapped. In atomic physics, nonadiabatic transitions in quadrupole magnetic traps seriously hindered the generation of the first Bose-Einstein condensates, as these spin flips made it impossible to reach the required ultracold regime ͓10,11͔. The trap losses associated with the presence of the zero field at the trap center were eliminated by implementing the time-averaged orbiting potential ͑TOP͒ trap on the one hand ͓12͔ and by keeping the atoms away from the trap center with an optical plug on the other hand ͓13͔. Generally, trap loss due to nonadiabatic transitions can be completely suppressed by creating a nonzero field minimum in the trap center. Already in 1962, Ioffe introduced a special variant of a magnetostatic trap with a field offset in the center for nuclear physics experiments ͓14͔. Pritchard ͓15͔ suggested in 1983 to use such a trap for the confinement of neutral atoms. This type of magnetostatic trap is now widely known as the Ioffe-Pritchard ͑IP͒ trap.For trapped polar molecules, losses due to nonadiabatic transitio...

We report on the observation of magnetic dipole allowed transitions in the well-characterized A 2 + − X 2 band system of the OH radical. A Stark decelerator in combination with microwave Rabi spectroscopy is used to control the populations in selected hyperfine levels of both -doublet components of the X 2 3/2 , v = 0, J = 3/2 ground state. Theoretical calculations presented in this Communication predict that the magnetic dipole transitions in the A 2 + , v = 1 ← X 2 , v = 0 band are weaker than the electric dipole transitions by a factor of 2.58 × 10 3 only, i.e., much less than commonly believed. Our experimental data confirm this prediction.

We report on the Stark deceleration of a pulsed molecular beam of NO radicals. Stark deceleration of this chemically important species has long been considered unfeasible due to its small electric dipole moment of 0.16 D. We prepared the NO radicals in the

Focusing of molecular beams using an electrostatic hexapole is a mature technique to produce samples of state-selected molecules. The ability to efficiently focus molecules depends on the properties of the molecular species of interest, the length of the hexapole state selector, as well as on the maximum electric field strength that can be achieved in these devices. In particular for species with a small effective dipole moment such as nitric oxide (NO), hexapole state selectors of several meters in length are required to focus the beam. We report on a novel design for an electrostatic hexapole state-selector that allows for a maximum electric field strength of 260 kV/cm, reducing significantly the length of the hexapole that is required to focus the beam. We demonstrate the focusing of a molecular beam of NO radicals (X (2)Π1∕2, v = 0, J = 1∕2, f) using a hexapole of only 30 cm length. A beamstop is integrated inside the hexapole at the geometric center of the device where the molecular trajectories have the largest deviation from the beam axis, effectively blocking the carrier gas of the molecular beam at minimum loss of NO density. The performance of the hexapole state-selector is investigated by state-selective laser induced fluorescence detection, as well as by two-dimensional imaging of the focused packet of NO radicals. The resulting packet of NO radicals has a density of 9 ± 3 × 10(10) cm(-3) and a state purity of 99%.

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