ABSTRACT
Seeking States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 527 14.3 The Zeeman, Rydberg, and Optical Decelerator . . . . . . . . . . . . . . . . . . . . . . . . . . . 528 14.4 Trapping Neutral Polar Molecules. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 530
14.4.1 DC Trapping of Molecules in Low-Field Seeking States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 530
14.4.2 Storage Ring and Molecular Synchrotron . . . . . . . . . . . . . . . . . . . . . . . . . 532 14.4.3 AC Trapping of Molecules in High-Field
Seeking States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533 14.4.4 Trap Lifetime Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537
14.5 Applications of Decelerated Beams and Trapped Molecules . . . . . . . . . . . . . 537 14.5.1 High-Resolution Spectroscopy and Metrology . . . . . . . . . . . . . . . . . . . . 538 14.5.2 Collision Studies at a Tunable Collision Energy . . . . . . . . . . . . . . . . . . 540 14.5.3 Direct Lifetime Measurements of Metastable States . . . . . . . . . . . . . . 542
14.6 Conclusions and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 547 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 548
The importance of the role that atomic and molecular beams have played in physics and chemistry cannot be overstated [1]. Nowadays, sophisticated laser-based techniques exist to sensitively and quantum-state-selectively detect atoms and molecules in a beam. In the early days, such detection techniques were lacking and the particles in a beamwere detected, for instance, by a “hot wire” (Langmuir-Taylor) detector, by electron-impact ionization, or by deposition and ex situ inspection of the deposit on a substrate placed at the end of a beam-machine [2]. In order to achieve quantum-state selectivity of the detection process, these detection techniques were combined with inhomogeneous magnetic and/or electric fields to characteristically alter the trajectories of the particles on their way to the detector. This was the approach pioneered by Otto Stern andWalther Gerlach in 1922 [3]. The key concept of their experiment, that is, the sorting of quantum states via space quantization, has been extensively used ever since. The experimental geometries were devised to create strong magnetic or electric field gradients on the beam axis to efficiently deflect particles. In 1939, Isidor Rabi introduced the molecular beam magnetic resonance method by using two magnets in succession to produce inhomogeneous magnetic fields of oppositely directed gradients. In Rabi’s setup, the deflection of particles caused by the first magnet was compensated for by the second magnet such that the particles were directed on a sigmoidal path to the detector.A transition to “other states of space quantization,” induced between the two magnetic sections, could be detected via the resulting reduction of the detector signal [4]. Later, both magnetic [5,6] and electric [7] field geometries were designed to focus particles in selected quantum states onto a detector. An electrostatic quadrupole focuser, that is, an arrangement of four cylindrical electrodes alternately energized by positive and negative voltages, was used to couple a beam of ammonia molecules into a microwave cavity. Such an electrostatic quadrupole lens focuses ammonia molecules that are in the upper level of the inversion doublet while simultaneously defocusing those that are in the lower level. The inverted population distribution of the ammoniamolecules in themicrowave cavity that was thus produced led to the invention of the maser by James Gordon, Herbert Zeiger, and Charles Townes in 1954-1955 [8,9]. Apart from making it possible to observe a spectacular amplification of the microwaves by stimulated emission, these focusing elements made it possible to record, with high resolution and good sensitivity, microwave spectra in a molecular beam. By using several multipole focusers in succession, interlaced with interaction regions with electromagnetic radiation, versatile setups to unravel the quantum structure of atoms andmolecules were developed. In scattering experiments, multipole focusers were exploited to study the steric effect, that is, how the orientation of an attacking molecule affects its reactivity [10]. Variants of the molecular beam resonance methods as well as scattering machines that employed state selectors were implemented in many laboratories, and have yielded a wealth of detailed information on stable molecules, radicals, and molecular complexes alike.
