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Review of Scientific Instruments

Magnetic deflection analysis of supersonic metastable atom beams

C. Weiser, Peter E. Siska

Published Nov 1, 1987• 16 citations
Publisher

Abstract

Deflection of 3S1 and 3P2 metastable rare-gas atoms in an electron-impact-excited thermal supersonic expansion by an inhomogeneous magnetic field produces well-defined daughter beams for each M state. The deflection patterns are spatially resolved with a translatable detector and used to derive fine-structure populations in the 3P0 and 3P2 states, and to measure absolute efficiency of state selection by optical depletion. The fine-structure population ratio 3P2/3P0 is measured for electron excitation energies from 100–400 eV, showing a 20%–40% decline with increasing energy for all the rare gases. Radiative decay due to perturbation by the magnetic field appears to be negligible, but the M-state-resolved populations are unequal and asymmetric, perhaps due to Majorana transitions at the entrance to the field. The method may also be useful in polarized angular momentum studies of metastable atom–molecule collisions.

DOI
10.1063/1.1139474

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In this thesis, the study of cold gases of neon atoms in different metastable states is described. It contains measurements of the collisional parameters for both the 3s[3/2]2 and the 3s'[1/2]0 metastable state and the dependence of the inelastic loss on external fields. Furthermore, the investigation of frequency dependent laser-induced collisions, and the possibility to excite photoassociation resonances is presented. Based on previous measurements we have selected 22Ne for evaporative cooling. Although, we can experimentally achieve an increase in phase-space density with evaporative cooling, the relatively high inelastic collision parameters prevent the realization of a Bose-Einstein condensate in neon. For the measurements described here, neon atoms have been confined in a magneto-optical trap, in a magnetostatic trap, or in an optical dipole trap, respectively. By laser cooling inside the magnetic trap, atomic samples with more than 95 percent occupation of the magnetic substate m=+2 could be prepared. They have a typical temperature of 0.5 mK, central densities up to 1e11 cm^-3, and a central phase-space density of up to 2.2e-7. After loading the optical dipole trap from the magnetic trap, 2.5e6 atoms with typical temperatures of 0.1 mK, and central densities up to 5e10 cm^-3 were trapped. By evaporative cooling of the atoms in the magnetic trap we could increase the phase-space density by a factor of 200 to 5e-5. Also simulations of optimized evaporation for our experimental parameters show clearly that we are limited to a phase-space density on the order of 1e-5. From these simulations it became clear that a 5-fold increase in the good-to-bad ratio for evaporative cooling suffices to reach the quantum degenerate regime. Investigating the frequency dependence of laser-induced collisions did not reveal an experimental signature for the excitation of photoassociation resonances. For the 3D3 line a frequency dependence of laser enhanced Penning ionization was observed, which is interesting in itself. The absence of the collisional enhancement effect by laser light for the transition to the 3D2 line is intriguing and for an explanation calculations are required. Measurement of the two-body loss coefficient as function of the magnetic field showed a field dependence of the inelastic loss. These losses increase towards both small and large offset fields. In the magnetic trap, we are limited to offset fields < 50 G. In this range of fields, the two-body losses are too large to achieve a Bose-Einstein condensate of magnetically trapped metastable neon. The implementation of an optical dipole trap allowed us to trap the 3P0 metastable state. From the trap loss measurements we determined the two-body loss coefficient of the 3P0 metastable state for both bosonic isotopes 20Ne and 22Ne. For 20Ne we obtained s=6(+5,-4)e-10 cm^3/s and for 22Ne s=11(+7,-6)e-10 cm^3/s. These large two-body losses make it extremely unlikely to reach degeneracy with this metastable state. Nevertheless it is important that the 3P0 metastable state can be trapped to investigate other interesting physical effects. For example, it is essential to apply the STIRAP technique for trapped atoms and to realize the proposed coherent control of collisions. There is also a large interest in a precise determination of the lifetime of this metastable state, which is of importance for the verification of QED. We can also trap neon atoms in their energetically lowest magnetic substate 3P2(m=-2) with the perspective of reducing inelastic collisions in the energetically lowest state.

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•Jan 1, 2014•2 citations
Deflection of 3S1 and 3P2 metastable rare-gas atoms in an electron-impact-excited thermal supersonic expansion by an inhomogeneous magnetic field produces well-defined daughter beams for each M state. The deflection patterns are spatially resolved with a translatable detector and used to derive fine-structure populations in the 3P0 and 3P2 states, and to measure absolute efficiency of state selection by optical depletion. The fine-structure population ratio 3P2/3P0 is measured for electron excitation energies from 100–400 eV, showing a 20%–40% decline with increasing energy for all the rare gases. Radiative decay due to perturbation by the magnetic field appears to be negligible, but the M-state-resolved populations are unequal and asymmetric, perhaps due to Majorana transitions at the entrance to the field. The method may also be useful in polarized angular momentum studies of metastable atom–molecule collisions.
The optical pumping of atoms is a central process in many applications of atomic physics, such as laser cooling and the precise determination of physical constants. Popular methods for producing metastable helium atoms in supersonic expansions are discharge and electron-impact excitation. However, both approaches yield helium atoms in two metastable states, 21S0{2}^{1}{S}_{0}21S0​ and 23S1{2}^{3}{S}_{1}23S1​, which limits the applications of these highly energetic species. This study presents an experimental scheme in which the 21S0{2}^{1}{S}_{0}21S0​ state population is depleted via excitation to the 41P1{4}^{1}{P}_{1}41P1​ state, using diode-laser radiation. The results suggest nearly 100% depletion efficiency at all beam velocities studied.
Isotopes of barium are selectively excited to a metastable state using a single ring dye laser. The excited atoms are then deflected out of an atomic beam by an inhomogeneous magnetic field. The deflection of the beam is studied using a hot wire detector and a quadrupole mass spectrometer. Measurements are presented showing that nearly all atoms of a particle isotope can be excited from the 6s{sup 2} ground still to the 6s5d state and subsequently spatially separated. This method of isotope separation requires metastable states having lifetimes greater than about 100 {mu}sec. These states can be readily excited from the ground state in atoms such as the Group II elements.
Metastable argon atoms Ar*(3P2), produced by electron bombardment of a nozzle beam of ground state Ar atoms, are slowed down from their initial thermal velocity of 560 m/s down to a few tens of m/s, using a standard Zeeman slower. In this decelerator, a repulsive force is induced by a counter-propagating σ+- polarized laser beam, locked in frequency on the3P2-3D3closed transition (λ= 811.5 nm) and then detuned by 340 MHz. A special profile of longitudinal magnetic field is adjusted in order to maintain atoms in resonance with light all over the device, by compensating the variable Doppler shift by the convenient Zeeman shift. Low velocities, e.g. 55 m/s, are accessible but due to spontaneous emission randomly distributed recoil momentums enlarge both angular and velocity distributions of the beam. Nevertheless, by placing off axis collimating slits and grating, it is in principle possible to observe transmission and/or diffraction phenomena specific of low velocities.
In this thesis, the study of cold gases of neon atoms in different metastable states is described. It contains measurements of the collisional parameters for both the 3s[3/2]2 and the 3s'[1/2]0 metastable state and the dependence of the inelastic loss on external fields. Furthermore, the investigation of frequency dependent laser-induced collisions, and the possibility to excite photoassociation resonances is presented. Based on previous measurements we have selected 22Ne for evaporative cooling. Although, we can experimentally achieve an increase in phase-space density with evaporative cooling, the relatively high inelastic collision parameters prevent the realization of a Bose-Einstein condensate in neon. For the measurements described here, neon atoms have been confined in a magneto-optical trap, in a magnetostatic trap, or in an optical dipole trap, respectively. By laser cooling inside the magnetic trap, atomic samples with more than 95 percent occupation of the magnetic substate m=+2 could be prepared. They have a typical temperature of 0.5 mK, central densities up to 1e11 cm^-3, and a central phase-space density of up to 2.2e-7. After loading the optical dipole trap from the magnetic trap, 2.5e6 atoms with typical temperatures of 0.1 mK, and central densities up to 5e10 cm^-3 were trapped. By evaporative cooling of the atoms in the magnetic trap we could increase the phase-space density by a factor of 200 to 5e-5. Also simulations of optimized evaporation for our experimental parameters show clearly that we are limited to a phase-space density on the order of 1e-5. From these simulations it became clear that a 5-fold increase in the good-to-bad ratio for evaporative cooling suffices to reach the quantum degenerate regime. Investigating the frequency dependence of laser-induced collisions did not reveal an experimental signature for the excitation of photoassociation resonances. For the 3D3 line a frequency dependence of laser enhanced Penning ionization was observed, which is interesting in itself. The absence of the collisional enhancement effect by laser light for the transition to the 3D2 line is intriguing and for an explanation calculations are required. Measurement of the two-body loss coefficient as function of the magnetic field showed a field dependence of the inelastic loss. These losses increase towards both small and large offset fields. In the magnetic trap, we are limited to offset fields < 50 G. In this range of fields, the two-body losses are too large to achieve a Bose-Einstein condensate of magnetically trapped metastable neon. The implementation of an optical dipole trap allowed us to trap the 3P0 metastable state. From the trap loss measurements we determined the two-body loss coefficient of the 3P0 metastable state for both bosonic isotopes 20Ne and 22Ne. For 20Ne we obtained s=6(+5,-4)e-10 cm^3/s and for 22Ne s=11(+7,-6)e-10 cm^3/s. These large two-body losses make it extremely unlikely to reach degeneracy with this metastable state. Nevertheless it is important that the 3P0 metastable state can be trapped to investigate other interesting physical effects. For example, it is essential to apply the STIRAP technique for trapped atoms and to realize the proposed coherent control of collisions. There is also a large interest in a precise determination of the lifetime of this metastable state, which is of importance for the verification of QED. We can also trap neon atoms in their energetically lowest magnetic substate 3P2(m=-2) with the perspective of reducing inelastic collisions in the energetically lowest state.
Zeeman deceleration is an experimental technique which allows for the manipulation of open-shell atoms and molecules in a supersonic beam thus producing mK-cold, velocity-tunable beams of particles in selected quantum states. The method relies on the Zeeman interaction between paramagnetic particles and time-varying, inhomogeneous magnetic fields generated by pulsing high currents through an array of solenoid coils. This thesis describes the construction and implementation of a supersonic beam setup including a 12-stage Zeeman decelerator. The Zeeman decelerator follows an original design that makes it possible to replace individual deceleration coils. Using ground-state hydrogen atoms as a test system, it is shown that the transverse acceptance in a Zeeman decelerator can be significantly increased by generating a rather low, temporally varying quadrupole field in one of the solenoid coils. An electron-impact source was constructed and optimised enabling, for the first time, the Zeeman deceleration of metastable helium atoms in the 23S1 state, with an up to 40 % decrease in the kinetic energy of the beam. It is shown that the pulse duration for electron-impact excitation needs to be matched to the acceptance of the decelerator in order to attain a good contrast between the decelerated and undecelerated parts of the beam. Experimental results are rigorously analysed and interpreted using three-dimensional numerical particle trajectory simulations. A phase-space model provides, for the first time, a means to estimate the six-dimensional phase-space acceptance in a Zeeman decelerator and to find optimum parameter sets for improved Zeeman deceleration schemes. The approach also reveals a hitherto unconsidered velocity dependence of the phase stability which is ascribed mainly to the rise and fall times of the current pulses that generate the magnetic fields inside the deceleration coils. In the future, it is planned to combine the Zeeman decelerator with a source of cold atomic and molecular ions to study chemical collisions at low temperatures. A hybrid magnetic guide consisting of permanent magnet assemblies (Halbach arrays) in hexapole configuration and a set of current-carrying wires is proposed and simulated as an interface between these setups. The design promises very efficient velocity selection, a high degree of quantum-state selection and a nearly complete removal of residual carrier gas. Prospects for using magnetic hexapole focusing in front of the Zeeman decelerator are discussed. The work represents a major step towards the study and control of chemical reactivity of paramagnetic species in the low-temperature regime and it will help in the testing of fundamental chemical reaction theories.