H.C.W. Beijerinck

Bright thermal atomic beams by laser cooling::a 1400-fold gain in beam flux

Using a three-step transverse laser cooling scheme, a strongly diverging flow of metastable Ne(3s3P2] atoms is compressed into a well-collimated, small diameter atomic beam (e.g., 1.4 mrad HWHM divergence at 3.6 mm beam diameter) with an unmodified axial velocity distribution centered at 580 m/s. The maximum increase in beam flux 1.04 m downstream of the source is a factor 1400; the maximum increase in phase space density, i.e., brightness, is a factor 160. The laser power used is only 140 mW. The scheme is extendable to a large variety of atomic species and enables the application of bright atomic beams in many areas of physics.

Intermolecular potentials for the metastable Ne*-rare gas and Ne*-molecule systems

Abstract The absolute value and the velocity dependence of the total cross section Q ( g ) has been measured in a crossed beam machine for the Ne*-Ar, Kr, Xe and Ne*-O 2 , N 2 , CH 2 and CO 2 systems, using a mixed beam containing Ne*( 3 P( 0 ) and Ne * ( 3 P 2 ) fine structure states in a 1:5 ratio. The range of velocities is typically 1000 ⩽ g ⩽ 8000 m s −1 , always including the interesting N = 1 glory oscillation. The results for the Ne* -rare gas systems are in excellent agreement with the predictions of the ion-atom Morse-Morse-spline-van der Waals potentials of Gregor and Siska, both with regard to the absolute value (1.5%), position of the N = 1 glory maximum (2.7%) and the amplitude of the N = 1 glory maximum (4.3%). The predictions of the potentials proposed by Hausamann are less satisfactory, most likely due to the specific switchover function used to connect the well area at R/R M ≈ 1.1 to the van der Waals long-range attractive branch at R/R M ≈ 2 ( R M is the well position). By using a semiclassical scaling method the potential parameters ϵ (well depth), R M (well position) and C 6 (van der Waals constant) have been determined for the Ne*-molecule systems, using the Gregor and Siska IAMMSV potential for the Ne*-Xe system as a reference. The well parameters are (ϵ (meV), R M (A)) = (3.21, 5.43), (4.24, 5.17), (13.55, 4.74) and (7.08, 5.44) for the Ne*-N 2 , O 2 , CO 2 and CH 4 , systems, respectively. For the C 6 values we observe a fair scaling with the polarisibility α of the molecule. For the Ne*-CO 2 system we observe a damping of the amplitude of the glory oscillations, which increases rapidly with decreasing velocity. This damping is interpreted in terms of the probability for ionisation along the glory trajectory, providing useful information for determining a complex potential for this system.

Grating feedback in a 810 nm broad-area diode laser

Narrow linewidth, single spectral mode operation has been obtained in a high power, 810 nm broad-area diode laser in an extended cavity configuration with a grating as external reflector (grating feedback). For stable operation it was necessary to misalign the feedback slightly in the plane of the laser junction. Characteristics of the thus obtained laser system are a linewidth below 5 MHz, an output intensity of about 50% of the free running power, a large-scale tuning range of 15 nm and continuous scanning over 4 GHz. In the spatial domain, the laser remains multimode and astigmatic. To show the practical applicability of this system, saturated absorption of a krypton line is demonstrated.

Excitation transfer for Xe* (3P2) + N2 in the eV energy range: double potential crossing for N2 (B, υ1 =5)

Abstract In a crossed beam experiment the yield of N 2 (B 3 Π g , υ 1 = 5) excited product molecules was measured for Xe * /N 2 collisions over a kinetic energy 80-2000 meV. The observed threshold is 59.4 meV, nearly equal to the endothermicity of 59 meV for this process. The dynamics of this collision process is described with a Landau-Zener model in terms of two curve crossings of the entrance and exit potentials. For energies higher than 500 meV additional curve crossing with the repulsive branch of the potentials of the υ 1 ⩽ 4 final states can be reached, resulting in sharp decrease of the υ 1 = 5 production cross section.

Pure state Ne*(*) [ J,M>]-preparation in a magnetic field : polarization effects in ionizing collisions

Abstract A compact device (lengh 175 mm) has been built for the energy resolved detection of low energy (1–5 eV) Penning electrons with a 2π solid angle collection efficiency, based on the principle of adiabatic parallelisation of electron motion in a diverging magnetic field. A retarding field analysis is then used as a high pass filter to discriminate between Penning electrons released in collisions of rare gas atoms in metastable and shortlived, laser excited states. The overall detection efficiency is 0.13. The Zeeman-splitting of the atomic levels in the scattering center (maximum B = 222 G) allows the preparation of single magnetic substates |J, M⪢B. By rotating the detector in the collision plane, well defined |J, M⪢g states can be produced with respect to the relative velocity g, the quantization axis relevant for the collisions. The system has been tested by measuring the collision energy dependence of polarized-atom cross sections JQ|M| for the Ne* [3P2]-Ar and Ne** [3D3]-Ar systems. For the Ne* [3P2] metastable atoms we find 2Q0/2Q2 = 1.55 ± 0.06 and 1.05 ± 0.06 in the thermal and superthermal energy range, respectively, which should be compared to 1.30 of Bregel et al. at thermal energies. For the Ne** [3D3] state we find 3Q0,1/3Q2,3 = 1.65 ± 0.06 and 1.00 ± 0.10 for the same energy ranges.

FINE-STRUCTURE DEPENDENCE OF THE AR* (3P0,2) + N2(X) EXCITATION TRANSFER PROCESS

Using diode lasers for the preparation of state selected beams of metastable Ar∗(3P0, 3P2) atoms, the total cross section for the formation of the N2(C,v′) final state has been measured for v′ = 0, 1, 2, and 3, respectively, in a wide range of collision energies 80 < E (meV) < 2000. The energy dependence is very similar for both initial fine structure states and all final vibrational states. The experimental results are described in terms of a coupling of the initial and final state by the intermediate Ar+ + N2− ionic potential, using a Landau Zener model to calculate the transition probability. Due to the similarity in shape, all cross sections are described by a single set of crossing parameters. The observed threshold energy is Ex,i = 63 meV for both fine structure states: the contribution of the 3P0 state does not solve the discrepancy with earlier measurements of other groups. The initial state rotation of the N2 molecules has no influence on the shape of the cross section, as investigated by varying the rotational temperature of the supersonic target beam.

Magnetically Induced Laser Cooling for Ne*: Approaching the Recoil Limit

Magnetically induced laser cooling to temperatures close to the recoil limit is investigated in one dimension. For a metastable neon beam, we present high-precision measurements investigating the actual temperature limit in this cooling process. Using time-of-flight techniques to reduce the effect of the longitudinal velocity spread, we observe cooling at small magnetic field toward ν = 0 with an r.m.s. width of the distribution of 5.4 cm/s, well below the Doppler limit. At a larger magnetic field (0.4 Gauss) the velocity-selective resonances are extremely sharp. Here we find the r.m.s. width of the distribution to be 3.4 cm/s, only 1.1 times the recoil speed k/M, corresponding to a temperature T = 2.7 μK.

A factor 1600 increase in neutral atomic beam intensity using laser cooling

Using a three-step transverse cooling scheme, a strongly diverging flow of metastable Ne*(/sup 3/P/sub 2/) atoms is compressed into a small diameter (3 mm), well-collimated (1 mrad) atomic beam, using only 200 mW total laser power. This extremely valuable technique for frequency standard applications is applicable to many different atomic systems.<<ETX>>

Ar∗ (3P2) / Kr∗ (3P0,2) + N2(X) excitation transfer collisions: Final state rotational alignment

Abstract The alignment of the rotational angular momentum J ′ of the N 2 (C; v ′, J ′) final state is investigated in a wide range of collision energies 80 E (meV) ∗ + N 2 system the alignment is nearly energy-independent in the thermal range; the value A ≈ 0.4 indicates a preference for J ′ perpendicular to the initial relative velocity. At superthermal energies we observe a decrease to A ≈ 0 at E ≈ 2 eV, indicating an isotropic distribution of the direction of J ′. The alignment is independent of the final vibrational state for v ′ = 0 and 1. For the endothermic Kr ∗ + N 2 system, where the different thresholds for the 3 P 0 and 3 P 2 states allow us to obtain the fine structure dependence of the alignment, we observe A ≈ 0.15 for 3 P 0 and A ≈ 0.25 for 3 P 2 . For energies far above threshold we observe a decrease to A = 0.05 at E = 5 eV, which is attributed to the behaviour of the 3 P 2 state.

Production of bright, cold beams for atomic collision experiments

Laser manipulation techniques for neutral atoms can be used to produce atomic beams which are more intense, brighter and colder than can be achieved by any other means. These beams will have a tremendous impact on the experimental study of low‐energy atomic collisions. We first describe the design and operation of an intensifier for a thermal (axial velocity 600 m/s) beam of metastable neon atoms. The intensifier produces a gain in beam brightness of a factor 160 and a typical gain in usable flux of a factor 1400. Next, we discuss the design, construction and preliminary tests of a setup to produce an atomic beam which is slow and cold as well as bright and intense. The setup is expected to produce a beam of metastable atoms with an axial velocity of 100 m/s, a spread therein of 1.5 m/s, a diameter of 1 mm, a residual divergence of 1 mrad and a flux of 1012 atoms/s.