A new scheme for temperature measurements in a magneto-optical trap is reported.It allows non-destructive on-line measurements of spring constant, friction coefficient, and temperature in a magneto-optical trap. By this method, sub-Doppler temperatures as low as 20pK have been measured on-line in a rubidium trap.Introduction. -For many purposes in spectroscopy and atomic physics in general it is desirable to have atomic samples with very low velocities confined to a well-defined small volume. Thus in the recent past tremendous efforts have been undertaken in laser cooling and trapping. Raab et al. [l] reported the first magneto-optical trap (MOT), which employs the Zeeman-and Doppler-effects to exert position-and velocity-dependent light pressure on atoms. Since the introduction of vapour cell MOTS by Monroe et al. E23 these devices have gained widespread interest. The lower bound for the temperature T of optically trapped two-level atoms is the so-called Doppler limit TD = h .I'/2kB (140 pK for Rb) where r is the natural linewidth of the cooling transition (Rb: r = 2x-5.9 MHz). Sub-Doppler temperatures, first measured in molasses by Lett et al. [3], have been explained by Dalibard and Cohen-Tannoudji[43 by the non-adiabatic motion of (real) multilevel atoms through the optical-polarization gradient in molasses. The first measurements of sub-Doppler temperatures in a caesium MOT were published by Steane and Foot[5].We report here about the operation of a magneto-optical rubidium trap. We found sub-Doppler temperatures using a new simple technique of measuring the mechanical properties of the MOT.Apparatus. -A stainless-steel vacuum chamber with a capacity of 11 is pumped by a 21 / s ion getter pump and a 300 cm2 ST 707 getter strip. The pressure inside the ion getter pump is (*) Now with Deutsche Forschungsanstalt fiir LUR und Raumfahrt (DLR), I"-AUG, Sudstral3e 125, D-W 5300 Bonn 2, Germany.

Using laser light tuned to a repulsive molecular potential, we have been able to suppress inelastic ground-state hyperfine-changing collisions between ultracold R b atoms. Adiabatic excitation to the repulsive curve alters the atomic trajectories and prevents the atoms from approaching close enough for the hyperfine change to occur. Experimental results show suppressions up to -50% and are in reasonable agreement with a simple Landau-Zener model. Our ability to control collisional trap loss processes may have important implications for the achievement of high densities in laser cooled samples.

Two different isotopes-(85)Rb and (87)Rb-have been captured simultaneously in one magneto-optical trap. The trap parameters could be chosen nearly independently for the two isotopes. The trap loss rates that are due to collisions between atoms of different isotopes have been measured in the low-intensity limit.

We demonstrate a prototype of a laser-cooled *' Rb fountain clock and measure the frequency shift due to cold collisions. The shift is hctionally 30 times smaller than that in a lasercooled CS clock. We observe a density dependent pulling by the microwave cavity and use it to cancel the collision shift. We have also demonstrated a juggling atomic fountain to study cold collisions and we discuss the importance of juggling for future fountain ClOCks.

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