Trap-depth determination from residual gas collisions

Abstract

We present a method for determining the depth of an atomic or molecular trap of any type. This method relies on a measurement of the trap loss rate induced by collisions with background gas particles. Given a fixed gas composition, the loss rate uniquely determines the trap depth. Because of the "soft" long-range nature of the van der Waals interaction, these collisions transfer kinetic energy to trapped particles across a broad range of energy scales, from room temperature to the microkelvin energy scale. The resulting loss rate therefore exhibits a significant variation over an enormous range of trap depths, making this technique a powerful diagnostic with a large dynamic range. We present trap depth measurements of a Rb magneto-optical trap using this method and a different technique that relies on measurements of loss rates during optical excitation of colliding atoms to a repulsive molecular state. The main advantage of the method presented here is its large dynamic range and applicability to traps of any type requiring only knowledge of the background gas density and the interaction potential between the trapped and background gas particles.The particle loss rate from a trap is one of the primary observables for probing the collisional physics of trapped gases. Measurements of trap loss are routinely made in a variety of experiments to deduce elastic and inelastic collision cross sections for intratrap collisions and for collisions between trapped species and externally introduced particles. Trap depth often plays an important role in the interpretation of these measurements. A well-studied example of this is the large intensity-dependent variation displayed by the two-body intratrap loss-rate coefficient for atoms trapped in a magnetooptical trap (MOT). This variation results from an interplay of trap depth and the energy imparted to trapped atoms due to hyperfine or fine structure changing collisions, as well as radiative escape [1][2][3][4][5][6][7][8][9][10][11][12]. More recently, inelastic and elastic collision rates in dipole traps have been of interest, particularly for metastable species [13,14]. The fraction of elastic collisions resulting in an evaporated atom depends on trap depth, which is a key parameter for evaporative cooling [13,15,16]. The lifetime dependence of a state-insenstive dipole trap on trap depth for cesium atoms has also recently been investigated [17]. Of course the most fundamental role of trap depth is that it be large enough to provide sufficient confinement, which has been an issue for experiments with buffer-gas-cooled atoms or molecules [18,19].In recent years there has been interest in making precision determinations of collision cross-sections from measurements of particle loss rates due to externally introduced particles. These measurements involve collisions of trapped neutral particles with neutral atoms and molecules [20][21][22][23][24], electron beams [25][26][27], and trapped ions [28][29][30]. Photoionization cross sections have also been investigated though mea...

Magnetic Trapping of Cold Methyl Radicals

“…This value is necessarily less than but within an order of magnitude of the total cross section due to the finite trap depth. From previous works on trap loss rates of cold atoms due to collisions with foreign gases [35,36], it is known that the trap loss rate varies quite strongly with trap depth especially near the energy scale for quantum diffractive collisions, ϵ d ¼ 4πℏ 2 =mσ T , where m is the mass of trapped particles and σ T is the total collision cross section [37]. This energy scale is on the order of ∼100 mK for the present case.…”

“…Under such conditions, the number of trapped radicals, N CH 3 , may obey a first order differential equation dN CH 3 =dt ¼ −ð1=τÞN CH 3 , where the lifetime, τ, is inversely proportional to the average of the product of the cross section, σ, velocity, v, and the density, n, of background gas particles [35]. The measurement of background gases by a residual gas analyzer revealed that the major component of the background gas in our vacuum chamber was H 2 (or H) with some N 2 as a minor component (less than 1=3 of H 2 ).…”

Magneto-optical trap loading rate dependence on trap depth and vapor density

“…The steady state number of atoms in the MOT is In the regime where β n ss is small compared to the losses due to background vapor, Γ≃ (a few s −1 to 0.01 s −1 ), Eq. (10) can be approximated by [33] Nt N ss 1 − e −γt .…”

“…For rubidium atoms in a magnetic trap, the trap depth can readily be estimated based on the magnetic field gradient, the magnetic properties of the trapped species, and the geometry of the vacuum cell [33]. That is, atoms in a magnetic trap with sufficient energy to reach the wall of the vacuum container will be heated via contact with the wall and escape the trap.…”

“…When this energy is sufficient for the atoms to escape the trap, loss is observed, providing a measure of the average MOT depth. The method employed is described fully in [33] and, for convenience, outlined in Appendix A of this paper. Table 1 summarizes the cooling laser settings (detunings and intensities) for each MOT measured along with deduced trap depths.…”