Session K6: Atom-Atom Collisions

Neutron impact ionization of helium

Low-energy ($\sim$keV) collisions of neutrons with atoms result in a momentum transfer to the atomic nuclei while the direct interaction with the electronic degrees of freedom is negligible. In the frame of a target nucleus, this represents a simultaneous \emph{kick} of all electrons, i.e., a sudden boost of the $N$-electron wave function in momentum space giving rise to excitation and ionization with a broad distribution of final states. As the kick operator is a true many-electron operator, multiply-excited and correlated final states can be readily accessed that are precluded by selection rules in the case of photoabsorption. We study neutron impact ionization of helium for kinetic energies of the neutrons up to a few tens of keV, corresponding to a momentum transfer of up to half an atomic unit for the electrons. We will present single and double ionization yields as a function of neutron energy, as well as ionization spectra which clearly show Fano resonance lines for many doubly excited states that are strongly suppressed in photoionization or electron-impact ionization.   

Simulations of the Spatial Dependence of Populations in High Field Optical Pumping

Optical pumping of alkali atoms forms the basis for many modern experiments including atomic clocks, magnetometers, and hyperpolarization of noble gases and solids. The alkali atoms in these experiments interact with other alkali atoms, the optical pumping laser, buffer gas or noble gas targets, and the glass cell walls or a coating. Recent experimental results at high magnetic fields have shown that ground-state sublevel populations in a cesium vapor exhibit spatial diffusion, each with a different effective diffusion length. At high magnetic fields, each ground-state sublevel can be individually probed with a weak $D1~(S_{1/2}\rightarrow P_{1/2})$ laser while a stronger $D2 ~(S_{1/2}\rightarrow P_{3/2})$ laser depopulates a single sublevel. The probe beam is physically translated to measure the populations at different positions in the vapor cell. To try and understand some unexpected features observed in the sublevel populations undergoing optical pumping, we present a numerical model of the density matrix of alkali atoms as a function of position within the vapor cell. Steady-state sublevel populations are shown for atoms undergoing optical pumping, alkali-alkali collisions, alkali-buffer gas collisions, and depolarization at the cell walls, and these results are compared to experimental observations.   

Positive nonlinear pressure shift of Cs in Ne

We demonstrate that the hyperfine resonance frequency of ground state Cs atoms have a nonlinear dependence on the pressure of the buffer gas Ne at a fixed temperature. The hyperfine resonance frequency of alkali-metal atoms is shifted by an amount, which had long been assumed to be linear with the buffer gas pressure until Fei Gong discovered that the shift of Rb and Cs hyperfine resonance frequency has a nonlinear dependence on the pressure of the buffer gas Ar and Kr. While the nonlinear pressure shift of Cs in Ar and Kr is negative, we found that the nonlinear pressure shift of Cs hyperfine frequency in Ne is positive. The reason of the nonlinear shift is the three body collision(eg: Cs-Ne-Ne) and the formation of Van der Waals molecules of a Cs atom and a buffer gas atom of Ar, Kr, or Ne. The hyperfine precession rate of a Cs atom bound in molecule has a shift respect to a free Cs atom. The reversal sign of this nonlinear pressure shift of Cs in Ne respect to Ar and Kr demonstrate that the shift of the hyperfine precession rate of Cs in CsNe is reversed respect to CsAr and CsKr.   

Collisional narrowing with optically trapped atoms

Cold atoms trapped in a conservative potential can be used for quantum information processing, where their high density can be employed to achieve high efficiency in quantum operations. The high atomic density, on the other hand, alters the atomic coherence dynamics and leads to a surprising prolongation of the coherence times, a phenomenon we call collisional narrowing in analog to the well known motional narrowing effect in NMR. Here we present our theoretical and experimental study of collisional narrowing in optically trapped atoms. We show that elastic collisions reduce the spread of the atomic phase distribution and modify its time evolution from ballistic to diffusive. We perform Ramsey experiments on cold $^{87}Rb$ atoms trapped in a far-off-resonance laser, and observe a decrease of the dephasing rate for an increasing collision rate. We also show that the new emerging dephasing timescale depends only on the atomic phase space density of the ensemble.   

Disorientation of the Ne$^{\ast }$(2p$_{i}$ [J=1]) atoms due to He atom collisions in glow discharges at 10 K $<$ T $<$ 3000 K

We calculate the disorientation of the Ne$^{\ast }$(2p$_{i}$ [J=1]) atoms in a wide temperature range using a quantum close-coupling many-channel approach and the model potential for the interaction between Ne$^{\ast }$(2p$_{i}$ [J=1]) and $\mbox{He}\,(\mbox{1s}^{\mbox{2}})$ atoms in a gaseous mixture at thermal equilibrium proposed in [1]. Comparison with experiments of disorientation of the Ne$^{\ast }$(2p$_{2})$ atoms [2] for 17 K $<$ T $<$ 300 K is reported, and excellent agreement is found above 35 K. Below 35 K our rate coefficients are larger than the experimental data from [2]. The difference is possibly due to a stronger influence of the nuclear rotational motion in the molecular Hamiltonian than the electrostatic interaction at large atomic separation. This assumption is in agreement with our analysis of disalignment of the Ne$^{\ast }$(2p$_{i}$ [J=1]) atoms done in [3], but is in disagreement with the measurements of depolarization at low temperatures reported in [2]. We explain the variation of the disorientation rates with the temperature based on the anisotropy of the collisional channels associated to the Ne$^{\ast }$(2p$_{i}$ [J=1]) states. [1] Bahrim C and Khadilkar V V 2009 \textit{Phys. Rev. A} \textbf{79} 042715. [2] Nimura M, Hasuo M and Fujimoto T 2004\textit{ J. Phys. B. At. Mol. Opt. Phys. }37 4647. [3] Bahrim C and Khadilkar V V 2008 \textit{J. Phys. B: At. Mol. Opt. Phys.} \textbf{41} 035203.   

Limits on anomalous short-range spin-dependent interactions from spin-exchange collisions

Spin-exchange between light alkali atoms and noble gases proceed mainly due to the Fermi contact interaction in binary collisions, leading to small spin-exchange cross sections ($\sim $10$^{-24}$ cm$^{2})$. We investigate constraints on short-range ($\sim $1 nm) anomalous spin-dependent forces between nuclei based on the agreement between theoretical predictions and experimental measurements of spin-exchange cross sections for collisions between light alkali atoms and noble gases.   

Magnetically trapped erbium and thulium: rapid spin relaxation

Spin relaxation due to atom--atom collisions is measured for buffer-gas cooled and magnetically trapped erbium and thulium atoms at a temperature near 500~mK. The rate constants for Er--Er and Tm--Tm collisions are $3.0 \times 10^{-10}$ and $1.1 \times 10^{-10}$ cm$^3$ s$^{-1}$, respectively, 2--3 orders of magnitude larger than those observed for highly magnetic $S$-state atoms. This is strong evidence for an additional, dominant spin relaxation mechanism, electronic interaction anisotropy, in collisions between these ``submerged-shell," $L \neq 0$ atoms. This result is in contrast to the dramatic suppression of electronic interaction anisotropy in collisions between these atoms and helium. The large spin relaxation rates observed here imply that evaporative cooling of these atoms in a magnetic trap will be highly inefficient.   

Magnetic Trapping of NH Molecules with 20 s Lifetimes

Buffer gas cooling is used to trap NH molecules with $1/e$ lifetimes exceeding 20 s. Helium vapor generated by laser desorption of a helium film is employed to thermalize $10^{5}$ molecules at a temperature of 500 mK in a 3.9 T magnetic trap. Long molecule trapping times are attained through rapid pumpout of residual buffer gas. Molecules experience a helium background gas density below $1 \times 10^{12}$ cm$^{-3}$.   

Collision dynamics of ultracold atoms confined in 1D optical lattices

We present a formalism for rigorous multichannel scattering calculations of cross sections for inelastic and reactive collisions of atoms and molecules confined by 1D optical lattices. We obtain analytical expressions for the mean frequency of inelastic collisions in a confined gas in the temperature regime $T \sim \hbar \omega$ and at temperatures $T\gg \hbar \omega$, where $\omega$ is the oscillation frequency of trapped particles in the confining potential. Our numerical calculations for the gaseous mixture of Li and Rb atoms show that the threshold energy dependence of the collision cross sections can be tuned by varying the confinement strength and external magnetic fields at $T \ll \hbar \omega$ and that inelastic collisions in the temperature regime $T\sim \hbar \omega$ exhibit significant deviations from 3D scattering for systems with strong confinement and large scattering lengths. We find that the ratio of rate constants for inelastic scattering and elastic collisions is suppressed in the presence of a laser confinement and this suppression is significant for Li--Rb collisions at $T<40$~$\mu$K.   

The charm of trap loss: sensitive probe of collisions and trap depth

More than just a nuisance, atom trap loss due to collisions with background gas provides a sensitive probe of both the collisional physics between the target and background species and the trap depth. We discuss recent work on a technique for the determination of the differential and total cross sections from trap loss measurements. We also discuss the use of such loss measurements for the determination of the trap depth - a particularly difficult quantity to measure in optical traps.