Session I5: Collisions Involving Atoms, Molecules, or Ions

Magnetic Trapping and Zeeman Relaxation of NH Molecules

NH molecular radicals are magnetically trapped in the presence of a helium buffer gas and their Zeeman relaxation and energy transport collisional cross-sections with helium are measured. Molecules are loaded from a molecular beam into a cold buffer gas cell in a 4T anti-Helmholtz magnetic trap. The NH-He energy transport cross-section is measured to be 2.7$\pm$ 0.8 $\times$10$^{-14}$cm$^{2}$ at 710 mK. The inelastic (Zeeman state changing) cross-section is also measured to be 3.8 $\pm$ 1.1$\times$10$^{-19}$cm$^{2}$ at 710 mK, indicating a $\gamma$ (elastic to inelastic cross-section ratio) of 7$\times$10$^{4}$, in agreement with the theory of Krems et al (PRA 68 051401(R) (2003)). Cross-section measurements are obtained for the interaction of the molecular isotopes $^{14}$NH, $^{14}$ND, $^{15}$NH and $^ {15}$ND with the helium isotopes He-3 and He-4.   

Spin Relaxation of Cold Atomic Iron and Nickel in Collisions with $^3$He

We measure the ratio of transport to spin-projection changing collision cross sections ($\gamma$) in the Fe-He and Ni-He systems. $\gamma$ for Ni [$^3F_4$, $m_J=4$] is found to be $\gamma = 1.1 \pm.5\times 10^{4}$ at 1 K in a .8 T magnetic field. $\gamma$ for Fe [$^5D_4$, $m_J = 4$] was low enough such that only a bound could be measured, $\gamma < 5\times 10^{3}$. The Ni result extends the notion of submerged shell suppression of inelastic loss in non-S-state atoms to this group of transition metals.   

Spin-orbit interaction and large inelastic rates in bismuth-helium collisions

We present a combined experimental and theoretical study of cold collisions between bismuth and helium atoms in strong magnetic fields and demonstrate that measurements of collision-induced Zeeman relaxation provide a probe of the spin-orbit interaction couplings between different non- relativistic states of Bi. The Zeeman relaxation of Bi in the ground electronic state is found to be very efficient due to admixture of the electronic excited states and the electronic interaction anisotropy arising from the orbital angular momentum of the electrons in the excited states.   

Second-order calculations of the transfer-ionization process: multiple differential cross sections

In a recent set of papers we considered the transfer ionization process: $$ {\rm p + He \rightarrow H^0 + He^{2+} + e^-} $$ in different geometries where the collision fragments were detected in coincidence. We demonstrated that the triple differential cross section was sensitive to both radial and angular correlation in the target. The theoretical predictions have been confirmed by experiment. Our model was a first order one in the proton-helium interaction. In this work we extend the previous model to the 2$^{\rm nd}$ order in the projectile target interaction. The result of our calculations demonstrate that the 2$^{\rm nd}$ order effects do not destroy the effects of angular and radial correlation in the target atom for the kinematics and dynamics explored in the previous papers. We present results in both the first order and second order model. The theoretical results are in sensible agreement with the latest experimental data.   

H2 dissociation due to collision with He

Cross sections for dissociation of H$_2$ due to collision with He are calculated for highly excited rovibrational states using the quantum mechanical coupled states approximation. An $L^2$ Sturmian basis set with multiple length scales is used to to provide a discrete representation of the H$_2$ continuum which includes orbiting resonances and a non-resonant background. Cross sections are given over a range of translational energies for both resonant and non-resonant dissociation together with the most important bound state transitions for many different initial states. The results demonstrate that it is possible to compute converged quantum mechanical cross sections using basis sets of modest size. It is found that collision induced dissociation competes with inelastic scattering as a depopulation mechanism for the highly excited states. The relevance of the present calculations to astrophysical models is discussed.   

Measurement of pressure broadening of the 2s-3d transition of Li by Ne and Ar

Doppler-free two-photon spectroscopy offers a unique combination of advantages for line-broadening studies. The narrow lines make resolution of small changes in linewidth easily observable, while the fact that entire velocity distribution is excited ensures that a thermal distribution at the temperature of the sample is being studied. We have employed this technique in studying line broadening of the 2s-3d state of Li by Ne and Ar,. Experimental broadening rates are reported, and compared to those calculated in the impact approximation using available potentials.   

Nonlinear Pressure Shifts of $^{133}$Cs Hyperfine Frequencies

The hyperfine (microwave) magnetic-resonance frequencies of optically pumped alkali-metal atoms in buffer-gas have long been used in compact, portable frequency standards. Van der Waals molecules, consisting of an alkali-metal atom loosely bound to a buffer gas atom, can form in such vapor cells. The molecules strongly affect the spin relaxation of alkali metal atoms in Ar, Kr and Xe gases at pressures of a few Torr, where the collisionally limited lifetime of the molecules is comparable to the characteristic period of the spin-rotation interaction between the rotational angular momentum N of the molecule and the electron spin S of the alkali-metal atom. The hyperfine-shift interaction, the modification a nearby buffer-gas atom makes to the Fermi contact interaction between S and the nuclear spin I of the alkali atom, can contribute to the width of the microwave resonance line, and it is responsible for the pressure shifts of the hyperfine resonance frequencies that are so important for clocks. Major improvements have been done to the apparatus and the process of data taking since last time. The experimental results show that Van der Waals molecules also modify the effects of the hyperfine-shift interaction. For Ar or Kr pressures of a few tens of Torr or less, the shift of the microwave resonance frequency of Cs is not linear in the buffer gas pressure.   

Direct Observation of the Opening and Closing of Inelastic Scattering Channels

We observe the abrupt changes of scattering phase shifts as a function of collision energy and magnetic field. Scattering atoms form a multi-channel system, where a distinction can be made between energetically open and closed channels. When the collision energy increases, inelastic collision channels open, yielding an abrupt change in the scattering phase shift at the threshold. We directly observe the difference of s-wave scattering phase shifts in our juggling cesium fountain clock by detecting only the scattered part of the atomic wavefunctions \footnote{R. A. Hart, X. Xu, R. Legere, \& K. Gibble, Nature 446, 892-895 (2007).}. Here, we study the scattering of the cesium clock states scattering off of cesium atoms prepared in F=3,m$_F$.   

A Cold Atom Measurement of Charge Exchange Collisions between Trapped Yb$^{+}$ and Yb

We measure the collisional cross-section and rate constant of the $^{174}$Yb and $^{172}$Yb$^{+}$ charge-transfer process. The neutral atoms are trapped in a magneto-optical trap (MOT) resonant with their 399 nm, $^{1}S_{0}\rightarrow{^{1}P_{1}}$ transition and are near the Doppler-limited temperature of 680 $\mu$K. The ions are confined in a planar Paul trap with a secular frequency of 39 kHz, Doppler cooled, and spatially overlapped with the neutral atoms. The collisional energy is varied from 4 meV to 100s of neV by varying the micromotion energy of the ions by displacement from the center of the Paul trap. We report the rate constant in comparison to that derived from the Langevin cross-section.   

Electron transfer into the projectile continuum in near-relativistic ion-atom collisions

Fast ion-atom collisions permit in experiments on electron transfer into the projectile continuum to study the dynamics of ionisation and radiative and non-radiative capture close to threshold; this is an exceedingly sensitive test of theory. We have studied electron emission in forward direction in 2 systems with different projectile Compton profile, U$^{88+}$ + N$_{2}$ and Sn$^{47+}$ + N$_{2}$ , using the 0$^{0}$ electron spectrometer at the supersonic jet target of the ESR storage ring. We present first results for 90AMeV U$^{88+}$ und 300AMeV Sn$^{47+}$ + N$_{2}$ , for which coincidences between cusp electrons with v$_{e}\approx $v$_{Proj}$ and charge exchanged projectiles were measured.   

Sputtering by fast heavy ion bombardment in a shock wave model

Theoretical description of sputtering by fast and heavy ions is a challenging task. Besides, the process may proceed through emission of large intact clusters causing a considerable damage to a surface. Cluster emission is one of the least understood fields in energetic particle-solid interactions. Experimental studies of sputtering demonstrate that emission of large clusters with hundreds or even thousands of atoms in a cluster is very probable. However mechanisms of that phenomenon are not well understood. In this work we study sputtering as well as emission of large clusters by fast and heavy ion bombardment within the shock wave model. The model includes both nuclear and electronic stopping power mechanisms. Calculations have been carried out for sputtering from Au and Ag surfaces by bare ions with energies of 10 keV - 10 MeV. The results of these calculations demonstrate sensible agreement with the experimental data. Our calculations within the shock wave model also reproduce experimentally observed power-law dependence for the emission of clusters, i.e. $Y(n) \propto n^{-a}$. We analyze limitations of the model as well as further developments.