Bulletin of the American Physical Society
40th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 54, Number 7
Tuesday–Saturday, May 19–23, 2009; Charlottesville, Virginia
Session K2: Focus Session: Collision-induced States in Molecules |
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Chair: Paul Julienne, National Institute of Standards and Technology Room: Gilmer Hall 130 |
Thursday, May 21, 2009 10:30AM - 10:42AM |
K2.00001: Progress toward cold N-NH collisions in a magnetic trap Matthew Hummon, Hsin-I Lu, Edem Tsikata, John Doyle We report progress toward the observation of cold collisions of atomic nitrogen (N) and imidogen (NH) in a deep (4 Tesla) magnetic trap. The atoms and molecules are loaded from a room temperature molecular beam into the magnetic trap using the cryogenic helium buffer gas technique. In previous NH trapping experiments using buffer gas loading, the $1/e$ lifetime of NH in the trap has been limited to $\tau < 1$~s due to collisions with background helium gas. Here, we implement a new large aperture trapping cell and cryogenic pulsed valved for introduction of the buffer gas. This leads to a lower density of background helium, and we observe NH trap lifetimes of $\tau > 3$~s and observation times of trapped NH greater than 10~s. [Preview Abstract] |
Thursday, May 21, 2009 10:42AM - 10:54AM |
K2.00002: Multichannel quantum-defect theory of ultracold atom-ion collisions Zbigniew Idziaszek, Tommaso Calarco, Paul Julienne, Andrea Simoni We study atom-ion scattering in the ultracold regime. To this aim, an analytical model based on the multichannel quantum defect formalism is developed and compared to close-coupled numerical calculations. We apply our model to the specific systems of ${}^{40}$Ca$^{+}$ - Na and ${}^{138}$Ba$^{+}$ - ${}^{87}$Rb, and investigate the occurrence of magnetic Feshbach resonances. The presence of several resonances at experimentally accessible magnetic fields should allow the atom-ion interaction to be precisely tuned. A fully quantum-mechanical study of charge exchange processes shows that charge-exchange rates should remain small even in the presence of resonance effects. Most of our results can be cast in a system-independent form and are important for the realization of the charge-neutral ultracold systems. [Preview Abstract] |
Thursday, May 21, 2009 10:54AM - 11:24AM |
K2.00003: Rapid formation of H$_{3}^{+}$ from ammonia and methane following ionization by fast protons Invited Speaker: Bond rearrangement, specifically the formation of H$_{2}^{+}$ and H$_{3}^{+}$ after ionization of methane and ammonia by 4 MeV protons, is studied in both the common and deuterated isotopes of those molecules. Our coincidence-time-of-flight measurements show the relative probability of H$_{2}^{+}$ and H$_{3}^{+}$ production from ammonia was higher for the lighter isotope, contradicting the common intuition that the rearrangement occurs on the timescale of the dissociation. The isotopic effects in methane, while smaller, were in the opposite direction. As expected, the relative probability of bond rearrangement leading to H$_{2}^{+}$ increases with the number of hydrogen atoms in the target for H$_{2}$O, NH$_{3}$, and CH$_{4}$ targets. Formation of H$_{3}^{+}$, however, is less likely from a methane target than from ammonia. We examined this result by calculating the ionic potential energy surface in reduced coordinates corresponding to a symmetric stretch of a H$_{3}^{+}$ triangle away from the nitrogen or carbon atom. From both the experiment and the model calculation, we find that bond rearrangement is a two step processes. First, the nuclear wavepacket is projected onto the ionic potential energy surface by a sudden (vertical) ionization, and then it propagates to the final products. The details of both steps determine the amount of bond rearrangement. [Preview Abstract] |
Thursday, May 21, 2009 11:24AM - 11:36AM |
K2.00004: Feshbach resonances and the asymptotic bound state model K.W. Madison Information regarding the interatomic interaction potentials can be extracted from the positions and widths of experimentally observed magnetic Feshbach resonances (FRs). These potentials can, in turn, be used to predict the positions of other resonances. However, a full quantum scattering calculation is computationally intensive, and iteratively finding the proper model potentials to reproduce the experimentally observed resonances can be a lengthy process. To simplify this search and to gain some physical insight into the scattering properties, one can utilize the so-called ``asymptotic bound state model" to determine the energies of the last bound states of, for alkali mixtures, the triplet and singlet potentials consistent with the experimentally observed FRs. The potential curves can then be tuned to reproduce these bound state energies, and fine tuning of the potentials can be done using a full quantum scattering calculation. We will present the details, advantages, and limitations of this ABS model. We will also provide an example of its use in the recent determination of the Li--Rb interaction potentials from two experimentally measured FRs. [Preview Abstract] |
Thursday, May 21, 2009 11:36AM - 11:48AM |
K2.00005: Search for collisional exchange of ground-state atomic alignment between rubidium isotopes E.J. Bahr, D.F. Jackson Kimball, B. Coste, S.A. Rangwala, J.M. Higbie, M.P. Ledbetter, S.M. Rochester, V.V. Yashchuk, D. Budker The collisional transfer of pure atomic alignment (related to coherences between Zeeman sublevels with $\Delta $M=2) between isotopes of rubidium is searched for using time-dependent magneto-optical rotation. Alignment-exchange collisions are fundamentally different than the commonly studied orientation-exchange collisions: for example, spin-exchange collisions preserve the net orientation in an atomic vapor (because of angular momentum conservation) but do not conserve alignment. Collisional transfer of alignment in alkali atoms has seldom been studied because the cross-sections are expected to be three to four orders of magnitude smaller than the nominal spin-exchange cross-sections. This is due to the fact that ground-state alkali atoms have electronic angular momentum J=1/2 and so the electronic state cannot support a $\Delta $M=2 coherence. Thus collisional transfer of alignment is only possible because of hyperfine re-coupling during the collision. Implications of the measurement for searches for anomalous spin-dependent forces will be discussed. [Preview Abstract] |
Thursday, May 21, 2009 11:48AM - 12:00PM |
K2.00006: Geometric phase driven predissociation: Lifetimes of 2$^2A'$ levels of H$_3$ Juan Blandon, Viatcheslav Kokoouline We discuss the role of the geometric phase in predissociation dynamics of vibrational states near a conical intersection of two electronic potential surfaces of a $D_{3h}$ molecule. For quantitative description of the predissociation driven by the coupling near a conical intersection, we developed a method for calculating lifetimes and positions of vibrational predissociated states (Feshbach resonances) for X$_3$ molecule. The method takes into account the two coupled three-body potential energy surfaces, which are degenerate at the intersection. As an example, we apply the method to obtain lifetimes and positions of resonances of predissociated vibrational levels of the 2$^2A'$ electronic state of the H$_3$ molecule. The three-body recombination rate coefficient for the H+H+H $\to$ H$_2$+H process is estimated. [Preview Abstract] |
Thursday, May 21, 2009 12:00PM - 12:30PM |
K2.00007: Young-type interference in collisions between Helium and molecular Hydrogen ions Invited Speaker: The dissociative electron transfer from He into slow diatomic molecular ions was measured in a kinematically complete experiment by using cold target recoil ion momentum spectroscopy (COLTRIMS) in combination with a high resolution molecular fragment imaging technique. The electron transfer leads to neutral molecules in highly excited electronic states which dissociate in a second step of the reaction. With COLTRIMS we determine the energy which is taken from the motion of the projectile and transferred to internal degrees of freedom of the molecule. Furthermore we are able to determine the kinetic energy release (KER) of the dissociation as well as the spatial orientation of the molecular break up. When using H$_{2}^{+}$ as projectiles this enables us to select the direct population of the lowest dissociative state of H$_{2}$ b$^{3}\Sigma ^{+}_{u}$ from the measured data. This reaction channel was analyzed in an inverse kinematics scheme where the molecule is regarded as two closely lying scattering centers at which an incoming He wave scatters. The orientation and the distance of these two scattering centers are obtained by detecting the fragments after the reaction, as the internuclear vector can be determined using the reflection as well as the axial recoil approximation. We find a striking double slit interference pattern in the transverse momentum transfer between the He atom and the molecule which we can modulate by selecting different internuclear distances (i.e. KERs) in our offline analysis. Compared to an optical double slit, the interference's minima and maxima are swapped. The latter is the result of a phase shift in the electronic part of the wave function. [Preview Abstract] |
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