Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B26: Focus Session: Photophysics of Cold Molecules II |
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Sponsoring Units: DCP Chair: Roland Wester, University of Freiburg Room: Morial Convention Center 218 |
Monday, March 10, 2008 11:15AM - 11:51AM |
B26.00001: Molecular collision studies with Stark-decelerated beams Invited Speaker: Molecular scattering behaviour has generally proven difficult to study at low collision energies. We formed a molecular beam of OH radicals with a narrow velocity distribution and a tunable velocity by passing the beam through a Stark decelerator [1]. The transition probabilities for inelastic scattering of the OH radicals with Xe atoms were measured as a function of the collision energy in the range of 50 to 400 wavenumbers. The behaviour of the cross-sections for inelastic scattering near the energetic thresholds was accurately measured, and excellent agreement was obtained with cross-sections derived from coupled- channel calculations on ab initio computed potential energy surfaces [2]. For collision studies at lower energies, the decelerated beams of molecules can be loaded into a variety of traps. In these traps, electric fields are used to keep the molecules confined in a region of space where they can be studied in complete isolation from the (hot) environment. Typically, 10$^5$ state- selected molecules can be trapped for times up to several seconds at a density of 10$^7$ mol/cm$^3$ and at a temperature of several tens of mK [3]. The long interaction time afforded by the trap has been exploited to measure the infrared radiative lifetime of vibrationally excited OH radicals, for instance, as well as to study the far-infrared optical pumping of these polar molecules due to blackbody radiation [4]. As an alternative to these traps, we have demonstrated an electrostatic storage ring for neutral molecules. In its simplest form, a storage ring is a trap in which the molecules - rather than having a minimum potential energy at a single location in space - have a minimum potential energy on a circle. To fully exploit the possibilities offered by a ring structure, it is imperative that the molecules remain in a bunch as they revolve around the ring. This ensures a high density of stored molecules, moreover, this makes it possible to inject multiple - either co-linear or counter propagating - packets into the ring without affecting the packet(s) already stored. We have recently demonstrated a prototype molecular synchrotron, which will be used as a low-energy collider for neutral molecules in the future [5].\newline [1] H.L. Bethlem, G. Berden, and G. Meijer, Phys. Rev. Lett. 83, (1999) 1558-1561.\newline [2] J.J. Gilijamse, S. Hoekstra, S.Y.T. van de Meerakker, G.C. Groenenboom, and G. Meijer, Science 313, (2006) 1617-1620.\newline [3] S.Y.T. van de Meerakker, P.H.M. Smeets, N. Vanhaecke, R.T. Jongma, and G. Meijer, Phys. Rev. Lett. 94, (2005) Artn. 023004.\newline [4] S. Hoekstra, J.J. Gilijamse, B. Sartakov, N. Vanhaecke, L. Scharfenberg, S.Y.T. van de Meerakker, and G. Meijer, Phys. Rev. Lett. 98, (2007) Artn. 133001.\newline [5] C.E. Heiner, D. Carty, G. Meijer, and H.L. Bethlem, Nature Physics 3, (2007) 115-118. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B26.00002: Magnetoelectrostatic trapping of neutral OH molecules Brian Sawyer, Benjamin Stuhl, Benjamin Lev, Mark Yeo, Dajun Wang, Jun Ye Advances in cold molecule production promise to profoundly impact research on precision measurement, quantum information, and controlled chemistry. To this end, we employ a Stark decelerator to remove 99.5{\%} of the center-of-mass kinetic energy of a supersonic beam of ground-state OH molecules. We subsequently trap a 70 mK sample of the decelerated molecules at a density of $>$10$^{5}$ cm$^{-3}$ within a magnetic quadrupole whose center lies $\sim $1cm from the decelerator exit. Our magnetoelectrostatic trap (MET) design allows for the addition of an electric field of variable magnitude to the trapped sample to facilitate polar-molecule collision studies. We report progress toward observation of cold collisions between samples of polar molecules. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B26.00003: Photodissociation of SO$_2$ as a way to cold atoms and molecules Lisdat Christian, Oleg Bucicov, Marcin Nowak, Sebastian Jung, Eberhard Tiemann We discuss the possibility to use the photodissociation of cold SO$_2$ molecules to produce internally and translationally cold photofragments SO and O. It is expected from our measurements of the molecular Stark effect~[1] that the dissociation pathways and excess energies of the fragments are tunable by electric fields~[2]. Cold SO$_2$ molecules are produced by Stark deceleration. We have realized a Stark decelerator that is able to slow down packages SO$_2$ in weak-field seeking levels to a few 10~m/s center of mass velocity. A Stark decelerator with 326~stages is required for this purpose, since the ratio of Stark shift to initial kinetic energy is small for SO$_2$. The photofragments SO and O have triplet ground states, while the ground state of SO$_2$ is diamagnetic. In combination with the photodissociation at the threshold we want to employ this constellation to accumulate fragments in a magnetic trap by dissociating SO$_2$ as it is stopped by electric fields in the center of the trap. \newline [1] J. Phys. B \textbf{39}, S1085 (2006). \newline [2] Phys. Rev.~A \textbf{74}, 040701(R) (2006). [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B26.00004: Alternating gradient focusing and deceleration of large molecules Kirstin Wohlfart, Fabian Gr\"atz, Frank Filsinger, Gerard Meijer, Jochen K\"upper During the last decade, fascinating progress has been made in the spectroscopy of the ``molecular building blocks of life''. Meanwhile, our group has been developing methods to decelerate neutral, polar molecules using time varying inhomogeneous electric fields. Extending these techniques to bio-molecules would allow, for instance, to increase observation times for precision spectroscopy or to separate different conformers. However, for such large molecules all states are practically high-field seeking. Therefore, alternating gradient focusing has to be applied. Here, we demonstrate the focusing and deceleration of benzonitrile (C$_7$H$_5$N) from a molecular beam. Benzonitrile is prototypical for large asymmetric top molecules that exhibits rich rotational structure and a high density of states. It is decelerated in its absolute ground state from 320~m/s to 289~m/s, and similar velocity changes are obtained for excited rotational states. We are setting up a longer alternating gradient decelerator, which will enable us to decelerate benzonitrile or larger molecules to much lower velocities and to thereby completely separate the decelerated packet from the rest of the beam pulse. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 1:03PM |
B26.00005: Production and Trapping of Ultracold Polar RbCs Molecules Invited Speaker: Our group has recently demonstrated the ability to assemble ultracold, polar molecules from laser-cooled atoms. We use photoassociation followed by stimulated emission pumping to produce RbCs molecules in their absolute ground state, at temperatures $T\sim 100\mu \mbox{K}$. In recent work, we have moved towards the goal of accumulated large, high-density samples of ultracold RbCs. Here we present new results on the trapping and collisional properties of RbCs in levels of high vibrational excitation. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B26.00006: Multistage Zeeman deceleration of hydrogen S.D. Hogan, A. Wiederkehr, M. Andrist, H. Schmutz, B. Lambillotte, F. Merkt With the goals of: (i) performing ultra-high resolution spectroscopy with long interaction times between a cloud of cold atoms or molecules and a narrow bandwidth radiation field, and (ii) studying cold reactive collisions in which the kinetic energies and quantum states of the colliding particles may be controlled to a high degree, a multi-stage Zeeman decelerator for neutral radicals has recently been developed in our laboratory. This instrument relies on the same concept of phase stability employed in charged particle accelerators. It opens up the possibility to manipulate the translational motion of a wider range of species than has been demonstrated using other quantum-state-selective techniques such as multi-stage Stark deceleration, and applies to a very different class of species than those to which Rydberg Stark deceleration is appropriate. The results of a recent series of experiments in which we have decelerated ground state hydrogen will be presented along with progress toward three-dimensional magnetic trapping of the decelerated radicals. In these experiments magnetic fields of 1-2~T are pulsed in each of the coils which make up the decelerator for tens of microseconds, with rise and fall times shorter than 5~$\mu$s. We have characterized the decelerated part of the gas pulse and studied the effect of zero field time windows, in which electron spin flips can occur, on the deceleration process. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B26.00007: Kinetics of Cold Molecule Production in ``Kinematic'' Cooling Jeffrey Kay, Kevin Strecker, David Chandler ``Kinematic'' cooling is a general technique by which a vast array of molecules can be translationally cooled using crossed atomic and molecular beams. The success of the technique relies primarily on the existence of an approximate mass degeneracy between the molecule to be cooled and its atomic (or molecular) collision partner. Here, we discuss factors that affect the efficiency of cold molecule production by this method, as well as schemes that may allow tunability of the velocity and temperature of the cold molecules on a fine scale. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B26.00008: A new source for quantum optics with biomolecules and biomolecular clusters Markus Marksteiner, Philipp Haslinger, Hendrik Ulbricht, Markus Arndt We present recent progress towards matter wave experiments with amino acids, polypeptides and large biomolecular clusters. All successful experiments on macromolecule interferometry so far, with fullerenes, fullerene derivates and large perfluoroalkyl-functionalized azobenzenes used effusive beam sources. The combination of Stark deflectometry with quantum interferometry also allowed us to create a new device for precisely measuring electric susceptibilities of large molecules in the gas phase. In order to apply quantum interference to molecules of biological interest, we have now implemented a pulsed laser desorption source. The combination of UV laser desorption into an intense noble gas jet and single-photon ionization by a VUV excimer laser (157nm) allows us to observe intense neutral jets of amino acids (e.g. Tryptophan), nucleotides (e.g. Guanin) and polypeptides ranging from tri-peptides to Gramicidin. Remarkably, we also found a new method for producing large neutral amino acid clusters, such as for instance Trp$_{30}$, with masses exceeding 6000 amu: the addition of alkaline Earth salts in the desorption process leads to the inclusion of at least one metal atom per complex and is sufficient to catalyze the cluster formation process. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 2:15PM |
B26.00009: Collisions of ultracold molecules Invited Speaker: In our experiments we routinely produce ultracold trapped samples of dimer molecules out of a Cs atomic gas by exploiting the atom-dimer coupling near Feshbach resonances. We explore the rich molecular structure for the Cs dimers near the atomic threshold by consecutive state transfer after initial dimer production and produce atom-dimer mixtures for which we measure the atom-dimer collisional rate as a function of magnetic field at temperatures down to 40 nK. We find resonant enhancement of this rate for sufficiently small dimer binding energies for which coupling to an Efimov trimer state is possible. We also produce pure dimer samples for which we measure the collisional loss rate. For a weakly bound molecular s-state this rate depends strongly on temperature and on the applied magnetic field. We will also discuss first results from our experiment on producing ultracold ro-vibrational ground state molecules for the case of Cs dimers and RbCs starting from weakly bound molecules which initially are produced on a Feshbach resonance. [Preview Abstract] |
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