Session H26: Focus Session: Photophysics of Cold Molecules IV

Reactions of cold trapped anions

Interactions of negative ions with small organic molecules represent model systems for the investigation of reaction dynamics in few-body systems. Their corrugated potential energy landscape, originating in long-range attractive and short-range repulsive forces, requires the coupling of different degrees of freedom for reactions to occur. We have adopted two complementary approaches to study anion-molecule reaction dynamics. Using velocity map imaging in combination with crossed beams at low energy we study the differential cross section of negative ion reactions. For nucleophilic substitution reactions we have observed several distinct reaction mechanisms when varying the collision energy [1]. Total reaction rate measurements, which we carry out in the box-shaped potential of a 22pole ion trap [2], have revealed unexpected temperature-dependences for proton transfer and for cluster stabilisation at low temperatures. In addition, laser-induced photodetachment is studied in the trap to obtain absolute destruction cross sections for negative ions in light fields [3]. These results are relevant for the understanding of the negative ion abundances in interstellar molecular clouds. \newline [1] J. Mikosch et al., Science (in press) \newline [2] J. Mikosch et al., Phys. Rev. Lett. 98, 223001 (2007) \newline [3] S. Trippel et al., Phys. Rev. Lett. 97, 193003 (2006)   

Cold reactive collisions between laser-cooled ions and velocity-selected neutral molecules

The recent development of a range of techniques for producing ``cold'' molecules at very low translational temperatures T $<$ 1 K in the gas phase has provided the opportunity for studying molecular collisions in a new physical regime. We report a new experimental method to study reactive collisions between ions and neutral molecules at very low temperatures which allows for tunable collision energies and a variety of chemically diverse reaction partners. Our technique relies on the combination of a quadrupole-guide velocity selector for the generation of cold polar molecules with a facility to produce strongly ordered samples of laser-cooled ions in an ion trap, usually referred to as Coulomb crystals. Despite the low fluxes of neutral molecules obtained from the quadrupole-guide, the strong localization and long trapping times of the ions allows chemical reactions to be studied at the single-particle level. In a proof-of-principle experiment, we have studied the chemical reaction between translationally cold CH$_3$F molecules and laser-cooled Ca$^+$ ions in a collision energy range corresponding to 1-10 K. The characteristics of our cold-molecule sources and the performance of the new technique as well as perspectives for further developments will be discussed.   

Demonstration of a three-dimensional trap for state-selected Rydberg atoms

Recent progress in the development of methods by which to decelerate and manipulate the translational motion of Rydberg atoms and molecules in the gas phase using static and time-varying inhomogeneous electric fields has led to the experimental realization of Rydberg atom optics elements including a lens, a mirror and a two-dimensional trap. These experiments exploit the very large electric dipole moments associated with Rydberg Stark states, and have demonstrated the possibility to stop a seeded, pulsed, supersonic beam of atomic hydrogen traveling with an initial velocity of 700~ms$^{-1}$ within 3~mm and $5~\mu$s using electric fields of only a few kVcm$^{-1}$.\\ With the goal of achieving complete control of a cloud of Rydberg atoms or molecules in three-dimensions, we have recently designed and constructed a three-dimensional electrostatic trap for these particles. The design of this trap will be presented along with the results of a series of experiments in which we have used the trap to confine, in three dimensions, a cloud of atomic hydrogen Rydberg atoms in states with principal quantum numbers around $n=30$. The dynamics of the Rydberg atoms in the trap have been investigated by pulsed field ionization and imaging techniques. Under favorable conditions, trapping times on the order of 150~$\mu$s have been observed, corresponding to the radiative lifetimes of the excited states.   

Measuring the role of alignment in a molecule optical lens

Far off-resonant pulsed lasers have been used to deflect and focus molecules via the optical dipole force, which is proportional the effective polarisability of the species [1]. Molecules have an anisotropic polarisability, which in the presence of an intense linearly polarised optical field (10$^{12}$ Wcm$^{-2})$ causes the molecule to align with the field polarisation vector. This alignment occurs due to the creation of pendular states which are a superposition of the field free rotational states of the molecule[2]. This alignment of the molecule with the electric field can result in a higher effective polarisability leading to an increased dipole force that can be used to tailor the properties of molecular optical elements. Using this property, we are studying how the field polarisation can be used to modify the focal length of the molecule optical lens, created by a focused laser beam. We will present calculations of this process and a comparison with our experiments on cold (3 K) carbon disulphide molecules focused by a Nd:YAG laser beam. [1] H.S Chung, B.S Zhao, S.H. Lee \textit{et al., }J. Chem. Phys \textbf{114,} 8293 (2001) [2] B. Friedrich, D. Herschbach, J. Phys. Chem. \textbf{99,} 15686 (1995)   

Cold and ultracold polar molecules

Study of ultracold molecules promises important benefits such as novel control of chemical reactions and molecular collisions, precision measurement of fundamental physical properties, and new methods for quantum information processing and quantum simulations. We undertake two approaches aimed to produce cold, polar molecular samples. In the first approach, we work directly with ground-state polar molecules such as hydroxyl radicals (OH) or formaldehyde molecules (H$_{2}$CO). After Stark deceleration through an inhomogeneously distributed electric field, OH molecules are loaded into a magnetic trap at a density $\sim $3 x 10$^{5}$ cm$^{-3}$ and temperature of 50 mK. An important advantage of magnetically trapping OH molecules is the freedom in applying an external electric field without significantly affecting the trap dynamics. The open geometry of the trap will enable experimental studies of cold, dipolar collisions subject to an external electric field. We will report our latest progress towards this goal. In the second approach we explore the possibility of producing ultracold polar molecules via association of two different atoms from ultracold atom gas mixtures near quantum degeneracy. Specifically, an interspecies Feshbach resonance between bosonic $^{87}$Rb and fermionic $^{40}$K permits efficient creations of heteronuclear Feshbach molecules. Subsequent optical spectroscopy reveals promising paths to efficiently transfer populations from the weakly bound to more deeply bound states. Progress on the production of these ultracold fermionic polar molecules will be reported.   

Experimental demonstration of electrostatic surface guiding for cold polar molecules

We demonstrate an electrostatic surface guiding for cold polar molecules over a long distance of 44.5 cm on a substrate by using a hollow electrostatic field, which is generated by two parallel charged wires and a grounded metal-plate. We measure the transverse spatial distribution of the guided supersonic D$_{2}$O (including CH$_{3}$Br) molecular beam and its longitudinal velocity one, and study the dependence of the relative guiding efficiency and the transverse temperature of the guided molecular beam on the guiding voltage, also perform Mote-Carlo simulations and theoretical studies for the molecular guiding process, and our guiding scheme has some potential applications in molecule optics, such as molecular-beam splitter, integrated molecular optics, etc.   

Rotational Spectra of Methane in Helium-4

We extend correlated basis function (CBF) theory, in combination with diffusion Monte Carlo simulations, to spherical top molecules solvated in superfluid $^4$He droplets. Similarly to our previous CBF work on linear molecules, the rotational excitations of a spherical top molecule are renormalized by a self energy which contains the $^4$He density modulation around the molecule as coupling. Due to the high symmetry of this density in the case of solvated spherical molecules the rotation-$^4$He coupling turns out to be weak, and the corresponding reduction of the effective rotational constant is small. Furthermore, unlike for linear molecules, for spherical top molecules the symmetry of the gas phase rotational spectrum is not preserved. Instead, for excitations of total angular momentum $J\ge 2$, we find that the self energy induces a splitting of the rotational energies. We present results for the rotational spectrum of solvated CH$_4$ and CD$_4$ for several available He-CH$_4$ potential energy surfaces, and compare with experimental results measured recently. We propose to measure spectra of partially substituted methane (e.g. CDH$_3$) to investigate the effect of symmetry breaking on the rotation-$^4$He coupling strength.   

Water containing molecular complexes studied by superfluid helium droplet spectroscopy

Superfluid helium droplets offer an ideal environment for spectroscopic studies of molecular complexes by virtue of the controllable aggregation process of embedded molecules and its weak interaction as a matrix medium. Here, we report the infrared spectroscopy of Ar-H$_{2}$O, N$_{2}$-H$_{2}$O, and O$_{2}$-H$_{2}$O complexes picked up in He droplets. The observed spectra in the anti-symmetric stretching vibrational region ($\nu_{3}$) of water around 3750 cm$^{-1}$ indicated that the water molecule in complexes rotates nearly freely in Ar-H$_{2}$O and O$_{2}$- H$_{2}$O, while not in N$_{2}$-H$_{2}$O. The spectra of Ar-H$_{2}$O and O$_{2}$- H$_{2}$O exhibited the splitting of the rotational lines, which is due to the anisotropy of their intermolecular potential. We have analyzed the observed splittings in the spectra to determine the intermolecular potentials of Ar-H$_{2}$O and O$_{2}$- H$_{2}$O in helium droplets. These results are compared with the corresponding potentials previously studied in both experimentally and theoretically.   

Microwave spectroscopy of doped helium clusters and doped helium droplets

High resolution microwave and infrared spectroscopy of small to medium sized doped helium clusters, e.g. He$_{N}$-OCS with $N$ from 2 to 70, has given detailed insights into how superfluidity, a bulk phase property, evolves from the microscopic scale. Some of the most significant findings were oscillatory behavior of cluster rotational constant $B$ with number of helium atoms, $N$, and the observation of very narrow lines (15 kHz in microwave and 0.001 cm$^{-1}$ in the infrared region), even for the largest $N$. How can this be reconciled with the broad (up to several GHz wide) lines of rotational and ro-vibrational transitions of molecular dopants in helium droplets? Microwave experiments of molecular dopants embedded in helium nanodroplets can help answer this question. We have measured the pure tunneling inversion transition of ammonia in helium droplets at about 20.7 GHz. A complex line shape, consisting of a sharp (15 MHz wide) line on top of a broad background (1.5 GHz wide) was observed. The line shape could be simulated by assuming identical energy sublevels of the initial and final state of the transition. This provides direct evidence for the existence of an energy level substructure of molecular states in doped helium droplets. Microwave rotational transitions of carbonylsulfide, OCS, in helium droplets show increase in line width with increasing rotational quantum number $J$ and, in some cases, prominent fine-structures. Some of these features can be interpreted in terms of droplet size distribution.