Kevin E. Strecker

Collisions of electronically excited molecules: differential cross-sections for rotationally inelastic scattering of NO(A2Σ+) with Ar and He

The paper reports experimental measurements and theoretical calculations of rotational-state-resolved differential scattering cross-sections (DCS) for collisions between electronically excited NO(A2Σ+) molecules and rare gas atoms. The experimental NO(A2Σ+) + Ar and NO(A2Σ+) + He state-resolved product scattering distributions are determined using velocity-mapped ion imaging. The ion images are analysed to determine the state-resolved DCS, which are compared with new theoretical DCS calculated using quantum scattering methods on ab initio electronic potential energy surfaces. Both collision systems are imaged simultaneously; this constraint on the collision energies of the two systems aids the comparison to theory. The experimental and calculated DCS agree well for the NO(A2Σ+)/He system. For the NO(A2Σ+)/Ar scattering system, the experiments do not recover the degree of forward-scattering theoretically predicted and significant differences in the positions of the observed and predicted rotational rainbow features are apparent at large scattering angles, particularly for the most rotationally inelastic collision channels investigated: ΔN = 12,14.

Imaging the rotationally state-selected NO(A,n) product from the predissociation of the A state of the NO-Ar van der Waals cluster

The origin of the resonant structures in the spectrum of the predissociative part of the A state in the NO-Ar van der Waals cluster has been investigated. We have employed direct excitation to the predissociative part of the NO-Ar A state followed by rotational state selective ionization of the NO fragment. Velocity map imaging of the NO ion yields the recoil energy of the rotational state-selected fragment. A substantial contribution of rotational hotbands to the resonant structures is observed. Our data indicate that a centrifugal barrier as the origin of these resonances can be ruled out. We hypothesize that after the NO-Ar cluster is excited to the A state sufficient mixing within the rotating cluster takes place as it changes geometry from being T shaped in the NO(X)-Ar state to linear in the NO(A)-Ar state. This mixing allows the low energy and high angular momentum (J approximately = 4.5) tumbling motion of the initially populated hotbands in the ground state NO(X)-Ar complex to be converted into NO(A,n = 2) spinning rotation in the A state of the complex. The electronically excited spinning complex falls apart adiabatically producing rotationally excited NO(A,n = 2) at the energetic threshold. This interpretation indicates that the resonances can be attributed to some type of vibrational Feshbach resonance. The appearance energy for the formation of NO(A,n = 0)+Ar is found to be 44294.3+/-1.4 cm(-1).

Differential cross sections for rotational excitation of ND3 by Ne.

We report the first measured differential cross sections for rotationally inelastic collisions between ND(3) and Ne, obtained using velocity-mapped ion imaging. In these experiments, ND(3) molecules initially in the J = 0, K = 0 and J = 1, K = 1 quantum states collide with Ne atoms at a center-of-mass collision energy of 65 meV, leading to rotational excitation of ND(3). Differential cross sections are then determined from images of the rotationally excited scattered molecules using an iterative extraction method. These measurements complement and compare well with previous measurements of differential cross sections for the ammonia-rare gas system (Meyer, H. J. Chem. Phys. 1994, 101, 6697.; Meyer, H. J. Phys. Chem. 1995, 99, 1101.) and are also relevant to the production of cold ND(3) molecules by crossed-beam scattering (Kay, J. J.; van de Meerakker, S. Y. T.; Strecker, K. E.; Chandler, D. W. Faraday Discuss. 2009, DOI: 10.1039/B819256C).

Ultraviolet photodissociation of vinyl iodide: understanding the halogen dependence of photodissociation mechanisms in vinyl halides

The photodissociation of vinyl iodide has been investigated at several wavelengths between 193 and 266 nm using three techniques: time-resolved Fourier transform emission spectroscopy, multiple pass laser absorption spectroscopy, and velocity-mapped ion imaging. The only dissociation channel observed is C-I bond cleavage to produce C2H3 (nu, N) + I (2P(J)) at all wavelengths investigated. Unlike photodissociation of other vinyl halides (C2H3X, X = F, Cl, Br), in which the HX product channel is significant, no HI elimination is observed. The angular and translational energy distributions of I atoms indicate that atomic products arise solely from dissociation on excited states with negligible contribution from internal conversion to the ground state. We derive an upper limit on the C-I bond strength of D0(C2H3-I) < or = 65 kcal mol(-1). The ground-state potential-energy surface of vinyl iodide is explored by ab initio calculations. We present a model in which the highest occupied molecular orbital in vinyl halides has increasing X(np) non-bonding character with increasing halogen mass. This change leads to reduced torsional force around the C-C bond in the excited state. Because the ground-state energy is highest when the CH2 plane is perpendicular to the CHX plane, a reduced torsional force in the excited state correlates with a lower rate for internal conversion compared to excited-state C-X bond fission. This model explains the gradual change in photodissociation mechanisms of vinyl halides from the dominance of internal conversion in vinyl fluoride to the dominance of excited-state dissociation in vinyl iodide.

A compact molecular beam machine

We have developed a compact, low cost, modular, crossed molecular beam machine. The new apparatus utilizes several technological advancements in molecular beams valves, ion detection, and vacuum pumping to reduce the size, cost, and complexity of a molecular beam apparatus. We apply these simplifications to construct a linear molecular beam machine as well as a crossed-atomic and molecular beam machine. The new apparatus measures almost 50 cm in length, with a total laboratory footprint less than 0.25 m(2) for the crossed-atomic and molecular beam machine. We demonstrate the performance of the apparatus by measuring the rotational temperature of nitric oxide from three common molecular beam valves and by observing collisional energy transfer in nitric oxide from a collision with argon.

Dual-etalon frequency-comb cavity ringdown spectrometer

We have demonstrated a spectroscopic technique for simultaneously obtaining broad spectral bandwidth and high frequency resolution absorption measurements, with 5 μs temporal resolution, continuously for tens of microseconds in an apparatus with no active stabilization. The technique utilizes two passive air-gap etalons to imprint two frequency comb patterns onto a single pulsed light source. The air-gap etalons also serve as cavity ringdown cells increasing the sensitivity of the absorption spectroscopy by increasing the interrogation path length. Here, we demonstrate the operation of the spectrometer utilizing a ~0.15 cm(-1) bandwidth pulsed dye laser and two nearly identical 300 MHz free-spectral range confocal air-gap etalons each with a finesse of ~1 × 10(5), to investigate the (1,1,3) overtone of water and the R(7) transition of the O(2) b(1)Σ(g)(+)←X(3)Σ(g)(-) (2,0) band with high spectral resolution.

The Quest for Cold and Ultracold Molecules

Cool molecules: The cooling of molecules to sub-Kelvin temperatures promises to have a great impact in chemistry and physics. Recently, the first experimental realizations of samples of deeply bound molecules that are approaching the ultracold regime were reported. In this contribution, these interesting results are briefly discussed.

The kinematic cooling of molecules with laser-cooled atoms

We propose a new scheme for the production of milli-Kelvin molecules via kinematic cooling through collisions with atoms in a magneto-optical trap (MOT). We will discuss the kinematic conditions necessary for producing cold molecules, the limits of the final attainable temperatures and the experimental implementation of this technique. Finally, we will look at some specific physical systems and discuss the effectiveness of kinematic cooling inside a MOT.

Cold atoms by kinematic cooling

We report the preparation and observation of translationally cold atoms using kinematic cooling. In these experiments, krypton atoms are cooled to subkelvin temperatures by elastic collisions in crossed atomic beams. Two independent velocity measurements indicate an upper-bound mean velocity of 13 m/s (E{sub trans}/k=850 mK) and are consistent with a much lower mean velocity of 4 m/s (E{sub trans}/k=80 mK) (k is Boltzmanns constant). The density of the cold atoms is measured to be 10{sup 9} atoms/cm{sup 3}. Scattering calculations and diffusion models support these velocity and density measurements. The results demonstrate that cold, dense samples of ground-state atoms and molecules can be prepared by elastic collisions between identical collision partners.

Ultra-cold molecule production.

The production of Ultra-cold molecules is a goal of many laboratories through out the world. Here we are pursuing a unique technique that utilizes the kinematics of atomic and molecular collisions to achieve the goal of producing substantial numbers of sub Kelvin molecules confined in a trap. Here a trap is defined as an apparatus that spatially localizes, in a known location in the laboratory, a sample of molecules whose temperature is below one degree absolute Kelvin. Further, the storage time for the molecules must be sufficient to measure and possibly further cool the molecules. We utilize a technique unique to Sandia to form cold molecules from near mass degenerate collisions between atoms and molecules. This report describes the progress we have made using this novel technique and the further progress towards trapping molecules we have cooled.