Non-intuitive rotational reorientation in collisions of NO(A 2Σ+) with Ne from direct measurement of a four-vector correlation.

Stereodynamic descriptions of molecular collisions concern the angular correlations that exist between vector properties of the motion of the participating species, including their velocities and rotational angular momenta. Measurements of vector correlations provide a unique view of the forces acting during collisions, and are a stringent test of electronic-structure calculations of molecular interactions. Here, we present direct measurement of the four-vector correlation between initial and final relative velocities and rotational angular momenta in a molecular collision. This property, which quantifies the extent to which a molecule retains a memory of its initial sense of rotation, or handedness, as a function of scattering angle, yields insight into the dynamics of a molecular collision. We report non-intuitive changes in the handedness for specific states and scattering angles, reproduced by classical and quantum scattering calculations. Comparison to calculations on different ab initio potential energy surfaces demonstrates this measurement's exquisite sensitivity to the underlying intermolecular forces.

Perspective: Advanced particle imaging.

Since the first ion imaging experiment [D. W. Chandler and P. L. Houston, J. Chem. Phys. 87, 1445-1447 (1987)], demonstrating the capability of collecting an image of the photofragments from a unimolecular dissociation event and analyzing that image to obtain the three-dimensional velocity distribution of the fragments, the efficacy and breadth of application of the ion imaging technique have continued to improve and grow. With the addition of velocity mapping, ion/electron centroiding, and slice imaging techniques, the versatility and velocity resolution have been unmatched. Recent improvements in molecular beam, laser, sensor, and computer technology are allowing even more advanced particle imaging experiments, and eventually we can expect multi-mass imaging with co-variance and full coincidence capability on a single shot basis with repetition rates in the kilohertz range. This progress should further enable "complete" experiments-the holy grail of molecular dynamics-where all quantum numbers of reactants and products of a bimolecular scattering event are fully determined and even under our control.

Alignment of the hydrogen molecule under intense laser fields.

Alignment of the electronically excited E,F state of the H2 molecule is studied using the velocity mapping imaging technique. Photofragment images of H+ due to the dissociation mechanism that follows the 2-photon excitation into the (E,F; ν = 0, J = 0) electronic state show a strong dependence on laser intensity, which is attributed to the high polarizability anisotropy of the H2 (E,F) state. We observe a marked structure in the angular distribution, which we explain as the interference between the prepared J = 0 and Stark-mixed J = 2 rovibrational states of H2, as the laser intensity increases. Quantification of these effects allows us to extract the polarizability anisotropy of the H2 (E,F J = 0) state yielding a value of 312 ± 82 a.u. (46 Å3). By comparison, CS2 has 10 Å3, I2 has 7 Å3, and hydrochlorothiazide (C7H8ClN3O4S2) has about 25 Å3 meaning that we have created the most easily aligned molecule ever measured, by creating a mixed superposition state that is highly anisotropic in its polarizability.

Rotational alignment of NO (A2Σ+) from collisions with Ne.

We report the direct angle-resolved measurement of collision-induced alignment of short-lived electronically excited molecules using crossed atomic and molecular beams. Utilizing velocity-mapped ion imaging, we measure the alignment of NO in its first electronically excited state (A(2)Σ(+)) following single collisions with Ne atoms. We prepare A(2)Σ(+) (v = 0, N = 0, j = 0.5) and by comparing images obtained using orthogonal linear probe laser polarizations, we experimentally determine the degree of alignment induced by collisional rotational excitation for the final rotational states N' = 4, 5, 7, and 9. The experimental results are compared to theoretical predictions using both a simple classical hard-shell model and quantum scattering calculations on an ab initio potential energy surface (PES). The experimental results show overall trends in the scattering-angle dependent polarization sensitivity that are accounted for by the simple classical model, but structure in the scattering-angle dependence that is not. The quantum scattering calculations qualitatively reproduce this structure, and we demonstrate that the experimental measurements have the sensitivity to critique the best available potential surfaces. This sensitivity to the PES is in contrast to that predicted for ground-state NO(X) alignment.

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.

Communication: direct angle-resolved measurements of collision dynamics with electronically excited molecules: NO(A2Σ+) + Ar.

We report direct doubly differential (quantum state and angle-resolved) scattering measurements involving short-lived electronically excited molecules using crossed molecular beams. In our experiment, supersonic beams of nitric oxide and argon atoms collide at 90°. In the crossing region, NO molecules are excited to the A(2)Σ(+)state by a pulsed nanosecond laser, undergo rotationally inelastic collisions with Ar atoms, and are then detected 400 ns later (approximately twice the radiative lifetime of the A(2)Σ(+)state) by 1 + 1(') multiphoton ionization via the E(2)Σ(+) state. The velocity distributions of the scattered molecules are recorded using velocity-mapped ion imaging. The resulting images provide a direct measurement of the state-to-state differential scattering cross sections. These results demonstrate that sufficient scattering events occur during the short lifetimes typical of molecular excited states (∼200 ns, in this case) to allow spectroscopically detected quantum-state-resolved measurements of products of excited-state collisions.

Cold and ultracold molecules: spotlight on orbiting resonances.

There is great interest in the production of cold molecules, at temperatures below 1 K, and ultracold molecules, at temperatures below 1 mK. Such molecules have potential applications in areas ranging from precision measurement to quantum information storage and processing, and quantum gases of ultracold polar molecules are expected to exhibit novel quantum phases. In addition, cold molecules open up a new domain for collision physics, dominated by long-range forces and scattering resonances. There have been major recent advances both in cooling molecules from room temperature and in forming molecules in ultracold atomic gases. As these techniques mature, and cold and ultracold samples are more accessible, collision studies at previously unavailable energies will be possible. This spotlight article will highlight some of the background and motivation for studying collisions at low energies and will direct readers to recent articles on the recent experimental advancements.

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.

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).

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).

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.

Production of cold ND3 by kinematic cooling.

We have produced translationally cold ammonia (ND3) molecules in various quantum states by kinematic cooling. In these experiments, ND3 molecules are brought nearly to rest in the (J, K) = (2,0), (2,1), (2,2), (3,1), (3,2), and (3,3) rotational levels of the ground vibronic state by rotationally-inelastic collisions with Ne atoms. The cold molecules are produced in quantum-state-dependent velocity distributions whose laboratory frame velocities are measured to be between 21 m s(-1) (E(trans)/k = 530 mk) and 32 m s(-1) (E(trans)/k = 1.2 K), and are calculated to be between 7.5 m s(-1) (E(trans)/k = 70 mK) and 27 m s(-1) (E(trans)/k = 880 mK). Due to systematic experimental effects, the measured velocities are upper limits to the actual velocities. These temperatures are low enough that it should be possible to use electrostatic traps to confine cold molecules in many of these quantum states.

Blast injury of the ear: clinical update from the global war on terror.

The purpose of this study was to describe the effects of blast exposure on hearing status. This study retrospectively analyzed hearing thresholds and otologic complaints for >250 patients with blast-related injuries from the global war on terror. Of patients who received full diagnostic evaluations, 32% reported a history of tympanic membrane perforation, 49% experienced tinnitus, 26% reported otalgia (ear pain), and 15% reported dizziness. Expected hearing thresholds were computed by applying age-correction factors to hearing tests performed earlier in the service members' careers and before their most recent deployment. Expected hearing thresholds were significantly better than actual postdeployment thresholds, indicating that significant changes occurred in the patients' hearing that could not be accounted for by age. Results from this study underline the need for documentation of pre-and postdeployment hearing tests and prompt otologic evaluation for the blast-exposed population.

Modification of the velocity distribution of H(2) molecules in a supersonic beam by intense pulsed optical gradients.

We report the acceleration and deceleration of H(2) molecules in a supersonic molecular beam by means of its interaction with an intense optical gradient from a nanosecond far-off-resonant optical pulse. The strong optical gradients are formed in the interference pattern of two intense optical pulses at 532 nm. The velocity distribution of the molecular beam, before and after the applied optical pulse, is measured by a velocity-mapped ion imaging technique. Changes in velocity up to 202 m s(-1)+/- 61 m s(-1) are observed in a molecular beam initially travelling at a mean speed of 563 m s(-1). We report the dependence of this change in velocity with the strength of the optical gradient applied.

A comparison of the military entrance processing station screening audiogram with the Defense Occupational and Environmental Health Readiness System reference audiogram at Fort Sill, Oklahoma, in 2000.

BACKGROUND: The Department of Defense Hearing Conservation Program requires that a reference audiogram be performed at initial entry training (IET), before noise exposure. In the Army, only Fort Sill, home of the field artillery, and Fort Benning, home of the infantry, are in compliance. All military applicants receive a screening audiogram at a military entrance processing station (MEPS) to qualify for service. This audiogram does not meet the Defense Occupational and Environmental Health Readiness System-Hearing Conservation (DOEHRS-HC) standard. Nevertheless, it has been proposed that the MEPS screen be used as the reference because of limited resources and time during IET medical in-processing. METHODS: A total of 11,816 individual reference audiograms performed at Fort Sill 95th Adjutant General Recruit Reception Center in 2000 were identified in the DOEHRS-HC database. Results of the MEPS screening audiograms were found for 11,311 (96%) of these individuals. The two audiograms were compared by frequency and ear and by using the two Department of Defense criteria for threshold shift. RESULTS: A total of 14.49% (95% confidence interval, 14.48-14.50%) of audiograms using the three-frequency average difference and 23.19% (95% confidence interval, 23.18-23.20%) using the four-frequency difference in either ear demonstrated a threshold shift. The mean difference in intensity between the two audiograms ranged from 5 to 12 dB and varied by frequency and ear, with the greatest differences being seen at 500 and 6,000 kHz and in the left ear, compared with the right ear. The mean threshold level was higher for each frequency in the DOEHRS-HC audiogram, compared with the MEPS audiogram. CONCLUSIONS: Approximately 15% of soldiers at Fort Sill in 2000 showed a clinically significant threshold difference between their MEPS screening and the DOEHRS-HC baseline audiogram. Methodological variations in testing and interval noise-induced hearing loss could account for these differences. The results do not support the use of the MEPS screening audiogram as the reference audiogram. Compliance with the Hearing Conservation Program in the Army would require either improving MEPS testing to DOEHRS-HC standards or performing baseline audiograms at all five IET sites.

An investigation of nonadiabatic interactions in Cl(2Pj) + D2 via crossed-molecular-beam scattering.

We have determined limits on the cross section for both electronically nonadiabatic excitation and quenching in the Cl((2)P(j)) + D(2) system. Our experiment incorporates crossed-molecular-beam scattering with state-selective Cl((2)P(12,32)) detection and velocity-mapped ion imaging. By colliding atomic chlorine with D(2), we address the propensity for collisions that result in a change of the spin-orbit level of atomic chlorine either through electronically nonadiabatic spin-orbit excitation Cl((2)P(32)) + D(2)-->Cl(*)((2)P(12)) + D(2) or through electronically nonadiabatic spin-orbit quenching Cl(*)((2)P(12)) + D(2)-->Cl((2)P(32)) + D(2). In the first part of this report, we estimate an upper limit for the electronically nonadiabatic spin-orbit excitation cross section at a collision energy of 5.3 kcal/mol, which lies above the energy of the reaction barrier (4.9 kcal/mol). Our analysis and simulation of the experimental data determine an upper limit for the excitation cross section as sigma(NA)< or =0.012 A(2). In the second part of this paper we investigate the propensity for electronically nonadiabatic spin-orbit quenching of Cl(*) following a collision with D(2) or He. We perform these experiments at collision energies above and below the energy of the reaction barrier. By comparing the amount of scattered Cl(*) in our images to the amount of Cl(*) lost from the atomic beam we obtain the maximum cross section for electronically nonadiabatic quenching as sigma(NA)< or =15(-15) (+44) A(2) for a collision energy of 7.6 kcal/mol. Our experiments show the probability for electronically nonadiabatic quenching in Cl(*) + D(2) to be indistinguishable to that for the kinematically identical system of Cl(*) + He.