Direct observation of coherence transfer and rotational-to-vibrational energy exchange in optically centrifuged CO2 super-rotors.

Optical centrifuges are laser-based molecular traps that can rotationally accelerate molecules to energies rivalling or exceeding molecular bond energies. Here we report time and frequency-resolved ultrafast coherent Raman measurements of optically centrifuged CO2 at 380 Torr spun to energies beyond its bond dissociation energy of 5.5 eV (Jmax = 364, Erot = 6.14 eV, Erot/kB = 71, 200 K). The entire rotational ladder from J = 24 to J = 364 was resolved simultaneously which enabled a more accurate measurement of the centrifugal distortion constants for CO2. Remarkably, coherence transfer was directly observed, and time-resolved, during the field-free relaxation of the trap as rotational energy flowed into bending-mode vibrational excitation. Vibrationally excited CO2 (ν2 > 3) was observed in the time-resolved spectra to populate after 3 mean collision times as a result of rotational-to-vibrational (R-V) energy transfer. Trajectory simulations show an optimal range of J for R-V energy transfer. Dephasing rates for molecules rotating up to 5.5 times during one collision were quantified. Very slow decays of the vibrational hot band rotational coherences suggest that they are sustained by coherence transfer and line mixing.

Class II malocclusion correction with Invisalign: Is it possible?

INTRODUCTION: This research aimed to determine whether Class II malocclusion can be treated with clear aligners after completing treatment with the initial set of aligners. METHODS: A sample of 80 adult patients were divided into Group 1 with Class I molar malocclusions (n = 40 [11 men and 29 women]; 38.70 ± 15.90 years) and Group 2 with Class II molar malocclusions (n = 40 [11 men and 29 women]; 35.25 ± 15.21 years). All patients had finished treatment with the initial set of Invisalign aligners (Align Technology, Santa Jose, Calif) without known centric occlusion-centric relation discrepancies, issues of compliance, or overcorrection. The 7 measurements using the American Board of Orthodontics (ABO) Model Grading System and millimetric measurements for anteroposterior (AP) and vertical dimensions were assessed and compared between the 2 groups at pretreatment, posttreatment ClinCheck (Align Technology) prediction, and posttreatment. RESULTS: No improvements were observed in the AP correction. The amount of AP correction in patients with Class II malocclusion was 6.8% of the predicted amount. The amount of overbite correction achieved was 28.8% and 38.9% of the predicted amounts in patients with Class I and Class II malocclusion, respectively. Significant improvements in alignment and interproximal contact scores were observed, with only slight improvements in total ABO scores. An increase in mean occlusal contacts score was observed after treatment. No patient with Class II malocclusions would meet the ABO standards after Invisalign treatment. CONCLUSIONS: The Invisalign system successfully achieves certain tooth movements but fails to achieve other movements predictably. No significant Class II correction or overjet reduction was observed with elastics for an average of 7-month duration in the adult population. Additional refinements may be necessary to address problems created during treatment, as evidenced by a posterior open bite incidence.

1-D imaging of rotation-vibration non-equilibrium from pure rotational ultrafast coherent anti-Stokes Raman scattering.

We present one-dimensional (1-D) imaging of rotation-vibration non-equilibrium measured by two-beam pure rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS). Simultaneous measurements of the spatial distribution of molecular rotation-vibration non-equilibrium are critical for understanding molecular energy transfer in low temperature plasmas and hypersonic flows. However, non-equilibrium CARS thermometry until now was limited to point measurements. The red shift of rotational energy levels by vibrational excitation was used to determine the rotational and vibrational temperatures from 1-D images of the pure rotational spectrum. Vibrational temperatures up to 5500 K were detected in a CH4/N2 nanosecond-pulsed pin-to-pin plasma within 2 mm near the cathode. This approach enables study of non-equilibrium systems with 40 µm spatial resolution.

Generation of narrowband pulses from chirped broadband pulse frequency mixing.

We extend an approach based upon sum-frequency generation of oppositely chirped pulses to narrow the bandwidths of broadband femtosecond pulses. We efficiently generate near-transform-limited pulses with durations of several picoseconds, while reducing the pulse bandwidth by a factor of 120, which is more than twice the reduction reported in previous literature. Such extreme bandwidth narrowing of a broadband pulse enhances the effects of dispersion nonlinearities. Precise chirp control enables us to characterize the efficacy of frequency mixing broadband pulses with nonlinear temporal chirps. We demonstrate the use of these narrowband pulses as probes in coherent anti-Stokes Raman spectroscopy.

Pure-rotational H2 thermometry by ultrabroadband coherent anti-Stokes Raman spectroscopy.

Coherent anti-Stokes Raman spectroscopy (CARS) is a sensitive technique for probing highly luminous flames in combustion applications to determine temperatures and species concentrations. CARS thermometry has been demonstrated for the vibrational Q-branch and pure-rotational S-branch of several small molecules. Practical advantages of pure-rotational CARS, such as multi-species detection, reduction of coherent line mixing and collisional narrowing even at high pressures, and the potential for more precise thermometry, have motivated experimental and theoretical advances in S-branch CARS of nitrogen (N2), for example, which is a dominant species in air-fed combustion processes. Although hydrogen (H2) is of interest given its prevalence as a reactant and product in many gas-phase reactions, laser bandwidth limitations have precluded the extension of CARS thermometry to the H2 S-branch. We demonstrate H2 thermometry using hybrid femtosecond/picosecond pure-rotational CARS, in which a broadband pump/Stokes pulse enables simultaneous excitation of the set of H2 S-branch transitions populated at flame temperatures over the spectral region of 0-2200 cm-1. We present a pure-rotational H2 CARS spectral model for data fitting and compare extracted temperatures to those from simultaneously collected N2 spectra in two systems of study: a heated flow and a diffusion flame on a Wolfhard-Parker slot burner. From 300 to 650 K in the heated flow, the H2 and N2 CARS extracted temperatures are, on average, within 2% of the set temperature. For flame measurements, the fitted H2 and N2 temperatures are, on average, within 5% of each other from 300 to 1600 K. Our results confirm the viability of pure-rotational H2 CARS thermometry for probing combustion reactions.

Design, synthesis, and evaluation of NO-donor containing carbonic anhydrase inhibitors to lower intraocular pressure.

The antiglaucoma drugs dorzolamide (1) and brinzolamide (2) lower intraocular pressure (IOP) by inhibiting the carbonic anhydrase (CA) enzyme to reduce aqueous humor production. The introduction of a nitric oxide (NO) donor into the alkyl side chain of dorzolamide (1) and brinzolamide (2) has led to the discovery of NO-dorzolamide 3a and NO-brinzolamide 4a, which could lower IOP through two mechanisms: CA inhibition to decrease aqueous humor secretion (reduce inflow) and NO release to increase aqueous humor drainage (increase outflow). Compounds 3a and 4a have shown improved efficacy of lowering IOP in both rabbits and monkeys compared to brinzolamide (2).

Split-probe hybrid femtosecond/picosecond rotational CARS for time-domain measurement of S-branch Raman linewidths within a single laser shot.

We introduce a multiplex technique for the single-laser-shot determination of S-branch Raman linewidths with high accuracy and precision by implementing hybrid femtosecond (fs)/picosecond (ps) rotational coherent anti-Stokes Raman spectroscopy (CARS) with multiple spatially and temporally separated probe beams derived from a single laser pulse. The probe beams scatter from the rotational coherence driven by the fs pump and Stokes pulses at four different probe pulse delay times spanning 360 ps, thereby mapping collisional coherence dephasing in time for the populated rotational levels. The probe beams scatter at different folded BOXCARS angles, yielding spatially separated CARS signals which are collected simultaneously on the charge coupled device camera. The technique yields a single-shot standard deviation (1σ) of less than 3.5% in the determination of Raman linewidths and the average linewidth values obtained for N(2) are within 1% of those previously reported. The presented technique opens the possibility for correcting CARS spectra for time-varying collisional environments in operando.

Communication: Simplified two-beam rotational CARS signal generation demonstrated in 1D.

We explore a novel phase matching scheme for gas-phase rotational coherent anti-Stokes Raman spectroscopy (CARS). The scheme significantly simplifies the employment of the technique in general. Two laser beams, one broadband and one narrowband, are crossed at arbitrary angle and the generated rotational CARS signal, copropagating with the probe beam, is isolated using a polarization gating technique. The effect of phase-vector mismatch for various experimental implementations was measured experimentally and compared to calculations. The spatial resolution of the current technique is improved by more than an order of magnitude over standard gas-phase CARS experimental arrangements, providing an interaction length of less than 50 µm when desired. Both the pump and Stokes photons originate from the broadband pulse, and are therefore automatically overlapped temporally and spatially. Significantly improved signal levels are achieved because of both the ease of alignment and the higher pulse energy available to the pump and Stokes fields. We demonstrate the technique for single-laser-shot 1D rotational CARS signal generation over approximately a 1 cm field in a flame.

400 µJ 79 ns amplified pulses from a Q-switched fiber laser using an Yb(3+)-doped fiber saturable absorber.

We report a passively Q-switched all-fiber laser using a large mode area (LMA) Yb(3+)-doped fiber cladding-pumped at 915 nm and an unpumped single-mode Yb(3+)-doped fiber as the saturable absorber (SA). The saturable absorber fiber and gain fiber were coupled with a free-space telescope to optimize the coupling efficiency between the disparate fibers, preferentially bleaching the SA fiber before gain depletion in the pumped fiber. Using this scheme we first demonstrate a Q-switched oscillator with 40 µJ 79 ns pulses at 1026 nm, and show that pulses can be generated from 1020 nm to 1040 nm. The associated peak power of the oscillator alone is more than two orders of magnitude larger than that reported in previous experimental studies using an Yb(3+)-doped fiber as a saturable absorber. We further demonstrate an amplified pulse energy of 0.4 mJ using an Yb(3+)-doped cladding pumped fiber amplifier. Experimental studies in which the saturable absorber length, pump times, and wavelengths are independently varied reveal the impact of these parameters on laser performance.

High-power all-fiber passively Q-switched laser using a doped fiber as a saturable absorber: numerical simulations.

We report a design for a power-scalable all-fiber passively Q-switched laser that uses a large mode area Yb-doped fiber as a gain medium adiabatically tapered to an unpumped single-mode Yb-doped fiber, which serves as a saturable absorber. Through the use of a comprehensive numerical simulator, we demonstrate a passively Q-switched 1030 nm pulsed laser with 14 ns pulse duration and 0.5 mJ pulse energy operating at 200 kHz repetition rate. The proposed configuration has a potential for orders of magnitude of improvement in both the pulse energies and durations compared to the previously reported result. The key mechanism for this improvement relates to the ratio of the core areas between the pumped inverted large mode area gain fiber and the unpumped doped single-mode fiber.

Quantitative one-dimensional imaging using picosecond dual-broadband pure-rotational coherent anti-Stokes Raman spectroscopy.

We employ picosecond dual-broadband pure-rotational coherent anti-Stokes Raman spectroscopy (CARS) in a one-dimensional (1D) imaging configuration. Temperature and O(2):N(2) concentration ratios are measured along a 1D line of up to 12 mm in length. The images consist of up to 330 individual rotational CARS (RCARS) spectra, corresponding to 330 spatially resolved volume elements in the probe volume. Signal levels are sufficient for the collection of single-laser-pulse images at temperatures of up to approximately 1200 K and shot-averaged images at flame temperatures, demonstrated at 2100 K. The precision of picosecond pure-rotational 1D imaging CARS is assessed by acquiring a series of 100 single-laser-pulse images in a heated flow of N(2) from 410 K-1200 K and evaluating a single volume element for temperature in each image. Accuracy is demonstrated by comparing temperatures from the evaluated averaged spectra to thermocouple readings in the heated flow. Deviations from the thermocouple of <30 K in the evaluated temperature were found at up to 1205 K. Accuracy and single-shot precision are compared to those reported for single-point nanosecond dual-broadband pure-RCARS and nanosecond 1D vibrational CARS.

Suppression of Raman-resonant interferences in rotational coherent anti-Stokes Raman spectroscopy using time-delayed picosecond probe pulses.

We measure time-dependent pure-rotational coherent anti-Stokes Raman spectroscopy (CARS) spectra for room-temperature N(2), O(2), CO(2), C(2)H(4), and C(3)H(8), as well as in a C(3)H(8) diffusion flame, using picosecond lasers. Because Raman coherences for N(2) and O(2) persist significantly longer than those for the other species, delayed probing can significantly reduce unwanted resonant contributions to rotational coherent anti-Stokes Raman spectroscopy spectra, enabling temperature and relative O(2)/N(2) concentration determination in fuel-rich gas mixtures. Delayed probing also eliminates interference from smeared vibrational CARS. Probe delay affects both the temperature and relative O(2)/N(2) concentrations inferred from rotational spectra when using a standard frequency-domain analysis.

Picosecond time-resolved pure-rotational coherent anti-Stokes Raman spectroscopy for N(2) thermometry.

Time-resolved pure-rotational coherent anti-Stokes Raman spectroscopy using picosecond-duration laser pulses is investigated for gas thermometry. The use of picosecond laser pulses significantly reduces background caused by scattering of the probe beam, and delayed probing of the Raman coherence enables elimination of interference from nonresonant four-wave mixing processes. Temperatures inferred from rotational spectra are sensitive to the probe delay because of the rotational-level dependence of collisional dephasing of Raman coherences. The sensitivity decreases, however, with increasing temperature, and accurate temperature measurements in a flame are demonstrated using a standard frequency-domain analysis of the spectra.

Collisional quenching of NO A 2Sigma+(v' = 0) between 125 and 294 K.

We report measurements of the temperature-dependent cross sections for the quenching of fluorescence from the A (2)Sigma(+)(v(')=0) state of NO for temperatures between 125 and 294 K. Thermally averaged cross sections were measured for quenching by NO(X (2)Pi), N(2), O(2), and CO in a cryogenically cooled gas flow cell. Picosecond laser-induced fluorescence was time resolved, and the thermally averaged quenching cross sections were determined from the dependence of the fluorescence decay rate on the quencher-gas pressure. These measurements extend to lower temperature the range of previously published results for NO and O(2) and constitute the first reported measurements of the N(2) and CO cross sections for temperatures below 294 K. Between 125 and 294 K, a negative temperature dependence is observed for quenching by NO, O(2), and CO, implicating collision-complex formation in all three cases. Over the same temperature range, a constant, nonzero cross section is measured for quenching by N(2). Updated empirical models for the temperature dependence of the cross sections between 125 and 4500 K are recommended based on weighted least-squares fits to the current low-temperature results and previously published measurements at higher temperature. The results of over 250 measurements presented here indicate that the collisionless lifetime of NO A (2)Sigma(+)(v(')=0) is approximately 192 ns.

Comparison of nanosecond and picosecond excitation for interference-free two-photon laser-induced fluorescence detection of atomic hydrogen in flames.

Two-photon laser-induced fluorescence (TP-LIF) line imaging of atomic hydrogen was investigated in a series of premixed CH4/O2/N2, H2/O2, and H2/O2/N2 flames using excitation with either picosecond or nanosecond pulsed lasers operating at 205 nm. Radial TP-LIF profiles were measured for a range of pulse fluences to determine the maximum interference-free signal levels and the corresponding picosecond and nanosecond laser fluences in each of 12 flames. For an interference-free measurement, the shape of the TP-LIF profile is independent of laser fluence. For larger fluences, distortions in the profile are attributed to photodissociation of H2O, CH3, and/or other combustion intermediates, and stimulated emission. In comparison with the nanosecond laser, excitation with the picosecond laser can effectively reduce the photolytic interference and produces approximately an order of magnitude larger interference-free signal in CH4/O2/N2 flames with equivalence ratios in the range of 0.5< or =Phi< or =1.4, and in H2/O2 flames with 0.3< or =Phi< or =1.2. Although photolytic interference limits the nanosecond laser fluence in all flames, stimulated emission, occurring between the laser-excited level, H(n=3), and H(n=2), is the limiting factor for picosecond excitation in the flames with the highest H atom concentration. Nanosecond excitation is advantageous in the richest (Phi=1.64) CH4/O2/N2 flame and in H2/O2/N2 flames. The optimal excitation pulse width for interference-free H atom detection depends on the relative concentrations of hydrogen atoms and photolytic precursors, the flame temperature, and the laser path length within the flame.

Total synthesis of leucascandrolide a: a new application of the Mukaiyama aldol-Prins reaction.

A total synthesis of the marine natural product leucascandrolide A has been completed. The titanium tetrabromide-mediated Mukaiyama aldol-Prins (MAP) reaction with aldehydes developed in our group provided a highly convergent and stereoselective method for assembling the core of the molecule. A new class of MAP reactions with acetals is introduced, and mechanistic considerations for both MAP methods are described. The total synthesis was completed by coupling of the side chain through two avenues: a known Still-Gennari olefination and a new Z-selective aldol/dehydroselenation reaction. Both procedures were highly selective and provided the natural product.

Influence of connectivity and porosity on ligand-based luminescence in zinc metal-organic frameworks.

Applications of metal-organic frameworks (MOFs) require close correlation between their structure and function. We describe the preparation and characterization of two zinc MOFs based on a flexible and emissive linker molecule, stilbene, which retains its luminescence within these solid materials. Reaction of trans-4,4'-stilbene dicarboxylic acid and zinc nitrate in N,N-dimethylformamide (DMF) yielded a dense 2-D network, 1, featuring zinc in both octahedral and tetrahedral coordination environments connected by trans-stilbene links. Similar reaction in N,N-diethylformamide (DEF) at higher temperatures resulted in a porous, 3-D framework structure, 2. This framework consists of two interpenetrating cubic lattices, each featuring basic zinc carboxylate vertices joined by trans-stilbene, analogous to the isoreticular MOF (IRMOF) series. We demonstrate that the optical properties of both 1 and 2 correlate with the local ligand environments observed in the crystal structures. Steady-state and time-resolved spectroscopic measurements reveal that the stilbene linkers in the dense structure 1 exhibit a small degree of interchromophore coupling. In contrast, the stilbenoid units in 2 display very little interaction in this low-density 3-D framework, with excitation and emission spectra characteristic of monomeric stilbenes, similar to the dicarboxylic acid in dilute solution. In both cases, the rigidity of the stilbene linker increases upon coordination to the inorganic units through inhibition of torsion about the central ethylene bond, resulting in luminescent crystals with increased emission lifetimes compared to solutions of trans-stilbene. The emission spectrum of 2 is found to depend on the nature of the incorporated solvent molecules, suggesting use of this or related materials in sensor applications.

Temperature- and species-dependent quenching of NO A 2Sigma+(v'=0) probed by two-photon laser-induced fluorescence using a picosecond laser.

We report improved measurements of the temperature-dependent cross sections for the quenching of fluorescence from the A 2Sigma+(v'=0) state of NO. Cross sections were measured for gas temperatures ranging from 294 to 1300 K for quenching by NO(X (2)Pi), H(2)O, CO(2), O(2), CO, N(2), and C(2)H(2). The A 2Sigma+(v'=0) state was populated via two-photon excitation with a picosecond laser at 454 nm, and the decay rate of the fluorescence originating from A 2Sigma+(v'=0) was measured directly. Thermally averaged quenching cross sections were determined from the dependence of the fluorescence decay rate on the quencher gas pressure. Our measurements are compared to previous measurements and models of the quenching cross sections, and new empirical fits to the data are presented. Our new cross-section data enable predictions in excellent agreement with prior measurements of the fluorescence lifetime in an atmospheric-pressure methane-air diffusion flame. The agreement resolves discrepancies between the lifetime measurements and predictions based on the previous quenching models, primarily through improved models for the quenching by H(2)O, CO(2), and O(2) at temperatures less than 1300 K.

Branching ratios for quenching of nitric oxide A 2Sigma+ (nu' = 0) to X 2Pi(nu' = 0).

We describe experiments designed to measure the fraction of nitric oxide molecules that undergo quenching from A 2Sigma+ (nu' = 0) directly to X 2Pi(nu" = 0). This quenching channel was investigated for room temperature collisions with O2, CO, CO2, and H2O by measuring recovery of the ground-state population following intense laser excitation. Experiments were conducted in a room temperature flow cell containing dilute mixtures of NO, N2, and the quenching gases. An intense nanosecond laser pulse, tuned to the NO A 2Sigma(+) - X 2Pi(0,0) Q11 + pQ21 bandhead at 226.3 nm, depopulated more than 20% of the equilibrium population in the X 2Pi(nu'' = 0) manifold. A weak, time-delayed, picosecond laser pulse, tuned to the A 2Sigma(+) - 2Pi(1,0) Q11 + pQ21 bandhead at 214.9 nm, probed recovery of population in X(nu'' = 0) via subsequent LIF for each of the investigated quenchers. Remarkably large branching ratios were observed for direct quenching to X 2Pi(nu'' = 0). Water, carbon monoxide, and oxygen quench NO A 2Sigma+ (v' = 0) to X 2Pi(nu" = 0) with branching ratios that are approximately 0.3. The significantly higher branching ratio for quenching by carbon dioxide is 0.6. The results provide insight on the NO quenching process and represent an important step toward a detailed understanding of the effects of collisional energy transfer on saturated laser-induced fluorescence, which is necessary to properly model detection strategies based on high laser fluences.