Spin-Polarized Hydrogen Depolarization Rates at High Hydrogen Halide Pressures: Hyperfine Depolarization via the HY-H Complex.

We measure the magnetization quantum beats of spin-polarized hydrogen (SPH) and spin-polarized deuterium (SPD) with a pickup coil, from the UV photodissociation of HCl, HBr, and DI, in the 5-5000 mbar pressure range. The pressure-dependent depolarization rate is linear at low pressures and reaches a plateau at higher pressures. The high-pressure depolarization rate is observed to be proportional to the halogen nuclear electric quadrupole coupling constant. We also investigate how the presence of an inert gas, SF6 or N2, affects the depolarization rate. The results are explained using a model in which depolarization occurs predominantly through an HY-H intermediate species (Y = Cl, Br, I).

Photofragment spin-polarization measurements via magnetization quantum beats: dynamics of DI photodissociation.

We report the electron-spin polarization of D atoms from the photodissociation of DI, at 213 nm and 266 nm, by measuring the magnetization quantum beats of the D atoms with a pick-up coil. We determine that the polarization P is large at both wavelengths (|P|∼ 1), however it is positive at 213 nm, and negative at 266 nm. These results, in both cases, are of opposite sign to calculations, which assume adiabatic dissociation along the A1Π1 or a3Π1 states. We interpret these results as evidence that nonadiabatic coupling between these states needs to be included for the theoretical treatment of DI photodissociation.

Ultrahigh-Density Spin-Polarized H and D Observed via Magnetization Quantum Beats.

We measure nuclear and electron spin-polarized H and D densities of at least 10^{19} cm^{-3} with ∼10 ns lifetimes, from the photodissociation of HBr and DI with circularly polarized UV light pulses. This density is ∼6 orders of magnitude higher than that produced by conventional continuous-production methods and, surprisingly, at least 100 times higher than expected densities for this photodissociation method. We observe the hyperfine quantum beating of the H and D magnetization with a pickup coil, i.e., the respective 0.7 and 3 ns periodic transfer of polarization from the electrons to the nuclei and back. The 10^{19} cm^{-3} spin-polarized H and D density is sufficient for laser-driven ion acceleration of spin-polarized electrons, protons, or deuterons, the preparation of nuclear-spin-polarized molecules, and the demonstration of spin-polarized D-T or D-^{3}He laser fusion, for which a reactivity enhancement of ∼50% is expected.

Recoil Inversion in the Photodissociation of Carbonyl Sulfide near 234 nm.

We report the observation of recoil inversion of the CO (v=0, J_{CO}=66) state in the UV dissociation of lab-frame oriented carbonyl sulfide (OCS). This state is ejected in the opposite direction with respect to all other (>30) states and in absence of any OCS rotation, thus resulting in spatial filtering of this particular high-J rovibrational state. This inversion is caused by resonances occurring in shallow local minima of the molecular potential, which bring the sulfur closer to the oxygen than the carbon atom, and is a striking example where such subtleties severely modify the photofragment trajectories. The resonant behavior is observed only in the photofragment trajectories and not in their population, showing that stereodynamic measurements from oriented molecules offer an indispensable probe for exploring energy landscapes.

Highly Nuclear-Spin-Polarized Deuterium Atoms from the UV Photodissociation of Deuterium Iodide.

We report a novel highly spin-polarized deuterium (SPD) source, via the photodissociation of deuterium iodide at 270 nm. I(^{2}P_{3/2}) photofragments are ionized with m-state selectivity, and their velocity distribution measured via velocity-map slice imaging, from which the D polarization is determined. The process produces ∼100% electronically polarized D at the time of dissociation, which is then converted to ∼60% nuclear D polarization after ∼1.6 ns. These production times for SPD allow collision-limited densities of ∼10^{18} cm^{-3} and at production rates of ∼10^{21} s^{-1} which are 10^{6} and 10^{4} times higher than conventional (Stern-Gerlach separation) methods, respectively. We discuss the production of SPD beams, and combining high-density SPD with laser fusion, to investigate polarized D-T, D-^{3}He, and D-D fusion.

Chiral cavity ring down polarimetry: Chirality and magnetometry measurements using signal reversals.

We present the theory and experimental details for chiral-cavity-ring-down polarimetry and magnetometry, based on ring cavities supporting counterpropagating laser beams. The optical-rotation symmetry is broken by the presence of both chiral and Faraday birefringence, giving rise to signal reversals which allow rapid background subtractions. We present the measurement of the specific rotation at 800 nm of vapors of α-pinene, 2-butanol, and α-phellandrene, the measurement of optical rotation of sucrose solutions in a flow cell, the measurement of the Verdet constant of fused silica, and measurements and theoretical treatment of evanescent-wave optical rotation at a prism surface. Therefore, these signal-enhancing and signal-reversing methods open the way for ultrasensitive polarimetry measurements in gases, liquids and solids, and at surfaces.

Evanescent-wave and ambient chiral sensing by signal-reversing cavity ringdown polarimetry.

Detecting and quantifying chirality is important in fields ranging from analytical and biological chemistry to pharmacology and fundamental physics: it can aid drug design and synthesis, contribute to protein structure determination, and help detect parity violation of the weak force. Recent developments employ microwaves, femtosecond pulses, superchiral light or photoionization to determine chirality, yet the most widely used methods remain the traditional methods of measuring circular dichroism and optical rotation. However, these signals are typically very weak against larger time-dependent backgrounds. Cavity-enhanced optical methods can be used to amplify weak signals by passing them repeatedly through an optical cavity, and two-mirror cavities achieving up to 10(5) cavity passes have enabled absorption and birefringence measurements with record sensitivities. But chiral signals cancel when passing back and forth through a cavity, while the ubiquitous spurious linear birefringence background is enhanced. Even when intracavity optics overcome these problems, absolute chirality measurements remain difficult and sometimes impossible. Here we use a pulsed-laser bowtie cavity ringdown polarimeter with counter-propagating beams to enhance chiral signals by a factor equal to the number of cavity passes (typically >10(3)); to suppress the effects of linear birefringence by means of a large induced intracavity Faraday rotation; and to effect rapid signal reversals by reversing the Faraday rotation and subtracting signals from the counter-propagating beams. These features allow absolute chiral signal measurements in environments where background subtraction is not feasible: we determine optical rotation from α-pinene vapour in open air, and from maltodextrin and fructose solutions in the evanescent wave produced by total internal reflection at a prism surface. The limits of the present polarimeter, when using a continuous-wave laser locked to a stable, high-finesse cavity, should match the sensitivity of linear birefringence measurements (3 × 10(-13) radians), which is several orders of magnitude more sensitive than current chiral detection limits and is expected to transform chiral sensing in many fields.

Sensitivity enhancement for evanescent-wave sensing using cavity-ring-down ellipsometry.

We demonstrate a method to increase the sensitivity of the s-p phase shift under total internal reflection (TIR) for optical sensing. This is achieved by the introduction of two simple dielectric layers to the TIR surface of a fused silica prism. The enhanced sensitivity is demonstrated using evanescent-wave cavity-ring-down-ellipsometry by measuring the refractive index of liquid mixtures and by studying the adsorption of polymers to the TIR surface of the fused silica prism.

(2+1) laser-induced fluorescence of spin-polarized hydrogen atoms.

We report the measurement of the spin polarization of hydrogen (SPH) atoms by (2+1) laser-induced fluorescence, produced via the photodissociation of thermal HBr molecules with circularly polarized 193 nm light. This scheme, which involves two-photon laser excitation at 205 nm and fluorescence at 656 nm, offers an experimentally simpler polarization-detection method than the previously reported vacuum ultraviolet detection scheme, allowing the detection of SPH atoms to be performed more straightforwardly, from the photodissociation of a wide range of molecules and from a variety of collision experiments.

Broadband lasers to detect and cool the vibration of cold molecules.

By using broadband lasers, we demonstrate the possibilities for control of cold molecules formed via photoassociation. Firstly, we present a detection REMPI scheme (M. Viteau et al., Phys. Rev. A, 2009, 79, 021402) to systematically investigate the mechanisms of formation of ultracold Cs2 molecules in deeply bound levels of their electronic ground state X1sigma(g)+. This broadband detection scheme could be generalized to other molecular species. Then we report a vibrational cooling technique (M. Viteau et al., Science, 2008, 321, 232) through optical pumping obtained by using a shaped mode locked femtosecond laser. The broadband femtosecond laser excites the molecules electronically, leading to a redistribution of the vibrational population in the ground state via a few absorption-spontaneous emission cycles. By removing the laser frequencies corresponding to the excitation of the v = 0 level, we realize a dark state for the so-shaped femtosecond laser, leading, with the successive laser pulses, to an accumulation of the molecules in the v = 0 level, ie. a laser cooling of the vibration. The simulation of the vibrational laser cooling allows us to characterize the criteria to extend the mechanism to other molecular species (R. V. Krems, Int. Rev. Phys. Chem., 2005, 24, 99). We finally discuss the generalization of the technique to laser cooling of the rotation of the molecule.

Time-dependent depolarization of aligned HD molecules.

An aligned sample of HD(v = 1, J = 2, M(J) = 0) molecules is prepared under collision-free conditions using the S(0) stimulated Raman pumping transition. Subsequent coupling to the spins of the deuteron I(D) and the proton I(H) causes the initial degree of alignment to oscillate and decrease as monitored over the time range from 0-13 mus via the O2 line of the [2 + 1] REMPI E,F(1)Sigma-X(1)Sigma (0,1) band. The time dependence of the rotational alignment is also calculated using both a hierarchical coupling scheme in which the rotational angular momentum J is regarded first to couple to I(D), and then the resultant F(i) to couple to I(H), to form the total angular momentum F and a non-hierarchical coupling scheme in which the HD energy level structure is not assumed to be diagonal in the |I(H)(JI(D))F(i)FM(F)> basis set. The experimental data is in good agreement with the non-hierarchical calculation but not with the hierarchical calculation, as expected for this system. Additionally, we calculate the time dependence of the H and D nuclear spin polarizations.

Laser detection of spin-polarized hydrogen from HCl and HBr photodissociation: comparison of H- and halogen-atom polarizations.

Thermal HCl and HBr molecules were photodissociated using circularly polarized 193 nm light, and the speed-dependent spin polarization of the H-atom photofragments was measured using polarized fluorescence at 121.6 nm. Both polarization components, described by the a(0)(1)(perpendicular) and Re[a(1)(1)(parallel, perpendicular)] parameters which arise from incoherent and coherent dissociation mechanisms, are measured. The values of the a(0)(1)(perpendicular) parameter, for both HCl and HBr photodissociation, are within experimental error of the predictions of both ab initio calculations and of previous measurements of the polarization of the halide cofragments. The experimental and ab initio theoretical values of the Re[a(1)(1)(parallel, perpendicular)] parameter show some disagreement, suggesting that further theoretical investigations are required. Overall, good agreement occurs despite the fact that the current experiments photodissociate molecules at 295 K, whereas previous measurements were conducted at rotational temperatures of about 15 K.

Preparation of oriented and aligned H(2) and HD by stimulated Raman pumping.

Stimulated Raman pumping has been used to prepare oriented and aligned samples of H(2)(nu=1,J=1,2,3) and HD(nu=1,J=2) under collision-free conditions using the (1,0) S(0), S(1), Q(1), Q(2), and O(3) lines. The M-sublevel anisotropies were interrogated by polarized [2+1] resonance-enhanced multiphoton ionization via the (0,1) O(2), O(3), and S(1) lines of the E,F (1)Sigma(g) (+)-X (1)Sigma(g) (+) system. The optical excitation schemes employed in this study generate highly oriented and aligned molecular ensembles. We show that the H(2)(nu=1,J=2,M=0) and H(2)(nu=1,J=2,M=2) samples retain their initial polarization for greater than 100 ns and are therefore suitable candidates for targets or projectiles in future scattering experiments.

Optical control of ground-state atomic orbital alignment: Cl(2P3/2) atoms from HCl(v=2,J=1) photodissociation.

H(35)Cl(v=0,J=0) molecules in a supersonic expansion were excited to the H(35)Cl(v=2,J=1,M=0) state with linearly polarized laser pulses at about 1.7 microm. These rotationally aligned J=1 molecules were then selectively photodissociated with a linearly polarized laser pulse at 220 nm after a time delay, and the velocity-dependent alignment of the (35)Cl((2)P(32)) photofragments was measured using 2+1 REMPI and time-of-flight mass spectrometry. The (35)Cl((2)P(32)) atoms are aligned by two mechanisms: (1) the time-dependent transfer of rotational polarization of the H(35)Cl(v=2,J=1,M=0) molecule to the (35)Cl((2)P(32)) nuclear spin [which is conserved during the photodissociation and thus contributes to the total (35)Cl((2)P(32)) photofragment atomic polarization] and (2) the alignment of the (35)Cl((2)P(32)) electronic polarization resulting from the photoexcitation and dissociation process. The total alignment of the (35)Cl((2)P(32)) photofragments from these two mechanisms was found to vary as a function of time delay between the excitation and the photolysis laser pulses, in agreement with theoretical predictions. We show that the alignment of the ground-state (35)Cl((2)P(32)) atoms, with respect to the photodissociation recoil direction, can be controlled optically. Potential applications include the study of alignment-dependent collision effects.