Collisions Of NH Research Articles

Multichannel Quantum Defect Theory with a Frame Transformation for Ultracold Atom-Molecule Collisions in Magnetic Fields.

We extend the powerful formalism of multichannel quantum defect theory combined with a frame transformation to ultracold atom-molecule collisions in magnetic fields. By solving the coupled-channel equations with hyperfine and Zeeman interactions omitted at short range, the extended theory enables a drastically simplified description of the intricate quantum dynamics of ultracold molecular collisions in terms of a small number of short-range parameters. We apply the formalism to ultracold Mg+NH collisions in a magnetic field, achieving a 10^{4}-fold reduction in computational effort.

Active species densities in R/x%N2 and R/x%(N2-5%H2) (R = Ar or He) microwave early afterglows

Afterglows of R/x%N2 and R/x%(N2-5%H2) (R = Ar or He) flowing microwave discharges are characterized by optical emission spectroscopy. Absolute densities of N-atoms, N2(A) and N2(X,v>13) metastable molecules and N2+ ions and evaluated densities of NH and H are determined after calibration of the N-atom density by NO titration. New results on NH radical and H-atom relative densities are obtained by considering that the excitation of the NH(A) radiative state in the afterglow is produced by N2(X,v>13) + NH collisions. The interest of these results concerns the enhancement of surface nitriding by combined effects of N and H atoms inclusion in afterglow conditions.

Cold NH–NH collisions in a magnetic field: Basis set convergence versus sensitivity to the interaction potential

We explore the combined effect of the uncertainties due to the interaction potential and basis set convergence on low-temperature collisional properties of spin-polarized NH molecules in a magnetic field. We show that quantum scattering calculations with different rotational basis sets and λ-scaled interaction potentials produce qualitatively different ratios of elastic to inelastic cross sections for collision energies above 10−3 cm−1, leading to favorable (unfavorable) prospects for sympathetic cooling of NH molecules depending on the basis set cutoff parameter . The physical reason behind this effect is that the resonance widths, which determine the maximum variation of the scattering cross sections with λ, tend to depend strongly on . At ultralow collision energies, all basis sets produce highly uncertain (and statistically indistinguishable) elastic and inelastic cross sections; however, their ratio γ is much less sensitive to small variations of the interaction potential. Our results highlight the importance of basis set convergence in quantum scattering calculations and establish the existence of parameter regimes where unconverged calculations can still be used to make qualitatively accurate predictions of scattering observables.

Quantum Reactive Scattering of UltracoldNH(X Σ−3)Radicals in a Magnetic Trap

We investigate the ultracold reaction dynamics of magnetically trapped NH(X (3)Σ(-)) radicals using rigorous quantum scattering calculations involving three coupled potential energy surfaces. We find that the reactive NH+NH cross section is driven by a short-ranged collisional mechanism, and its magnitude is only weakly dependent on magnetic field strength. Unlike most ultracold reactions observed so far, the NH+NH scattering dynamics is nonuniversal. Our results indicate that chemical reactions can cause more trap loss than spin-inelastic NH+NH collisions, making molecular evaporative cooling more difficult than previously anticipated.

Scattering resonances in slow NH3–He collisions

We theoretically study slow collisions of NH(3) molecules with He atoms, where we focus in particular on the observation of scattering resonances. We calculate state-to-state integral and differential cross sections for collision energies ranging from 10(-4) cm(-1) to 130 cm(-1), using fully converged quantum close-coupling calculations. To describe the interaction between the NH(3) molecules and the He atoms, we present a four-dimensional potential energy surface, based on an accurate fit of 4180 ab initio points. Prior to collision, we consider the ammonia molecules to be in their antisymmetric umbrella state with angular momentum j = 1 and projection k = 1, which is a suitable state for Stark deceleration. We find pronounced shape and Feshbach resonances, especially for inelastic collisions into the symmetric umbrella state with j = k = 1. We analyze the observed resonant structures in detail by looking at scattering wavefunctions, phase shifts, and lifetimes. Finally, we discuss the prospects for observing the predicted scattering resonances in future crossed molecular beam experiments with a Stark-decelerated NH(3) beam.

Cold and ultracold NH–NH collisions: The field-free case

We present elastic and inelastic spin-changing cross sections for cold and ultracold NH(X (3)Σ(-)) + NH(X (3)Σ(-)) collisions, obtained from full quantum scattering calculations on an accurate ab initio quintet potential-energy surface. Although we consider only collisions in zero field, we focus on the cross sections relevant for magnetic trapping experiments. It is shown that evaporative cooling of both fermionic (14)NH and bosonic (15)NH is likely to be successful for hyperfine states that allow s-wave collisions. The calculated cross sections are very sensitive to the details of the interaction potential, due to the presence of (quasi)bound state resonances. The remaining inaccuracy of the ab initio potential-energy surface therefore gives rise to an uncertainty in the numerical cross-section values. However, based on a sampling of the uncertainty range of the ab initio calculations, we conclude that the exact potential is likely to be such that the elastic-to-inelastic cross-section ratio is sufficiently large to achieve efficient evaporative cooling. This likelihood is only weakly dependent on the size of the channel basis set used in the scattering calculations.

Production of Ultracold NH Molecules by Sympathetic Cooling with Mg

We show that sympathetic cooling of NH molecules by Mg atoms has a good prospect of success. We carry out calculations on M-changing collisions of NH (3Sigma-) molecules in magnetically trappable states with Mg, using a recently calculated potential energy surface. We show that elastic collision rates are much faster than inelastic rates for a wide range of fields at temperatures up to 10 mK and that the ratio increases for lower temperatures and magnetic fields.

Cross-sections for electron–imidogen radical (NH) collisions

In this work, we report a theoretical study on electron collisions with NH radicals in the low and intermediate energy ranges. Calculated elastic differential, integral and momentum-transfer cross-sections as well as grand-total (elastic+inelastic) and total absorption cross-sections for electron–NH collisions are reported in the (1–500)-eV range. A complex optical potential composed by static, exchange, correlation–polarization plus absorption contributions, derived from a fully molecular wave function, is used to describe the interaction dynamics. The Schwinger variational iterative method combined with the distorted-wave approximation is applied to calculate scattering amplitudes. Present calculated results are compared with the existing data for electron–NH scattering in the literature. Also, comparison made between our calculated cross-sections for elastic scattering with the theoretical and experimental results for electron–NH 3 collisions has revealed remarkable similarity even at incident energies as low as 1 eV.

Product Studies of Inelastic and Reactive Collisions of NH2 + NO: Effects of Vibrationally and Electronically Excited NH2†

The reaction between NH 2 and NO has been studied by time-resolved step-scan Fourier transform infrared emission spectroscopy. We observe time-dependent emission from vibrationally excited NO, N 2 O, and H 2 O arising from the interaction of NO with both relaxed and initially vibrationally and electronically excited NH 2 , produced by the 193 nm laser photolysis of ammonia. The excited NH 2 gives rise to a rapidly decaying emission signal. The correlated time dependences of the vibrationally excited NO and N 2 O signals suggest that they are formed by direct collisions of NO with the vibrationally excited NH 2 . There is sufficient excitation of the NH 2 to overcome the high barrier to N 2 O formation. The emission from vibrationally excited H 2 O shows that its formation is delayed, with a significant induction period. The time dependence of the H 2 O induction period matches well to the decay of emissions from the v 1 and v 3 bands of NH 2 , suggesting that the NH 2 is deactivated to its ground electronic and vibrational states before the water product is produced by reaction with NO.

An Experimental Investigation of Collisions of NH 3 with Para‐H 2 at the Temperatures of Cold Molecular Clouds

Experimentally measured cross sections for collisional broadening of ammonia by J = 0, para-H2 are presented for the (J,K) = (1, 1),(2, 2), and (3, 3) NH3 inversion transitions at temperatures from 12.5 to 40 K. The cross sections were obtained in a quasi-equilibrium environment utilizing the collisional cooling technique. These are the first laboratory measurements of interactions between NH3 and J = 0, para-H2 at the temperatures of cold molecular clouds. The measured cross sections are compared to theoretical predictions using two existing ab initio NH3-H2 potential surfaces and to previously measured He broadening cross sections. In comparison to He broadening cross sections at these temperatures, the para-H2 results are up to 4 times larger. This is in contrast to 300 K experimental results where para-H2 cross sections are only 50% larger than He.

State-to-state studies of ground state NH(X 3Σ−,v=0,J,N)+Ne

State-to-state rotational energy transfer of ground state NH(X 3Σ,v=0,J,N) in collisions with Ne is examined. NH is exclusively generated in the metastable NH(a 1Δ) state via photodissociation of hydrazoic acid at a wavelength of 266 nm. The strongly forbidden NH(a 1Δ→X 3Σ−) intercombination transition around 794 nm is used to generate single state NH(X 3Σ−,v=0,J,N) applying the stimulated emission pumping technique. The ground state radicals are detected after a certain delay time with laser induced fluorescence (LIF) using the intense NH(A 3Π←X 3Σ−) transition around 336 nm with respect to all quantum states. The collision induced energy flux between the different rotation and spin levels is studied in detail and a comprehensive set of state-to-state rate constants for inelastic collisions of NH(X 3Σ−,v=0,J,N) with Ne up to N=7 which include the effect of multiple collisions is given. The state-to-state rate constants are obtained by the use of an iterative integrated profiles method. We find a propensity for (ΔN=0, Δi=±1) and (ΔN=±1, Δi=0) transitions where N represents the quantum state for nuclear rotation and i represents the index of the spin component Fi. In most cases the energy transfer which changes the spin component and conserves the nuclear rotation quantum number N (ΔN=0, Δi=±1), is the most effective energy transfer in collisions with Ne. The energy dependence of the transition efficiency concerning only the nuclear rotation quantum number N obeys an energy-gap law (EGL).

Deactivation Rate Constants of OH(X2Πi, v = 1−4) by Collisions of NH3

Collisional deactivation of OH(X2Π, v=1−4) by NH3 has been studied using pulsed laser photolysis coupled with the pulsed laser probe technique. Mixtures of O3/NH3/N2 were photolyzed at 248 nm, and time-resolved OH populations were monitored via the Δv = 0 and −3 sequences of the A2∑+−X2Πi transition. The following deactivation rate constants at 298 ± 1 K were obtained: (2.9 ± 0.2) × 10-11 for v = 1, (1.1 ± 0.2) × 10-10 for v = 2, (3.4 ± 0.4) × 10-10 for v = 3, and (4.1 ± 0.3) × 10-10 for v = 4 in units of cm3 molecule-1 s-1 (the errors are 2σ). The present study is the first report on the rate constant for OH(v=4) + NH3 reaction. The deactivation of OH(v≤3) by NH3 can be elucidated by nonresonant V−V energy transfer caused by long-range interaction; that of v = 4 deviates from the gap law.

Single state NH(X 3Σ−,v=0,J,N) preparation for state-to-state studies

A new method is presented to examine state-to-state rotational energy transfer in ground state NH(X 3Σ−,v=0,J,N). NH(X 3Σ−) is generated via state selective stimulated emission pumping using the strongly forbidden NH(a 1Δ→X 3Σ−) intercombination transition around 794 nm after foregoing photodissociation of HN3 at a wavelength of 266 nm. Products are detected by laser induced fluorescence (LIF). Chemically relevant collision dynamics including spatial processes can be studied for the first time in v=0 of the electronic ground state. State-to-state rate constants for inelastic collisions of NH(X 3Σ−,v=0,J=3,N=3) with Ne are presented.

Absorption in the troughs of the far infrared spectra of NH 3 and mixtures of NH 3 and H 2

The absorption coefficients in the troughs between the J-multiplets of the far infrared rotation-inversion spectrum of NH 3 and NH 3 broadened by H 2 were measured at 296 K in the region from about 65–230 cm −1. Wave numbers were selected in the troughs that are removed from nearby weak absorption lines, and measurements of the absorption versus the density of NH 3 and of H 2 verified that the absorption was due to the wings of distant rather than nearby spectral lines. These measurements were compared with computations of the absorption arising from binary collisions of NH 3–NH 3 and NH 3–H 2 for uncoupled Lorentz lines. Except at the lowest and highest frequencies, the measured continuum absorption in the troughs was less than the computed absorption, a discrepancy that increased with increasing frequency. An improved representation of the measured absorption was obtained by fitting the Rosenkrantz line shape (Orton et al., unpublished) with the Ben-Reuven (Lightman and Ben-Reuven. J Chem Phys 1969;50:351–3) formulation of the line coupling coefficient of the inversion J-doublets. The application of these results to the spectrum of Jupiter shows clear differences between the spectrum of the above model and the Lorentz line shape, with the largest differences occurring in the clearest troughs.

Close coupling results for inelastic collisions of NH 3 and Ar. A stringent test of a spectroscopic potential

We have calculated state-to-state total cross sections for rotational excitation and inversion of NH 3 by collisions with Ar using the close coupling method. The Ar-NH 3 interaction potential has been obtained from a fit to the spectrum of this van der Waals molecule. The calculated cross sections agree to within about 30% with the measured values; the estimated error in the latter is 10% to 20%.

Quantum scattering studies of inelastic collisions of NH(A 3Π) with helium: Fine-structure and Λ-doublet propensities

The results of full close-coupled calculations of state-to-state cross sections for rotationally inelastic collisions of NH in its A 3Π electronic state with helium, based on the recently calculated ab initio potential energy surfaces of Jonas and Staemmler [Z. Phys. D 14, 143 (1989)], are presented. The calculated Λ-doublet resolved cross sections have been compared with predictions based on formal analyses of the scattering equations both in the Hund’s case (a) and (b) limits. For transitions involving low J levels, a strong propensity toward conservation of the e/f label was found, as expected in the case (a) limit. For higher J, the cross sections connecting related pairs of Λ-doublet levels were found to be unequal, reflecting a quantum mechanical interference between the two potential energy surfaces arising from the interaction of a molecule in a Λ>0 state with a perturber. For transitions connected by even l terms in the expansion of the potentials, a simple analysis, based on the relative strengths of the l=2 coupling matrix elements of the electrostatic potential, was found capable of explaining the relative ordering of the cross sections in most cases. A similar success for predicting transitions coupled by odd l terms in the potential was not found; this reflects the fact that the l=3 terms are relatively small for the NH(A 3Π)–He interaction. The calculated cross sections for large J also exhibit a propensity for conservation of the fine-structure label, as expected in the case (b) limit. As an indication of the reliability of the calculated interaction potential and our treatment of the collision dynamics, appropriately summed calculated cross sections reproduce well the experimental rates for transitions from selected f levels into all e levels, as measured by Stuhl and co-workers.