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ABSTRACT: We present an ab initio potential for the H-CO(X ^2 A') complex in which the CO bond length is varied and the long range interactions between H and CO are accurately represented. It was computed using the spin-unrestricted open-shell single and double excitation coupled cluster method with perturbative triples [UCCSD(T)]. Three doubly augmented correlation-consistent basis sets were utilized to extrapolate the correlation energy to the complete basis set limit. More than 4400 data points were calculated and used for an analytic fit of the potential: long range terms with inverse power dependence on the H-CO distance R were fit to the data points for large R, the reproducing kernel Hilbert space (RKHS) method was applied to the data at smaller distances. Our potential was compared with previous calculations and with some data extracted from spectroscopy. Furthermore, it was used in three-dimensional discrete variable representation (DVR) calculations of the vibrational frequencies and rotational constants of HCO, which agree very well with the most recently measured data. Also the dissociation energy D_0 = 0.623 eV of HCO into H + CO obtained from these calculations agrees well with experimental values. Finally, we made preliminary two-dimensional (2D) calculations of the cross sections for rotationally inelastic H-CO collisions with the CO bond length fixed and obtained good agreement with recently published 2D results.
The Journal of Physical Chemistry A 04/2013; · 2.95 Impact Factor
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ABSTRACT: 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.
Physical Review Letters 02/2013; 110(6):063201. · 7.37 Impact Factor
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ABSTRACT: Whereas atom-molecule collisions have been studied with complete quantum-state resolution, interactions between two state-selected molecules have proven much harder to probe. Here, we report the measurement of state-resolved inelastic scattering cross sections for collisions between two open-shell molecules that are both prepared in a single quantum state. Stark-decelerated hydroxyl (OH) radicals were scattered with hexapole-focused nitric oxide (NO) radicals in a crossed-beam configuration. Rotationally and spin-orbit inelastic scattering cross sections were measured on an absolute scale for collision energies between 70 and 300 cm(-1). These cross sections show fair agreement with quantum coupled-channels calculations using a set of coupled model potential energy surfaces based on ab initio calculations for the long-range nonadiabatic interactions and a simplistic short-range interaction. This comparison reveals the crucial role of electrostatic forces in complex molecular collision processes.
Science 11/2012; 338(6110):1060-3. · 31.20 Impact Factor
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ABSTRACT: We report on the observation of magnetic dipole allowed transitions in the well-characterized A (2)Σ(+) - X (2)Π band system of the OH radical. A Stark decelerator in combination with microwave Rabi spectroscopy is used to control the populations in selected hyperfine levels of both Λ-doublet components of the X (2)Π(3∕2), v = 0, J = 3∕2 ground state. Theoretical calculations presented in this Communication predict that the magnetic dipole transitions in the A (2)Σ(+), v = 1 ← X (2)Π, v = 0 band are weaker than the electric dipole transitions by a factor of 2.58 × 10(3) only, i.e., much less than commonly believed. Our experimental data confirm this prediction.
The Journal of chemical physics 09/2012; 137(10):101102. · 3.09 Impact Factor
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ABSTRACT: We report on the observation of magnetic dipole allowed transitions in the
well-characterized $A\,^2\Sigma^+ - X\,^2\Pi$ band system of the OH radical. A
Stark decelerator in combination with microwave Rabi spectroscopy is used to
control the populations in selected hyperfine levels of both $\Lambda$-doublet
components of the $X\,^2\Pi_{3/2},v=0,J=3/2$ ground state. Theoretical
calculations presented in this paper predict that the magnetic dipole
transitions in the $\nu'=1 \leftarrow \nu=0$ band are weaker than the electric
dipole transitions by a factor of $2.58\times 10^3$ only, i.e., much less than
commonly believed. Our experimental data confirm this prediction.
08/2012;
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ABSTRACT: The observation of the isotope effect in the high-order-harmonic generation (HHG) of H2 presents a challenge for time-dependent density-functional-theory (TDDFT) methods, since this effect is related to the dynamics of the ion created in the tunneling ionization step of HHG and it depends on the harmonic order. As an initial step toward describing this effect within current computational capacity, we benchmark a method in which the nuclear and electronic degrees of freedom are separated and both treated quantum mechanically. For the electrons two TDDFT formalisms are adopted. Although the ion-dynamics effect is not described in our method, it reproduces the measured D2-to-H2 HHG ratios up to the 25th harmonic when the 35th is the classical cutoff. Beyond the 25th harmonic, however, our results show substantial deviation and are sensitive to the laser intensity. A higher intensity reproduces the experimental results. Analysis reveals an R-dependent phase factor as the cause of the isotope effect in our calculation. We isolate this phase factor and propose a strong-field-approximation-phase model, which reproduces experimental data, including those for which the ion-dynamics model has to be further modified. We show that the model that we propose is intrinsically related to the ion-dynamics model. Our model provides a correction to the TDDFT approach when the ion-dynamics effect becomes significant. It also indicates that the isotope effect is not only a probe for the ion created by the external field but is ultimately a more useful probe for the ground-state nuclear wave function. For all molecules whose vertical ionization potential strongly depends on the nuclear geometry, HHG may serve as a sensitive ultrafast probe of nuclear dynamics.
Phys. Rev. A. 04/2012; 85(5).
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ABSTRACT: We present detailed calculations on resonances in rotationally and spin-orbit inelastic scattering of OH (X(2)Π, j = 3/2, F(1), f) radicals with He and Ne atoms. We calculate new ab initio potential energy surfaces for OH-He, and the cross sections derived from these surfaces compare well with the recent crossed beam scattering experiment of Kirste et al. [Phys. Rev. A 82, 042717 (2010)]. We identify both shape and Feshbach resonances in the integral and differential state-to-state scattering cross sections, and we discuss the prospects for experimentally observing scattering resonances using Stark decelerated beams of OH radicals.
The Journal of chemical physics 04/2012; 136(14):144308. · 3.09 Impact Factor
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ABSTRACT: 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.
The Journal of chemical physics 02/2012; 136(7):074301. · 3.09 Impact Factor
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ABSTRACT: We present detailed calculations on resonances in rotationally and spin-orbit
inelastic scattering of OH ($X\,^2\Pi, j=3/2, F_1, f$) radicals with He and Ne
atoms. We calculate new \emph{ab initio} potential energy surfaces for OH-He,
and the cross sections derived from these surfaces compare favorably with the
recent crossed beam scattering experiment of Kirste \emph{et al.} [Phys. Rev. A
\textbf{82}, 042717 (2010)]. We identify both shape and Feshbach resonances in
the integral and differential state-to-state scattering cross sections, and we
discuss the prospects for experimentally observing scattering resonances using
Stark decelerated beams of OH radicals.
01/2012;
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ABSTRACT: Carbon monoxide molecules in their electronic, vibrational, and rotational ground state are highly attractive for trapping experiments. The optical or ac electric traps that can be envisioned for these molecules will be very shallow, however, with depths in the sub-milliKelvin range. Here, we outline that the required samples of translationally cold CO (X(1)Σ(+), v'' = 0, N'' = 0) molecules can be produced after Stark deceleration of a beam of laser-prepared metastable CO (a(3)Π(1)) molecules followed by optical transfer of the metastable species to the ground state via perturbed levels in the A(1)Π state. The optical transfer scheme is experimentally demonstrated and the radiative lifetimes and the electric dipole moments of the intermediate levels are determined.
The Journal of chemical physics 09/2011; 135(11):114201. · 3.09 Impact Factor
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ABSTRACT: This paper has been withdrawn by the authors because the wave packet
propagation used in the ion-dynamics calculation did not allow for
electron-nuclei correlation. Hence, the conclusion that the ion-dynamics model
is not in agreement with experiment is not substantiated.
09/2011;
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ABSTRACT: We present elastic and inelastic spin-changing cross sections for cold and ultracold NH(X 3Σ−) + NH(X 3Σ−) 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 14NH and bosonic 15NH 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.
The Journal of Chemical Physics 03/2011; 134(12):124309-124309-9. · 3.33 Impact Factor
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ABSTRACT: We present a detailed analysis of the role of the magnetic dipole-dipole
interaction in cold and ultracold collisions. We focus on collisions between
magnetically trapped NH molecules, but the theory is general for any two
paramagnetic species for which the electronic spin and its space-fixed
projection are (approximately) good quantum numbers. It is shown that dipolar
spin relaxation is directly associated with magnetic-dipole induced avoided
crossings that occur between different adiabatic potential curves. For a given
collision energy and magnetic field strength, the cross-section contributions
from different scattering channels depend strongly on whether or not the
corresponding avoided crossings are energetically accessible. We find that the
crossings become lower in energy as the magnetic field decreases, so that
higher partial-wave scattering becomes increasingly important \textit{below} a
certain magnetic field strength. In addition, we derive analytical
cross-section expressions for dipolar spin relaxation based on the Born
approximation and distorted-wave Born approximation. The validity regions of
these analytical expressions are determined by comparison with the NH + NH
cross sections obtained from full coupled-channel calculations. We find that
the Born approximation is accurate over a wide range of energies and field
strengths, but breaks down at high energies and high magnetic fields. The
analytical distorted-wave Born approximation gives more accurate results in the
case of s-wave scattering, but shows some significant discrepancies for the
higher partial-wave channels. We thus conclude that the Born approximation
gives generally more meaningful results than the distorted-wave Born
approximation at the collision energies and fields considered in this work.
03/2011;
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ABSTRACT: In this paper we report slice imaging polarization experiments on the state-to-state photodissociation at 42,594 cm(-1) of spatially oriented OCS(v(2) = 1|JlM = 111) → CO(J) + S((1)D(2)). Slice images were measured of the three-dimensional recoil distribution of the S((1)D(2)) photofragment for different polarization geometries of the photolysis and probe laser. The high resolution slice images show well separated velocity rings in the S((1)D(2)) velocity distribution. The velocity rings of the S((1)D(2)) photofragment correlate with individual rotational states of the CO(J) cofragment in the J(CO) = 57-65 region. The angular distribution of the S((1)D(2)) velocity rings are extracted and analyzed using two different polarization models. The first model assumes the nonaxial dynamics evolves after excitation to a single potential energy surface of an oriented OCS(v(2) = 1|JlM = 111) molecule. The second model assumes the excitation is to two potential energy surfaces, and the OCS molecule is randomly oriented. In the high J region (J(CO) = 62-65) it appears that both models fit the polarization very well, in the region J(CO) = 57-61 both models seem to fit the data less well. From the molecular frame alignment moments the m-state distribution of S((1)D(2)) is calculated as a function of the CO(J) channel. A comparison is made with the theoretical m-state distribution calculated from the long-range electrostatic dipole-dipole plus quadrupole interaction model. The S((1)D(2)) photofragment velocity distribution shows a very pronounced strong peak for S((1)D(2)) fragments born in coincidence with CO(J = 61).
Physical Chemistry Chemical Physics 03/2011; 13(18):8549-59. · 3.57 Impact Factor
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ABSTRACT: Elastic and spin-changing inelastic collision cross sections are presented for cold and ultracold magnetically trapped NH. The cross sections are obtained from coupled-channel scattering calculations as a function of energy and magnetic field. We specifically investigate the influence of the intramolecular spin-spin, spin-rotation, and intermolecular magnetic dipole coupling on the collision dynamics. It is shown that 15NH is a very suitable candidate for evaporative cooling experiments. The dominant trap-loss mechanism in the ultracold regime originates from the intermolecular dipolar coupling term. At higher energies and fields, intramolecular spin-spin coupling becomes increasingly important. Our qualitative results and conclusions are fairly independent of the exact form of the potential and of the size of the channel basis set.
Phys. Rev. A. 02/2011; 83(2).
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ABSTRACT: The OH + CH(3) product channel for the photodissociation of CH(3)OH at 157 nm was investigated using the velocity map imaging technique with the detection of CH(3) radical products via (2+1) resonance-enhanced multiphoton ionization (REMPI). Images were measured for the CH(3) formed in the ground and excited states (v(2) = 0, 1, 2, and 3) of the umbrella vibrational mode and correlated OH vibrational state distributions were also determined. We find that the vibrational distribution of the OH fragment in the OH + CH(3) channel is clearly inverted. Anisotropic distributions for the CH(3) (v(2) = 0, 1, 2, and 3) products were also determined, which is indicative of a fast dissociation process for the C-O bond cleavage. A slower CH(3) product channel was also observed, that is assigned to a two-step photodissociation process, in which the first step is the production of a CH(3)O(X (2)E) radical via the cleavage of the O-H bond in CH(3)OH, followed by probe laser photodissociation of the nascent CH(3)O radicals yielding CH(3)(X (2)A(1), v = 0) products.
Physical Chemistry Chemical Physics 02/2011; 13(6):2350-5. · 3.57 Impact Factor
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ABSTRACT: We present a combined experimental and theoretical study on the rotationally
inelastic scattering of OH ($X\,^2\Pi_{3/2}, J=3/2, f$) radicals with the
collision partners He, Ne, Ar, Kr, Xe, and D$_2$ as a function of the collision
energy between $\sim 70$ cm$^{-1}$ and 400~cm$^{-1}$. The OH radicals are state
selected and velocity tuned prior to the collision using a Stark decelerator,
and field-free parity-resolved state-to-state inelastic relative scattering
cross sections are measured in a crossed molecular beam configuration. For all
OH-rare gas atom systems excellent agreement is obtained with the cross
sections predicted by close-coupling scattering calculations based on accurate
\emph{ab initio} potential energy surfaces. This series of experiments
complements recent studies on the scattering of OH radicals with Xe [Gilijamse
\emph{et al.}, Science {\bf 313}, 1617 (2006)], Ar [Scharfenberg \emph{et al.},
Phys. Chem. Chem. Phys. {\bf 12}, 10660 (2010)], He, and D$_2$ [Kirste \emph{et
al.}, Phys. Rev. A {\bf 82}, 042717 (2010)]. A comparison of the relative
scattering cross sections for this set of collision partners reveals
interesting trends in the scattering behavior.
01/2011;
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ABSTRACT: Larmor precession of a quantum mechanical angular momentum vector about an applied magnetic field forms the basis for a range of magnetic resonance techniques, including nuclear magnetic resonance spectroscopy and magnetic resonance imaging. We have used a polarized laser pump-probe scheme with velocity-map imaging detection to visualize, for the first time, the precessional motion of a quantum mechanical angular momentum vector. Photodissociation of O(2) at 157 nm provides a clean source of fast-moving O((1)D(2)) atoms, with their electronic angular momentum vector strongly aligned perpendicular to the recoil direction. In the presence of an external magnetic field, the distribution of atomic angular momenta precesses about the field direction, and polarization-sensitive images of the atomic scattering distribution recorded as a function of field strength yield 'time-lapse-photography' style movies of the precessional motion. We present movies recorded in various experimental geometries, and discuss potential consequences and applications in atmospheric chemistry and reaction dynamics.
Nature Chemistry 01/2011; 3(1):28-33. · 20.52 Impact Factor
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The Journal of chemical physics 09/2010; 133(10):109902. · 3.09 Impact Factor
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ABSTRACT: The first fully allowed spectroscopic transition in O2 is the transition comprising the well-known Schumann–Runge bands and continuum. We report a velocity-map imaging study in which the O(1 D) angular momentum polarisation and O(3 P) spin–orbit branching ratios arising from this process have been measured. We show that direct 157 nm excitation into the Schumann–Runge continuum leads to extremely strong angular momentum polarisation in the O(1D) product. Comparison with previous studies indicates that this is a general feature of dissociation via the B state. The fine structure branching ratios in the co-fragment O(3P J=2,1,0) were measured to be 88.5 ± 1.6 : 9.7 ± 1.4 : 1.9 ± 0.4. Based on a consideration of the Massey parameter for the system, the data were modelled using theoretical calculations based on adiabatic and diabatic models of the dissociation. While both models were able to describe some aspects of the dissociation accurately, neither was able to predict both the fine structure branching ratios of the O(3P) products and the O(1D2) alignment. We have also investigated O(1D2) alignment arising from 203.8 and 205.5 nm photodissociation via the state of O2 vibrationally excited to v=6–11. As in the 157 nm photodissciation of vibrationally ground state O2, strong polarisation of the O(1D2) photofragments is observed.
Molecular Physics 04/2010; 108(Nos. 7-9):1145-1157. · 1.82 Impact Factor
