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Journal ArticleDOI

Rotationally Inelastic Scattering of Quantum-State-Selected ND3 with Ar.

08 Jan 2015-Journal of Physical Chemistry A (American Chemical Society)-Vol. 119, Iss: 23, pp 5979-5987

TL;DR: The angular distributions are dominated by forward scattering for all measured final rotational and vibrational inversion symmetry states, in contrast to recent results for inelastic scattering of ND3 with He, where the differences between He and Ar collision partners are explained by differences in the potential energy surfaces that govern the scattering dynamics.
Abstract: Rotationally inelastic scattering of ND3 with Ar is studied at mean collision energies of 410 and 310 cm–1. In the experimental component of the study, ND3 molecules are prepared by supersonic expansion and subsequent hexapole state selection in the ground electronic and vibrational levels and in the jk± = 11– rotational level. A beam of state-selected ND3 molecules is crossed with a beam of Ar, and scattered ND3 molecules are detected in single final j′k′± quantum states using resonance enhanced multiphoton ionization spectroscopy. State-to-state differential cross sections for rotational-level changing collisions are obtained by velocity map imaging. The experimental measurements are compared with close-coupling quantum-mechanical scattering calculations performed using an ab initio potential energy surface. The computed DCSs agree well with the experimental measurements, confirming the high quality of the potential energy surface. The angular distributions are dominated by forward scattering for all me...

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Tkac, O., Saha, A. K., Loreau, J., Parker, D. H., van der Avoird, A., &
Orr-Ewing, A. J. (2014). Rotationally Inelastic Scattering of Quantum-
State-Selected ND3 withAr.
Journal of Physical Chemistry A
,
119
(23),
5979 - 5987. https://doi.org/10.1021/jp5115042
Peer reviewed version
Link to published version (if available):
10.1021/jp5115042
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Rotationally Inelastic Scattering of Quantum-State Selected ND
3
with Ar
Ondřej Tkáč,
a
Ashim K. Saha,
b
Jérôme Loreau,
c
David H. Parker,*
b
Ad van der Avoird,*
b
and
Andrew J. Orr-Ewing*
d
a
Laboratorium für Physikalische Chemie, ETH Zürich, CH-8093 Zürich, Switzerland
b
Radboud University Nijmegen, Institute for Molecules and Materials, Toernooiveld 1,
6525ED Nijmegen, The Netherlands; E-mail: parker@science.ru.nl, avda@theochem.ru.nl
c
Service de Chimie Quantique et Photophysique, Université Libre de Bruxelles (ULB) CP
160/09, 50 av. F.D. Roosevelt, 1050 Brussels, Belgium
d
School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK; E-mail:
a.orr-ewing@bris.ac.uk

2
Abstract
Rotationally inelastic scattering of ND
3
with Ar is studied at mean collision energies of 410
and 310 cm
-1
. In the experimental component of the study, ND
3
molecules are prepared by
supersonic expansion and subsequent hexapole state selection in the ground electronic and
vibrational levels and in the
rotational level. A beam of state-selected ND
3
molecules
is crossed with a beam of Ar, and scattered ND
3
molecules are detected in single final 
󰆒
quantum states using resonance enhanced multiphoton ionization spectroscopy. State-to-state
differential cross sections for rotational-level changing collisions are obtained by velocity map
imaging. The experimental measurements are compared with close-coupling quantum-
mechanical scattering calculations performed using an ab initio potential energy surface. The
computed DCSs agree well with the experimental measurements, confirming the high quality
of the potential energy surface. The angular distributions are dominated by forward scattering
for all measured final rotational and vibrational inversion symmetry states. This outcome is in
contrast to our recent results for inelastic scattering of ND
3
with He, where we observed
significant amount of sideways and backward scattering for some final rotational levels of ND
3
.
The differences between He and Ar collision partners are explained by differences in the
potential energy surfaces that govern the scattering dynamics.

3
I. Introduction
Prior study of rotational energy transfer in collisions of ammonia isotopologues with
He and H
2
has been motivated by astrophysical applications,
1-4
as discussed in our recent
paper.
5
To complement such studies, and to examine the effects of changes to the properties
of the collision partner on the collision dynamics, inelastic scattering of NH
3
with Ar has also
been the subject of extensive previous experimental investigation.
6-9
This paper presents the
first quantum state-to-state resolved differential cross sections (DCSs), as well as calculated
integral cross sections (ICS) for the ND
3
+ Ar system. The objectives are to explore the
dynamics of translational to rotational energy transfer for this system, and thereby to derive
insights into the intermolecular interactions between these colliding species. The state-
resolved ICSs and DCSs are sensitive to the anisotropy of the intermolecular interactions and
can be used to test computed potential energy surfaces (PESs) and quantum mechanical (QM)
scattering calculations that simulate the collision dynamics. ICSs do not allow ready
distinction of the influence of the short range repulsive and the long range attractive parts of
the PES,
3, 8
so important additional insights are gained from measurement of the angular
distribution of particles scattered into a specific final state.
A DCS determination offers more detailed information about the PES governing the
collisions than is contained in the ICS, since the form of the DCS can reveal the dependence
of the collision dynamics on the impact parameter, . The 
󰇛
󰇜


relationship, where
󰇛
󰇜
is interaction potential and

is the collision energy, illustrates that forward scattering
(corresponding to small deflection angles
) originates from large impact parameters and hence
samples the attractive long-range part of the potential. Rainbow scattering occurs when a
trajectory samples the minimum of the potential, which may correspond to a well resulting
from van der Waals interactions, and the large deflections characteristic of backward scattering
originate from collisions at small impact parameters that probe the short range part of the PES.
In addition to extending the understanding of the intermolecular interaction between
ND
3
and Ar, the results presented in this paper are contrasted with the collisional scattering
behaviour of ND
3
with Ne,
10
and He.
5
In this way, the effect of mass, polarizability and duration
of the interaction can be explored for collisions of ND
3
with He, Ne and Ar. With selection of
ND
3
in a single vibrational and rotational level and the antisymmetric component of the
umbrella vibrational inversion doublet prior to collisions, we are able to make precise
measurements of the scattering that are not degraded by averaging over an initial distribution
of states.
Prior determinations of DCSs for rotationally inelastic scattering were reported, for
example, for H
2
O collisions with He
11-12
and H
2
,
13
OH radical with Ar and He,
14
HCl with
various colliders,
15
and NO with Ar and He.
16-20
Recently, the inelastic scattering dynamics of
methyl radical with He,
21
H
2
and D
2
,
22
and Ar
23
were examined using crossed molecular beam

4
methods in combination with velocity map imaging (VMI). Measured DCSs were contrasted
with theoretical DCSs calculated using quantum mechanical close-coupling scattering
calculations on newly computed ab initio PESs. Excellent agreement lends confidence to the
quality of the calculated PESs. These studies also explored the effects of anisotropies in the
intermolecular potential associated with the polar and azimuthal angles of approach of the
collision partner, defined with respect to the three-fold rotational symmetry axis of the methyl
radical. Comparisons have been made between the scattering dynamics of the planar, open-
shell CD
3
radical and the pyramidal, closed-shell ND
3
molecule in collisions with He on the
basis of rigorous close-coupling scattering calculations.
24
There are many similarities between
the DCSs for ND
3
He (for collisions that conserve the ± symmetry) and CD
3
He scattering,
nevertheless observed differences can be linked to interaction terms in the expansion of the
PES which directly couple transitions between initial and final rotational levels.
The ND
3
molecule is an important candidate for potential applications in experimental
studies of cold collisions. In crossed beam scattering experiments, a Stark decelerator can be
used to decelerate neutral polar molecules with a time-varying electric field. The inelastic
scattering can be studied over a wide range of collision energies. In this way, the details of
scattering processes that remain hidden in conventional crossed beam scattering experiments
may be revealed. For example, the effects of Feshbach and shape resonance can be observed,
as can the diffraction oscillation present in the small angle scattering of the DCSs, which are
beyond the resolution of the current experiments. Diffraction oscillations were recently
resolved in inelastic scattering experiments of a Stark decelerated beam of NO with He, Ne
and Ar.
25
The signatures of scattering resonances have been studied theoretically for the NH
3
He system.
26
In this paper, results are presented for the state-to-state scattering of ND
3
, prepared in
its ground electronic and vibrational levels and in the
rotational level, with Ar at two
collision energies 410 ± 40 cm
-1
and 310 ± 30 cm
-1
. Initial state selection is achieved by
supersonic expansion and hexapole state selection. In addition to the experiments performed
with the hexapole state selected ND
3
(
) scattered by Ar, velocity map images were measured
for ND
3
Ar without use of the hexapole, for scattering into final levels with ' = 2 and 3, with
the initial state averaged over several rotational levels populated in the molecular beam
expansion. Experimental DCSs are compared to theoretical DCSs calculated using the close-
coupling method on an accurate ab initio PES.
II. Method
A. Experimental apparatus

Figures (8)
Citations
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Abstract: We present theoretical studies on the scattering resonances in rotationally inelastic collisions of NH3 and ND3 molecules with H2 molecules. We use the quantum close-coupling method to compute state-to-state integral and differential cross sections for the NH3/ND3–H2 system for collision energies between 5 and 70 cm−1, using a previously reported potential energy surface [Maret et al., Mon. Not. R. Astron. Soc. 399, 425 (2009)]. We identify the resonances as shape or Feshbach resonances. To analyze these, we use an adiabatic bender model, as well as examination at the scattering wave functions and lifetimes. The strength and width of the resonance peaks suggest that they could be observed in a crossed molecular beam experiment involving a Stark-decelerated NH3 beam.

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TL;DR: It is shown that some systems exhibit a large ratio of elastic to inelastic cross sections in the cold regime, which is promising for sympathetic cooling experiments and investigates the possibility of sympathetic cooling of ammonia using cold or ultracold rare gas atoms.
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References
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Sheldon Green1Institutions (1)
Abstract: The formalism for describing rotational excitation in collisions between symmetric top rigid rotors and spherical atoms is presented both within the accurate quantum close coupling framework and also the coupled states approximation of McGuire and Kouri and the effective potential approximation of Rabitz. Calculations are reported for thermal energy NH3-He collisions, treating NH3 as a rigid rotor and employing a uniform electron gas (Gordon-Kim) approximation for the intermolecular potential. Coupled states are found to be in nearly quantitative agreement with close coupling results while the effective potential method is found to be at least qualitatively correct. Modifications necessary to treat the inversion motion in NH3 are discussed.

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TL;DR: A Stark decelerator is used to form a beam of state-selected and velocity-controlled NO radicals and measure state-to-state differential cross-sections for inelastic collisions of NO with He, Ne and Ar atoms using velocity map imaging to fully resolving quantum diffraction oscillations.
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