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 withAr.

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