1. Jet-cooled infrared laser spectroscopy in the umbrella 2 vibration region of NH3:
improving the potential energy surface model of the NH3-Ar van der Waals complex
Pierre Asselin1, Atef Jabri1, Alexey Potapov2, Jérôme Loreau3, Ad van der Avoird4
1Laboratoire MONARIS , CNRS, Université Pierre et Marie Curie, Paris, France
2University Jena Institute of Solid State Physics, Jena, Germany
3Université Libre de Bruxelles, Bruxelles, Belgique
4Institute of Theoretical Chemistry, Nijmegen, Nederlands
ISMS 72nd meeting, Champaign-Urbana, June 23, 2017

2. NH3-Ar : state-of-the art
Experimental
Microwave and mmw spectroscopies
model of nearly isotropic NH3-Ar interaction with nearly free internal rotation of the NH3 subunit
within the complex
Evidence for inversion in the ground state of the complex
 Benchmark to examine the effects of the vdw interaction on the NH3 inversion tunneling
In free NH3 : tunneling splitting  0.8 cm-1 for v=0,  36 cm-1 for v2=1
In NH3 complexes , tunneling generally quenched … except for NH3-Ar
Description of internal rotor states of NH3-Ar (J,K) by their correlation with NH3 (j,k) + Ar
Two distinct nuclear-spin modifications ortho and para for NH3-Ar
Lowest ortho level  (j=0,k=0) (Nelson et al. JCP, 85, 5512 (1986))
Analysis like a linear molecule ( J total angular momentum on the axis of the vdw bond)
Lowest para levels  and  (j=1,k=1) (Zwart et al. JCP 95, 793 (1991))
For  states Inversion tunneling splitting nearly same as the value in free NH3 (0.76 cm-1)
For  states tunneling quenched due to asymmetry of the NH3 inversion potential.
only Coriolis mixing between  and  states weakly allows inversion for  states
Inversion 2 N H Ar
ISMS 72nd meeting, Champaign-Urbana, June 23, 2017

3. NH3-Ar : state-of-the art
Experimental
Infrared spectroscopy in the 2 region (Fraser et al)
mw-sideband CO2 laser spectroscopy
Observation of s (j=1, k=0), v2=1  a (j=0, k=0) v2=0
1.3 cm-1 red shifted / NH3 monomer (2)
 Theoretical v2=10 spectrum
One lower ortho state : a (j=0, k=0) v2= Three lower para states : s, a, s (j=1, k=1) v2=0
v2=1 s,
mw-IR double resonance spectroscopy
NH3-Ar lines detected at , and cm-1
Tentative assignment to the lower and higher frequencies of inversion doublet splitting
 32 and 36 cm-1 for v2=1
Q-branch
Fully resolved !
a (j=0, k=0)
s (j=1, k=0)
s (j=1, k=0)
s, s (j=1, k=1)
a, a (j=1, k=1)
s, s (j=2, k=1)
v2=1
ISMS 72nd meeting, Champaign-Urbana, June 23, 2017

4. NH3-Ar : state-of-the art
Theoretical
Since 1990’s, many PES of NH3-Ar to interpret mw , mmw and mid- IR experiments
Chalasinski et al. JCP,91,7809 (1989) MP2 theory
Bulski et al. JCP 94,491 (1990) semi-empirical PES (dependence on the 2 inversion angle)
Tao et al. JCP 101,1129 (1994)
Schuttenmaier et al. JCP 101,146 (1994) fitting of spectroscopic data
The most advanced theoretical work is the 4D PES by Loreau et al. JCP, (2014).
CCSD(T) coupled to large basis sets taking into account explicitly the umbrella motion of NH3
Ortho states (v2=0)
Para states
Band origins of ortho and
para states reproduced
to 0.8 cm-1
B ground state constants
differ by at most 2%
Experimental data needed
in the 2 band region to
test the PES.
ISMS 72nd meeting, Champaign-Urbana, June 23, 2017

5. Objectives of the present HR infrared study
Investigation of the 2 band region of NH3-Ar with our QCL spectrometer
coupled to a pulsed jet
Observation of transitions starting :
- from the lowest ortho state a (j=0, k=0)
- from the lowest para states a, s, s (j=1, k=1)
Rovibrational analyses
- to derive excited-state parameters in the umbrella mode region
- to evidence possible -  Coriolis couplings
- to confirm the splitting inversion doublet in the 2 state
ISMS 72nd meeting, Champaign-Urbana, June 23, 2017

6. (Calibration frequency)
The SPIRALES set-up and timing sequence of the pulsed experiment
Reference cell
(Calibration frequency)
Ge etalon
(Frequency scaling)
External cavity QCL
Gas mixture
Pulsed valve
Supersonic jet
Astigmatic cavity
QCL in
QCL out
EC-QCL tunable MHF : cm-1
Conditions of supersonic expansion :
NH3/He/Ar=1/33/66 P0 = 3 bar
pulsed slit nozzle : 30 mm x 50 µm
Mutipass optics : 70 passes
bkg
jet
Valve rate (1Hz)
EC-QCL ramp
Trigger output
Data acquisition
Driver valve pulse 1 2 3 4 5
Time /ms 20 40 60 d1 d2
Pulse sequence
5 ms
Gas pulse
Valve driver
Gas
Pinhole or slit jet
Etalon
Reference cell
12-bit 4-channel acquisition card
Counter card PC
EC-QCL
Piezo driver
IR detector
Beamsplitter
Iris
Experiment control
100Hz
1Hz
 0.8 cm-1 scan / 5ms
 data acquisition (5 MHz)
delay to open the gas valve
Background and jet spectra
Ge etalon and reference cell for frequency calibration
Interleaved scans for recording expanded IR spectra
ISMS 72nd meeting, Champaign-Urbana, June 23, 2017

7. Calculated internal rotor energy levels in Ar-NH3
Σa(j=0,k=0)
Challenging experimental study
De  100 cm-1
2 transitions observed from the
lowest ortho state
3 para states within 1.5 cm-1
12.5 cm -1 higher than ortho state :
Rel. population  % at Tv=10-20 K
Several transitions observed from
para states but tricky assignments :
three possible ground states and
probable  Coriolis couplings in
the excited state .
~971
Σs, Πs(j=2,k=1)
~983
v2=1
Σs(j=1,k=0)
Σa, Πa(j=1,k=1) ~
~960
Πa(j=1,k=0)
Σs, Πs(j=1,k=1)
~938
~950
Πs (j=1,k=1)
Σa (j=1,k=1)
Σs (j=1,k=1)
v2=0
1.5
12.5
Σa(j=0,k=0)
Ortho
Para
ISMS 72nd meeting, Champaign-Urbana, June 23, 2017

8. Q-branch, absence of P(1) Q-branch degraded to the low
Jet-cooled spectrum of the s (j=1,k=0)  a(j=0,k=0) v2=10 transition of NH3-Ar
Evidence of  band :
Q-branch, absence of P(1)
Q-branch degraded to the low
frequency side : B’eff=B’+q/2 < B’’
set of excited-state constants
including Q lines (width 1.5-3 MHz)
of Fraser et al
P(2) 5
R(0) 10
ISMS 72nd meeting, Champaign-Urbana, June 23, 2017

9. TR = 3 K derived from the fit Experimental widths : 75(5) MHz
Jet-cooled spectrum of the s (j=1,k=0)  a(j=0,k=0) v2=10 transition of NH3-Ar *
P(1) 5
R(0) 10
Evidence of  band :
No Q-branch
TR = 3 K derived from the fit
Experimental widths : 75(5) MHz
ISMS 72nd meeting, Champaign-Urbana, June 23, 2017

10. Jet-cooled spectrum of the s,s (j=1,k=1)  s,a,s (j=1,k=1) v2=10 transitions of NH3-Ar
Σa, Πa(j=1,k=1)
Σs, Πs(j=1,k=1)
Πs (j=1,k=1)
Σa (j=1,k=1)
Σs (j=1,k=1)
938
974.5
R(0) 5
R(0) 5
Transitions to both symmetric and antisymmetric tunneling component of the j=1,k=1 of v2=1
Evidence for Coriolis coupling in both states : abnormal spacing of R(J) branches
Two rotational lines at and cm-1observed from mw-IR double resonance
studies (Fraser et al, Bizzari et al)  estimate of inversion splitting for v2=1 (36 cm-1)
 This work aims to definitely validate this interpretation only based on two lines observed
ISMS 72nd meeting, Champaign-Urbana, June 23, 2017

11. Conclusion and outlooks
The SPIRALES set-up (tunable QCL spectrometer coupled to pulsed jet) has been exploited to probe the region of the 2 inversion mode ( cm-1) of the van der Waals NH3-Ar complex.
Four rovibrational bands , two starting from the lowest ortho state, two starting from the stacking of three para states, have been observed and currently analyzed.
The rovibrational analysis of ortho transitions s(j=1,k=0)  a(j=0,k=0) and s(j=1,k=0)  a(j=0,k=0) provided a reliable set of 2 state molecular parameters About the sa transition, our data are much less accurate than the parameter set of Fraser et al (observed linewidths 1-3 MHz).
The rovibrational analysis of para transitions s,s (j=1,k=1)  s,a,s (j=1,k=1) in the 938 cm-1 and a,a (j=1,k=1)  s,a,s (j=1,k=1) is made delicate by the possible overlap of transitions starting from nearly degenerate levels of the v2=0 state (s(a),s(a) for ex) and by the probable Coriolis coupling between s(a) and s(a) in the v2=1 state.
Precise determination of the inversion splitting in the v2=1 state of NH3-Ar and refinement of the 4D PES of Loreau et al. to be expected from jet-cooled mid-IR laser spectroscopy.
ISMS 72nd meeting, Champaign-Urbana, June 23, 2017