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

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

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=0 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 938.6893, 970.5498 and 974.6248 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

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, 224303 (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

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

(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 : 930-990 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

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  15-40 % 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) ~967-975 ~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

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

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

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 938.6893 and 974.6248 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

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 (935-975 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