Spectroscopy of He-, Ne-, and Ar - C 2 D 2 complexes Mojtaba Rezaei, Nasser Moazzen-Ahmadi Department of Physics and Astronomy University of Calgary A.R.W. McKellar National Research Council of Canada Berta Fernández University of Santiago de Compostela David Farrelly Utah State University

TDL Jet Trigger Ref. Gas 12 bit DAQ Card Timer Controller Card (CTR05) Laser Sweep Trigger DAQ Trigger Gas Supply Jet Signal Jet Controller (Iota One) Jet Controller IR Detectors TDL Controller (L5830) Monochromator pulsed supersonic jet / tunable diode laser apparatus at The University of Calgary Etalon

Mol. Phys. 77, 111 (1992) Experimental spectra of helium – acetylene have never been published

J. Chem. Phys. 102, 8385 (1995)

This is the best He-HCCH potential currently available, so we asked Berta Fernández to calculate energy levels for He-HCCH and He-DCCD.

He – C 2 D 2 energy levels calculated by Fernández & Farrelly from the Munteneau & Fernández CCSD(T) potential

Coriolis model. Used by Brian Howard for Rg – SiH 4 complexes and by the Köln group for Rg – CH 4. For the moment, we are concerned only with j = 0 and 1 levels, where j is the DCCD rotation. There is one stack of rotational levels for j = 0, denoted 0 σe. There are three stacks for j = 1, denoted 1  f, 1  e, and 1 σe. E(0 σe) = B(0) J(J + 1) – D(0) [J(J + 1)] 2 E(1  f) = E vr +  + B(1  ) J(J + 1) – D(1) [J(J + 1)] 2 E(1 σe) = E vr –  + B(1σ) J(J + 1) – D(1) [J(J + 1)] 2  connected by off-diagonal Coriolis coupling: [  J(J +1)] 1/2  E(1  e) = E vr +  + B(1  ) J(J + 1) – D(1) [J(J + 1)] 2 This model represents the ab initio levels fairly well.

He – C 2 D 2

He – C 2 H 2 He – C 2 D 2 Theory Moszynski et al. Theory Munteanu & Fernández Experiment B(0) D(0) E vr b B(1σ) B(1  )   1/ D(1) j*j* j* is dimensionless; j* = 1 in the free rotation limit Coriolis model parameters (cm -1 ) positive  means linear negative  means T-shaped zero  means free rotation

This is our predicted He – HCCH spectrum for the j = 1 – 0 region, near R(0) of the HCCH 3 band

Microwave spectra of He – acetylene have never been reported. We predict R(0) for the j = 0 stack of He – C 2 D 2 to lie around MHz. So far, our He – C 2 D 2 analysis is limited to the j = 1  0 region. There is also a j = 0  1 spectrum, but in our current data it is mostly obscured by (C 2 D 2 ) 2 transitions.

Ne – C 2 D 2 j = 1  0 spectrum, near C 2 D 2 R(0)

Ne – C 2 D 2 He – C 2 D 2 B(0) E vr B(1σ) B(1  )   1/ j*j* Coriolis model parameters (cm -1 ) negative  means T-shaped positive  means linear includes j = 1 energy and vibrational shift j* is dimensionless j* = 1 in free rotation limit

Ne – C 2 D 2 We think the correct assignment here may be:  1 state

Ne – C 2 D 2 j = 2  1 spectrum, near C 2 D 2 R(1) j = 3  2 spectrum, near C 2 D 2 R(2) not assigned yet

Ar – C 2 D 2 K = 1  0 subband

Ar – C 2 D 2 K = 2  1 subband

Ar – C 2 D 2 part of bending combination band, v 2 = 1  0, K = 0  0 plus other unassigned structure

Conclusions First assignment for He – acetylene. Fernández potential works fairly well, but the real complex is even closer to the free rotation limit. Ne – C 2 D 2 is also close to the free rotor limit and hence tricky to assign, especially for j > 1. Data are incomplete: we need spectra in the j = 0  1 region. Ar – C 2 D 2 is more like a ‘normal’ molecule and relatively easy to assign. Data also incomplete. Asymmetric rotor fit is possible, but not very good. Better to treat each ‘state’ (v, K) separately (similar to Ar – CO).