1. M. Rezaei, J. George, L. Welbanks, and N. Moazzen-Ahmadi
Fundamental and combination bands of CO2-C2H2 and CO2-C2D2 in the mid-infrared region
M. Rezaei, J. George, L. Welbanks, and N. Moazzen-Ahmadi
Institute for Quantum Science and Technology
Department of Physics and Astronomy, University of Calgary
I would like to acknowledge my graduate students for their hard work and the funding agencies to make this work possible.

2. Measuring intermolecular forces
A systematic study of small clusters formed from CO2, N2O, OCS, CS2 and C2H2 using high resolution spectroscopy.
With the help of ab initio calculations, the aim is to understand the role played by various components of the intermolecular interactions.
Study of binary systems is made for constructing very high-level ab initio pair potential.
Ternary systems provide information on non-additive effects in the interaction energy.
Study of larger clusters provide information on the number of stable geometries for a cluster size and on possible condensation pathways.
In the last few years, we have been making a systematic study of small clusters formed from CO2, N2O, OCS, CS2, and C2H2 using high resolution spectroscopy. With the help of ab initio calculations (of course done by others), the aim is to understand the role played by various components of intermolecular interactions.
In this respect, study of the binary systems (dimers) is made for constructing an accurate PES, whereas ternary systems provide information on non-additive effects in the interaction energy and finally larger clusters provide information on the number of stable geometries for a cluster size and on possible condensation pathways.
I guess you could say that Richard Sakally has been doing precisely this for water clusters.

3. Emerging themes
Possibility of determining low-frequency intermolecular vibrational modes by means of mid-infrared combination bands.
Increasing number of complexes for which more than one structural form, or isomer, is observed.
The growing amenability of even ‘large’ clusters (say, tetramers and larger) to spectroscopic study.
All three factors help to probe potential energy surfaces more completely in regions away from the global minimum.
Three themes which has emerged from the studies are: First is the …. Then there are an…., and finally, there is a growing …..

4. Homodimers
The homodimers studied by us and others using high resolution spectroscopy are shown here. In some cases two isomers have been observed.

5. Heterodimers
Here is the structures for dimers of unlike pairs studied by high resolution spectroscopy. For many of these dimers we have measured their intermolecular vibrational frequencies by mid-infrared combination bands. In this talk I would like to concentrate on CO2-C2H2, CO2-C2D2 complex.

6. CO2–C2H2 complex a C2V symmetry R = 3.289(1) Å b
Microwave study: J.S. Muenter, J. Chem. Phys. 90, 4048 (1989).
Infrared study in the 3 micron region : Prichard et al., J. Chem. Phys. 89, 1245 (1988); Z.S. Huang and R.E. Miller, Chem. Phys. 132, 185 (1989).
Ab initio: Bone and Handy, Theor. Chim. Acta 78, 133 (1990);W.B. Dealmeida, Chem. Phys. 141, 297 (1990) 297.
Infrared study in the CO2 3 region : C. Lauzin et al., J. Mol. Spectrosc., 267, 19 (2011).
Overtone region of C2H2: C. Lauzin et al., Mol. Phys. 109, 2105 (2011); K. Didriche et al. Mol. Phys. 110, 2773 (2012).
The CO2–C2H2 complex has been previously investigated experimentally in both the microwave, infrared, overtone region and theoretically by ab initio methods. All of these studies are consistent with a planar structure consisting of two parallel entities, with C2v symmetry. The distance between the centers of mass of the two monomers was determined to be 3.289(1) Å.

7. Symmetry of the combination bands and band type
Concentrating on the combination bands involving intermolecular modes, here are the symmetries and band types we expect to observed in the CO2 nu3 or in acetylene or C2D2 nu3 region.

8. What do we already know about the intermolecular modes?
Torsional frequency: 45 cm-1 on the basis of microwave hyperfine splittings1
Torsional frequency: (10) cm-1 from CO2 3 + torsion2
van der Waals stretch: 75 cm-1 on the basis of microwave hyperfine splittings2
No modern high level ab initio calculations!
What do we know about the frequencies of the intermolecular modes?
There is an estimate of 45 cm-1 on the basis of microwave hyperfine splittings. This estimate is in very good agreement with the torsional frequency deduced from torsional combination of CO2 3 excitation + torsional mode. The estimate for van der Waals stretch band is 75 cm-1. There are no modern high level ab initio calculations for CO2-C2H2 complex. Hoping this will change in the near future.
1J.S. Muenter, J. Chem. Phys. 90, 4048 (1989).
2C. Lauzin et al., J. Mol. Spectrosc., 267, 19 (2011).

9. Our OPO is the commercial Lockheed…..

10. Our spectrometer measures direct IR absorption.

11. We now have a fourth channel which minimizes the power fluctuation of the source.

12. New observations 1. C2D2 3 for CO2-C2D2 (0 = 2436.42182(5) cm-1),
b-type
2. C2D2 3 + torsion CO2-C2D2 (0 = (9) cm-1)
c-type
3. CO2 3 + torsion CO2-C2D2 (0 = (8) cm-1)
4. CO2 3 + an in-plane bend of CO2-C2D2 (0 = (7) cm-1)
a-type
5. CO2 3 + an in-plane bend of CO2-C2H2 (0 = (5) cm-1)
We have observed 5The measured bands, C2D2 nu3 fundamental for CO2-C2D2, and four combination band.

13. Assignment, simulation and analysis of spectra were done using Colin Western’s versatile and powerful computer programme PGOPHER.
Here, I am showing a portion of the spectrum for CO2-C2D2 fundamental and its simulation.
Assignment, simulation and analysis of spectra were done using Colin Western’s versatile and powerful computer programme PGOPHER.

14. CO2 3 + torsion C2D2 3 + torsion
Spectra for torsional combination bands, one for CO2 nu3+torsion and the other for C2D2 nu3 + torsion. They look remarkably similar.
C2D2 3 + torsion

15. CO2 3 + an in-plane bend CO2 3 + an in-plane bend
Two other pieces of spectra and their simulations are shown here. One for CO2-C2H2 and the other for CO2-C2D2. Because the relevant fundamentals are already known or can be estimated to better than 0.5 cm-1. The intermolecular modes can be easily deduced.
CO2 3 + an in-plane bend

16. Species
CO2 ν3 +
C2D2 ν3+ or C2H2 ν3+
torsion
in-plane bend
CO2-C2D2
39.5(5)
54.5(5)
39.961(1) -
CO2-C2H2
44.385(10)
61.408(1)
It is not possible to identify which in-plane bend because both geared and anti-geared give rise to a-type bands.
the effect of the monomer vibrations on the intermolecular forces is small.

17. Comparison between different torsional vibrational frequencies (in cm-1).
In conclusion, we have been able to employ an OPO in rapid scan signal averaging mode and have been able to observe weak combination bands for CO2-C2H2 complex. Two of the four intermolecular modes have now been measured.