1. CO2 dimer: Five intermolecular vibrations observed via infrared combination bands
Jalal Norooz Oliaee, Mehdi Dehghany, Mojtaba Rezaei, Nasser Moazzen-Ahmadi
Department of Physics and Astronomy University of Calgary
A.R.W. McKellar
National Research Council of Canada

2. TDLS 2009, Zermatt, Switzerland
pulsed supersonic jet / tunable laser apparatus at The University of Calgary
Jet Controller (Iota One)
Jet
Trigger
Jet Controller
Gas Supply
IR Detectors
TDL
Jet Signal
Laser Controller
Laser
Sweep
Trigger
Ref. Gas
Timer Controller
Card (CTR05)
DAQ
Trigger
12 bit
DAQ Card
Etalon 2

3. CO2 dimer was one of the very first van der Waals complexes to be observed by (truly) high-resolution IR spectroscopy

4. as well as the dimer spectrum in the 2350 cm-1 region (3)
This CO2 dimer spectrum in the 3700 cm-1 region (1 + 3 / 22 + 3) was assigned a few years later
Jucks, Huang, Dayton, Miller, and Lafferty, J. Chem. Phys. 86, 4341 (1987).
Jucks, Huang, Miller, Fraser, Pine, and Lafferty, J. Chem. Phys. 88, 2185 (1988).
as well as the dimer spectrum in the 2350 cm-1 region (3)
Walsh, England, Dyke, and Howard, Chem. Phys. Lett. 142, 265 (1987).

5. Here are two older works of significance here
There are many calculations of CO2 intermolecular forces (empirical, ab initio, …)
Here are two older works of significance here

6. The current work was guided with help from new calculations by Xiao-Gang Wang, Tucker Carrington, and Richard Dawes
C2h point group
Intermolecular mode
Symmetry
Chen and Light (2000) (cm-1)
Dawes et al. (2016) (cm-1)
Geared bend (in-plane) Bu
19.7
20.64
Torsion (out-of-plane) Au
22.6
24.44
2 x geared bend Ag
29.5
32.34
vdW stretch
40.8
46.07
Anti-geared bend (in-plane) 90
92.30

7. Symmetry is important! There are two intramolecular dimer modes in the 3 region: Ag = forbidden out-of-phase vibration of the CO2 monomers ( cm-1?) Bu = allowed in-phase vibration of the CO2 monomers ( cm-1)
Intermolecular mode
Symmetry
Combined with Ag intramolecular mode
Combined with Bu intramolecular mode
Geared bend (in-plane) Bu Ag
Torsion (out-of-plane) Au Bg
2 x geared bend
vdW stretch
Anti-geared bend (in-plane)
Only Au and Bu states have allowed transitions (from the dimer ground state)

8. Symmetry is broken in the mixed dimer, giving new information
A few years ago, we studied (13CO2)2 and 12CO2 – 13CO2 and also searched for dimer combination bands in the 3 region
Symmetry is broken in the mixed dimer, giving new information

9. Combination bands tell us about the interesting low frequency intermolecular vibrations
In 2010, we analyzed one combination band. But its vibrational assignment was not completely certain.
The new Wang calculations predict the (2 x geared bend) combination band at = cm-1. So this is certainly the correct assignment for our previously observed cm-1 band.

10. We have long been puzzled by this mystery CO2 cluster band
- Mol. Phys. 108, 2195 (2010)
By laboriously checking CO2 dimer ground state combination differences, we have finally figured it out!

11. There are two dimer combination bands here
There are two dimer combination bands here. Their upper states have a strong Coriolis interaction, which can be fitted very well.
Geared bend
Torsion 0
(1)
(1) A
(27)
(26) B
(48)
(45) C
(40)
(40) Ga
(32) Gb
(33)

12. Bu Ag Au Bg Intermolecular mode Symmetry
Combined with Ag intramolecular mode  cm-1
Combined with Bu intramolecular mode = cm-1
Geared bend (in-plane) Bu Ag
Torsion (out-of-plane) Au Bg
2 x geared bend
vdW stretch
Anti-geared bend (in-plane)

13. Another mystery region, and here, too, we found many CO2 dimer ground state combination differences.
There are two bands here with origins as indicated, a stronger one at about and a weak one at cm-1. Both have Bu symmetry, and both are very perturbed.

14. The stronger band (2443.02 cm-1) is almost certainly the anti-geared bend combination.
Intermolecular mode
Symmetry
Combined with Ag intramolecular mode  cm-1
Combined with Bu intramolecular mode = cm-1
Geared bend (in-plane) Bu Ag
Torsion (out-of-plane) Au Bg
2 x geared bend
vdW stretch
Anti-geared bend (in-plane)
The weaker one ( cm-1) is probably a ‘background’ state which gains intensity from the stronger.

15. Theory and experiment agree very well!!
Note of course that the experiment involves the dimer in the excited (CO2 3) state, while the calculations are for the ground state.
Intermolecular mode
Symmetry
Theory Chen & Light (2000)
Theory Wang et al. (2016)
Present experiment
Geared bend (in-plane) Bu
19.7
20.64
22.26
Torsion (out-of-plane) Au
22.6
24.44
23.24
2 x geared bend Ag
29.5
32.34
31.509
vdW stretch
40.8
46.07 ?
Anti-geared bend (in-plane) 90
92.23
92.25
Interestingly, simple empirical estimates of 32 cm-1 for the stretch and 90 cm-1 for the anti-geared bend were made in 1988, based on centrifugal distortion constants.

16. Conclusions
We have observed three of the four intermolecular (van der Waals) modes of the CO2 dimer by means of IR combination bands in the CO2 3 region, plus an intermolecular overtone.
The experimental frequencies agree very well with a new ab initio calculation by Wang et al.
The geared bend and torsional modes near 23 cm-1 are highly mixed by Coriolis interactions which can be fitted very well.
The anti-geared bend mode at 92 cm-1 is highly perturbed in ways that defy analysis. This could be (partly?) due to tunneling shifts (which have not previously been observed for CO2 dimer).