1. Infrared spectrum of CO – O2 a new weakly bound complex
A.R.W. McKellar
National Research Council of Canada
A. Barclay, N. Moazzen-Ahmadi
Department of Physics and Astronomy University of Calgary
K.H. Michaelian
CanmetENERGY, Natural Resources 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
QCL
Jet Signal
Laser Controller
Laser
Sweep
Trigger
Ref. Gas
Timer Controller
Card (CTR05)
DAQ
Trigger
12 bit
DAQ Card
Etalon 2

3. Only a few weakly-bound complexes containing O2 have been studied by hi-res spectroscopy
O2 – O2
Campargue, Biennier, Kachanov, Jost, Bussery-Honvault, Veyret, Churassy, Bacis, CPL 288, 734 (1998).
Ar – O2
Mettes, Heymen, Verhoeve, Reuss, Lainé, Brocks, CP 92, 9 (1985). [rf Zeeman spectra]
HF – O2 DF – O2
Fawzy, Lovejoy, Nesbitt, Hougen, JCP 117, 693 (2002).
Wu, Sedo, Grumstrup, Leopold, JCP 127, (2007).
H2O – O2
Kuma, Slipchenko, Momose, Vilesov, JPC A 114, 9022 (2010).
N2O – O2
Qian, Seccombe, Howard, JCP 107, 7658 (1997).
Song, Rui, Chuan-Xi, Chinese Phys. B 23, (2014).
SO2 – O2
Walker, Fraser, Hougen, Suenram, Fawzy, Novick, Columbus (1997, 1998, 2016).

4. Only a few weakly-bound complexes containing O2 have been studied by hi-res spectroscopy
O2 – O2
Campargue, Biennier, Kachanov, Jost, Bussery-Honvault, Veyret, Churassy, Bacis, CPL 288, 734 (1998).
Ar – O2
Mettes, Heymen, Verhoeve, Reuss, Lainé, Brocks, CP 92, 9 (1985). [rf Zeeman spectra]
HF – O2 DF – O2
Fawzy, Lovejoy, Nesbitt, Hougen, JCP 117, 693 (2002).
Wu, Sedo, Grumstrup, Leopold, JCP 127, (2007).
H2O – O2
Kuma, Slipchenko, Momose, Vilesov, JPC A 114, 9022 (2010).
N2O – O2
Qian, Seccombe, Howard, JCP 107, 7658 (1997).
Song, Rui, Chuan-Xi, Chinese Phys. B 23, (2014).
SO2 – O2
Walker, Fraser, Hougen, Suenram, Fawzy, Novick, Columbus (1997, 1998, 2016).

5. Observed spectrum, He + CO + O2 expansion mixture

6. Observed spectrum, He + CO + O2 expansion mixture Observed spectrum, He + CO expansion mixture

7. We assign a K = 1 - 0 subband, as shown, for CO-O2!

8. The K = subband of CO-O2 was immediately recognizable because of the similarity with CO-Ar (shown), CO-Ne, CO-N2, etc.

9. We assigned:
K = 10 band
K = 01 band
K = 00 band

10. We assigned: K = 10 band K = 01 band K = 00 band K = 21 band
Plus 2 more
K = 00 bands
(no doubt the K = 2, 0', and 0" levels are also present in the ground vibrational state, but are not sufficiently populated to observe)

11. But wait, there’s more!

12. But wait, there’s more!

13. We assigned a whole new set of levels
We assigned a whole new set of levels! No transitions are observed between the old and new sets. So we don’t know their relative energies.
(the new set is analogous to the previous one, but with K = K  2)

14. O2 molecule 3g J = N + S How to understand CO – O2 ?
N = 1, 3, 5, 7, etc. (nuclear spin symmetry)
S = 1 S N
S N
S N

15. CO molecule 1g+
J = N
N = 0, 1, 2, 3, etc.

16. Free rotor limit E(CO-O2) = E(CO) + E(O2) + B L(L + 1)

17. Free rotor limit E(CO-O2) = E(CO) + E(O2) + B L(L + 1)
We think our first set of levels correlates with the red ones, and our second set with the blue ones

18. Here is how the correlation works out
Truly “allowed” transitions have j(CO) =  1 as shown, because the transition dipole moment is on the CO.
The red and blue sets of levels are separate because they have different orientations of S, and changing this orientation is “forbidden”.

19. The free rotor model is only a limiting case!
Probably the electron spin couples to the intermolecular axis, rather than remaining coupled to the O2 rotation. In this case the results can still be rationalized in a similar way.
This is shown in the table, where the M quantum numbers represent projections of n(O2), S, and j(CO) on the intermolecular axis. Here we follow the spirit of van der Avoird’s paper on Ar – O2 [J. Chem. Phys. 79, 1170 (1983)].
We remain a bit confused about parity!!
(n(O2), j(O2), j(CO))
n(O2), j(CO); Mn(O2), MS, Mj(CO) 
(1, 0, 0), K = 0
2-½ (1, 0; 1, -1, 0 + 1, 0; -1, 1, 0)
(1, 0, 1), K = 1
2-½ (1, 1; 1, -1, 1  1, 1; -1, 1, -1) or 2-½ (1, 1; -1, 1, 1  1, 1; 1, -1, -1)
(1, 0, 1), K = 0' and 0"
2-½ (1, 1; 1, -1, 0 + 1, 1; -1, 1, 0) and 2-½ (1, 1; 1, 0, -1 + 1, 1; -1, 0, 1) or 2-½ (1, 1; 0, 1, -1 + 1, 1; 0, -1, 1)
(1, 0, 2), K = 2
2-½ (1, 2; 1, -1, 2  1, 2; -1, 1, -2) or 2-½ (1, 2; -1, 1, 2  1, 2; 1, -1, -2)
(1, 2, 0), K = 2
2-½ (1, 0; 1, 1, 0  1, 0; -1, -1, 0)
(1, 2, 1), K = 3
2-½ (1, 1; 1, 1, 1  1, 1; -1, -1, -1)
(1, 2, 1), K = 1
2-½ (1, 1; 1, 1, -1  1, 1; -1, -1, 1)
(1, 2, 2), K = 4
2-½ (1, 2; 1, 1, 2  1, 2; -1, -1, -2)
(1, 2, 2), K = 2'
2-½ (1, 2; 1, 1, 0  1, 2; -1, -1, 0)

20. How to analyze/fit the observed levels?
Try a conventional triplet asymmetric rotor Hamiltonian.
Basically this does not seem to work for CO-O2!
Not surprising, since a singlet asymmetric rotor Hamiltonian does not work for CO-N2.
Works for CH2!

21. How to analyze/fit the observed levels?
Use Fawzy/Hougen or Qian/Low/Seccombe/Howard
These Hamiltonians are equivalent (we think).
They depend crucially on the (fixed) angle  between the O-O axis and the a-inertial (intermolecular) axis.
They seem to work for N2O-O2 and HF-O2, but not (we think!) for CO-O2.
A fixed  -value is not realistic for CO-O2 (?).
And, as mentioned, even CO-N2 cannot be fit with a ‘normal’ Hamiltonian.

22. How to analyze/fit the observed levels?
Simply fit each stack separately with effective parameters.
Origin B b
105xD
105xd
v(CO) = 0
K = 0e
0.000
2.2
K = 1e,f
2.820
3.7
-0.4
v(CO) = 1
3.6
-0.2
K = 2e,f
2.6
K = 0'e
5.1
K = 0"e
-65
0.000*
-2.6
3.1
K = 3e,f
2.992*
-2.2
2.910*
-0.6
3.3 *
-2.7 * *
0.3
3.5
K = 4e,f *
-0.9
K = 2'e,f *
-4.4
18.8

23. Conclusions
Extensive IR spectra of CO-O2 are observed using a tunable QCL to probe a pulsed slit-jet supersonic expansion (Teff  2.5 K).
Spectra are assigned on the basis of 10 stacks of rotational levels with well defined K-values ranging from 0 to 4.
The stacks fall into two groups, with no observed transitions between the groups.
The groups correspond to different projections of the unpaired O2 electron spin, S, and correlate with the two lowest O2 rotational levels: (n(O2), j(O2)) = (1, 0) and (1, 2).
The relative energy of the groups is not determined precisely, but from intensities it appears that the (1, 2) group lies about 2 cm-1 above the (1, 0) group. Presumably there is a (1, 1) group at higher energies.