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

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

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, 204315 (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, 123301 (2014). SO2 – O2 Walker, Fraser, Hougen, Suenram, Fawzy, Novick, Columbus (1997, 1998, 2016).

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, 204315 (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, 123301 (2014). SO2 – O2 Walker, Fraser, Hougen, Suenram, Fawzy, Novick, Columbus (1997, 1998, 2016).

Observed spectrum, He + CO + O2 expansion mixture

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

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

The K = 1 - 0 subband of CO-O2 was immediately recognizable because of the similarity with CO-Ar (shown), CO-Ne, CO-N2, etc.

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

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)

But wait, there’s more!

But wait, there’s more!

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)

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

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

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

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

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”.

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)

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!

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.

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 0.07724 2.2 K = 1e,f 2.820 0.07911 -0.0037 3.7 -0.4 v(CO) = 1 2142.694 0.07729 2145.482 0.07921 -0.0043 3.6 -0.2 K = 2e,f 2152.571 0.07540 2.6 K = 0'e 2151.824 0.07015 5.1 K = 0"e 2152.794 0.04203 -65 0.000* 0.07322 -2.6 3.1 K = 3e,f 2.992* 0.07347 -2.2 2.910* 0.07394 -0.0026 -0.6 3.3 2142.694* 0.07323 -2.7 2145.666* 0.07379 2146.878* 0.07411 0.3 3.5 K = 4e,f 2152.773* 0.07196 -0.9 K = 2'e,f 2152.488* 0.07281 -4.4 18.8

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.