5314027763

POSTER SESSION. H17

THE MYSTERY OF THE CO DIMER: VARIABLE TEMPERATURĘ JET-COOLED INFRARED SPECTRA OF (¹²C¹⁶0)₂ AND ^l3C^ieO-

^l2c^l6o

M.D. BROOKES AND A.R.W. McKELLAR. Steacie Institute Jor Molecular Sciences, National Research Council of Canada, Ottawa, On-tańo KIA 0R6, Canada.

The CO dimer represents an outstanding unsolved problem in spectroscopy. It was first detected alinost 20 years ago in the microwave region by Klemperer’s group.¹ Morę recently the IR spectrum, which appears in the 2140 cm"¹ region close to the origin of the CO monomer fundamental band, was observed in Bonn.² Theoretical calculations³ indicate that the CO-CO intermolecular potential surface has a ruimber of minima differing in energy by smali arnounts (10-20 cm¹) and separated by smali barriers (10-20 cm^-1), so it may not be meaningful to think of a unique geoinetrical structure for this species.

We have extended the study of the IR spectrum,⁴ using a rapid-scan diodę laser spec* trometer together with both planar and axisymmetric jet expansions. By probing the jet at varying distances downstream, we observe the (CO)₂ spectra over a rangę of effective rotational tcmperatures from about 15 K down to 1 K. The temperaturo dependencc of the transitions allows us to identify one perpendicular and two parallel subbands with comraon ground State combination differences, arising frorn the lowest rotational levels of the dimer. Only even lower-state ./-values are present due to nuclear spin statistics, and there are thus no allowed ground State pure rotational transitions ainong the levels we have assigned. The observed rotational level spacings correspond to a surprisingly large effective intermolecular distance of about 4.35 A , as compared for example to 4.03 A for the rather similar CO-N₂ complex.⁵ Our assignment is incompatible with the earlier analysis of Havenith et. al.²

Using a 13-C enriched sample, we have now been able to detect two of the same three subbands in the mixed dimer ^,3C^lf’0-¹²C^I60. These are only slightly shifted from their positions in the symmetric isotope, confirming our expectation that they correspond to transitions whcre vibrational and rotational excitation take place on the same CO con-stituent. Both even and odd ./-values are present for the isotopically mixed dimer, and we are thus able to predict the positions of allowed pure rotational transitions in the 6-20 GHz region accessible to sensitive FT rnicrowave spectroineters.

1

   P.A. Vanden Bout, J.M. Steed, L.S. Bernstein, and W. Klemperer, Astrophys. J. 234, 503 (1979).

2

   M. Havenith, M. Petri, C. Lubina, G. Hilpcrt, and W. Urban. Mol. Phys. 167, 248 (1994).

3

   A. van der Pol, A. van der Avoird, and P.E.S. Wormer, J. Chern. Phys. 92. 7498 (1990); P.R. Bunker, P. Jensen, S.C. Althorpe. and D.C. Clary, J. Mol. Spectrosc. 157, 208 (1993); A.W. Meredith and A.J. Stone, Mol. Phys., to be published.

4

   M.D. Brookes and A.R.W. McKellar. Chem. Phys. Lett. 287, 365 (1998).

5

   Y. Xu and A.R.W. McKellar, J. Chem. Phys. 104, 2488 (1996).