1. The Cyclic CO 2 Trimer: Observation of two parallel bands and determination of intermolecular out-of-plane torsional frequencies Steacie Institute for Molecular Sciences Mahin Afshari, Mehdi Dehghany, N. Moazzen-Ahmadi Department of Physics & Astronomy, University of Calgary A. R. W. McKellar Steacie Institute for Molecular Sciences, National Research Council of Canada

2. Outline Experimental setup  Supersonic jet apparatus with a tunable diode laser probe Spectra and analysis  Previous works on CO 2 trimers  Parallel bands of the planar CO 2 trimer  Future works

3. Experimental setup TDL Jet Trigger Ref. Gas 12 bit DAQ Card Timer Controller Card (CTR05) Laser Sweep Trigger DAQ Trigger Gas Supply Jet Signal Jet Controller (Iota One) Jet Controller IR Detectors TDL Controller (L5830) Etalon Monochromator Supersonic jet apparatus with a tunable diode laser probe

4. Current ramp to TDL from L5830 Trigger from L5830 DAQ Card records each is 4000 points @ 4MHz Jet trigger to Iota One from CTR05 Actual jet opening (approx.) d1 d2 Background Dummy Jet signal Time sequence for data acquisition 1 kHz ~ 2.5 ms 1 ms

5. Conclusion Background Jet signal

6. Multi-channel Jet nozzle 30 mm 2 2 2 1.5 1 1 2 Side view G. Hilpert et al. CPL 219, 384 (1994). Top view 25 μm

7. Experimental condition Carrier GasConcentration of the CO 2 Backing Pressure Line widthRotational Temperature He~0.8%8 atm0.0015 cm -1 ~2.5 K

8. Previous studies on the CO 2 trimer High-resolution infrared diode laser spectroscopy of (CO 2 ) 3 : Vibrationally averaged structures, resonant dipole vibrational shifts, and tests of CO 2 –CO 2 pair potentials J. Chem. Phys. 103 (18), 9 November 1995 Geometric isomerism in clusters: High resolution infrared spectroscopy of a noncyclic CO 2 trimer J. Chem. Phys. 105 (23), 15 December 1996 J. Chem. Phys. 87, 1502 August 1987 Sub-Doppler infrared spectrum of the carbon dioxide trimer G. T. Fraser a), A. S. Pine a), W. J. Lafferty a), and R. E. Miller b) a) Molecular Spectroscopy Division, National Bureau of Standards, Gaithersburg, Maryland 20899 b) Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27514

9. Symmetry of the vibrational fundamentals of the cyclic (CO 2 ) 3  vib =  tot -  rot -  trans = 5 A  5 E  2 A   2 E  E C 3 C 3 2  h S 3 S 3 2  tot 27 0 0 9 0 0  tot =6 A  6 E  3 A   3 E   rot = A  E   trans = E  A 

10. 8 Intramolecular vibrations  vib = 5 A  5 E  2 A   2 E  2A : Symmetric in-phase combination of 3 monomers stretches ( 3 & 1 ) A  : Symmetric combination of 3 out of plane monomers bends ( 2 ) A : Symmetric combination of 3 in-plane monomers bends ( 2 ) 2E : Asymmetric out of phase combination of 3 monomers stretches ( 3 & 1 ) E : Asymmetric combination of 3 in-plane monomers bends ( 2 ) E  : Asymmetric combination of 3 out of plane monomers bends ( 2 ) 6 Intermolecular vibrations A : Symmetric combination of 3 in- plane van der Waals stretch A : Symmetric combination of 3 in- plane bends A  : Symmetric combination of 3 out of plane torsion E : Asymmetric combination of 3 in-plane van der Waals stretch E : Asymmetric combination of 3 in-plane bends E  : Asymmetric combination of 3 out of plane torsion Symmetry of the vibrational fundamentals of the cyclic (CO 2 ) 3

11. Selection rules for the cyclic (CO 2 ) 3 vibrations in the region of monomer 3 fundamental A E =  Ta &  Tb Perpendicular band Not Allowed A EA A A  =  Tc Parallel band A  A  =A  Not Allowed A A  E  =E  A E  E  = 2A  E  A  =  Tc Parallel band Vibrational fundamentals: Vibration + Torsion combinations:

12. Lower frequency parallel combination band of the cyclic (CO 2 ) 3 Observed Simulated

13. torsion = 12.3 cm -1

14. The higher frequency parallel combination band of the cyclic (CO 2 ) 3 Low backing pressure Simulated torsion = 18.8 cm -1 High backing pressure

15. Find the exact monomer tilt angle in the cyclic CO 2 trimer from isotopic effects Future works on the cyclic CO 2 trimer 

16. Summary of the observed CO 2 cluster bands and their band origins (in the region of CO 2 3 fundamental) (cm -1 ) 2349.14 3 2350.772 1987 Howard 2351.719 1995 Nesbitt 2343.291 1996 Nesbitt 2352.721 1996 Nesbitt 2364.020 2007 Our group 2370.489 Perpendicular band Parallel bands Not published torsion = 12.3 cm -1 torsion = 18.8 cm -1

17. Thank you!

18. N 2 O centrosymmetric dimer spectrum in the 1 band region

19. ..... and several other bands with similar structures. B=0.005 cm -1 Q-branchB=0.0035 cm -1 Q-branch? B=0.003 cm -1 Q-branch? Future works on CO 2 clusters Higher clusters?