Strange combination band of the cross-shaped complex CO2 – CS2 Nasser Moazzen-Ahmadi Department of Physics and Astronomy University of Calgary A.R.W. McKellar National Research Council of Canada

CO2-CS2 observed by Beaudet group TDLS 2009, Zermatt, Switzerland CO2-CS2 observed by Beaudet group 2

only Ka = even levels in ground state TDLS 2009, Zermatt, Switzerland C2v point group only Ka = even levels in ground state 3

TDLS 2009, Zermatt, Switzerland cross-shaped minimum, but slipped-parallel is only 2.64 cm-1 higher (or 8.25 cm-1 higher zpe) 4

TDLS 2009, Zermatt, Switzerland We observe the same fundamental band, with c-type selection rules, in good agreement with Beaudet Beaudet (1998) Present experiment Theory (Brown et al.) A” 0.08590 0.08593 0.08591 B” 0.04634 0.04638 0.04643 C” 0.03546 0.03542 ΔK” 1.06 e-7 -4.32 e-7 -1.12 e-7 ΔJK” -1.01 e-7 3.59 e-7 1.03 e-7 ΔJ” -1.37 e-7 4.65 e-7 1.09 e-7 ν0 2346.5448 2346.5464 5

We also observe two CO2-CS2 combination bands: TDLS 2009, Zermatt, Switzerland We also observe two CO2-CS2 combination bands: b-type, origin at 2361.838 cm-1 (+15.292 from fundamental) a-type, origin at 2388.426 cm-1 (+41.880 from fundamental) Clearly the 15.292 cm-1 combination band is the torsion, and 41.880 cm-1 is the CO2 bend!! The b- and a-type selection rules are consistent with this. 6

Central portion of the CO2-CS2 torsional combination band TDLS 2009, Zermatt, Switzerland Central portion of the CO2-CS2 torsional combination band 7

Torsion Theory (Brown et al.) TDLS 2009, Zermatt, Switzerland The torsional band seems to be fairly well-behaved (however note ΔK and ΔJK) Torsion Ground state Torsion Theory (Brown et al.) A 0.08547 0.08593 0.08608 B 0.04600 0.04638 0.04605 C 0.03498 0.03542 0.03497 ΔK -69.5 e-7 -4.32 e-7 -13. e-7 ΔJK -6.61 e-7 3.59 e-7 20. e-7 ΔJ 6.25 e-7 4.65 e-7 2.8 e-7 ν0 2361.8378 Recall that we observe this band at +15.292 cm-1 from the fundamental, and calculated torsion was 15.261 cm-1 !!! 8

But the CO2 bend band is bizarre! TDLS 2009, Zermatt, Switzerland But the CO2 bend band is bizarre! We can fit it (quite well), but only by having A’ ~ 0.7 cm-1 and ΔK’ ~ 0.03 cm-1, which is crazy! Recall this is an a-type band, and only even values of Ka are possible 9

CO2 bend, theory (Brown et al.) TDLS 2009, Zermatt, Switzerland Crazy parameters are required to fit this band. But the fit is pretty good; intensities and positions of some ΔK = 2 transitions are even predicted well CO2 bend Ground state CO2 bend, theory (Brown et al.) A 0.7084 0.08593 2.484 B 0.04443 0.04638 0.0453 C 0.03443 0.03542 0.0331 ΔK 0.0287 -4.32 e-7 ΔJK 0.000251 3.59 e-7 ΔJ -80. e-7 4.65 e-7 ν0 2388.4264 [41.880] 41.918 10

TDLS 2009, Zermatt, Switzerland Showing good agreement of observed and simulated spectrum, including intensities. Note ΔK = 2 transitions. 11

TDLS 2009, Zermatt, Switzerland 12

TDLS 2009, Zermatt, Switzerland Interestingly, crazy parameters were also predicted for this vibration by Brown et al. In their paper, they only reported J = Ka = 1 levels (which are forbidden). But now we have their higher calculated levels, and agreement is amazing! JKa Observed Calculated 00 41.880 41.918 11 44.441 22 44.329 44.277 33 48.573 44 46.039 45.942 (Recall that Calc is for pure intermolecular vibrations, while Obs is for intermolecular + CO2 2 vibration.) 13

TDLS 2009, Zermatt, Switzerland JKaKc Observed Calculated Obs -Calc 000 41.880 41.918 -0.038 101 41.960 41.996 -0.036 202 42.117 42.154 -0.037 221 44.329 44.281 +0.048 220 44.274 +0.055 303 42.353 42.391 322 44.564 44.516 321 44.563 44.515 404 42.669 42.709 -0.040 423 44.878 44.821 +0.057 422 44.875 44.827 441 46.039 45.942 +0.097 440 14

TDLS 2009, Zermatt, Switzerland Conclusions We have excellent agreement between experiment and theory, so this combination band is not so strange, after all! But what does it mean? Theory is just a black box, in a sense. Is this what Phil Bunker calls “big computer, small brain”? At least with theory we can look at wavefunctions and fractional parentage. Can we blame the strange energy level pattern on multiple tunneling paths? Is there a simple hindered rotation model which might fit the observations? 15