Sara E. Ray and Anne B. McCoy Spectroscopic Comparisons of Helium Di-halogen Complexes: What Dictates the Spectral Structure? Sara E. Ray and Anne B. McCoy The Ohio State University with: John J. Glennon and Richard A. Loomis Washington University – St. Louis, MO

Experimental Spectrum HeBr2 0.91 K 17576 manually offset for comparison

Experimental Spectrum HeI2 0.92 K T-shaped Feature Linear Feature 0 cm-1 is the I2 band origin

Potential Energy Surfaces Ground (X ) State Potentiala Fit to ab initio points calculated at the CCSD(T) level of theory SDB-aug-cc-pVTZ basis for I aug-cc-pV5Z basis for He Excited (B ) State Potentialb Sum of He-I Morse potential interactions and I2 RKR potential Morse Parametersc: D = 16.5 cm-1, α = 1.4 Å-1, r = 4.0 Å a: R. Prosmiti, A. Valdes, P. Vallareal & G. Delgado-Barrio, J. Phys. Chem. A, 108, 6065 (2004) b: R. F. Barrow & K. K. Yee, J. Chem. Soc. Faraday Trans. 2, 69, 684 (1973) c: R. L. Waterland, M. I. Lester & N. Halberstadt, J. Chem. Phys., 92, 4261 (1990)

B-State Potential Energy Surfaces I I I I T-shaped Global Minimum De’ = 36.01 cm-1 Re’ = 3.627 Å Linear Saddle Points De’= 18.38 cm-1 Re’ = 5.390 Å Linear Global Minimum De’’= 43.51 cm-1 Re’’ = 4.847 Å T-shaped Local Minimum De’’ = 39.26 cm-1 Re’’ = 3.898 Å

Calculating Energies and Wave Functions Express the Hamiltonian in Jacobi coordinates Average H over a vibrational state (v’=20) of I2 Where…

HeI2 Energies – X-State Energies

Adiabatic X-state PES and Wave Function Probability Amplitudes pot n”=0 n”=1 n”=2 n”=3

Adiabatic B-state PES and Wave Function Probability Amplitudes pot n’=0 n’=1 n’=2 n’=3 n’=4 n’=5 n’=6

HeI2 Experimental vs. Calculated Spectra Calc. 0.92 K Expt. 0.92 K Experimental linear feature (black) manually shifted with respect to the calculated (purple).

HeI2 Energies – B-State

Comparing Linear Features The linear features of HeI2 and HeBr2 are structurally different. HeI2 Expt. Spectrum 0.92 K HeBr2 Expt. Spectrum 0.91 K HeI2 Calc. Spectrum 0.92 K HeBr2 Calc. Spectrum 0.91 K

Comparing Linear Features – Switching Surfaces The linear features of HeI2 and HeBr2 are structurally different. How can we figure out what dictates the structure of the linear features? Is it the B-state potential? Is it the halogen? To answer these two questions we decided to perform a switch. We calculated the HeI2 spectrum (I2 masses and rotational constants) using the potential parametersd from HeBr2 and vice-versa. d: D.S. Boucher, D.B. Strasfeld, R.A. Loomis, J.M. Herbert, S.E. Ray & A.B. McCoy, J. Chem. Phys., 123, 104312-1 (2005)

Switching Surfaces – Spectra Comparison Expt. HeI2 Expt. HeBr2 I2 mass & bv Br2 mass & bv Calc. HeI2 Calc. HeBr2 Top two plots have been manually offset to illustrate agreement

Switching Surfaces – What do we know so far? When we used the I2 masses and rotational constants on the HeBr2 surface, the “switched spectrum” showed better agreement with the HeI2 experimental and calculated spectra than with the HeBr2 spectra. Switching the B-state potential energy surface has little effect on the structure of the linear feature. Where do we go from here? We can further test this theory by taking our HeI2 B-state potential and simplifying it even further. We can then calculate the HeI2 spectrum using this simplified model and compare it to the original.

Elliptical B-state Potential The PES is a function of the sum of the distance of the helium atom from the two focal points that define an ellipse. De and α are chosen to reproduce the J’=0 energy levels. The focal points are chosen to reproduce the minimum energy separations of the linear and T-shaped conformations. X X

Elliptical Diagram & Energies

Linear Assignments of the Calculated and Elliptical Spectra – HeI2 The stick spectra assignments only show transitions to and from J2 and were calculated at 0.45 K. The experimental spectrum was taken at 0.92 K.

Linear Assignments of the Calculated and Elliptical Spectra – HeBr2 The stick spectra assignments only show transitions to and from J2 and were calculated at 0.45 K. The experimental spectrum was taken at 0.91 K.

He-I2 complexes display definite structure. Conclusions He-I2 complexes display definite structure. The linear band contour of HeI2 is distinctly different than the linear band contour of HeBr2. The HeI2 spectrum calculated using the potential parameters of HeBr2 shows better agreement with the original HeI2 spectrum rather than the HeBr2 spectrum. The HeI2 spectrum obtained after simplifying the B-state potential to an elliptical form also shows good agreement with the original HeI2 spectrum. The above two points lead us to believe the kinetic energy plays a more significant role in determining the spectral structure than the potential energy surface.

Loomis Group – Rich Loomis and John Glennon Acknowledgements NSF for Funding McCoy Group – Anne McCoy, Gé Vissers, Samantha Horvath, Charlotte Hinkle, Eric Egnot – graduated José López and Lindsay Johnson John Herbert Loomis Group – Rich Loomis and John Glennon

Acknowledgements NSF for Funding José López Anne McCoy John Herbert Samantha Horvath Gé Vissers Charlotte Hinkle Rich Loomis John Glennon Lindsay Johnson Eric Egnot