1. Experimental Characterization of the He + I 35 Cl(E,v † =11,12) and He + I 35 Cl( ,v † =0-2) Intermolecular Potential Energy Surfaces Joshua P. Darr and Richard A. Loomis* Washington University in St. Louis Department of Chemistry June 22, 2005 The Ohio State University: 60 th International Symposium on Molecular Spectroscopy

2. Background: The dihalogen molecules have a number of low lying valence electronic states whose asymptotes correlate with atomic fragments At higher energies, there are several ion-pair states clustered close in energy whose asymptotes correlate with ionic fragments For ICl, the 3 lowest ion-pair states are the E, , and D (  =0, 1, and 2, respectively) states

3. ICl Potential Energy Curves I 2 P 3/2 + Cl 2 P 1/2 I + 3 P 2 + Cl – 1 S 2 I 2 P 3/2 + Cl 2 P 3/2 3.03.54.02.5 0 1000 2000 17500 41000 Transition Energy (cm –1 ) I–Cl Distance (Å) X 1+X 1+ B 3  0+ Z 1 B 0 + E 0+ 18000 39000

4. Summary of Previous Work: Rg + XY Ion-Pair State Intermolecular Interactions Collision-induced non-adiabatic transitions –T.A. Stephenson, A.M. Pravilov, and A.A. Buchachenko Ne···ICl(  ) van der Waals complexes –M.I. Lester, T.A. Stephenson Rg···I 2 (f,F,E) complexes –A.M. Pravilov Ar + I 2 (E,D,D, , ,  ) Potential –A.A. Buchachenko

5. Experiment: Pump: excite transitions associated with linear or T-shaped He···ICl in the A–X or B–X region Probe: promote metastable He···ICl(A,B) complexes to the E-state or  -state Conditions: –~10 mm downstream, 200 psi, 0.89 mm nozzle –laser powers: ~20 mJ pump, 10-80  J probe

6. Excitation: I 35 Cl B–X, 2–0 and A–X, 15–0 Region

7. He···ICl Probability Densities 1601208040 5 7 3 (º)(º) R (Å) He + I 35 Cl(X,v  =0) 1601208040 5 7 3 (º)(º) -15.1-18.3 He + I 35 Cl(B,v) -15.60-9.30-7.53-6.36-4.93 n=4 n=3n=2n=1n=0 n  =0n  =1 R (Å) McCoy, et al., JCP 120, 2677 (2004)

8. Observation of He···I 35 Cl  –A, 1–15 and 2–15 Features

9. (nb,ns)(nb,ns) Resonant Two-Photon Spectra in the I 35 Cl  –A and E–B Regions

10. Determination of  E from He···I 35 Cl(  )

11.   EE Determination of  E from He···ICl(  )  E = (h pump,L + h probe,L ) – (h pump,T + h probe,T ) = 5.8 cm –1 After correction for different rotational levels used:  E = 5.4(3) cm –1 h pump,L h pump,T h probe,L h probe,T

12. Binding Energies of Different He···I 35 Cl Electronic States T-shapedLinear He···ICl(X,v  =0) 22.0(2) cm –1 16.6(3) cm –1 He···ICl(A,v=15) He···ICl(B,v=2) -- 13.4(4) -- 13.4(3) He···ICl( ,v † =1) He···ICl(E,v † =12) -- 41.0(1.0) -- 39.2(4)

13. Relative Intensity Changes with v † (nb,ns)(nb,ns)

14. Possible Explanations? Non-adiabatic transitions among the ion-pair states –More efficient detection of E–X fluorescence

15. Relative Intensity Changes with v † ?

16. Change in the potential energy surface with change in v † –Change in I–Cl distance changes potential anisotropy Possible Explanations? –This will change the Franck-Condon factors of the probe transition with the transition from the n=0 level to the (0,0) level being the most sensitive

17. Acknowledgements Prof. Loomis, Dave Boucher, John Glennon, and other previous group members Prof. Anne McCoy, OSU

18. Extrapolation of  E from He···ICl(  )

19. T-shaped Linear Determination of  E from He···ICl(  ) Energy h pump,T h pump,L h probe,T h probe,L 7.0 cm –1 EE A-state  -state X-state  E = h pump,L – h pump,T – 7.0 = 16,228.0 – 16,215.1 – 7.0 cm –1 = 5.9(8) cm –1

20. Features in the ICl A–X Spectral Region Broader than ICl B–X features Linear feature shifted to higher energy More collisional relaxation than in B–X region –smaller energy gap –larger internuclear distance Same downstream distance dependence of T-shaped and linear complexes