1. High-resolution threshold photoionization and photoelectron spectroscopy of propene and 2-butyne Julie M. Michaud, Konstantina Vasilatou and Frédéric Merkt Laboratorium für Physikalische Chemie, ETH Zürich, Switzerland

2. 2 Outline Motivation Previous work on 2-butyne and propene Photoion and photoelectron spectra at 1 cm -1 resolution Photoelectron spectra at <1 cm -1 resolution Summary and outlook

3. 3 Motivation Study of large molecules with high resolution photoelectron spectroscopy; resolve partially or fully the rotational structure; Investigate the spectral signatures of internal rotation in the cations; Effect of different functional groups; Effect of 1 versus 2 internal rotors.

4. 4 2-butyne a C. di Lauro, P. R. Bunker, J. W. C. Johns and A. R. W. McKellar, J. Mol. Spectrosc. 162, 142-151 (1993). Large molecule with high symmetry (A = 2.66 cm -1 ; B = C = 0.1119 cm -1 ) a 2-rotor system analogous to ethane  methyl groups much further apart could mean a less restricting potential many theoretical and experimental studies of the neutral can be deuterated (d 6 and 1,1,1-d 3 )

5. 5 Internal rotation in neutral 2-butyne 1939: Infrared and Raman spectroscopic study of the vapour and liquid of 2-butyne. a 1941: Free rotation in dimethylacetylene was first suggested. b 1954: Free rotation was confirmed by I. M. Mills and H. W. Thompson from the spacing of the Q-branches in the infrared spectrum. c 1964: Bunker and Longuet-Higgins revisited the spectrum of Mills and Thompson and determined the torsional barrier to be not greater than 10 cm -1. d a B. L. Crawford Jr. J. Chem. Phys. 7, 555 (1939); b D. M. Yost, et al. J. Am. Chem. Soc. 63, 3492 (1941); c I. M. Mills, et al., Proc. Roy. Soc. A, 226, 306 (1954); d P. R. Bunker, et al., Prod. Roy. Soc. A, 280, 340 (1964).

6. 6 In 2-butyne: -first vibrational progression (2110 cm -1 ): 2 ; C  C stretch (2313 cm -1 in neutral) -second progression (1270 cm -1 ): 3 ; C-H bending, parallel to the axis of the CH 3 group (1380 cm -1 in neutral) P. Carlier, J. E. Dubois, P. Masclet, G. Mouvier, J. Electron Spectrosc. 7, 55 (1975).

7. 7 Propene - prototypical system with single rotor next to double bond A. B. Burrill et al. J. Chem. Phys. 115, 133, 2001. MATI spectrum E i : 78587± 4 cm -1

8. 8 Propene - torsional barrier in cation a A. B. Burrill et al. J. Chem. Phys. 115, 133, 2001; b J. R. Durig et al. J. Chem. Phys. 93, 3487, 1989; c M. L. Senent et al. J. Mol. Struct. 372, 257, 1995. Potential for pure torsional rotation Barrier height: in neutral: 693.7 b - 721 c cm -1 In cation: 429 a cm -1 429 cm -1

9. 9 Photoelectron spectroscopy Neutral state Ionic states h E kin

10. 10 Propene - ZEKE photoelectron spectrum; X + (a”)  X(a’/a”) Wavenumber (cm -1 ) A. M. Schulenburg, F. Merkt, Unpublished results. +0.5 V -0.8 V -2 V -5 V Spectrum shown from -2 V pulse

11. 11 Experimental set-up - 1 cm -1 resolution

12. 12 Excitation scheme 94092.87 cm -1 Kr resonance Single-photon excitation from the neutral ground state of 2-butyne or propene. VUV radiation generated by difference frequency mixing in Kr using the 94092.87 cm -1 two-photon resonance.

13. 13 2-Butyne - origin band X + ( 2 E u )  X ( 1 A) Ion signal Electron signal; +1V, -1.5V, +2.4 kV MCP, 0.05 nm/step Wavenumber (cm -1 )

14. 14 Wavenumber (cm -1 ) +0.5 V / -0.8 V pulse 2-Butyne - X + ( 2 E u )  X ( 1 A) Ground vibrational rotational constants a A v=0 2.66 cm -1 B v=0 0.112 cm -1 a P. R. Bunker et al. J. Mol. Spectrosc. 162, 142 (1993).

15. 15 2-butyne - X + ( 2 E u )  X ( 1 A) Wavenumber (cm -1 ) +0.5 V / -1.5 V pulse 2169 cm -1 = 2 P. Carlier, et al. J. Electron Spectrosc. 7, 55 (1975).

16. 16 Experimental set-up - 0.1 cm -1 resolution

17. 17 a G. Wlodarczak et al. J. Mol. Spectrosc. 167, 239 (1994). Fixed a Fit T rot 10 K E i 78600.96 cm -1 A e 1.5438 cm -1 A e + 1.330 cm -1 B e 0.3105 cm -1 B e + 0.305 cm -1 C e 0.2712 cm -1 C e + 0.276 cm -1 Ionization from a p  + d  orbital Propene E i (Burrill et al.): 78 587 cm -1

18. 18 Propene-d 6 - ZEKE spectrum; X + (a”)  X(a’/a”) A. M. Schulenburg, F. Merkt, Unpublished results. +0.5 V -0.8 V -2 V -5 V Spectrum shown from -2 V pulse Wavenumber (cm -1 )

19. 19 Propene-d 6 - Excited vibrational bands Wavenumber (cm -1 ) 789057891078915 79080 79095 Excited vibrational bands give additional insight into the torsional motion. Spectra are complicated by the lower intensity and nuclear spin of 6 deuterium atoms.

20. 20 Summary and outlook Continue the analysis of the propene and d 6 - propene photoelectron spectra; Obtain spectra of the perdeuterated and 2- butyne-1,1,1-d 3 isotopologues; Study 2-butyne with the ring laser set-up to resolve more of the rotational structure; Analyze the spectra and extract information regarding the torsional motion in 2-butyne.

21. 21 Acknowledgements Thank you to the Merkt group; Thank you to the technical staff at ETH Zürich, especially Hansjürg Schmutz and Guido Grassi; Funding: ETH Fellowship and Swiss National Science Foundation.

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