1. 1 - as a function of:electron energy scattering angle - wide angular range - accurate - elastic & inelastic N 2, CH 4, cyclopropane The Art of Measuring Cross Sections

2. 2 Motivation J. Phys. B, in press theory: Nishimura & Gianturco 2002 “The experimental data are not in very good agreement with each other, … by as much as 50%. No experimental integral cross section data exist at very low energies just above threshold and therefore we have no confirmation of the peak shown there by the calculations”

3. 3 Excitation function: RESONANCES (states of the negative ion)

4. 4

5. 5

6. 6 The Magnetic Angle Changer

7. 7 Problems with measuring angular distribution -beam position and energy scale drift as a function of  -the drift has a “memory effect” -thermal broadening and translational excitation depend on , energy and sample -repetitive sweep with stepping motor unpractical Strategies: -record angular spectra in segments (0° - 90°, 45° - 135°, 90° - 180°) and piece results together -record areas under bands, not signal intensity

8. 8 He standard Thermal Doppler broadening and translational excitation are not negligible at high angles and energies in He

9. 9 Normalizing to He

10. 10 Response functions

11. 11 Absolute inelastic values

12. 12 Joining the segments and absolute values

13. 13 The final result dashed: d  wave red: experiment

14. 14 Summary of results in N 2

15. 15 up triangles: Sohn et al. squares: Bundschu et al. down triangles: Tanaka circles: Shyn & Cravens CH 4 elastic diamonds: Newell et al.

16. 16 CH4: elastic DCS green circles: Sohn et al. 1986 green squares: Rohr 1980 green triangles: Bundschu et al. 1997 downward pointing triangles: Tanaka 1982

17. 17 CH4: VE DCS triangles: Bundschu et al. 1997

18. 18 cyclopropane

19. 19 Conclusion the ‚art‘ of DCS measurements has been improved