1

- as a function of: electron energyscattering angle

- wide angular range

- accurate

- elastic & inelastic

N2, CH4, cyclopropane

The Art of Measuring Cross Sections

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

Excitation function: RESONANCES (states of the negative ion)

4

5

6

The Magnetic Angle Changer

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

He standard

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

9

Normalizing to He

10

Response functions

11

Absolute inelastic values

12

Joining the segments and absolute values

13

The final result

dashed: d wave

red: experiment

14

Summary of results in N2

15

up triangles: Sohn et al.

squares: Bundschu et al.

down triangles: Tanaka

circles: Shyn & Cravens

CH4 elastic

diamonds: Newell et al.

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

CH4: VE DCS

triangles: Bundschu et al. 1997

18

cyclopropane

19

Conclusion

the ‚art‘ of DCS measurements has been improved