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Centre of Mass Energy Release

Photofragment ions undergo a small change in velocity due to the release of excess energy from the fragmentation process. The width of the momentum profile is a complex function of the momentum (and hence energy) release distribution, the electromagnet's geometry and the fragment's angular distribution. However, the average centre of mass energy release can be approximated to [69]:

\begin{displaymath}
{ \rm T(eV)=\frac{m_2}{16m_{3}}\times\left({\frac{\Delta E}{ \rm E}}\right)^{2}\times {\rm V}_{\rm accn}}
\end{displaymath} (2.3)

Where, m $_{1}^{+}\rightarrow$m2+ + m3, (m2 is the mass of the daughter ion detected, m3 is the mass of the other fragment), $\Delta$E is the energy release recorded in the lab (calculated from the FWHM of the momentum release), E is the energy of the daughter ions and ${\rm V}_{\rm accn}$ is the acceleration voltage.

The profiles of energy releases yield valuable information about the upper state of a particular transition. Fragment ions can be thought of as ejected parallel or perpendicular to the laser polarisation, depending on whether $\Delta$J= 0 (a Q transition) or $\Delta$J=$\pm$1 (P and R transitions) were induced. Fragment ions arising from a Q transition are oriented along the ion beam axis more strongly than fragments arising from a P or R transition (orientated along the laser polarisation axis). In a fast ion beam laboratory experiment which records energy (or momentum) releases using an electron multiplier, Q lines are observed as a doublet (or in the case of lower resolution, a small dip in the centre of the profile), whereas P or R lines exhibit a singlet profile [4].


next up previous contents
Next: Resolution of the Electromagnetic Up: Energy Releases Previous: Conversion of momentum to   Contents
Tim Gibbon
1999-09-06