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Lifetimes and Line shapes

In order for photofragment ions to arrive at the electron multiplier, the excited state must dissociate in the time taken for the molecule to traverse the flight region. This places an upper limit on the possible lifetimes observable in an ion beam apparatus. The lower limit is governed by the laser line width; states with small lifetimes (seen as very broad lines) are inefficiently pumped due to the narrow line width of the dye laser, making detection difficult.
The line shape in the absence of Doppler broadening is a Lorentzian, where $\Gamma$, the Full Width at Half Maximum (FWHM) is related to the lifetime ($\tau$) through the Heisenberg Uncertainty Principle:

\begin{displaymath}
\Gamma({\rm cm}^{-1}) \approx \frac{5.3\times10^{-12}}{\tau(s)}
\end{displaymath} (2.2)

The lifetime of the upper state can therefore readily be measured by photofragmentation techniques and the mechanism for dissociation inferred. Broadening occurs due to the geometry and potentials of the source. This shape can be approximated to a Gaussian and so the overall profile is approximated to a convolution of a Gaussian line shape with a Lorentzian. This line shape is known as a Voigt profile [66] and all recorded spectroscopic lines were fitted to a Voigt profile using the PEAKFIT or GNUPLOT programs.


next up previous contents
Next: Source Conditions Up: Features of fast ion Previous: Kinematic Compression   Contents
Tim Gibbon
1999-09-06