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Doppler Shift

If ions are travelling at high velocities in the same direction as the laser radiation, a positive Doppler shift occurs (i.e. the molecular transition frequency is lower than the observed laboratory transition frequency.) In general, the transition frequency $\nu_{t}$ for a particle travelling at velocity, v is given by:


\begin{displaymath}
{\rm\nu_{t} = \nu_{o} \left({\frac {1 - \frac {v}{c}} {1 + \frac {v}{c}}}\right)^\frac{1}{2}}
\end{displaymath} (2.1)

where $ \nu_{o}$ is the observed frequency and c is the speed of light in a vacuum. For a molecule such as GeH+, at an acceleration voltage of 1,425 V, this corresponds to a positive shift of approximately 3.5 cm-1  at 17,500 cm-1.



Tim Gibbon
1999-09-06