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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
for a particle travelling at velocity, v is given by:
 |
(2.1) |
where
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