Doppler

Doppler

Whether radiation is being absorber or emitted the frequency at which is takes place depends on the velocity of the atom or molecule relative to the detector. This is for the same reason that an observer hears the whistle of a train travelling towards him or her as having a frequency apparently higher than it really is, and lower when it is travelling away from him or her. The effect is known as the Doppler effect

If an atom or molecule is traveling towards the detector with a velocity, then the frequency at which a transitions is observed to occur is related to the actual transicion frequency in a stationary atom or molecule by

Where m is the mass of the atom or molecule; this is normally far greater than the natural line width. the broadening is inhomogeneous, since not all atoms or molecules in aparticular sample behave in the same way, and results in a line shape known as Gaussian

When collisions occur between gas phase atoms or molecules there is an exchange of energy, which leads effectively to a broadening of energy levels. If is the mean time between collisions and each collision results in a transition between two states there is a line broadening of the transition, where derived from the uncertainty principle of equation. This broadening is, like natural line broadening, homogeneous and usually produces a lorentzian line shape except for transitions at low frequencies, when an unsymmetrical line shape results.

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