Knowledge Bank

In the positive column of a DC discharge, ions drift in the direction of the electric field, while the random motion of neutrals are not affected by the field. This causes a Doppler shift in frequency for the ion emission spectra but not for the neutrals. By subtracting two emission spectra taken at opposite polarity of the electrodes, neutral transitions are canceled, however, the ion transitions are not because of the frequency shifts. Therefore, a discrimination between ionic and neutral spectra can be obtained. We have developed this technique and applied it to several molecular ion systems such as $N2+,O2+,CO2+,CS2+,N2O+,C4H2+$, etc. All the emission spectra have been recorded by a Bruker Fourier Transform Spectrometer.