Knowledge Bank

Polarity-dependent, Doppler shifts of emissions from neutral species have been observed for the firsttime in a positive column of a DC discharge. We have studied discharge emissions from $H2$ and its isotopomers using a Fourier-transform spectrometer. Experimental evidence supports the hypothesis that the polarity-dependent, Doppler shifts observed for emission lines derive from momentum transferred during the electron-molecule collisions that produce the excited emissive states. We have formulated a theoretical model to relate Doppler shifts to the drift velocity of electrons in discharge. Predictions based on the model are consistent with the experimental observations. For example, we have found experimentally that the Doppler shift of emission from a triplet state is typically larger than that from a singlet state. This result can be interpreted quantitatively in terms of a model which predicts different momentun transfer for singlet-singlet and singlet-triplet electron-molecule excition collisions.