During this transfer, the electron typically winds up in an excited state (high energy level) of the impurity ion. As the electron decays down to the ground state (minimum energy level), energy conservation requires radiation to be emitted by the impurity ion. This emission has discrete values of energy, or wavelength, which correspond to the energy differences between the initial and final atomic levels of a particular electron transition. For example, consider charge exchange between a deuterium atom and a C+6 ion: if the electron is transferred to the "n"=7 energy level of the carbon ion, then the ion will emit radiation at discrete energies given by the difference in energy between the "n"=7 and "n"=6 levels, the "n"=6 and "n"=5 levels, the "n"=5 and "n"=4 levels, and so on (down to "n=1"). This line emission is Doppler-broadened as a result of ion thermal motion, and Doppler-shifted as a result of ion flow. The Doppler shift causes the emission to be blue-shifted (towards shorter wavelength/higher frequency) if the ions are moving towards the point of observation, or red-shifted (towards longer wavelength/lower frequency) if the flow is away from the point of observation. Measurements of the carbon emission line shape are therefore used to extract values for the impurity ion temperature and velocity.