If there are negative ions in a plasma, they can be detected by photodetachtment. Photodetachment is the process in which an electron is released from a negative ion by irradiating the ion with photons of the appropriate wavelength (the photon energy should be larger than the detachment energy). In this process, an extra free electron is created. That electron can be detected by various diagnostics, but in our group we have (up to now) mostly used microwave resonance spectroscopy. The electrons modify the refractive index of the plasma, and that can be measured by microwave cavity resonance. Of course, after the end of the laser pulse, the extra electrons start to re-attach again, so one has to be quick in order to quantitatively measure the negative ion density.