The spectroscopic methods used to probe the monohydrates may be distinguished into action and linear techniques. In action techniques, the effect of the photons on the monohydrates is probed, e.g. ionisation, dissociation or fluorescence. In linear techniques, the effect of the monohydrates on the photons is probed, e.g. direct infrared absorption or inelastic (Raman) scattering . Action techniques often offer much higher sensitivity, they can be size-selective and conformationally selective .