 The difficulty is in distinguishing a genuine hidden state from one which is simply out of thermal equilibrium.Probably the first instance of a photoinduced state is described for the organic molecular compound TTF-CA, which turns from neutral to ionic species as a result of excitation by laser pulses.However, a similar transformation is also possible by the application of pressure, so strictly speaking the photoinduced transition is not to a hidden state under the definition given in the introductory paragraph.