Submillimetre-wave lines of H2D+ and D2H+ as probes into chemistry in cold dark clouds.

We discuss past and recent progress of our continuing project of submillimetre-wave spectroscopic investigations of H2D+ and D2H+. Three new lines of H2D+ in the 2.5-3.5 THz range are measured with a tunable far-infrared laser system. Since these molecules are very light asymmetric molecules, analysis based on a conventional effective Hamiltonian is not very useful in predicting the transition frequencies to the accuracy of the order of several MHz or better. In this respect, any addition of new accurate measurements of transition frequencies is important. In this paper, some discussions will be made on H5+ and its deuterated species as probable interstellar species in cold dark clouds. In particular, D3+, which is predicted to be abundant in cold dark clouds, can be (indirectly) detected by observing D3+ x H2.