Knowledge Bank

Formaldehyde is one of the most abundant interstellar molecules. Protonated formaldehyde is a precursor of $H2CO$ and at the same time is formed from $H2CO$ through proton transfer reactions with ions such as $H3+,HCO+,HN2+$. In early theoretical models of the formation of interstellar molecules, $H3CO+$ was assumed to be the most stable form of protonated formaldehyde. Several recent ab initio calculations, however, have predicted that $H2COH+$ is the most stable isomcric form, and have predicted the rotational transition frequencies to assist in astronomical searches. Nevertheless, the predictions have not been accurate enough to warrant astronomical searches. The v1 fundamental band (O-H stretch) of $H2COH+$ has been observed in the $3-\mu m$ region with a difference frequency laser as a radiation source. Protonated formaldehyde was generated in a hollow cathode discharge through a gas mixture of $H2CO(\sim 10$ mTorr) and $H2(\sim 1$ Torr). The band was found perturbed, and only the $Ka=O$ and $Ka=1$ lines have been assigned at the present stage. The spectroscopic parameters for the ground state, however, have been derived from the combination differences. The rotational transition frequencies, that are relevant for search for the interstellar lines, are calculated from these molecular constants.