A search for NaH in dense molecular clouds : evidence against formation on grains.

The J = 1-0 rotational transition of NaH, at 289.864 GHz, was searched for but not detected toward five dense molecular clouds. The most stringent upper limit on the NaH fractional abundance is found for Orion A, where forbidden NaH/forbidden H2 is less than 1/10 to the 11th. This upper limit is consistent with the predictions of ion-molecule chemistry but is several orders of magnitude smaller than would be expected if NaH were formed efficiently on grains. The 6(0)-5(0) E transition of methanol, at 289.939 GHz, was detected toward Orion A and Sgr B2 during the observations.