Observation of the 4f -3ds transition of the ArH molecule

The emission spectrum of ArH contains a band near 10 110 cm-1 that appears to be the analogue of the 3ds - 4p, v = 0 - 0, band of ArD, observed and analysed near 10 230 cm-1. However, previous attempts to assign the rotational structure of this band of ArH were unsuccessful. Here we observe and analyse the 4f - 3ds band of ArH near 4400 cm-1, and are then able to calculate the rotational structure of the 3ds - 4p transition entirely from known data. The observed band is similar but not identical to the calculated band. We speculate that the observed spectrum is a v - v sequence band of 3ds - 4p, where the v ~0 upper state is populated through some mechanism peculiar to this isotopomer.