Observation of the 4f -> 3d sigma transition of the ArH molecule

The emission spectrum of ArH contains a band near 10 110 cm(-1) that appear s to be the analogue of the 3d sigma - 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 - 3d sigma band of ArH near 4400 cm(-1), and are then a ble to calculate the rotational structure of the 3d sigma - 4p transition e ntirely 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 seq uence band of 3d sigma - 4p, where the v not equal 0 upper state is populat ed through some mechanism peculiar to this isotopomer.