The ultra-violet spectra of HF and DF

Abstract

Successful analyses have been m ade of th e ultra-violet spectra excited by electric discharges in m ixtures of helium with HF or DF. The spectra, which consist of large numbers of irregularly spaced lines, are shown to arise from transitions V 1 Σ + - X 1 Σ + in neutral HF and DF. State X is the ground state: state V , the lowest stable excited singlet state, probably correlates with H + + F - . The potential minimum of state V lies at 10·5 eV above that of the ground state. The internuclear distance in this state —in agreement with theoretical predictions — is large, more than twice that in the ground state, so that the bands observed consist of transitions from low vibrational levels in V to high vibrational levels in X . In HF, the observed levels are 9 ≤ v " ≤ 19: predissociation by rotation is observed in the levels 10 ≤ v " ≤ 19. The last vibrational state which may have a few stable rotational levels is v " = 20, so that, by combining the present observations with those of the vibration-rotation bands, there is now available information about the vibrational and rotational levels over the greater part of their range of stable existence in the ground state. The dissociation energy, D" 0 , of HF is found to be 5·86 ±0·01 eV.