Experimental studies of the 200 nm diffuse band in zinc vapour and its interpretation

Abstract

The absorption spectrum of zinc vapour is measured under various conditions in the 190–224 nm region and the absolute absorption coefficient per pair of atoms is derived. A relatively narrow diffuse band at 200.22 nm (in a vacuum) is observed and its profile is carefully determined for several temperatures. It is found that the band maximum slightly decreases with a temperature increase. This indicates that the band is generated by transitions occurring within the range of negative energy values of the ground-state potential. The experimental results are utilized to derive information concerning the upper-state potential and the oscillator strength function via semiclassical calculations of the band profile and its comparison with the experimental one. It is shown that the theoretical potentials for the lowest Rydberg states available cannot be responsible for the band formation, and some part of the previously unknown excited-state potential near its minimum as well as the f(R) function reproducing the experimental band profile are found. The equilibrium position, Re, and the energy minimum, E(Re), for this state are established to be 3.94 ± 0.15 Å and 49 660 ± 50 cm−1, respectively. It is also shown that f(R), being equal to zero at large distances, for R lower than about 4 Å increases rapidly to the maximal value of 0.61 ± 0.03 at R = 3.65 ± 0.05 Å. Such a behaviour indicates that the upper state here can be coupled with a state characterized by strongly allowed transition to the ground state. A discussion concerning this matter and identification of the upper-state asymptote is presented.