Vacuum ultraviolet resonance spectrum of Cl₂ molecule

Abstract

A new series of resonance doublets in the region 1850-1400 A has been identified in the spectrum of Cl₂. The spectra were obtained in electrical excitation of chlorine vapor, and photographed with a Hilger 3-meter normal incidence vacuum grating spectrograph having a resolving power of about 20,000 and a reciprocal plate dispersion of 5.9 A/mm. Analysis of the spectra suggests that the molecules in the rotational level J" = 23, v" = 0 of the ground state go over to J_r = 22 rotational level of v' vibrational level of an upper electronic state by absorbing the chlorine atomic line, 73,980.2 cm⁻¹ and that the observed resonance series results from transitions to the different levels of the normal state of the molecule. The vibrational and rotational constants (in cm⁻¹) of the normal state of Cl₂ obtained by least squares treatment of the data are . The vibrational quanta have been obtained for all the levels up to the dissociation limit and the value of the dissociation energy determined from them is 20,062 ± 10 cm⁻¹ as against the previous value, 19,969 ± 20 cm⁻¹. The potential energy curve of the normal state of Cl₂ has been computed by using the Rydberg-Klein-Rees method and compared with the Hulbert-Hirschfelder curve as well as the Morse curve calculated with the above constants. An anomalous bend has been observed in the rmin limb of the RKR curve and explained as probably due to the mutual perturbation of and levels. Possible assignments to the upper state of the transition involved are discussed. The emitter of the observed resonance series has been concluded to be Cl₂³⁵.