The ultraviolet absorption spectra of HCN and DCN have been investigated below 2000 Å. Four band systems have been found of which two, α—X and β—X, are described and analyzed in the present paper. At the long wavelength end the α—X system in both HCN and DCN consists of simple progressions of sharp bands whose fine structure has been measured. At shorter wavelengths diffuseness (predissociation) sets in gradually but at somewhat different energies in HCN and DCN. Both the vibrational and rotational analyses lead independently and unambiguously to the conclusion that in the upper state, α, unlike in the ground state, the molecule is non-linear. The geometrical parameters obtained from the rotational fine structure are r0(CH) = 1.140 Å, r0(CN) = 1.297 Å, and [Formula: see text]H—C—N = 125.0°. Two vibrational frequencies in the α state are established: for HCN, ν1 = 1506, ν2 = 949; for DCN, ν1 = 1506, ν2 = 735 cm−1. The CH stretching frequency has not been found.The β—X system has been clearly observed only for DCN. In HCN it is apparently so strongly predissociated that its presence among the diffuse α—X bands is difficult to establish. In the β state, as in the α state, the molecule is bent. The somewhat fragmentary rotational analysis leads to r0(CN) = 1.334 Å and [Formula: see text]D—C—N = 114.5° when r0(CD) = 1.140 Å is assumed. Only one vibrational frequency, ν2 = 731 cm−1, has been established in the β state of DCN. The vibrational quantum number v2 is uncertain since no isotope effect is available to establish the position of the 0–0 band.In the rotational analysis, particularly of the α—X system, the effects of the asymmetry of the molecule in the upper state are clearly demonstrated by the doubling of the levels with K = 1 and K = 2 and the large contributions of the asymmetry to the terms in J2(J + 1)2. From the sign of the K-type doubling for K = 1 it follows unambiguously that both the α and the β state belong to the species A″, i.e. the electronic eigenfunctions are antisymmetric with respect to the plane of the molecule. It appears probable that both transitions α—X and β—X correspond to forbidden transitions (1Δ—1Σ+ and 1Σ−—1Σ+) of the linear case. The non-linear structure of the excited states is briefly discussed in terms of electron configurations.A number of interesting features of the predissociation in the α state are pointed out. It is shown that the dissociation products are in all probability H + CN(2Π).
ULTRAVIOLET ABSORPTION SPECTRA AT REDUCED TEMPERATURES