ABSORPTION-SPECTRA OF ALKALI CYANIDE MOLECULES IN THE VACUUM-ULTRAVIOLET REGION - TRANSITIONS TO DISSOCIATIVE AND PREDISSOCIATIVE STATES

Absorption spectra of alkali cyanide molecules, MCN (M = Na, K, Rb), i n the gas phase were measured in the energy range from 4.1 to 11.4 eV by use of synchrotron radiation. A windowless absorption cell was spec ially, designed for the measurement of absorption spectra at temperatu res less than 1000 K. A major portion of the spectra was ascribable to electronic transitions to repulsive states and to excited ion-pair st ates of MCN. The bands at 8.0, 7.4, and 7.1 eV in the spectra of NaCN, KCN, and RbCN, respectively, are associated with the transitions to t he lowest excited ion-pair states of MCN correlating to M(+) + CN- ((3 ) Sigma(+)). The rest of the bands having a fwhm of about 1 eV were as signed to bound-free transitions to repulsive Rydberg states correlati ng to M(S-2,P-2) + CN(X(2) Sigma(+),A(2) Pi,B-2 Sigma(+)). The bands a ssociated with the higher excited ion-pair states seem to be piled up with the bands associated with Rydberg states higher than these repuls ive Rydberg states. It seems that the repulsive Rydberg states and the excited ion-pair states are related to the dissociative and the predi ssociative states in the reactions of MCN with metastable rare gas ato ms, respectively.