New spectra of the CH molecule

Abstract

Absorption spectra of CH and CD molecules have been studied in the ultraviolet and vacuum ultra- violet by means of the flash photolysis of diazomethane. The three Iaiown systems A-X, B-X, and C-X are fairly strong in absorption. Of these the last two, B-X and C-X, have been extended. In par- ticular, in the B-X system diffuseness of the higher rotational lines has been observed. This diffuseness is a result of the predissociation previously derived from the breaking-off of the rotational structure in emission. The difference in the predissociation limits as observed in absorption and emission indicates the presence of a slight maximum in the potential function of the B 2~- state, However, it appears that sufficient tunneling at the base of the barrier arises for the predIssociation limit as observed in emission to be equal to the dissociation limit, and as a result the dissociation energy of CII (and CD) now in the literature is not appreciably changed by the recognition of the potential maximum. The revised value is D00(CH) = 3.454 ± 0.012 eV. A predissociation of the C 2~+ state was recognized early in this work and has already been confirmed by the lifetime measurements of Hesser and Lutz. A number of new electronic transitions have been found in the vacuum ultraviolet, including a Rydberg series starting at 1370 A and yielding an ionization potential of 10.64 eV. At longer wavelengths new electronic transitions D 2H1-X 211 E 211_x 211, F 2~+_x 2fl, and D 211j-B 2~ occur at 1690, 1560, 1540, and 3007 A. The first two and the last are clearly predissociated (diffuse) for all observed J-values, while the third is predissociated for higher f-values. All vacuum-ultraviolet transitions are likely to be of importance in stellar and interstellar spectra in this spectral region. Moreover, the predissociations established in the C, D, and E states are so strong that almost every light quantum beyond 3200 A absorbed by CHin the interstellar medium will lead to its dissociation, thus greatly limiting the lifetime of a given CH molecule