Photoinduced charge-transfer dissociation in van der Waals complexes. V. Na⋯XCH3 (X=F, Cl, and Br)

Abstract

van der Waals complexes Na⋯XCH3 (X=F, Cl, and Br) have been generated by crossing a beam of sodium with the expansion region of a supersonic jet of the appropriate halide, seeded with a rare gas. The identity of these complexes was determined by photoionization time-of-flight mass spectrometry. The primary route for photodepletion of these complexes is thought to be the excitation of the Na chromophore followed by a charge-transfer dissociation: Na⋯XCH3+hν→[Na*⋯XCH3]‡→[Na+⋯XCH3−]‡→NaX+CH3. Measurement of the photo-depletion cross section as a function of the excitation wavelength provided an approach to the study of these harpooning reactions starting in selected transition states. The action spectra for the three complexes consisted of up to four broad peaks. An assignment, made by ab initio calculations, was based on the electronically excited states of Na* perturbed by the halide molecule in the complex. Peaks, ranging from the red (∼700 nm) to the blue (∼400 nm), were assigned to a superposition of Na* states 3 2Px,y, and to successively higher excited states 3 2Pz, 4 2S, and 3 2D. The transition probabilities computed for the various Na⋯XCH3→[Na*⋯XCH3]‡ transitions were generally in qualitative accord with experiment. Vibrational progressions of the covalent excited states 3 2Px,y,z were observed and analyzed.