Velocity- and mass-resolved REMPI spectroscopy of van der Waals molecules. A technique for determining the cluster size responsible for spectral features

An experimental method is presented for assigning vibronic spectra of van der Waals molecules based on the measurement of the velocity- and mass-resolved resonance enhanced multiphoton ionisation (VMR-REMPI) spectrum. The basis of the technique is that at each wavelength one measures the velocity map image of the ions produced at a chosen mass. If this mass corresponds to the parent ion for the transition, the image shows a central spot only. However, if this mass is from a fragment ion, the image is enlarged due to the velocity gained by the ion during dissociation. VMR-REMPI spectra consist of plots of the integrated signal in an appropriate image area versus laser wavelength or frequency. The technique is demonstrated with two examples, benzene–acetylenen (n=1–3) and p-difluorobenzene–Ar. Application of the technique to the former system leads to a reassignment of the spectrum.