Molecular Photofragmentation Using AN Argon-Fluoride Excimer Laser

A photofragment spectrometer capable of recording the kinetic energy spectra of ionic photofragments via time-of-flight (TOF) spectroscopy and measuring time resolved dispersed fluorescence has been used to study the dissociation/ionization of small molecules. The fragmentation laser is a line -narrowed, tunable ArF* excimer laser. The laser is tuned to enhance multiphoton processes. We have used the photofragment spectrometer to study the competition between multiphoton dissociation and ionization in carbon monoxide. Our results imply that two photon dissociation channel(s) dominate the three photon dissociation and ionization channels. We have inferred the steps for the dissociation process, i.e., eqalign{&rm CO(X^1Sigma^+) {buildrel{hnu_{193 }}overlongrightarrow} CO(a^3Pi) {buildrel{h nu_{193}}overlongrightarrow } CO(c^3Pi) tocr&qquadqquad C(2s^2 2p^4 ^1D) + O(2s^2 2p^3 ^3P) cr}. From our spectra, we have seen that no fragmentation takes place unless the laser is coincident with the rm a^3Pi >=ts X^1Sigma ^+ transition (Cameron bands). The c ^3Pi state is predissociated due to perturbations by valence ^3Pi states. Our observations suggest a predissociation lifetime for the c^3Pi state of less than 1 ps. We have used the fluorescence or ion spectrum to determine the wavelength of the ArF* laser by comparison to the known Cameron band transitions. Furthermore, because the Cameron band transitions have a narrow linewidth, the laser linewidth can be directly measured. Our experiments have shown that our ArF* laser linewidth is 0.75 cm ^{-1}. We have compared the photofragmentation of CO to the photofragmentation of CO_2. We have determined that CO_2 does not dissociate into ground state CO when irradiated by intense 193 nm radiation.