A 193 nm laser photofragmentation time-of-flight mass spectrometric study of CS2 and CS2 clusters

Abstract

A crossed laser and molecular beam photofragmentation apparatus is described. The apparatus is equipped with a rotatable molecular beam source and a translationally movable ultrahigh vacuum mass spectrometer for time-of-flight (TOF) measurements. Using this apparatus we have measured the TOF spectra of S and CS resulting from the photofragmentation processes, CS2+hν(193 nm)→CS(X,v)+S(1D or 3P). The translational energy distributions of photofragments derived from the S and CS TOF spectra are in good agreement. This observation, together with the finding that the TOF spectra of S and CS are independent of laser power in the 25–150 mJ range, shows that the further absorption of a laser photon by CS to form C(3P)+S(3P) within the laser pulse is insignificant. The TOF spectra of S obtained at electron ionization energies of 20 and 50 eV are indiscernible, indicating that the contribution to the TOF spectrum of S from dissociative ionization of CS is negligible at electron impact energies ≤50 eV. The thermochemical thresholds for the S(1D) and S(3P) channels are determined to be 18.7 and 45.0±0.4 kcal/mol, respectively, consistent with literature values. Structures found in the translational energy distribution can be correlated with vibrational structures of CS(X,v=0–5) associated with the S(1D) channel. The translational energy distribution supports the previous observation that the vibrational state distribution of CS(X,v) is peaked at v=3. The TOF experiment is also consistent with the S(3P)/S(1D) ratio of 2.8±0.3 determined in a recent vacuum ultraviolet laser induced fluorescence measurement on the S photofragment. Photofragments from CS2 clusters are observed at small laboratory angles with respect to the CS2 beam direction and are found to have velocity distributions peaked at the CS2 cluster beam velocity.