Multiphoton ionization and dissociation dynamics of Freon-113 induced by femtosecond laser pulse

Abstract

The ozone layer which absorbs harmful solar UV radiation is an essential umbrella for human. However, a large number of exhausts of Freon released by human activity into the atmosphere pose a great threat to the ozone layer. The UV sunlight radiation induced Freon dissociation produces chlorine radicals, which are found to be the main culprit for destroying the atmospheric ozone. In this paper, multiphoton ionization and dissociation dynamics of Freon-113 (CF2ClCFCl2) induced by femtosecond laser pulse are studied by time-of-flight mass spectrometry coupled with velocity map imaging technique. Fragment mass spectra of Freon-113 are measured by time-of-flight mass spectrometry. No parent ions are discovered in the time-of-flight mass spectra, and all the detected ions are from the fragmentation induced by the laser pulse. Daughter ions CFCl2+, CF2Cl+, C2F3Cl2+ are found to be the three major fragmentation ions in the multi-photon ionization and dissociation. Several photodissociation channels are discussed and concluded by further analysis and calibration (via the ratio of mass to charge) of the measured time-of-flight mass spectra. Three main photodissociation mechanisms are found as follows: 1) C2F3Cl3+→C2F3Cl2++Cl with breaking C--Cl bond and directly producing the Cl radical; 2) C2F3Cl3+ →CFCl2++CF2Cl with breaking the C--C; 3) C2F3Cl3+ →CF2Cl++CFCl2 with breaking the C--C bond. Ion images of the three main fragments C2F3Cl2+, CFCl2+ and CF2Cl+ are measured by the velocity map imaging setup. The speed distributions of these three fragment ions are obtained from the velocity map imaging. The speed distribution of C2F3Cl2+ with breaking C--Cl bond can be fitted by two Gaussian distributions while the speed distributions of both CFCl2+ and CF2Cl+ with breaking the C--C bond can be well fitted by one Gaussian distribution. The different fittings reflect different production channels. The detailed photodissociation dynamics is obtained by analyzing the kinetic energy distribution and angular distribution of the fragment ions. Additionally, density functional theory calculations on high-precision level are also performed on photodissociation dynamics for further analysis and discussion. An in-depth understanding of dissociation dynamics of freon can provide theoretical reference and experimental basis for further controlling the dissociation process that can do destruction to the ozone layer.