An investigation of the molecular properties of 1,1,1-trichloroethane using laser spectroscopy

Abstract

FT-IR and FT-Raman spectra of 1,1,1-trichlorethane (CH3CCI3) were recorded in the regions 400 - 3500 cm-1 and 200 - 3500 cm-1 respectively. The observed vibrational bands were analysed and assigned to different normal modes of vibration of the molecule. Density functional calculations were performed to support wave number assignment of the observed bands. The equilibrium geometry and harmonic wave numbers of TCE were calculated with the DFT B3LYP method [Spartan, 2004]. The vibrational wave numbers were compared with IR experimental data. The discrepancies between the calculated and observed spectra is that the rotational energy levels cause splitting or broadening of infrared absorbance peaks and this refinement was not included in the calculations using Spartan [2004]. Ultraviolet-visible absorption spectroscopy was used to determine the wavelength needed for excitation and ionization of TCE and it was confirmed that the absorption of energy by TCE is in the deep UV region (from 300 nm increasing strongly down to 200 nm, which is the experimental limit). The time of flight mass spectra of ion products formed from TCE were recorded after excitation by nanosecond and femtosecond laser pulses at various wavelengths (dye laser: 425 nm and 212.5 nm; Nd:YAG laser: 532 nm and 266 nm; femtosecond laser: 795 nm and 397.5 nm) and at various different conditions. The mass spectra obtained at different conditions (wavelength, pulse energy and pulse duration) with both lasers were compared in order to find information about ionization and dissociation of the molecule. The parent ion was not detected in either nanosecond or femtosecond experiments, probably due to the molecule being dissociated easily. The main

difference between nanosecond and femtosecond laser pulse ionization of TCE is that more larger fragments are observed when using femtosecond laser pulses, due to ladder climbing being dominant, while ladder switching is dominant in the nanosecond regime.