Femtosecond photodissociation dynamics of 1,1,1-trifluoro-2The iodoethane in the Franck-Condon region

Abstract

We report resonance Raman spectra of 1,1,1-trifluoro-2-iodoethane in cyclohexane taken with excitation wavelengths within the A-band absorption. The experimental A-band resonance Raman intensities and absorption spectrum were simulated using time-dependent wavepacket calculations and a simple model. The results of the time-dependent wavepacket calculations were used in conjunction with the normal mode descriptions found from normal coordinate calculations to obtain the short-time photodissociation dynamics (at 15 fs) of 1,1,1-trifluoro-2iodoethane. The short-time photodissociation dynamics of 1,1,1-trifluoro-2-iodoethane has a large amount of multidimensional character and is qualitatively consistent with an impulsive ‘semi-rigid’ radical model of the photodissociation dynamics that shows the CF₃ CH₂ radical going towards a more planar geometry about the CH₂ carbon atom as the C–I bond is being broken. Our resonance Raman intensity analysis indicates that in the beginning stages of the photodissociation in the Franck–Condon region the C–C bond becomes longer, the CCI angle becomes smaller, the HCH and CCH angles become larger, two of the FCC angles (out of the CCI plane) become larger while the third FCC angle (in the CCI plane) becomes smaller, and there is some motion in the FCF angles and C–F bond lengths that is either positive or negative in sign, depending on the sign combination of the normal mode displacements. We also compare the photodissociation dynamics of 1,1,1-trifluoro-2-iodoethane with that of iodoethane.