Abstract
Photodissociation of pyrrole molecules with IR excited N-H bond stretching vibration (v 1 = 1, ν IR = 3532 cm -1 ) was investigated in a velocity map imaging experiment upon photodissociation at λ UV = 243 nm. The velocity map images show the bimodal structure of fast and slow H-fragments.
In the UV+IR experiment, the fast peak is shifted to 1.04eV compared to 0.81eV of the UV-only photodissociation, which means that approximately 53% of additional vibrational energy (0.44eV) is deposited into H-fragment kinetic energy. The IR pre-excitation also exhibits enhanced H-fragment signal with respect to UV-only experiment due to a more favourable Franck-Condon factor of transition from the vibrationally excited pyrrole.
The UV+IR spectra were further compared with isoenergetic UV-only experiments. Upon excitation at λ UV = 224 nm, the fast peak maximum shifts 0.15eV towards lower energies and only 61 % of total available energy converts into H-fragment kinetic energy in respect to 72% of UV+IR photodissociation, which demonstrates efficient coupling of the excited N-H vibration to the bond dissociation and fragment kinetic energy.
The observation corresponds to spectra of Cronin et al. measured with excitation of λ UV = 220 nm and λ UV = 228 nm with similar lower conversion to fragment kinetic energy of 56% and 70%, respectively. Finally, we measured the time-resolved nanosecond experiment by pump-probe approach.
The H-fragment signal shows similar intensification and kinetic energy distribution independently on the delay of IR pulse up to 150 ns, which illustrates preservation of the vibrational excitation in various degrees of freedom of pyrrole that enhances the UV photodissociation.