Abstract
We investigate photodissociation of vibrationally excited pyrrole molecules in a velocity map imaging experiment with IR excitation of N–H bond stretching vibration v1 = 1, νIR= 3532 cm−1, and UV photodissociation at λUV= 243 nm. In the IR+UV experiment, the H-fragment signal is enhanced with respect to the 243 nm UV-only photodissociation due to a more favorable Franck-Condon factor for the vibrationally excited molecule. In the measured H-fragment kinetic energy distribution, the maximum of the fast peak in the IR+UV experiment is shifted by 0.23 eV compared to the UV-only photodissociation which corresponds to 53 % of the vibrational energy deposited into the fragment kinetic energy. We compare our results with an isoenergetic UV-only photodissociation at λUV= 224 nm. About 72 % of the total available energy, is released into the fragment kinetic energy in the IR+UV experiment, while it is only 61 % in the UV-only photodissociation. This can be substantiated by the coupling of the N–H bond stretching vibration into the kinetic energy of the departing H-fragment. We also probe the time-dependent dynamics by a nanosecond pump-probe experiment. The IR excitation enhances the N–H bond dissociation even when the UV photodissociation is delayed by 150 ns. This enhancement increases also the yield of the fast fragments at the position of the peak corresponding to the IR+UV excitation, i.e. even 150 ns after the IR vibrational excitation, the same amount of the IR excitation energy can be converted into the H-fragment velocity as immediately after the excitation.
Highlights
Pyrrole C4H5N is a simple five-membered ring heteroaromatic molecule, representing an example of UV-active chromophore in such biomolecules as hemes and chlorophylls
At photon energies relevant to our present investigation, the following N–H bond dissociation dynamics can be drawn from previous studies: Upon the excitation to the first πσ∗ (1A2) state at λ ≈ 240–236 nm, just above the barrier, the dissociation proceeds along the N–H coordinate to a conical intersections (CI) with the ground state S0 where the reaction routes split
We investigate the photodissociation of vibrationally excited pyrrole in a velocity map imaging (VMI) experiment with IR+UV excitation at νIR= 3532 cm−1 and λUV = 243 nm
Summary
Pyrrole C4H5N is a simple five-membered ring heteroaromatic molecule, representing an example of UV-active chromophore in such biomolecules as hemes and chlorophylls. At photon energies relevant to our present investigation, the following N–H bond dissociation dynamics can be drawn from previous studies: Upon the excitation to the first πσ∗ (1A2) state at λ ≈ 240–236 nm, just above the barrier, the dissociation proceeds along the N–H coordinate to a CI with the ground state S0 where the reaction routes split. The total kinetic energy release (TKER) spectra show similar bimodal distributions with an increase in the intensity of the slow H-atoms compared to the fast ones This can be rationalized by the involvement of other excited states and CIs. through which the population can be funneled into the ground state. The H-fragment kinetic energy distributions from our experiments are compared to previous single UV-photon dissociation experiments at comparable excitation energies. we introduce a time delay between IR and UV pulses, which provides information about the IVR dynamics
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