Mode-specific excited-state dynamics of N-methylpyrrole Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cp00113a

Abstract

State-selective deactivation rates of N-methylpyrrole in the S-1 state have been measured by using the picosecond pump-probe method. The S-1 decay time leading to the N-CH3 bond dissociation is found to be strongly mode-dependent as manifested in both S-1 decay and methyl-fragment growth dynamics. Time-resolved velocity-map ion images of the CH3 fragment, as far as the fragment of the Gaussian-shaped high kinetic energy distribution is concerned, suggest that the N-CH3 cleavage reaction might occur through an intermediate. Sudden decrease of the S-1 lifetime at similar to 700 cm(-1) above the S-1 origin is accompanied by the fragmentation of the Boltzmann-type low kinetic energy distribution. The appearance rate of this low-kinetic energy fragment turns out to be quite slow to give tau similar to 5 ns compared to the S-1 lifetime of similar to 174 ps at the +806 cm(-1) band, for instance, confirming previous findings that the S-1 decay process starts to be overwhelmed by a new fast nonradiative transition in the corresponding excitation energy region. The lifetime at the S-1 origin accessed by the two-photon absorption is firstly measured to give tau similar to 8 ns. Using one and two photon absoption processes, a number of S-1 vibronic bands are identified to give mode-dependent lifetimes spanning an enormously wide temporal range of 8 ns-5 ps in the quite narrow excitation energy region of 0-1800 cm(-1) above the S-1 origin. Understanding of the N-methylpyrrole dynamics on multidimensional excited-state potential energy surfaces governing energy dissipating processes will get much benefit from our detailed mode-specific lifetime measurements.