Mode-specific excited-state dynamics of N-methylpyrrole

Abstract

State-selective deactivation rates of N-methylpyrrole in the S₁ state have been measured by using the picosecond pump–probe method. The S₁ decay time leading to the N–CH₃ bond dissociation is found to be strongly mode-dependent as manifested in both S₁ 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 S₁ lifetime at ∼700 cm⁻¹ above the S₁ 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 τ ∼ 5 ns compared to the S₁ lifetime of ∼174 ps at the +806 cm⁻¹ band, for instance, confirming previous findings that the S₁ decay process starts to be overwhelmed by a new fast nonradiative transition in the corresponding excitation energy region. The lifetime at the S₁ origin accessed by the two-photon absorption is firstly measured to give τ ∼ 8 ns. Using one and two photon absoption processes, a number of S₁ 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 S₁ 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.