Ultrafast photodynamics of pyrazine in the vacuum ultraviolet region studied by time-resolved photoelectron imaging using 7.8-eV pulses

摘要

The ultrafast electronic dynamics of pyrazine (C4N2H4) were studied by time-resolved photoelectron imaging (TRPEI) using the third (3 omega, 4.7 eV) and fifth harmonics (5 omega, 7.8 eV) of a femtosecond Ti:sapphire laser (omega). Although the photoionization signals due to the 5 omega - 3 omega and 3 omega - 5 omega pulse sequences overlapped near the time origin, we have successfully extracted their individual TRPEI signals using least squares fitting of the observed electron kinetic energy distributions. When the 5 omega pulses preceded the 3 omega pulses, the 5 omega pulses predominantly excited the S-4 (pi pi*, B-1(1u) + B-1(2u)) state. The photoionization signal from the S-4 state generated by the time-delayed 3 omega pulses was dominated by the D-3(B-2(2g)) <- S-4 photoionization process and exhibited a broad electron kinetic energy distribution, which rapidly downshifted in energy within 100 fs. Also observed were the photoionization signals for the 3s, 3p(z), and 3p(y) members of the Rydberg series converging to D-0((2)A(g)). The Rydberg signals appeared immediately within our instrumental time resolution of 27 fs, indicating that these states are directly photoexcited from the ground state or populated from S-4 within 27 fs. The 3s, 3p(z), and 3p(y) states exhibited single exponential decay with lifetimes of 94 +/- 2, 89 +/- 2, and 58 +/- 1 fs, respectively. With the reverse pulse sequence of 3 omega - 5 omega, the ultrafast internal conversion (IC) from S-2(pi pi*) to S-1(n pi*) was observed. The decay associated spectrum of S-2 exhibited multiple bands ascribed to D-0, D-1, and D-3, in agreement with the 3 omega-pump and 6 omega-probe experiment described in our preceding paper [T. Horio et al., J. Chem. Phys. 145, 044306 (2016)]. The electron kinetic energy and angular distributions from S1 populated by IC from S2 are also discussed. Published by AIP Publishing.