Abstract
This article describes our recent experimental studies on internal conversion via a conical intersection using photoelectron spectroscopy. Ultrafast S2(ππ*)–S1(nπ*) internal conversion in pyrazine is observed in real time using sub-20 fs deep ultraviolet pulses (264 and 198 nm). While the photoelectron kinetic energy distribution does not exhibit a clear signature of internal conversion, the photoelectron angular anisotropy unambiguously reveals the sudden change of electron configuration upon internal conversion. An explanation is presented as to why these two observables have different sensitivities to internal conversion. The 198 nm probe photon energy is insufficient for covering the entire Franck-Condon envelopes upon photoionization from S2/S1 to D1/D0. A vacuum ultraviolet free electron laser (SCSS) producing 161 nm radiation is employed to solve this problem, while its pulse-to-pulse timing jitter limits the time resolution to about 1 ps. The S2–S1 internal conversion is revisited using the sub-20 fs 159 nm pulse created by filamentation four-wave mixing. Conical intersections between D1(π−1) and D0(n−1) and also between the Rydberg state with a D1 ion core and that with a D0 ion core of pyrazine are studied by He(I) photoelectron spectroscopy, pulsed field ionization photoelectron spectroscopy and one-color resonance-enhanced multiphoton ionization spectroscopy. Finally, ultrafast S2(ππ*)–S1(ππ*) internal conversion in benzene and toluene are compared with pyrazine.
Highlights
The quantum mechanical equation of motions of the nuclei and electrons is not exactly solvable, soBorn and Oppenheimer approximated the rigorous equation by separating it into the sets of equations of the nuclei and the electrons [1]
The electronic characters of the S2 and S1 states gradually change along the reaction pathway. This is contrasted with the case of pyrazine, which has a conical intersection near the minimum of the diabatic S2 surface and the subsequent dynamics primarily occur in the planar geometry (Figure 16)
Theoretical studies have been performed on the S2–S1 internal conversion in pyrazine as a benchmark system, its real-time observation was only enabled by the development of sub-20 fs ultrafast lasers operating in the deep
Summary
The quantum mechanical equation of motions of the nuclei and electrons is not exactly solvable, so. Eppink and Parker have modified the ion optic electrodes and enabled two-dimensional space focusing of ions to improve the imaging resolution [20] Their method is called velocity map imaging, because the arrival position of the ion on the detector plane is proportional to the velocity perpendicular to the flight axis and independent of the ionization point [21]. The detector consists of microchannel plates, a phosphor screen and a charge-coupled device (CCD) or a complementary-metal-oxide-semiconductor (CMOS) camera, and it records the arrival positions of the photoelectrons on the detector plane Since both the pump and probe laser polarizations are parallel to each other and to the detector face, the original 3D distribution has axial symmetry around the polarization direction [21,26]. Where t, θ, and E are the pump-probe time delay, the electron ejection angle from the laser polarization direction, and the photoelectron kinetic energy.
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