Time resolved photoexcitation and photoionization scheme of quadricyclane. 208 nm photons are used to populate the 3s, 3p x,y, and 3p z states, from where photoionization is achieved using 416 nm photons. Internal conversion leads from 3p to 3s (green arrow). Quadricyclane, containing only sigma bonds, undergoes significant structural changes upon photoexcitation, resulting in large amplitude motion along certain vibrational degrees of freedom. The vibrational wave packet dissipates towards other vibrational degrees of freedom. The vibrational energy available to quadricyclane in the 3s Rydberg state is sufficient to cause isomerization of to norbornadiene.
Rydberg electron binding energy spectra of quadricyclane (black solid line) and norbornadiene (red dashed line). The 3-photon, one-color ionization spectra were obtained by ionizing with 416 nm photons. Peaks centered at 2.88 eV, 2.29 eV, and 2.24 eV are assigned to the 3s, and the different 3p Rydberg states, respectively. The small peak near 2.7 eV is likely due to an unidentified impurity in the QD sample. Photoionization of NB using 416 nm photons proceeds through the 3s Rydberg state with binding energy of 2.78 eV. A broad peak centered at 2.4 eV likely originates from a valence state.
Photoelectron binding energy spectra obtained upon two-color photoionization with 208 nm pump pulses and 416 nm probe pulses. The one-color contributions to the spectra are subtracted. (a) False-color map of the time-resolved electron binding energy spectrum. The intensity is encoded in the color as shown in the color bar, with a (natural) logarithmic intensity scale. (b) Photoionization spectra at the pump-probe time delays of −153 fs and 580 fs.
Centers of the Rydberg peaks and their FWHM as a function of time delay between laser pulses. Promotion of an electron from the ground state to the Rydberg states results in a significant change in equilibrium structure of the molecule, thus depositing energy in certain vibrational degrees of freedom causing large amplitude anharmonic motions that broaden the Rydberg peaks. As the vibrational energy gets redistributed among various vibrational degrees of freedom the amplitude of motion decreases so that the widths and positions of the Rydberg peaks shift.
Projection of the photoelectron spectrum on a time coordinate. The dots are the experimental data, while the solid black lines show the fitted curves (see the Appendix for details). Black squares show the evolution of the 3p Rydberg states in QD. Red crosses are the population of the 3s Rydberg states in QD. Blue triangles are the population of the 3s Rydberg states in NB.
Parameters of the quadricyclane peak shifts and peak width change.35
Article metrics loading...
Full text loading...