Schematic of the femtosecond pump-probe setup. QPD1, QPD2 are quad photo-detectors used for the alignment of the beam overlap; SPD1, SPD2 are sandwich photo-detectors; NDFW1, NDFW2 are neutral density filter wheels; M1, M2, M3 are computer-controlled mirrors; BS1 is a computer-controlled beam splitter. SM1 is a spherical focusing mirror; SH1, SH2 are computer-controlled shutters; SHG BBO is a frequency doubling BBO crystal.
1PA (blue solid line), 2PA (red symbols) spectra, and fluorescence anisotropy as a function of one-photon excitation wavelength (solid line, upper panels) of H2TBTAC (left panel) and H2TtBuPc (right panel). Vertical dashed lines indicate locations of the quasi g–g and g–u transitions (see text for details).
Prevalent theoretical π-conjugation pathway (thick lines) in H2TBTAC (a) and H2TtBuPc (b). Numbers shown next to the bonds symbols in H2TtBuPc depict relative induced current density calculated in Ref. 28 .
ESA spectra (symbols) of H2TBTAC (a) and H2TtBuPc (b). Linear absorption spectra are shown for comparison (blue line). Vertical arrows indicate the positions of the tentative S 1 → S f transitions relevant to the 2PA spectra.
Experimental 2PA spectra (red symbols) and 2PA spectra simulated as a stepwise GS + ESA absorption (solid line) of H2TBTAC (a) and H2TtBuPc (b) using the parameters summarized in Table I . Insets show the energy level diagrams with the corresponding degenerate 2PA (red arrows) and singlet-singlet ESA with the pump photon shown by a blue arrow and the probe photon shown by a maroon arrow. Two states labeled as Sf1 and Sf2 in the case of H2TtBuPc correspond to two different final states of the 2PA transition, as described in the text. The solid brown curve in panel (b) shows the result obtained by assuming that both the 1st and 2nd peak having Ag → Ag symmetry; whereas the dashed brown line corresponds to the Ag → B1g character of the 2nd peak.
Transition frequencies calculated as a sum of GS and ESA peaks ( ) and 2PA ( ), dipole moments, symmetry factor A with the symmetry of the final state shown in parentheses, and calculated and measured peak 2PA cross sections of the g–g transition.
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