# Ultraviolet Transient Absorption Spectrometer with Sub-20-fs Time Resolution
## 4 Experimental Results
The first \"bright\" excited singlet state of pyrene is of B\({}_{\mathrm{2u}}\) symmetry (S\({}_{2}\) or 1\({}^{1}\)B\({}_{\mathrm{2u}}\) state) and the S\({}_{0}\)\(\rightarrow\)S\({}_{2}\) transition peaks at 3.7 eV; a second bright excited state is the S\({}_{3}\) state (or 2\({}^{1}\)B\({}_{\mathrm{2u}}\) state) with the S\({}_{0}\)\(\rightarrow\)S\({}_{3}\) transition peaking at 4.55 eV (270 nm). Following excitation of the bright S\({}_{2}\) or S\({}_{3}\) states, rapid internal conversion (IC) to the dark S\({}_{1}\) state occurs, mediated by conical intersections. While the S\({}_{2}\)\(\rightarrow\)S\({}_{1}\) process has been studied in detail [83; 84; 85], the S\({}_{3}\)\(\rightarrow\)S\({}_{1}\) process is less explored. Here we exploit our tunable pump and broadband probe to follow the dynamics of both these processes. Figure 7 shows TA maps obtained by pumping with SFG at 340 nm and probing in the UV and in the visible range respectively. We start by discussing the UV range (Figure 7a). Around time zero we observe a short-lived signal that is assigned to the solvent response and is due to TPA of one pump and one probe photon. After this we note a strong positive peak at 334 nm, corresponding to photo-bleaching of the S\({}_{0}\)\(\rightarrow\)S\({}_{2}\) transition, resonant with our pump, with the well resolved vibronic replicas at 320 and 306 nm. At longer wavelengths around 360-370 nm we observe a photoinduced absorption (PA) band, assigned to the S\({}_{1}\) state, which is not formed instantaneously, but rather grows on the 200-fs timescale. In the visible range we observe a PA band, assigned to the S\({}_{2}\) state, which peaks at 580 nm, forms instantaneously and decays on the 200 fs timescale, and two long-lived PA bands peaking at 515 and 470 nm, assigned to S\({}_{1}\). Taken together, these data confirm ultrafast IC from the photoexcited S\({}_{2}\) to the S\({}_{1}\) state; the S\({}_{2}\) PA dynamics at 585 nm, displayed in Figure 8b, shows a decay with \(\approx\)85-fs time constant, in very good agreement with previous results [83; 84; 85]. Figure 5: Solid lines: spectra of the WLC generated in CaF\({}_{2}\) with the SH pump (**a**), in CaF\({}_{2}\) with the FW pump (**b**) and in sapphire with the FW pump (**c**). Shaded areas: shot-to-shot wavelength dependent rms of the WLC fluctuations. Figure 9a shows the 2D TA map, as a function of probe wavelength and delay, following excitation of the S\({}_{3}\) state at 270 nm. At early times we observe a negative \(\Delta\)T/T signal which peaks at 550 nm and is assigned to a PA from the photoexcited S\({}_{3}\) state; this band rapidly shifts to the red at 580 nm (red spectrum in Figure 9b), matching the PA band of S\({}_{2}\); at longer delays, the PA spectrum of S\({}_{1}\) reappears, with characteristic peaks at 370, 470 and 515 nm. These data are consistent with a two-step S\({}_{3}\)\(\rightarrow\)S\({}_{2}\)\(\rightarrow\)S\({}_{1}\) IC process; the TA dynamics, displayed in Figure 10, show that this cascaded process occurs, as expected, on a longer 500 fs timescale. Finally, Figure 11a shows the \(\Delta\)T/T dynamics at 385 nm, corresponding to the PA from the S\({}_{1}\) state; the signal is clearly modulated by a long-lived oscillation, with 85-fs period (corresponding to a 392 cm-1 frequency, as evidenced in panel (b)), which is observed thanks to the high temporal resolution of our setup. A wavelength dependent Fourier transform (panel (c)) reveals a second mode at 410 cm-1 at wavelengths between 335 and 342 nm, which closely matches the Raman ground state mode of 408 cm-1 [86]. Since the 392 cm-1 mode shows a clear amplitude minimum at a wavelength of 370 nm, which corresponds to the peak of the S\({}_{1}\) PA band, we assign the oscillation to a vibrational wavepacket on the S\({}_{1}\) state, which survives the IC process and is indicative of a vibrationally coherent photophysics in pyrene [87; 88; 89; 90].