Ultraviolet Transient Absorption Spectrometer with Sub-20-fs Time Resolution

First of all, the UV range is resonant with electronic transitions of many small molecules with fairly simple excited state energy level structure, whose photoinduced dynamics can be accurately modeled using ab initio computational approaches; ultrafast TA spectroscopy in the UV can thus be used to benchmark the accuracy of such methods [16,17]. [...]molecular alignment-based (M-) FROG, which employs the impulsive alignment of gaseous molecules as gate function, was utilized for the measurement of weak UV femtosecond pulses around 200 nm [68]. [...]Figure 11a shows the ∆T/T dynamics at 385 nm, corresponding to the PA from the S1 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 S1 PA band, we assign the oscillation to a vibrational wavepacket on the S1 state, which survives the IC process and is indicative of a vibrationally coherent photophysics in pyrene [87,88,89,90].