Two-dimensional Fourier transform spectroscopy in the ultraviolet with sub-20 fs pump pulses and 250–720 nm supercontinuum probe

Abstract

Experimental realizations of two-dimensional (2D) electronic spectroscopy in the ultraviolet (UV) must so far contend with a limited bandwidth in both the excitation and particularly the probe frequency. The pump bandwidth is at best 1500 cm ^−1 (full width at half maximum) at a fixed wavelength of 267 nm or 400 cm ^−1 for tunable pulses. The use of a replica of the pump pulse as a probe limits the observation of photochemical processes to the excitation region and makes the disentanglement of overlapping signal contributions difficult. We show that 2D Fourier transform spectroscopy can be conducted in a shaper-assisted collinear setup comprising fully tunable UV pulse pairs and supercontinuum probe spanning 250–720 nm. The pump pulses are broadened up to a useable spectral coverage of 2000 cm ^−1 (25 nm at 316 nm) by self-phase modulation in bulk CaF _2 and compressed to 18 fs. By referencing the white light probe and eliminating pump stray light contributions, high signal-to-noise ratios even for weak probe intensities are achieved. Data acquisition times as short as 4 min for a selected population time allow the rapid recording of 2D spectra for photolabile biological samples even with the employed 1 kHz laser system. The potential of the setup is demonstrated on two representative molecules: pyrene and 2,2-diphenyl-5,6-benzo(2H)chromene. Well-resolved cross-peaks are observed and the excitation energy dependence of the relaxation processes is revealed.