Ultrafast Solution-Phase Photochemistry Revealed by Multioctave Continuum Probe

A transient absorption (TA) spectroscopy experiment using a multioctave supercontinuum probe simultaneously covering the visible, near and deep ultraviolet is developed and employed to reveal new photochemical phenomena for two aqueous phase systems. Although this spectral region contains key absorptions associated with amino acids and DNA building blocks, supercontinua extending to 5.5 eV with sufficient bandwidth and photon-flux have been difficult to achieve for solution phase spectroscopy. This technical challenge is surmounted by utilizing gas-filled hollow-capillary fibers to generate bright ultrafast superbroadband soliton probes using both Ti:sapphire and Yb:KGW amplifiers. Here, we demonstrate soliton-probe continuum generation spanning 2.1-5.5 eV for liquid-phase TA spectroscopy. We apply this innovation to the continuous two-photon absorption of liquid water, revealing new polarization information on preresonant states, and to the UV photohydration reaction of RNA derivative 1,3-dimethyluracil (DMU). In DMU, we directly detect a <2% channel, demonstrating that the photohydration reaction occurs via a twisted ground-state intermediate.