KMno4 solutions are colored deep purple by a structured mid-visible light absorption which leads with marginal quantum efficiency to decomposition into oxygen and MnO2 (s). Despite the chemical importance of this compound, full appreciation of MnO4- electronioc structure and photochemical dynamics are still unclear. Clarifying these points with ultrafast spectroscopy is our objective. Transient transmission spectra of MnO4-(aq) after sample irradiation with ~10 fs pump pulses centered at 515 nm and probing with a supercontinuum in the range of 400 - 850 nm are presented in figure. A negative difference absorption spectrum in the range of 470-580nm, appears promptly after a coherent artifact at t=0 and is assigned to ground state bleach (GSB). In addition three new absorption bands are observed. The first appears instantly at 400-470nm and 600-650nm within ~3ps, and decay exponentially with t=16 ps leading to GSB recovery. This spectral chronology was fit by a kinetic scenario including two excited state species. The first is the initial T2 bright state with the latter assigned as a lower lying triplet. Throughout the probed region strong and long lived wave packet signatuures are observed. A preliminary frequency analysis of the coherent vibrational data ishows only a single frequency which matches the frequency of the ground state breathing mode previously recorded with Resonance Raman Scattering.[4] This result is surprising, since theory predicts significant displacements along other than the Mn-O bond length coordinate. Extension to photoexcitation higher up in the singlet manifold is under way to open up the observation of nascent products or longer lived reactive intermediates. Theoretical modelling will be required to understand the curious absence of coherences at other than the breathing vibration near 800 cm-1.

Femtosecond photochemistry of aqueous permanganate

256th National Meeting (2018)

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