Femtosecond transient absorption to reveal spectroscopically dark dynamic processes

• Main Author: Kraatz, Ingvar Tobias
• Other Authors: Whitaker, Ben J. ; Nix, Michael G. D.
• Published: University of Leeds 2016
• Subjects: 541
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.692418

A femtosecond transient absorption spectrometer has been constructed and was used to investigate the carrier dynamics in CuInS2/ZnS quantum dots and the excited state dynamics of pyrrole, 2-ethylpyrrole and pyrrole-2-carboxaldehyde in a wide excitation range. The time evolution of absorbing population was probed between 320 nm to 1070 nm. N-H bond fission was observed in pyrrole and its derivatives upon accessing the 11πσ∗ state. The onset for N-H bond fission in pyrrole-2-carboxaldehyde was determined to be at 222 nm for both the solution phase (ethanol) and experiments conducted in the gas phase with H (Rydberg) atom photofragment translational spectroscopy (PTS). Excitation below the 11πσ∗ onset in pyrrole-2-carboxaldehyde resulted in triplet state formation with time constants of 2 ps and 4 ps. The observed onset for N-H fission in pyrrole and 2-ethylpyrrole in ethanol solution coincided with the N-H bond fission observed in gas phase studies [23, 48]. Contrary to the H atom PTS data [23, 48], but in agreement with recent dynamics calculations [69, 98], the molecular radical cofragments (pyrrolyl, 2-ethylpyrrolyl) indicate the formation of electronically excited radical fragments in the D1(B1) state. Spectral signatures were assigned to the radical co-fragments and a decay time constant of 1.2 ps (pyrrolyl) and 2.5 ps (2-ethylpyrrolyl) was determined that correspond to the population flow out of the excited radical state into the radical ground state D0(A2) state via a conical intersection. Femtosecond transient absorption and pump-dump-probe spectroscopy was used to follow the carrier dynamics in CuInS2/ZnS quantum dots (suspended in n-hexane). Evidence is provided that the long-lived photoluminescence is originating from a highlying intraband donor state. Three time constants were extracted: a 0.5 ps component marking the filling time of the emitting sub-bandgap state, a 1.8 ps time constant associated to the de-excitation via surface sites and a longer 27 ps decay component assigned to Auger recombination.