60th Annual Report on Research 2015

During the first year of this project, we focused on two things. The first was developing a theoretical understanding of pulse propagation effects in optical 2D Fourier-transform spectroscopy. The second was performing initial experiments on colloidal quantum dots as a warm up for experiments on molecular systems.

To understand the pulse propagation effects, a solution to Maxwell’s equations in the three-dimensional frequency domain was used to calculate rephasing two-dimensional Fourier transform (2DFT) spectra of the D2 line of atomic rubidium vapor in argon buffer gas. Experimental distortions from the spatial propagation of pulses through the sample were simulated in 2DFT spectra calculated for the homogeneous Bloch line shape model. Spectral features that appeared at optical densities of up to 3 were investigated. As optical density increases, absorptive and dispersive distortions started with peak shape broadening, progressed to peak splitting, and ultimately resulted in a previously unexplored coherent transient twisting of the split peaks. In contrast to the low optical density limit, where the 2D peak shape for the Bloch model depends only on the total dephasing time, these distortions of the 2D peak shape at finite optical density vary with the waiting time and the excited state lifetime through coherent transient effects. Experiment-specific conditions were explored, demonstrating the effects of varying beam overlap within the sample and of pseudo-time domain filtering. For beam overlap starting at the sample entrance, decreasing the length of beam overlap reduces the line width along the ωτ axis but also reduces signal intensity. A pseudo−time domain filter, where signal prior to the center of the last excitation pulse is excluded from the FID-referenced 2D signal, reduces propagation distortions along the ωt axis. It is demonstrated that 2DFT rephasing spectra cannot take advantage of an excitation−detection transformation that can eliminate propagation distortions in 2DFT relaxation spectra. Finally, the high optical density experimental 2DFT spectrum of rubidium vapor in argon buffer gas [J. Phys. Chem. A 2013, 117, 6279−6287] was quantitatively compared, in line width, in depth of peak splitting, and in coherent transient peak twisting, to a simulation with optical density higher than that reported.

Coherent exciton dynamics of colloidal quantum dots are sensitive to fundamental excitonic processes of interest for several applications. Two dimensional coherent spectroscopy was performed in CdSe/ZnS colloidal quantum dots, revealing the presence of a Stokes shifted exciton resonance that is the convolution of the zero phonon line with neighboring phonon sidebands that result from the emission of acoustic phonons. By tracking the resonance linewidth over a temperature range of 10K to 300K and a waiting time delay up to 300 fs, we observed a contribution to the linewidth that stems from the absorption of phonons at higher temperatures and longer waiting times.