Pulse Propagation Effects in Optical 2D Fourier-Transform Spectroscopy: Theory

A solution to Maxwell's equations in the three-dimensional frequency domain is used to calculate rephasing two-dimensional Fourier transform (2DFT) spectra of the D-2 line of atomic rubidium vapor in argon buffer gas. Experimental,distortions from the spatial propagation of pulses through the sample are simulated in 2DFT spectra calculated for the homogeneous Bloch line shape model. Spectral features that appear at optical densities of up, to 3 are investigated. As optical,density increases, absorptive and,dispersive distortions start with peak shape broadening, progress to peak splitting, and ultimately result 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,:thee:,distortion 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 are 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 omega(tau), 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 FED-referenced 2D signal, reduces propagation distortions along the omega(tau) axis. It is - dedemonstratedhat 2DFT rephasing spectra cannot take advantage of an excitation detection transformation that can eliminate propagation distortions in 2DFT:relaiation spectra. Finally, the high optical density experimental 2DFT spectrum of rubidium vapor in argon buffer gas [J. Phys, Chem. A 2013, 117, 6279-6287] is quarititatively compared; in-line-width, in depth of Peak splitting, and,in coherent transient peak twisting, to a siniulation with optical density higher than :that reported.