Advances in Multicolor Ultrafast Spectroscopies: Utilizing Theory and Experiment to Explore Vibronic Coupling in Molecular Systems from the Infrared to the X-ray

Abstract

Multicolor ultrafast spectroscopies enable the simultaneous and/or sequential interrogation of dissimilar degrees of freedom relevant to the inter- and intramolecular interactions that drive natural and artificial chemical phenomena on the atto- to nanosecond timescale. Two-dimensional electronic-vibrational (2D EV) and vibrational-electronic (2D VE) spectroscopies expand on the multidimensional spectroscopic landscape as the cross of infrared (IR) and electronic spectroscopies. They provide a window into the coupling of nuclear and electronic degrees of freedom. Each peak position and amplitude gives a direct measure of how specific IR active modes are coupled to specific electronic transitions. Both of these spectroscopies can be described by the same model Hamiltonian that consists of multiple high-frequency vibrational modes, each uniquely coupled in both the ground and electronically excited state, giving rise to a coupling between the electronic states. Polarization-selective spectra and modeling can determine shifts in the electronic excited state potential, and measure dipole orientations, Duschinsky mixing, and non-Condon effects. Spectroscopic and therefore chemical information can be isolated through a host of time-frequency analysis tools. Model analysis demonstrates the strength of these techniques used in tandem. This generalized approach can be applied to a host of molecular systems including mass and charge transfer systems. Because 2D EV is specifically sensitive to the couplings between nuclear and electronic degrees of freedom it is an effective method for uncovering non-Condon, non-adiabatic, and non-Born-Oppenheimer effects. A model proton transfer system (10-hydroxybenzo[h]quinoline) exhibits non-Born-Oppenheimer behavior, manifested through a coherence transfer following optical excitation. Rather than propagating a coherence between the ground and excited state enol geometries, there is experimental evidence that the coupled behavior of the proton and electron motions creates a coherence between the ground state enol geometry and the excited state keto geometry. This experimental evidence of a coherence transfer presents in multiple ways. First, 2D EV spectra show an excited state emission at a pump frequency lower than the electronic transition frequency. Second, coherent oscillations of that signal in the time domain match the character of the vibrational motions previously shown to be coupled to the proton transfer. Multicolor X-ray pump X-ray probe transient absorption spectroscopy of molecular systems in solution directly reports on valence--core interactions by probing a core-to-core absorption measurement in a spectrally isolated area. Specifically, in third-row transition metals, the 1s\rightarrow3p transition can undergo an absorption event in the wake of an Auger-Meitner cascade following the removal of a 1s electron which produces the necessary vacancy in the 3p. The first of two coincident X-ray pulses removes a 1s electron. The second pulse monitors the 1s\rightarrow3p transition energy --- the absorption corollary to K\beta fluorescence. Markov-chain Monte-Carlo simulations can predict the core-excited electronic states present during the cascades. Further, they can describe the time evolution of those states, providing explanations for the types of peaks present and their temporal response profile. In the presence of additional core and valence holes, each orbital relaxes in energy due to the change in electrostatic shielding. Additional interactions due to 3p--3d exchange, spin--orbit coupling, and crystal--field effects further perturb these orbitals. Time-dependent density functional theory calculations predict that the 1s\rightarrow3p transition energies shift ~2 eV with each sequential hole in the valence (t2g), reporting on the strength of core-valence interactions. These predictions are tested and confirmed with measurements made on K4FeII(CN)6 and K3FeIII(CN)6 in H2O. We utilize both the density functional theory and Markov-chain Monte-Carlo simulations in peak assignment, and experimentally measure the predicted ~2 eV shift in 1s\rightarrow3p transition energy as a function of valence hole density. This technique's first demonstration in solution is chronicled here.