Electron-phonon dynamics in organic solids

Motivation:

Both electrons and protons interact with a background of vibrations in their environment. Intimate interaction with the surrounding leads to trapping of charges and creation of kinetic barriers for motion. Furthermore, upon absorption of light, the rapidly perturbed electronic charge density excites vibrations in the surrounding. Therefore, in light induced electron-proton coupling, it is important to study the dynamics of the environmental vibrations that are generated by light excitation. Coupling to vibrations is relevant in other fields such as exciton transport, photovoltaics, conductivity, and superconductivity. The universal importance of this problem requires fundamental studies in model systems that exhibit the coupling between excitations over large energy scales.

Coupling Between Charge Transfer, Low and High Frequency Phonons

The problem of electron-proton transfer naturally led us to the ubiquitous proton-bearing redox couple hydroquinone-benzoquinone. We recognized that a charge transfer crystal made from this couple, known as quinhydrone, forms an impressive system that combines several dynamical ingredients; electronic absorption, charge transfer, light-induced low-frequency vibrations, and hydrogen bonding, all in one system. Our experiments revealed the coupling between three degrees of freedom of vastly different energy scales - electronic transitions, low frequency lattice motion, and high frequency OH stretch. Our investigation of this system led to the discovery of a range of new phenomena including vibrational coherence transfer, periodic modulation of the hydrogen bonding OH group by impulsively excited low frequency vibrations, and generation of acoustic waves. Here are links to some of our work on this project. Paper1, Paper2.

Apart from revealing the coupling between the electronic excitation and vibrations involved in hydrogen bonding, our work also introduced the use of broad-band vibrational coherence spectroscopy in such systems in the mid-IR range. The concept behind this approach is that low frequency vibrations modulate both the amplitude and the center frequency of higher frequency modes. When the entire spectrum of the high frequency mode is observed in time-domain by broad-band transient spectroscopy, amplitude modulation and frequency modulation lead to two distinct effects. Amplitude modulation uniformly changes the absorption across the spectrum, while modulation of the center frequency creates periodic changes in the absorption on the left and right side of the central frequency with opposite phase. Therefore, after Fourier Transforming the transient spectra, analyzing the phase of the oscillatory component across the probe spectrum reveals whether the coupling of high frequency mode to the low frequency mode was of amplitude modulation or frequency modulation type. Such level of detail cannot be obtained by any other conventional method. This type of insight in organic crystals is quite rare, and to our knowledge, we are the only group that has applied this analysis in the mid-IR range.