Jonas Research Group

University of Colorado


 : Time Resolved Studies of Non-Adiabatic Electronic Dynamics

A time-resolved pump-probe spectroscopy experiment can tell a story about electronic dynamics on multiple electronic and vibronic states. In such an experiment, we use short pulses from a Ti:Sapphire laser to “watch” the electronic dynamics on a chromophore of interest. In short, the pump pulse (typically 20-30 femtoseconds long) excites the sample and launches vibronic wavepackets (a coherent superposition of eigenstates) on the excited and ground states. The probe pulse follows the pump, and we monitor the evolution of those wavepackets prepared by the pump by measuring the differential absorption of the probe.

For instance, the excitation of an electron into an excited state by the pump will deplete the ground state, which will cause less of the probe pulse to be absorbed than if there had been no pump. As the delay between pump and probe is scanned, we get information about the details of wavepacket motion, such as how the absorption frequency of the chromophore changes after excitation.
    Changing the relative electric field polarization between pump and probe pulses fills in the story even further. With a pump-probe polarization anisotropy measurement, we can see the alignment of the wavepacket change, giving information about vibrations that are relevant to electronic dynamics.

The major focus of my study has been of the electronic dynamics of four-fold symmetric or nearly four-fold symmetric molecules. The symmetry of these molecules requires that they have a doubly degenerate electronic state. The Jahn-Teller theorem says that motion along an asymmetric nuclear coordinate must split the degeneracy of such a state. This leads to a very interesting situation in which the character of an electronic wavepacket can change very quickly when driven by relatively small amplitude nuclear motions. Thus, such a system is well-suited for study by pump-probe polarization anisotropy.

Students Involved: Ryan Smith

Method Development: 2D-Spectroscopy

Multidimensional spectroscopy is an increasingly common tool for physical chemists. Optical analogs of the NMR COSY and NOESY experiments have noe been performed. These measurements are more susceptible to artifacts and distortion than their one-dimensional counterparts. The differences between optical and NMP spectroscopy make it difficult to detect and quantify distortion.

The focus of my research has been on understanding and/or eliminating distortions of 2D optical spectra, once understood, can easily be generalized to furter understanding of one-dimensional signals and their distorions as well. Distortions considered are due to absorption of the sample, beam crossing geometry and spectrometer/detector imerfections. Knowledge of distortions and their corrections will enable the study of even weak couplings between strong or widely separated optical transitions. Such coupling are thought to be important for an understanding of photosynthesis, solar energy conversion, and conical intersection dynamics.

Students Involved: Michael Yetzbacher, Rob Hill, Trevor Courtney

Surface Enhanced Raman Spectroscopy

The Raman scattering efficiency from molecules adsorbed to thin noble metal films (Ag, Au, etc.) is measured as the films undergo a variety of morphological and chemcial changes. Quantitaion of the responsewill help determine the mechanism of the enormous enhancement factors as well as the chemical identity of any active species involved. This research is designed with the end goeal of producing substrates capable of single molecule detection via SERS.

Future directions of this project will include interrogation of fabricated periodic nano- feature arrays to determine the rold of Plasmon assisted transmission in theSERS effect.

Stundents Involved: Jordan Corbman, Allion Kanarr

Multi-Exciton Generation Studies

Current photovoltaic devices have a limit on how much energy can be collected from sunlight: photons with insufficient energy are not absorbed and photons with excess energy create excitons that thermalize before being collected. One way to increase this limit would be to convert the high-energy photons into more than one exciton each.

There is experimental evidence that multiple exciton generation can happen in semiconductor quantum dots, but the mechanism is unclear. A semiconductor quantum dot has a size large compared to lattice constants but small or comparable to the bulk Bohr exciton radius. In this situation an excited electron is delocalized with respect to its parent atom but confined to the quantum do. The same is true for the empty orbital left behind, which is called a hole. The electron and the hole together are termed an exciton.

When forming an exciton by optical excitation, any photon energy above the band gap Eg is distributed between the hole and the electron. This excess energy puts the electron and hole far from thermal equilibrium with the crystal, and potentially even with each other. The cooling of these "hot" carriers can occur by a variety of methods. Two methods of interest in quantum dot multiple exciton generation theories are phonon emission and impact ionization. In phonon emission an exciton excites a vibration and scatters as a lower-energy exciton. In impact ionization an electron or hole loses excess energy by exciting another electron into the conduction band, thereby creating another exciton. Studies in the Jonas group include a variety of nonlinear spectroscopies in order to measure the timescales and importance of these mechanisms.

Students Involved: Byung, Rob, Bill, Trevor, and Danielle

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