UC Santa Barbara

A molecule’s ability to survive upon irradiation is intrinsically tied to how quickly it can release the energy it has absorbed, as prolonged excitation leads to damaging reactions with its surroundings. To probe how unique structural moieties of various biomolecules and organic dye molecules mechanistically relax after photoexcitation in a solvent-free environment, Resonance Enhanced Multi-Photon Ionization (REMPI) was used. REMPI is a powerful spectroscopy technique that describes photon events involving two to three photons sequentially absorbed by the sample to excite and ionize the molecule of interest along various trajectories. These spectroscopic studies employ lasers ranging from the UV-IR with either picosecond or nanosecond pulse width to analyze the absorption spectrum, tautomeric structure, and the excited-state dynamics of the molecules. In collaboration with theoretical calculations modelling the excited-state potential energy surface and dynamics, it becomes possible to draw conclusions on how electronically and structurally a molecule relaxes.Upon excitation, a molecule can utilize various mechanisms to reach the ground state. For example, the canonical nucleobases utilize fast internal conversion to reach conical intersections leading back to the ground state. A study of the alternative nucleobase, isocytosine, was performed comparing its dynamics to guanine and cytosine. Another relaxation mechanism used is excited-state intramolecular hydrogen, proton, and charge transfer (ESIHT, ESIPT, and ESICT respectively), which describes the movement of a charge or hydrogen atom from a donor to acceptor site. This process produces new photoproducts, each with new available relaxation pathways to the ground state. Intramolecular charge and atom transfer are typically ultrafast processes and are too fast to measure with the picosecond laser systems available but are slowed by several orders of magnitude when quantum tunneling is involved. Measurements of the photoproducts post transfer can also provide insight into the mechanism. These studies focus on how various structural moieties in organic dyes, nucleosides, and polypeptides and substituents such as methylation, deuteration, halogenation, and changes in secondary structure affect the rate of excited-state intramolecular transfer. Selective placement of the substituent groups at or near the transfer site provides precise control of the dynamics that when combined with theoretical calculations can develop detailed models of the mechanism. REMPI is a powerful chemical characterization tool that provides both highly specific mass, absorption, and structural information of a sample. However, the technique cannot distinguish spatial features on the surface of the sample material. The development of a novel sub-micron sampling technique, Tip Enhanced Laser Desorption (TELD), is described below. TELD is an off-line sampling process in which a sample is first imaged with an AFM, then a laser is focused and fired at the probe desorbing the surface beneath it, and then the vaporized material is collected and redeposited onto a new substrate for further analysis. Separation of the sampling and chemical characterization techniques results in independent optimization of the samples collected depending on the analytical technique chosen. Detailed experiments into various parameters of the technique such as laser power, number of pulses fired, tip material, and the forced applied between the tip and surface providing insight into the underlying mechanism of energy transfer. The synthesis and characterization of InGaN multi-quantum wells and the lasing wavelength of the material was also studied. Pumping of the material with a 355 nm pulsed laser revealed a pumping threshold of 1.5 MW/cm2 and a lasing wavelength of 568 nm, a spectral region previously not producible with semi-conducting laser materials.