Jeremy Johnson and Dr. Eric Sevy, Chemistry and Biochemistry
When chemical reactions take place, energy is transferred between the different atoms and molecules as they collide. The energy transfer in these collisions is the rate-limiting step of reactions. Therefore, to be able to accurately model reactions we need a good understanding of how energy is transferred. The probability distribution function, P(E,E’), is fundamental to predicting the efficiency of specific energy transfer events. It has not been possible, until recently, to measure this function experimentally because energy transfer studies only obtained measurements of the average energy transfer. Using an ultra high-resolution IR diode (~0.0003 cm-1), we can measure energy transfer in a state resolved fashion. Probabilities measured using this technique can be resorted as a function of energy to obtain P(E,E’). Some trends have been observed and several empirical models proposed, but additional data is necessary to develop an accurate theoretical model to describe this process.
The experiment used a UV pump, IR diode laser probe technique. The probability distribution functions have been measured for collisions involving the large donor molecules pyrazine, pyrimidine, and pyridine, and the smaller, less-complex bath molecule, carbon dioxide. The donor and bath molecules flowed through a long glass cell in equivalent amounts. Using an excimer laser (λ = 248 nm), the donor molecules were pumped to an excited electronic state, which rapidly converts to an excited vibrational state in the ground electronic level. Energy was then transferred to the bath molecules during collisions, whose discrete rovibrational states were observed with the ultra high-resulotion IR diode.
Donor molecules are to be excited with various amounts of energy using an excimer pumped doubled dye laser. New insight into energy transfer will be gained as the rates of these energy transfer events are measured as a function of initial donor energy. Data collection is a lengthy process and measurements have been made for the three donor molecule systems at an initial energy. Analysis of the data has shown that there is a correlation in the energy gain in the carbon dioxide bath between rotational and translational energy. As the amount of rotational energy gained in the carbon dioxide increases, there is a corresponding increase in the translational temperature. Preliminary analysis also shows a correlation between the proximity to the dissociation energy in the donor molecule to the efficiency of energy transfer.
This data collection is producing promising and interesting results. But in order to provide a more complete picture of collisional energy transfer, more information is needed. Data must still be collected at other initial energies by utilizing the excimer pumped doubled dye laser. It would also be useful to collect data with other donor/bath systems in order to test other factors that may be involved in energy transfer. As more data is collected and analyzed, a more complete understanding of collisional energy transfer will expand and a theory for energy transfer probability distribution functions will be developed.