Project Report

A chemical reaction can be imagined as having three steps. First, two reactants remain separate from one another. Then, they interact in what is called a "transition state." Finally, they separate as products. An example is shown: A? + B → A?·B → A + B? (1) Typically, the transition state is not stable, and cannot be isolated. However, we are able to create a very close approximation called a cluster anion in the gas phase, and can force the cluster to dissociate to products by putting energy into the system using laser radiation (hν). This is a process known as photodetachment. One such cluster that has been studied extensively is I?·CH3I. Photodetachment of this species proceeds as follows: I?·CH3I + hν → I(ground state) + CH3I(v) +e? → I(excited state) + CH3I(v) +e? (2) In order to see the transition, the laser must be of enough energy to overcome the electron binding energy(eBE) of the cluster. Here, there are two accessible states of I separated in energy by 0.94 electron-Volts(eV). The CH3I (v) indicates that some energy can go into vibrational motion of the CH3I molecule. Figure 1 shows the photodetachment spectra of I?·CH3I in red. The blue trace shows the photodetachment of I?, shifted in energy to illustrate how the two spectra overlap. The two strongest features in the I?·CH3I spectra correlate well with the I? spectra, although the I?·CH3I spectra is broadened due to the vibrational excitation of CH3I. Photodetachment of the Cl?·CH3I cluster anion is presumably similar. There are also two neutral states of Cl? that can be accessed, although these are separated by 0.11 eV. We would then expect to see two strong features, separated by 0.11 eV, with some vibrational excitation of the CH3I molecule. Figure 2 shows the Cl?·CH3I photodetachment spectra in red, and the Cl? spectra in blue, shifted to show the expected overlap. Where the excited state channel of the cluster should appear( labeled as 2P1/2 in Figure 2), there is a minima in the spectra. Further calculations indicate that the excited state channel may actually appear at 0.13 eV away from the ground state, corresponding to the peak at approximately 4.2 eV in the cluster spectra. However, the second vibrationally excited CH3I state occurs at 0.131 eV from the ground state. The resolution of these spectra is not sufficient to differentiate between the two energy transfer mechanisms. Higher resolution experiments are currently underway at the Australian National University.

National Science Foundation (NSF)
Office of International and Integrative Activities (IIA)
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Carter Kimsey
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Lasinski Joshua S
United States
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