<p>This thesis focuses on femtosecond studies of relaxation and photo-induced dissociation processes in the liquid environment. Measurements are performed using both polarization sensitive and magic angle transient absorption spectroscopy with excitation wavelengths of 387 nm and 258 nm and a white light continuum probe.</p><p>In the first three papers the photodissociation of the trihalides I<sub>3</sub><sup>-</sup> in acetonitrile and methanol as well as I<sub>2</sub>Br<sup>-</sup> in acetonitrile solution is investigated. These studies address such issues as the time scale for the production of the main photoproduct I<sub>2</sub><sup>-</sup>, rotational dynamics of the formed diatomic anions, the subsequent wavepacket dynamics of the coherently excited diiodide anion and vibrational relaxation in, and the geminate recombination of, the I<sub>2</sub><sup>- </sup>fragment. A nearly equal, bent geometry for the parent anion at the moment of bond breakage is proposed in all three solutions. However, the rotational temperature of the diiodide anion produced in the various solutions, reveals that motion along the bending coordinate of the dissociating triatomic anions plays an important role. The first signs of I<sub>2</sub><sup>-</sup> fragments can already be observed at delays of approximately 130 – 190 fs which indicates a faster dissociation than suggested in earlier publications. The production of I<sub>2</sub><sup>-</sup> seems fastest for I<sub>3</sub><sup>-</sup> in methanol, followed by I<sub>2</sub>Br<sup>-</sup> in acetonitrile and is slowest for I<sub>3</sub><sup>-</sup> in acetonitrile. It appears that vibrational relaxation of newly formed I<sub>2</sub><sup>-</sup> fragments happens on a short time scale of a few hundred femtoseconds from initially excited vibrational states centered around v = 60 to v = 20. This fast relaxation was never directly observed before in solution. After that it relaxes with a slower time constant of approximately 2 ps which is shorter than most former reported values. This biexponential behavior agrees well with earlier molecular dynamics simulations. The dependence of the dissociation product formation on excitation energy, parent anion and solvent is found to be relatively strong. These findings lead us to believe that the photo-induced dissociation of the triatomic anions I<sub>3</sub><sup>-</sup> and I<sub>2</sub>Br<sup>-</sup> in solution may very well resemble the gas phase process more than previously thought.</p><p>In paper IV electronic and vibrational relaxation rates of the cyanine dye Methyl-DOTCI are determined after excitation to high lying electronic states. The measurements are performed with two different excitation wavelengths and in various solvents. They reveal a fast electronic relaxation to the second excited electronic state which subsequently relaxes to the first excited electronic state with a time constant of about 10 ps. This relatively long relaxation time may partly be explained by the badly overlapping electronic wavefunctions obtained from theoretical calculations. Vibrational relaxation proceeds with a similar time constant of 10 ps but shows a marked solvent dependence with faster relaxation rates in alcohol solutions.</p>
| Identifer | oai:union.ndltd.org:UPSALLA/oai:DiVA.org:su-1374 |
| Date | January 2006 |
| Creators | Salén, Peter |
| Publisher | Stockholm University, Department of Physics, Stockholm : Fysikum |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Doctoral thesis, comprehensive summary, text |
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