Studies of Ultrafast Relaxation and Photodissociation Processes in Solution

Description

<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 I₃^- in acetonitrile and methanol as well as I₂Br^- in acetonitrile solution is investigated. These studies address such issues as the time scale for the production of the main photoproduct I₂^-, 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 I₂^-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 I₂^- 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 I₂^- seems fastest for I₃^- in methanol, followed by I₂Br^- in acetonitrile and is slowest for I₃^- in acetonitrile. It appears that vibrational relaxation of newly formed I₂^- 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 I₃^- and I₂Br^- 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>

Links & Downloads

1. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-1374
2. ISBN 91-7155-352-5

Tags

ultrafast spectroscopy

liquid phase

Physics

Fysik

Additional Fields

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