Licentiate Thesis: Time-resolved photoelectron spectroscopy studies of excited states dynamics of molecules
by
Ting Geng(Stockholm University, Department of Physics)
→
Europe/Stockholm
FA 32
FA 32
Description
This thesis is mainly focused on using time-resolved photoelectron spectroscopy to study the
dynamics of electronically excited states in molecules. These dynamics are induced
(“pumped”) by absorption of a UV/visible-energy photon, exciting the molecule from its
ground state into an excited state. This evolution is followed (“probed”) by a time-delayed
subsequent laser pulse which creates a photoion and a photoelectron. The kinetic energy of
the photoelectron is determined, and a photelectron energy spectrum as a function of delay
time between the pump and probe pulses.
With respect to the molecules studied here, the dynamics of the simple molecule, ethylene,
serve as a reference. Here, C=C bond torsion happens on the ππ* state, and when the
molecules transfer from the excited state back to the ground state, this is accompanied with
pyramidalization of CH2 group or hydrogen migration from one carbon atom to the other.
With stepwise methyl-substitution of the ethylene molecules, the dynamics changes with
increased methylation.
With this template of the dynamics of ethylene, the dynamics will also be influenced by (1)
adding hetero atoms. Alkyl vinyl ether, which adds an alkoxy group to ethylene, opens up a
new reaction channel on the πσ* state, which is cleavage of the –OCH3 group, while also
retaining the C=C bond torsion on the ππ* state, as in the dynamics of ethylene. (2) making
cyclic systems. 1,3-cyclohexadiene, in which a second C=C double bond (“ethylene”) is
added to form a six-carbon ring system. Compare with ethylene, the molecules only have one
potential energy surface involved in the dynamics. And molecule switches from the excited
ππ* state to the ground state without any pyramidalization. The dynamics proceed to the
ground state either via a ring-opening or back to the cyclohexa-1,3-diene ground state.
Combining time-resolved photoelectron spectroscopy, i.e. measuring the kinetic energy of
photoelectron, in combination with detailed computational studies, the dynamics in
increasingly complex molecules, for examples, acrylonitrile, furan, pyrrole, etc, can be
studied.
