Abstract
This thesis is part of a project that deals with photochemistry in the
condensed phase and is embedded in the collaborative research center
"Analysis and Control of Ultrafast Photoinduced Reactions" (SfB 450).
Halogens and interhalogens (I2, Cl2 und ClF) in
rare gases solids are investigated as model systems
by femtosecond-pump-probe spectroscopy.
Coherent wave packet dynamics of I2 with a period of T ~ 350 fs can
be observed for more than 10 picoseconds after photoexcitation, despite the
strong interactions of the molecule near the dissociation limit with the crystalline Kr matrix. With
this system, a novel evaluation scheme for pump-probe spectra is developed
and tested, which permits the construction of effective one dimensional
potentials of excited electronic states (B and E). An average trajectory
of I2 is measured to visualize the dissipative wave packet dynamics,
and vibrational relaxation rates are determined from different signatures of
the pump-probe spectra. Near the minimum of the potential, the energy loss is
less than 1 % per period, but it grows beyond 50 % above the gas phase
dissociation limit.
The molecules ClF and Cl2 are examined in Ar and Kr matrices with
pump-probe spectroscopy for the first time. The wave packet dynamics of an F
fragment that exits the solvent cage is observed in real time. The time for
direct cage exit is measured to be texit = 250 fs. Besides the dissociation,
the competing recombination of the molecular fragments displays rich
dynamics. Above the dissociation limit, the ClF molecule loses more than
35 % of its kinetic energy in the first period, whereas the rate slows
down to 0.1 % near the minimum of the potential. Experimental results
evidence a strong coupling of singlet and triplet states, which forces the
molecule to recombine into the lowest electronically excited states.
Although the spin-orbit coupling of the light atoms Cl and F is
weak, the spin-flip occurs in less than tf = 500 fs. Wave packet dynamics
persist despite these strong interactions. The scattering of fragments by the cage
is compared for molecules with similar electronic states but different isolation geometries in the lattice.
A photoselected orientation of the ClF bond in the isotropic Ar cage (single substitutional)
is destroyed within τd = 1,2 ps, whereas I2 remains aligned in
the fixed cylindrical Kr cage (double substitutional). In co-doped Cl2/ClF/Ar matrices, the ratio
of excited Cl+Cl- vs. Cl+F- is controlled with a contrast of
1:250, using a double-pulse sequence.
The systematic variation of pump and probe wavelength allows for a definite
interpretation of the experiments without the aid of calculations, which
makes the results particularly valuable for the comparison to the simulations which are
simultaneously developed within the SfB 450.
In advance, the spectroscopy of ClF in Ar and Kr was clarified and
dissociation yields were measured. Two NOPAs were constructed to provide
tunable fs-pulses. The commercial design was improved to double their
efficiency. In addition, the implementation of a flexible variant of the FROG technique
permits the characterization of the pulse duration and
phase of fs-pulses from the IR to the UV. |
