Abstract
This work is dedicated to the gas phase photochemistry of several prototypical
molecular systems on the femtosecond time scale. The main goal of the investigations
is the elucidation of important elementary ultrafast reaction mechanisms. As experimental
techniques femtosecond pump-probe spectroscopy in various spectral regions, molecular
beam methods, TOF mass spectrometry and photoion-photoelectron-coincidence spectroscopy
are deployed. All results are discussed in terms of available ab initio calculations.,
the dynamics and in part also the time-dependent energetics (by photoelectron
spectroscopy) of different types of unimolecular reactions are revealed. After
the direct photodissociation dynamics of methyl nitrite and oxygene ultrafast
relaxation and indirect photodissociation processes are studied: excitation energy-dependent
predissociation and quantum beats of carbon disulfide, isomerization and internal
conversion in ethylene and its chlorinated derivatives, conical intersections
and internal conversion in the benzene derivatives toluene and pyrazine, competitive
decay channels like three-body decay and molecular photodetachment in diiodo-difluoro-methane.,
the dynamics and energetics of femtosecond bimolecular reactions in small clusters
is investigated. The first real time study of a photoinitiated harpooning reaction
in a metal atom molecule complex is presented:\thinspace in barium methyl fluoride
the two competitive channels of a charge transfer reaction and a non-reactive
dissociation of the complex are analyzed. The product formation is detailed and
the transition state is characterized. Concerning the latter the role of a secondarily
occupied state is confirmed by direct excitation., in small ammonia clusters for
the first time an H-atom transfer reaction has been demonstrated by fs time-resolved
photoelectron spectroscopy. The ammonia clusters in their secondarily populated
H-transfer configuration are further excited by a time-delayed third laser pulse
and the induced photodissociation dynamics is uncovered. Different cluster-size-dependent
decay mechanisms are established. Based on theses studies, the first 'pump and
control' experiment in molecular clusters is performed using a suitable control
pulse in order to reduce the fragmentation of small ammonia cluster ions and hence
to control the product distribution.
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