Setup for the Real-Time MPI Experiments

The principles of two experimental setups are described in this section. In Sect. 2.1.1 the setup for the real-time MPI experiments is introduced. With this setup, the investigations discussed in Chaps. 3 and 4 were carried out. The experimental basics are concluded with a brief description (Sect. 2.1.2) of the setup used for the real-time studies of charge reversal processes discussed later in Chap. 5. 

Resonantly enhanced multiphoton ionization (REMPI), more generally called multiphoton ionization (MPI), of small molecules and clusters has been studied in recent years by a variety of techniques and is rather well understood. Different alkali molecules and clusters have been investigated by applying real-time MPI spectroscopy. The time evolution of the MPI signals is obtained by means of either picosecond or femtosecond pump probe techniques followed by mass-selective detection of the ionized aggregates. 

The principle of real-time MPI spectroscopy is summarized schematically in Fig. 2.1. The figure shows the two-photon ionization (TPI) of a molecule for the simplest case. Three potential-energy surfaces (PESs) of the molecule, the ground and excited electronic states of the neutral molecule and the ground state of the cation, are shown. A bound excited electronic state is populated by the first (pump) pulse. As a result of the rather broad spectral width of the 

To realize the real-time MPI spectroscopy of small alkali molecules and clusters an experimental setup as shown schematically in Fig. 2.2 is used. First, the main components of the setup, the laser systems used, the source for the molecules and clusters, and the mass-selective detection system are briefly described. An explanation of a few further details and the technique of the measuring process follows. 

The Ultrafast Laser Systems. Since several modifications of ultrafast laser systems are employed to perform real-time MPI investigations on different molecules and clusters, the special features of the laser systems used in the various experiments are given in the sections where the particular experiments are discussed. 

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