Isolated aromatic cations and their aggregates with water are analyzed in molecular beams by the combination of time-of-flight mass spectrometry and IR/UV (InfraRed/UltraViolet) laser spectroscopy in order to get information about structures of the different species, their vibrational couplings, and their reactivity (e.g. proton transfer processes or rearrangement reactions). The structure of the first solvation shell of dissolved particles (process of micro solvation) is simulated by the successive addition of water molecules to a solute molecule.

The investigation of structure and reactivity of ionic species requires spectroscopic methods which are mass-, isomer-, and vibrational-selective. In order to perform such a selective analysis of reactions and vibrational couplings directly on isolated ions, the triple resonance IR/PIRI (InfraRed/Photo Induced Rydberg Ionization) method as well as a variant of IR/PD (PhotoDissociation) spectroscopy are used (cf. list of publications). A high energy (pulse energies of >10 mJ) and at the same time high resolution (<0.04 cm^-1) IR laser system in the nanosecond regime has been developed to investigate the extremely structure-sensitive OH, NH, and CH stretching vibrations within the range from 2700 to 4000 cm^-1. The laser principle is based on a difference frequency mixing (dye laser output and 1064 nm) and an OPA system. Furthermore we built up a narrow bandwidth IR laser system in the mid-infrared down to 600 cm^-1.

The combination of all IR/UV and UV/UV methods lead to a comprehensive analysis of structure, dynamics, and reactivity of the cluster in ionic states. The electronic ground state (S0) and the first excited state (S1) of the neutral species can be investigated by further techniques. The cluster of 4-aminophenol with water is an example for which we have shown that a combination of a number of modern techniques, in particular the IR/R2PI, UV/IR/UV IR/PIRI, and IR photodissociation spectroscopy, yield an analysis of structure and reactivity in the S0, S1, and in the ionic states. By means of the NH and OH stretching vibrations, a rearrangement reaction in the cation could be examined in a vibrational selective manner. Further applications of the IR/PIRI method to vibrational couplings of inter- and intramolecular cluster vibrations are described e.g. in our publication on indole(H2O)1+. Since the OH stretching vibrations of the water molecule are strongly influenced by the kind of bonding in the cluster the analysis of solvation shells of aromatic molecules and ions becomes possible.

Figure 3:

(a) IR/PIRI method

This technology is a triple resonance spectroscopy: the origin or a vibrational level of the electronically excited state is selectively excited by a UV photon. This method guarantees an isomer selective selection of neutral precursors of the ionic state. Then, individual vibrational levels of the ion are selectively excited by a second UV photon which is achieved by exciting high-lying Rydberg states converging to the ionization potential or a vibration of the cation, respectively. Via the excitation of a further vibration (e.g. an OH, NH, or CH stretching vibration) by means of an IR photon, autoionization of the Rydberg states takes place. Therefore it is possible to excite two vibrations of the ion selectively and investigate e.g. their vibrational coupling or the reactivity along selected coordinates. Thus the explicit excitation of a vibrational mode can lead to proton transfer or rearrangements, which can be analyzed by an IR photon. A detailed description of the IR/PIRI method can be found in the publication list (cf. references on ZEKE =ZEro Kinetic Energy [Müller-Dethlefs, Sander, Schlag, Chem. Phys. Lett. 112, 291 (1984)] and MATI = Mass Analyzed Threshold Ionization [Zhu, Johnson, J. Chem. Phys. 94, 5769 (1991)] methods).

(b) IR photodissociation spectroscopy

With the help of this method ions are produced mass and isomer selective by two UV photons. Fragmentation of the cluster is obtained via resonant excitation of an OH, NH, or CH stretching vibration of the cluster ion by means of an IR photon, so that the IR active vibration is determined by the depletion of the cluster signal and an increase of the fragment ion signal.