| Summary |
Resonance-enhanced two-photon ionization with Fourier transform-limited narrow-band laser pulses in a cold molecular beam has been shown to be a versatile Doppler-free technique for the investigation of resolved molecular states in the electronic ground state, the first electronically excited state, and even of very high Rydberg states close to the ionization threshold /1/. In this contribution we demonstrate that this technique leads to UV spectra with partial rotational resolution (100 MHz FWHM laser linewidth) and mass selection of vibronic bands in the S1 ← S0 electronic transition of isolated flexible molecules and clusters in a supersonic molecular beam and thus to information on their structure and dynamics. Weak interactions determine the conformational structure and hence the properties and functionality of biologically important molecules.
Results are presented for several systems including styrene (ST, C8H8) and fluorostyrene (FST), and for biologically relevant molecules like 2-phenylethanol (2-PE, C8H10O), the neurotransmitter molecules, ephedrine (C10H15NO), pseudoephedrine, and their complexes with H2O, acetylene (C2H2), and Ar. In that way weak interactions encompassing intramolecular hydrogen bonds, intermolecular σ- and π- hydrogen bonds, and dispersion interactions are explored at molecular level.
We have investigated in detail the existence and stability of an OH***π hydrogen bonding between the side chain and the π-electrons of the aromatic ring of 2-phenylethanol when the π-electron density is reshaped by i) fluorine substitution at different positions of the benzene ring /2/, ii) ionization of the molecule /3/. In order to obtain information on the structure of the ionic molecule, we have performed experiments with the mass selected threshold ionization (MATI) method.
The experimental results on the structure and the vibrational frequencies of the neutral and ionic molecules are compared to data provided by high level ab initio quantum chemistry calculations performed in our group.
/1/ H. J. Neusser and K. Siglow, Chem. Rev., 2000, 100, 3921 - 3942.
/2/ R. Karaminkov, S. Chervenkov, and H. J. Neusser, Phys. Chem. Chem. Phys., 2008, 10, 2852-2859.
/3/ R. Karaminkov, S. Chervenkov, and H. J. Neusser, PCCP, in press.
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