C. P. Schulz

The visible photoabsorption spectrum of Ar+3

The photodissociation cross section of Ar+3 was measured at a number of wavelengths between 1064 and 320 nm. A single broad and featureless band was observed peaking near 520 nm with a width of ≈2600 cm−1 and a peak cross section of ≈10−16 cm2. Consideration of the electronic structure of Ar+3 indicates that the measured spectrum is equivalent to the photoabsorption spectrum. Two ionic products, Ar+ and Ar+2, were observed in the photodissociation of Ar+3, indicative of at least two exit pathways and suggestive of two electronic transitions.

Free sodium-water clusters: photoionisation studies in a pulsed molecular beam source

Neutral Na·(H2O)n clusters are studied by near-uv one-photon ionisation and time-of-flight mass spectroscopy. The clusters are formed in a “pickup source” by injection of a beam of Na atoms into the expansion zone of a pulsed nozzle beam of water vapour seeded into an argon carrier gas. The performance of this novel technique for studying cold aggregates of potentially reactive species is discussed in detail. The photoion efficiency (PIE) spectrum of the monomer near its ionisation threshold (4.379(2) eV) shows a rich structure. Vibrational frequencies of the ion can be deduced and some indication of molecular Rydberg states is seen. Ionisation potentials for larger clusters and the binding energies of the neutral clusters up ton=5 are reported.

Laser separation of geometrical isomers of weakly bound molecular complexes.

Molecular assemblies held together by weak intermolecular bonds exhibit a rich variety of geometries. Even a simple complex formed by only two molecules can adopt several conformations corresponding to different geometrical isomers. Isomers of small polar dimers can be isolated nondestructively by taking advantage of a selective and reversible ionization process, with the use of a mass spectrometry method that allows the determination and control of the geometrical configuration of neutral or negatively charged molecular complexes in supersonic beams. Here, the method is applied to isolated nucleic acid base pairs that can be selected in stacked or H-bonded configurations.

Electronically excited states in size-selected solvated alkali metal atoms. III. Depletion spectroscopy of Na(NH3)n-clusters

The first electronically excited state of small Na(NH3)n clusters up to n=22 is studied by means of depletion spectroscopy. A drastic decrease of the excitation energy from the 3s→3p transition of the Na atom (16 950 cm−1) down to 6000 cm−1 for the Na(NH3)4 cluster, the closing of the first solvation shell, is observed. For larger clusters the excitation energy increases slightly toward the bulk value (6300 cm−1) which represents the absorption of the “solvated” electron. For all Na(NH3)n clusters with n⩾3 a strong absorption peak is observed near 6600 cm−1. By comparison with deuterated sodium–ammonia clusters this absorption can be assigned to an intramolecular vibrational overtone of the ammonia molecule. This indicates a strong coupling between electronical and vibrational excitation in the Na(NH3)n clusters.

Electronically Excited States of Sodium-Water-Clusters

The lowest electronically excited state of small Na(H2O)n clusters has been investigated experimentally and theoretically. The excitation energy as determined by the depletion spectroscopy method drops from 16 950 cm−1 for the sodium atom down to 9670 cm−1 when only three water molecules are attached to the Na atom. For larger clusters the absorption band shifts back towards higher energies and reaches 10 880 cm−1 for n=12. The experimental data are compared to quantum-chemical calculations at the Moeller–Plesset second-order perturbation and multireference single and double excitation configuration interaction levels. We found that the observed size dependence of the transition energy is well reproduced by the interior structure where the sodium atom is surrounded by water molecules. The analysis of the radial charge distribution of the unpaired electron in these interior structures gives a new insight into the formation of the “solvated” electron.

Quantum Interference Spectroscopy of Rubidium-Helium Exciplexes Formed on Helium Nanodroplets

Femtosecond multiphoton pump-probe photoionization is applied to helium nanodroplets doped with rubidium (Rb). The yield of Rb+ ions features pronounced quantum interference (QI) fringes demonstrating the coherence of a superposition of electronic states on a time scale of tens of picoseconds. Furthermore, we observe QI in the yield of formed RbHe exciplex molecules. The quantum interferogram allows us to determine the vibrational structure of these unstable molecules. From a sliced Fourier analysis one cannot only extract the population dynamics of vibrational states but also follow their energetic evolution during the RbHe formation.

Electronically excited states in size selected solvated alkali metal atoms. II. Isotope effects in the spectroscopy of sodium water and sodium ammonia complexes

The change of spectroscopic properties of sodium-water and sodium-ammonia complexes upon deuteration is investigated. The ionization potential of NaD2O is shifted by 70 cm−1 towards lower energies compared to NaH2O. A shift of 81 cm−1 between NaND3 and NaNH3 is observed again towards lower energies. From these shift the vibrational frequencies for the ground state of the neutral and ionic complex have been estimated. The first electronically excited state (A2E) of NaND3 has been investigated by resonant two color two photon ionization. The comparison with the formerly observed NaNH3 spectrum enables us to do an unequivocal assignment of the vibrational structure.

Photoionization studies of free sodium ammonia clusters

Free sodium ammonia clusters Na(NH3)n up ton=45 were generated in a pickup source by injecting a beam of neutral sodium atoms into the expansion zone of a piezo driven pulsed nozzle. The clusters thus formed are studied by one-photon ionisation in the region of 266 nm to 520 nm, time-of-flight mass spectrometry as well as photoelectron spectrometry. Ionisation thresholds for clusters up ton=18 and dissociation energies for the neutral Na(NH3)n up ton=6 are reported.

Femtosecond pulse shaping as analytic tool in mass spectrometry of complex polyatomic systems

An additional dimension to mass spectrometric studies on building blocks of proteins is discussed in this paper. The present approach is based on tailored femtosecond laser pulses, using the concept of strong-field pulse shaping in an adaptive feedback loop. We show that control strategies making use of coherent properties of the electromagnetic wave allow one to break pre-selected backbone bonds in amino acid complexes that may be regarded as peptide model systems. Studies on different chromophores, such as phenylalanine and alanine, while keeping the backbone structure unchanged elucidates the effect of the excitation dynamics on the relaxation pathways. The observation of protonated species in the corresponding mass spectra indicates that optimal control of ultrafast laser pulses may even be useful to study intramolecular reactions such as hydrogen- or proton-transfer in particular cases. This opens new perspectives for biophysical and biochemical research, since these photochemical reactions are suggested to explain, e.g. photostability of DNA.

C60 in intense short pulse laser fields down to 9fs: Excitation on time scales below e-e and e-phonon coupling

The interaction of C60 fullerenes with 765-797 nm laser pulses as short as 9 fs at intensities of up to 3.7 x 10(14) W cm(-2) is investigated with photoion spectroscopy. The excitation time thus addressed lies well below the characteristic time scales for electron-electron and electron-phonon couplings. Thus, energy deposition into the system is separated from energy redistribution among the various electronic and nuclear degrees of freedom. Insight into fundamental photoinduced processes such as ionization and fragmentation is obtained from the analysis of the resulting mass spectra as a function of pulse duration, laser intensity, and time delay between pump and probe pulses, the latter revealing a memory effect for storing electronic energy in the system with a relaxation time of about 50 fs. Saturation intensities and relative abundances of (multiply charged) parent and fragment ions (C60(q+), q=1-6) are fingerprints for the ionization and fragmentation mechanisms. The observations indicate that for final charge states q>1 the well known C60 giant plasmon resonance is involved in creating ions and a significant amount of large fragments even with 9 fs pulses through a nonadiabatic multielectron dynamics. In contrast, for energetic reasons singly charged ions are generated by an essentially adiabatic single active electron mechanism and negligible fragmentation is found when 9 fs pulses are used. These findings promise to unravel a long standing puzzle in understanding C60 mass spectra generated by intense femtosecond laser pulses.