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Two color resonant mass analyzed threshold ionization (MATI) spectroscopy was applied in order to investigate the ionic properties of the structural isomers of trichlorobenzene (TCB) above the ionization threshold. For the first time, vibrational spectra of the 35 Cl 3 and 35 Cl 2 37 Cl isotopomers of 1,2,3-, 1,2,4- and 1,3,5-trichlorobenzene cations in their electronic ground state have been measured via different S 1 intermediate states by mass analyzed threshold ionization (MATI) spectroscopy. Additionally ab initio calculations at DFT (density functional theory) and TDDFT (time-dependent density functional theory) have been performed to compare experimental findings with theory. From the MATI spectra the adiabatic ionization energies of the three isomers 1,2,3-TCB, 1,2,4-TCB and 1,3,5-TCB could be determined to 74289 ± 6 cm −1 , 72779 ± 6 cm −1 and 74900 ± 100 cm −1 , respectively. Several vibrational modes of the isotopomers have been assigned by comparison of the experimental and theoretical results.

The phenol(+)...Ar(2) complex has been characterized in a supersonic jet by mass analyzed threshold ionization (MATI) spectroscopy via different intermediate intermolecular vibrational states of the first electronically excited state (S(1)). From the spectra recorded via the S(1)0(0) origin and the S(1)β(x) intermolecular vibrational state, the ionization energy (IE) has been determined as 68,288 ± 5 cm(-1), displaying a red shift of 340 cm(-1) from the IE of the phenol(+) monomer. Well-resolved, nearly harmonic vibrational progressions with a fundamental frequency of 10 cm(-1) have been observed in the ion ground state (D(0)) and assigned to the symmetric van der Waals (vdW) bending mode, β(x), along the x axis containing the C-O bond. MATI spectra recorded via the S(1) state involving other higher-lying intermolecular vibrational states (σ(s)(1), β(x)(3), σ(s)(1)β(x)(1), σ(s)(1)β(x)(2)) are characterized by unresolved broad structures.

For the first time, vibrational spectra of the 35Cl2 and 35Cl37Cl isotopomers of o-, m-, and p-dichlorobenzene cations in the electronic ground state have been measured via S1 intermediate states by mass analyzed threshold ionization (MATI) spectroscopy. Additionally, ab initio calculations at DFT (density functional theory), CIS (configuration interaction singles), and CASSCF (complete active space self-consistent field) levels of theory have been conducted to compare experimental findings with theory. From the MATI spectra, adiabatic ionization energies of the ortho, meta, and para isomers have been determined to be the same for each pair of investigated isotopomers to 73,237 +/- 6, 72,191 +/- 6, and 73,776 +/- 6 cm(-1), respectively. Several vibrational modes, including fundamentals, combinations, and progressions have been assigned by comparing the experimental and theoretical results. The appearance of overtone progressions involving the 7a mode could be explained by a geometry change of all three isomers during ionization in the direction of this mode by retraining the symmetry of the molecules. Although the general spectral features of the investigated isotopomers are similar, frequencies of some vibrations are slightly different up to a few wavenumbers depending on the involvement of the chlorine atoms in the molecular motion.

Vibrational spectra of the pyrazine and the pyrazine-Ar cation in the electronic ground state (D 0) have been measured via several intermediate states of the first excited state (S 1) by mass-analyzed threshold ionization spectroscopy. Additionally, ab-initio calculations at the DFT, CIS and MP2 level of theory have been conducted to compare experimental findings with theory. For the pyrazine monomer a negative anharmonic behavior for combination bands containing the 16a or the 1 vibration could be observed for the first time. It was found that the 16a vibration has to be considered when discussing the nature of the vibronic coupling in the first excited state (S 1) as well as in the ionic ground state (D 0). For some vibrations a new assignment is given in the first excited state (S 1) and the ionic ground state (D 0). An earlier discussed appearance of internal vibrational redistribution (IVR) by activating the 516a1 vibrational state in the first electronically excited state (S 1) is rejected due to the results of the presented measurements. For the pyrazine-Ar complex the ionization energy, as well as the binding energy in the ionic and the neutral ground state could be determined. These values are given by 74632 ± 5 cm-1, 630 cm-1 ± 20 cm-1 and 349 cm-1 ± 20 cm-1, respectively. The van-der-Waals vibrations in the first excited state could be reassigned and the frequencies of these vibrations have been found to be 12 cm-1, 27 cm-1 and 39 cm-1.

This article gives a brief impression of how high-resolution photoion and photoelectron spectroscopy can be applied to investigate molecular clusters. A short theoretical description of the nature of noncovalent interactions is given. In this context, we discuss which properties can best be investigated by applying a variety of spectroscopic techniques. The principles of the main photoion/photoelectron spectroscopy techniques are explained and examples of investigations of molecular clusters are presented. Van der Waals (vdW) clusters that are composed of a molecule and one or more rare-gas atoms as well as hydrogen-bonded hetero dimers are discussed. Complexes containing aniline or phenol are interesting because of two principal ligand binding sites, thus allowing either hydrogen bonding via the NH2 or OH group (H-bound) and vdW bonding via interaction with the aromatic π system (π-bound). For phenol…Arn and aniline…Arn (n = 1, 2), we discuss the very recently observed phenomenon of π H isomerization in the cation cluster. The interplay between different types of intermolecular bonds during fragmentation is discussed for the example of phenol…water…argon. For the fluorobenzene…Ar cluster, we demonstrate how structural information from rotational constants for the ionic state can be obtained from partially rotational-resolved zero-electron kinetic energy (ZEKE) spectra. Keywords: photoionization; photoelectron spectroscopy; molecular cluster; noncovalent interactions; REMPI; ZEKE; MATI; van der Waals; H-bound; isomerization

The dissociation energetics in the phenol(+)⋯Ar(2)(2π) cluster ion have been investigated using photoionization efficiency and mass analyzed threshold ionization spectroscopy. The appearance energies for the loss of one and two Ar atoms are determined as ∼210 and ∼1115 cm(-1), respectively. The difference between the appearance energy for the first Ar ligand in phenol(+)⋯Ar(2)(2π) and the dissociation energy of the phenol(+)⋯Ar(π) dimer (535cm(-1)) is explained by the isomerization of one π-bound Ar ligand to the OH binding site (H-bond) upon ionization. The energy difference between phenol(+)⋯Ar(2)(2π) and phenol(+)⋯Ar(2)(H/π) could also be estimated to be around 325cm(-1), which corresponds roughly to the difference of the binding energy of a π-bound and H-bound Ar ligands. The binding energy of the H-bound Ar atom in phenol(+)⋯Ar(2)(H/π) is derived to be ∼905cm(-1).

The various dissociation thresholds of phenol(+)···Ar(3) complexes for the consecutive loss of all three Ar ligands were measured in a molecular beam using resonant photoionization efficiency and mass analyzed threshold ionization spectroscopy via excitation of the first excited singlet state (S(1)). The adiabatic ionization energy is derived as 68077 ± 15 cm(-1). The analysis of the dissociation thresholds demonstrate that all three Ar ligands in the neutral phenol···Ar(3) tetramer are attached to the aromatic ring via π-bonding, denoted phenol···Ar(3)(3π). The value of the dissociation threshold for the loss of one Ar ligand from phenol(+)···Ar(3)(3π), ∼190 cm(-1), is significantly lower than the binding energy measured for the π-bonded Ar ligand in the phenol(+)···Ar(π) dimer, D(0) = 535 ± 3 cm(-1). This difference is rationalized by an ionization-induced π → H isomerization process occurring prior to dissociation, that is, one Ar atom in phenol(+)···Ar(3)(3π) moves to the OH binding site, leading to a structure with one H-bonded and 2 π-bonded ligands, denoted phenol(+)···Ar(3)(H/2π). The dissociation thresholds for the loss of two and three Ar atoms are also reported as 860 and 1730 cm(-1). From these values, the binding energy of the H-bound Ar atom can be estimated as 870 cm(-1).

A novel laser desorption system, with improved signal stability and extraordinary long lifetime, is presented for the study of jet-cooled biomolecules in the gas phase using vibrationally resolved photoionization spectroscopy. As a test substance tryptophane is used to characterize this desorption source. A usable lifetime of above 1 month (for a laser desorption repetition rate of 20 Hz) has been observed by optimizing the pellets (graphite/tryptophane, 3 mm diameter and 6 mm length) from which the substance is laser-desorbed. Additionally, the stability and signal-to-noise ratio has been improved by averaging the signal over the entire sample pellet by synchronizing the data acquisition with the rotation of the sample rod. The results demonstrate how a combination of the above helps to produce stable and conclusive spectra of tryptophane using one-color and two-color resonant two-photon ionization studies.

Resonance-enhanced multi-photon ionization (REMPI) and mass-analyzed threshold ionization (MATI) spectroscopic investigations were applied to the van der Waals complexes of the three dichlorobenzene isomers with argon. From the REMPI spectra it is concluded that the argon atom is shifted towards the chlorine atoms during excitation for the ortho and the meta isomers while it stays in the middle of the ring for the para isomer. From the MATI spectra it was possible to determine the binding energies in the ion ground state to 617 cm−1 ± 15 cm−1, 529 cm−1 ± 125 cm−1 and 581 cm−1 ± 76 cm−1 for the para, the meta and the ortho isomer, respectively. Together with theoretical calculations binding energies in the neutral ground state were determined to be 426 cm−1 ± 16 cm−1 for all the three isomers.

REMPI and MATI spectroscopic investigations were applied to van-der-Waals complexes of the three dichlorobenzene isomers with argon. From the REMPI spectra it is concluded that the argon atom is shifted towards the chlorine atoms during excitation for the ortho and the meta isomer while it stays in the middle of the ring for the para isomer. From the MATI spectra it was possible to determine the binding energies in the ion ground state as 617 +/- 15 cm(-1), 529 +/- 125 cm(-1) and 581 +/- 76 cm(-1) for the para, the meta and the ortho isomer, respectively. Together with theoretical calculations binding energies in the neutral ground state were determined to be 426 +/- 16 cm(-1) for all the three isomers.

For the first time, vibrational spectra of the pyridine cation in the electronic ground state have been measured via several intermediate states (0(0), 16b0(2), 16b0(4), 6a0(1), 6b(1), 16a0(1), 10a0(1) and 12(1)) by Mass-Analyzed Threshold Ionization (MATI) spectroscopy. From the MATI spectra, the adiabatic ionization energy of pyridine has been determined to be 74,185 +/- 6 cm(-1) (9.1978 +/- 0.0008 eV). Several vibronic modes in the ionic ground state could be assigned for the first time. An intensity gain of vibrations having b1 symmetry could be observed by activating the ion ground state. Also, a breakdown of the "delta nu = 0 propensity rule" for the excitation via the 16b(2) and 16b(4) states of the first excited states are displayed in the recorded spectra. In conjunction with ab initio calculations these observations can be explained by a strong geometrical distortion along the 16b vibration in the first excited state, leading to a "boat distortion".

A laser spectroscopic investigation of phenol...Ar(n) (n = 1-6) clusters in the first electronically excited state (S(1)) and the cationic ground state (D(0)) is reported. Resonance enhanced two-photon ionisation (R2PI) spectra have been recorded for the investigation of the S(1) state. The origins of S(1)← S(0) (S(1)0(0)) transition of phenol...Ar(n) (n = 1, 2,4-6) are all red shifted compared to the S(1)0(0) state of the monomer by 33 cm(-1), 67 cm(-1), 10 cm(-1), 20 cm(-1), 44 cm(-1), respectively. However, the origin of the phenolAr(3) cluster is blue shifted by 25 cm(-1). For the investigation of the ionic ground state photoionization efficiency (PIE) and mass-analyzed-threshold ionization (MATI) spectroscopy have been applied. The spectra of phenol...Ar(3) and phenol...Ar(4) yield values for the ionization energy (IE) of 68,077 ± 15 cm(-1) and 67,948 ± 15 cm(-1). With the combination of theoretical methods and R2PI, PIE and MATI spectroscopy, the major species present have been positively identified.