The structure and low-frequency vibrations of jet-cooled molecules of diphenyl ether are studied with the resonance-enhanced two-photon ionization technique. The origin for the S1←S0 transition is assigned at 35,873 cm−1 and, within 400 cm−1, the vibrational progressions suggest the existence of different conformations proposed for this molecule. A potential energy surface for the ground state, obtained with the use of group theory, is checked against the conformational energy surface given by molecular mechanics simulations.

The ease of fragmentation of various charge states of protonated polypropylenamine (POPAM) dendrimers is investigated by surface-induced dissociation. Investigated are the protonated diaminobutane propylenamines [DAB(PA) n ] DAB(PA)8 (1+ and 2+), DAB(PA)16 (2+ and 3+), and DAB(PA)32 (3+ and 4+). These ions have been proposed to fragment by charge-directed intramolecular nucleophilic substitution (Ni) reactions. Differences in relative fragment ion abundances between charge states can be related to the occupation of different protonation sites. These positions can be rationalized based on estimates of Coulomb energies and gas-phase basicities of the protonation/fragmentation sites. The laboratory collision energies at which the fragment ion current is ∼50% of the total ion current were found to increase with the size, but to be independent of charge state of the protonated POPAM dendrimers. It is suggested that intramolecular Coulomb repulsion within the multiply protonated POPAM dendrimers selected for activation does not readily result in easier fragmentation, which is in accordance with the proposed fragmentation mechanism.

The dissociative scattering of CF+, CF2+ and CF3+ ions from a perfluoropolyether (PFPE) surface is explored across the hyperthermal incident energy range 50 to 220 eV. A comparative study on internal energy uptake efficacy and on translational energy transfer to the surface in scattering from the PFPE surface is presented. In addition, measurements are performed on C+ scattering in order to examine the energy transfer to the PFPE liquid surface in more detail. The CF3+ ions take up more internal energy as a function of translational energy transfer in the collision than the CF2+ and the CF+ ions. Furthermore, a detailed analysis and discussion is presented on the surface effective mass as a function of incident (fluoro)carbon ion mass. The analysis is based on elastic scattering from a single group. The results are remarkably different from ion/metal surface scattering.

The (dissociative) scattering of CF+ and CF2+ ions from a perfluoropolyether is analyzed on the basis of the conservation laws of energy and momentum. The ion–surface collisions are treated as binary collisions between the primary ions and a representative group of the perfluoropolyether molecule. The energy partition between the two particles is analyzed on the basis of a statistical distribution between harmonic oscillators. The energy transfer between the individual atoms is treated as inelastic rovibrational excitation. This method not only allows us to draw some general conclusions concerning the dynamics of this type of molecule–surface collision, but also to estimate the dissociation probability of the scattered molecules as a function of the scattering angle and the energy loss.

The primary fragmentation of diphenyl ether and further secondary dissociation of its fragments have been studied by surface-induced dissociation (SID). In a tandem linear time-of-flight mass spectrometer, the dissociation of ions is studied as a function of the collision energy with a liquid perfluorinated polyether (PFPE) surface. In a tandem quadrupole instrument, an alkyl terminated (C18) and a perfluoroalkyl terminated (FC10) self-assembled monolayer surface have been used. The differences in the spectra obtained with the TOF and quadrupole instruments are attributed to the different time frames available for observation of the fragments. Deconvolution of the collision energy resolved SID spectra of diphenyl ether shows that above 50 eV collision energy, the energy deposition efficiency is similar in the two experimental set-ups, in spite of the different geometries. Ion/surface reactions are observed upon collisions of the diphenyl ether radical cation with the C18 surface but not with the FC10 surface The rearrangement fragments, obtained from perdeuterated diphenyl ether, C11D10+ and C11D9+, react with both the C18 and FC10 surface. Hydrogen exchange with the surface combined with hydrocarbon loss from C12D10O+, C11D10+, and C11D9+ is observed.

The dissociative scattering of CF3+ ions from a clean Ag(111) and a barium-covered Ag(111) surface was explored in the energy range 100–600 eV. Both negative (CF3−, CF2− and F−) and positive (CF+ and C+) ions were observed. The complete dissociation of the CF3+ ions into C+ fragments was found at low (normal) incoming energies. The breaking of three CF bonds is due to concerted electronic and mechanical energy transfer. The azimuthal orientation of the crystal has a large influence on the degree of dissociation, which indicates the importance of direct momentum transfer. For the barium-covered Ag(111) surface the negatively charged scattering products only showed F− and the positives showed CF+ and C+ fragments. No negatively charged polyatomic ions were observed. The lowering of the work function facilitates resonant neutralization into the excited 3s Rydberg state of the neutral molecule. Consequently, the spectral features associated with Auger neutralization are not observed.

The dissociative scattering of CF3+ ions from a liquid insulating perfluorinated polyether surface was explored in the incident energy range 100–500 eV. We have measured the angular and energy distributions of the positive and negative fragment ions. The impulsive energy transfer to the liquid molecules and to internal energy of the scattered ions has been determined as a function of incoming energy and total scattering angle. The energy transfer in the ion/surface collision indicated scattering off the –CF3 groups of the liquid molecules. The dissociation of the CF3+ ions was due to a single impulsive collision with these groups. The parent ion was observed as well as CF2+, CF+, and C+ ions. The degree of dissociation depends only on the total scattering angle and the incoming energy, as in gas phase scattering. Remarkably, negatively charged F− fragment ions were also observed. These were formed in a process referred to as collisionally-induced “ion-pair” formation from neutrals leaving the surface. The important parameter which determined the internal energy uptake of the scattered ions in the collision was the translational energy loss ΔE the parent ion suffered. Finally, a comparison has been made between scattering from a liquid perfluorinated polyether surface and a metal Ag(111) surface.

The dissociative scattering of CF3+ ions from a clean Ag(111) surface was explored in the energy range 100-600 eV. Both negative (CF3-, CF2-, and F-) and positive ions (CF+ and C+) were formed,The complete dissociation of the CF3+ molecular ions into C+ fragments was found to be feasible at low (normal) incoming energies. We conclude that the breaking of three C-F bonds is due to concerted electronic and mechanical energy transfer. The azimuthal orientation of the crystal has a large influence on the degree of dissociation, which indicates the importance of direct momentum transfer.

A perfluorinated polyether (PFPE) surface is used to enhance the fragment ion yield for surface-induced dissociation (SID) in a tandem time-of-flight mass spectrometer. Selection of the parent ion by separation of M+ from (M — H)+ is obtained with unit resolution by pulsing the voltage on the collision surface. The benzene radical cation is used as a model compound to characterise the instrument. Fragment ions are collected with a mass resolution of 100. The distribution of recoil energies of the SID fragments leaving the collision surface is not sufficient to clarify the limited mass resolution. We estimate an average conversion efficiency of kinetic into internal energy of 30±7% between 20 and 40 eV collision energy by analysing the relative intensities of the SID fragments as a function of collision energy.

The dissociative scattering of CF+3 ions from a barium-covered Ag(111) surface has been studied for incident energies between 100 and 500 eV. The negatively charged scattering products only showed F− and the positives showed CF+ and C+ fragments. No negatively charged molecular ions are observed. The results are compared with those on scattering from a clean Ag(111) surface. The lowering of the work function facilitates resonant neutralization into the excited 3s Rydberg state of the neutral molecule. Consequently, the spectral features associated with Auger neutralization are not observed.

To evaluate the necessity for cooling laser-desorbed molecules to obtain molecular ions and minimal fragmentation, mass spectra of a series of methoxybenzenes were measured in two different experimental configurations. In one geometry, laser-desorbed molecules were entrained in a pulsed supersonic jet before ionization. In the other, the molecules were ionized directly after laser desorption. The samples were ionized with laser-generated vacuum ultraviolet radiation in a single-photon process or ultraviolet radiation in a multi-photon process.

Resonance-enhanced multiphoton ionization (REMPI) as a means of specifically ionizing aromatic compounds is extended to the analysis of lignin pyrolyzates. First, mass-resolved excitation spectra of phenolic model compounds of increasing complexity were measured to determine optimal resonant wavelengths for jet-cooled phenol, guaiacol, 4-methylguaiacol, homovanillic acid, and syringol. The obtained set of resonant wavelengths is unique for each benzene derivative, depending on the number and types of substituents on the aromatic ring, different isomeric conformations, and stable rotational conformers which are frozen out in the supersonic jet. In this work, the one-color REMPI results are used to identify and quantify the main pyrolysis products of a cottonwood milled wood lignin. The insoluble lignin preparation is introduced directly inside the ion source of a time-of-flight (TOF) mass spectrometer and analyzed without any sample pretreatmeat. Flash pyrolysis is achieved by means of a pulsed CO2 laser. Analytical calibration curves are linear over a dynamic range of 4 orders of magnitude. Limits of detection obtained at a resonant wavelength of homovanillic acid are homovanillic acid, 400 pg, coniferyl alcohol, 3 ng, and sinapyl alcohol, 5 ng. Results are compared with high-resolution in-source pyrolysis mass spectrometry data obtained under EI conditions.

The mass-resolved excitation spectra of jet-cooled guaiacol (2-methoxyphenol) and 4-methylguaiacol (2-methoxy-4-methylphenol) have been measured. For guaiacol, two rotational isomers were identified. The potential barriers for internal rotation of the methoxy group were determined by molecular mechanics calculations for the ground state S-0 and by fitting to the experimental data for the first excited state S-1. The potential energy barrier for hindered rotation of the methoxy group is found to be dependent on the hydroxy group orientation with respect to the methoxy group and increases upon excitation. The potential energy barrier is described by a term V-1 due to the interaction with the hydroxy group and a term V-2 due to the interaction with the ring. When the hydroxy group is pointing away from the methoxy group, the values are V-1 = 146 cm(-1), V-2 = 580 cm(-1) for S-0 and V-1 = 238 cm(-1), V-2 = 560 cm(-1) for S-1. When the hydroxy group is pointing toward the methoxy group, these values are V-1 = 1074 cm(-1), V-2 = 550 cm(-1) for S-0 and V-1 = 1100 cm(-1), V-2 = 500 cm(-1) for S-1.

We present absorption spectra of the rotational band contours of the 610 and 710 bands in benzene, 12C6H6 and 13C12C5H6. These spectra were measured using resonant two-photon ionization in combination with time of flight mass spectrometry. A rotational temperature of 2.0 K was obtained by coexpanding the benzene in a supersonic jet of argon. We have resolved different J levels in the 610 band. Contours of the bands are simulated using a rigid-rotor model and accounting for Coriolis splitting. For the isotopomer, the vibrational modes 6 and 7 each consist of two nondegenerate modes a and b and the positions of these band origins relative to the band origin of the twofold degenerate state of benzene have been determined. The splitting between bands of mode 6 is in good agreement with values reported in literature which were calculated with force field methods for the ground state. The splitting in band 7 is dominated by either a difference in anharmonicity or a shift due to a Fermi resonance.

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Laser desorption with either multiphoton or single photon post-ionization is combined with time of flight mass spectrometric detection. In the first step large molecules are desorbed intact from a surface. In the second step laser ionization is employed to produce the ions of interest. Soft ionization, which yields primarily the molecular ion, and hard ionization, which yields primarily fragment ions, are both demonstrated. Instrumental details are shown with results from recent laser desorption multiphoton and single photon ionization experiments on a perbenzoylated sugar and on two porphyrins.

The combination of laser desorption followed by multiphoton ionization (MPI) with time of flight mass spectrometry (TOF/MS) produces a versatile method of high sensitivity and selectivity. The first step is laser desorption in which molecules are desorbed from the sample intact or with little degradation. Laser ionization is employed as the second step to produce the ions and also induce fragmentation. Soft ionization, which yields primarily the parent ion, and hard ionization, which yields primarily the fragments, are both possible.Instrumental details are described and preliminary results from recent laser desorption MPI-TOF/MS experiments on a perbenzoylated sugar and on a porphyrin are presented. Examples from single photon ionization (SPI) experiments on two alkanes are also shown.

Single crystals of α-glycine with two, three, and five of the protons replaced by deuterium are studied at 4.2 K using electron–nuclear double resonance (ENDOR) spectroscopy to detect the NMR transitions of the distant deuterons on diamagnetic molecules. Complete deuteron quadrupole interaction tensors, and indirectly the electric field gradient tensors, are determined for all the labeled positions. Proton dipolar coupling tensors are also obtained for the methylene hydrogens in the case of glycine-d3. The relationship between the deuterium quadrupole coupling constants and the inverse cube of the bond length is refined and discussed. The resulting structural information including X–D bond lengths and bond angles is compared with data obtained by neutron diffraction. In addition, extensive calculations at both the semiempirical and ab initio levels are carried out to determine the electric field gradient at the hydrogen positions. Both intramolecular and lattice contributions are considered.