Zsolt Gengeliczki

Imaging photoelectron photoion coincidence spectroscopy with velocity focusing electron optics

An imaging photoelectron photoion coincidence spectrometer at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source is presented and a few initial measurements are reported. Monochromatic synchrotron VUV radiation ionizes the cooled or thermal gas-phase sample. Photoelectrons are velocity focused, with better than 1 meV resolution for threshold electrons, and also act as start signal for the ion time-of-flight analysis. The ions are accelerated in a relatively low, 40-80 V cm(-1) field, which enables the direct measurement of rate constants in the 10(3)-10(7) s(-1) range. All electron and ion events are recorded in a triggerless multiple-start/multiple-stop setup, which makes it possible to carry out coincidence experiments at >100 kHz event frequencies. As examples, the threshold photoelectron spectrum of the argon dimer and the breakdown diagrams for hydrogen atom loss in room temperature methane and the chlorine atom loss in cold chlorobenzene are shown and discussed.

IR-UV double resonance spectroscopy of xanthine.

We present resonant two-photon ionization (R2PI), UV-UV, and IR-UV double resonance spectra of xanthine seeded in a supersonic jet by laser desorption. We show that there is only one tautomer of xanthine which absorbs in the wavelength range of 36 700 to 37 700 cm(-1). The IR-UV double resonance spectrum shows three strong bands at 3444, 3485, and 3501 cm(-1), all of which we assign as N-H stretching vibrations. Comparison of the IR-UV double resonance spectrum with frequencies and intensities obtained from density functional theory (DFT) and second order Møller Plesset (MP2) calculations suggests that the observed xanthine is the diketo N(7)H tautomer.

Structure of 2,4-Diaminopyrimidine-Theobromine Alternate Base Pairs

We report the structure of clusters of 2,4-diaminopyrimidine with 3,7-dimethylxanthine (theobromine) in the gas phase determined by IR-UV double resonance spectroscopy in both the near-IR and mid-IR regions in combination with ab initio computations. These clusters represent potential alternate nucleobase pairs, geometrically equivalent to guanine-cytosine. We have found the four lowest energy structures, which include the Watson-Crick base pairing motif. This Watson-Crick structure has not been observed by resonant two-photon ionization (R2PI) in the gas phase for the canonical DNA base pairs.

Dissociative Photoionization of X(CH3)3 (X = N, P, As, Sb, Bi): Mechanism, Trends, and Accurate Energetics

Threshold photoelectron photoion coincidence spectroscopy is used to study the dissociation of energy-selected X(CH(3))(3)(+) ions (X = As, Sb, Bi) by methyl loss, the only process observed up to 2 eV above the ionization energy. The ion time-of-flight distributions and the breakdown diagrams are analyzed in terms of the statistical RRKM theory to obtain accurate ionic dissociation energies. These experiments complement previous studies on analogous trimethyl compounds of the N group where X = N and P. However, trimethylamine was observed to lose only an H atom, whereas trimethylphosphine was shown to lose methyl radical, H atom, and, to a lesser extent, methane in parallel dissociation reactions. Both kinetic and thermodynamic arguments are needed to explain these trends. The methyl radical loss has two channels: either a H transfer to the central atom, followed by CH(3) loss, or a direct homolytic bond cleavage. However, the H transfer channel is blocked in trimethylamine by an H loss channel with an earlier onset, and, thus, the methyl loss is not observed. Bond energies are defined based on ab initio reaction energies and show that the main thermodynamic reason behind the trends in the energetics is the significantly weakening C=X double bond in the ion in the N --> As direction. The first adiabatic ionization energies of Sb(CH(3))(3) and Bi(CH(3))(3) have also been measured by ultraviolet photoelectron spectroscopy to be 8.02 +/- 0.05 and 8.08 +/- 0.05 eV, respectively.

Dissociation of Energy-Selected 1,1-Dimethylhydrazine Ions

The unimolecular dissociation of 1,1-dimethylhydrazine ions was studied by threshold photoelectron photoion coincidence spectroscopy (TPEPICO). Time-of-flight distributions and breakdown curves were recorded in the photon energy range of 9.5-10.4 eV. The 0 K appearance energies of the fragment ions were extracted by modeling the experimental data with rigid activated complex (RAC-) RRKM theory. It was found that the data could be well-reproduced with a single TS for each dissociation channel if two different H-loss channels were assumed, one corresponding to a C-H and the other to a N-H bond dissociation. Once the appearance energies were established, heats of formation of the fragment ions could be derived. The heat of formation of the neutral molecule was computed by applying composite ab initio methods (G3, CBS-APNO, W1U) on a series of isodesmic reactions between methyl hydrazines and methyl amines.

Resonant Two-Photon Ionization Mass Spectrometry of Jet-Cooled Phenolic Acids and Polyphenols

A method for analyzing phenolic acids and polyphenols by means of resonant two-photon ionization (R2PI) mass spectrometry coupled with laser desorption and supersonic jet cooling is described. The R2PI spectra of gallic acid, 3-O-methylgallic acid, protocatechuic acid, syringic acid, vanillic acid, and trans-resveratrol are vibronically resolved and distinct to allow for unambiguous identification. For vanillic acid, its R2PI spectrum can be separated into contributions of two rotational isomers based on UV-UV and IR-UV double-resonance spectroscopy. Since R2PI spectra display sharp and well-resolved peaks, the laser wavelength can be tuned for selective ionization of targeted molecules. The mass spectrum recorded under jet-cooled conditions and at the resonant wavelength displays only the molecular ion peak with no fragmentation or background peaks. Picogram sensitivity and linear response over a nanogram range allows trace quantitative measurements of target molecules in complex matrixes. These techniques were applied to detect syringic acid in a model archaeological wine vessel.