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It is a long-standing dream of scientists to capture the ultra-fast dynamicsof molecular or chemical reactions in real time and to make a molecular movie.With free-electron lasers delivering extreme ultraviolet (XUV) light atunprecedented intensities, in combination with pump-probe schemes, it is nowpossible to visualize structural changes on the femtosecond time scale inphoto-excited molecules. In hydrocarbons the absorption of a single photon maytrigger the migration of a hydrogen atom within the molecule. Here, such areaction was filmed in acetylene molecules (C2H2) showing a partial migrationof one of the protons along the carbon backbone which is consistent withdynamics calculations on ab initio potential energy surfaces. Our approachopens attractive perspectives and potential applications for a large variety ofXUV-induced ultra-fast phenomena in molecules relevant to physics, chemistry,and biology.

Author(s): M. Uiberacker (corresponding author) [1, 2]; Th. Uphues [3]; M. Schultze [2]; A. J. Verhoef [2, 4]; V. Yakovlev [1]; M. F. Kling [5]; J. Rauschenberger [1, 2]; N. [...]

Surface induced dissociation of the protonated ethanol molecular ion, protonated ethanol dimer, and protonated ethanol trimer on a stainless steel surface covered by hydrocarbons was investigated over the incident ion energy range 10-80 eV. Mass spectra of the product ions were recorded and from them the dependencies of the relative abundance of the product ions on the collision energy (CERMS, collision energy resolved mass spectra) were obtained. The dissociation kinetics follows the unimolecular decomposition model of a surface-excited polyatomic projectile behaving as a statistical ensemble. The CERMS curves of the three projectiles, normalized to the same number of internal degrees of freedom, fit each other over the respective energy regions, and matched together provide information on the break-down pattern of the surface-excited protonated ethanol trimer over a wide energy range. This clearly shows that surface collisions can be used to impart in a defined manner a certain excitation energy to the projectile ion. The binding energy of an ethanol monomer in the protonated trimer and in the protonated dimer was estimated from the data as 0.95 and 1.6 eV, respectively, in good agreement with earlier data. [ABSTRACT FROM AUTHOR]

Dissociations and reactions induced by impact of acetonitrile monomer ions (CH[sub 3]CN[sup +], CD[sub 3]CN[sup +]), dimer ions [(CH[sub 3]CN)[sub 2, sup +], (CD[sub 3]CN)[sub 2, sup +]] and trimer ions [(CD[sub 3]CN)[sub 3, sup +]] on a hydrocarbon-covered stainless-steel surface were investigated over the projectile energy range of 3-70 eV. Both simple dissociations of the projectile ion and chemical reactions of H-atom transfer from the surface material (followed by dissociations of the protonated projectile ion formed) were observed for the monomer ions. Results obtained for the dimer ions (CD[sub 3]CN)[sub 2, sup +] indicate the formation of the protonated acetonitrile ions via surface-induced reactions in two ways: (i) an intracluster ionmolecule reaction followed by dissociation to form CD[sub 3]CND[sup +], and (b) a hydrogen pick-up reaction from the surface material during the interaction of the dimer ion with the surface leading to CD[sub 3]CNH[sup +]. A simple model based on the Brauman double-well potential—suggested earlier to explain the occurrence of analogous reactions in acetone cluster ion/surface interactions—accounts well for the formation of both product ions. Moreover, in adition to these protonated species, considerable amounts of nondissociated dimer ions were observed after acetonitrile dimer cation/ surface collisions with energies up to 25 eV. Similarly, both trimer ions (up to 20 eV) and dimer ions (up to 30 eV) were observed in acetonitrile trimer cation/surface interactions. This indicates that unimolecular dissociation kinetics governs the product formation for these cluster ion/surface interactions. [ABSTRACT FROM AUTHOR]

We report the observation and characterization of a new nonresonant strong field ionization mechanism in polyatomic molecules: Nonadiabatic multi-electron (NME) dynamics. The strong field response of a given molecule depends on important properties such as molecular geometry and bonding, the path length of delocalized electrons and/or ionization potential as well as on basic laser pulse parameters such as wavelength and intensity. Popular quasi-static tunnelling models of strong field molecular ionization, based upon the adiabatic response of a single active electron, are demonstrated to be inadequate when electron delocalization is important. The NME ionization mechanism greatly affects molecular ionization, its fragmentation and its energetics. In addition, multi-electron effects are shown to be present even in the adiabatic long wavelength limit. [ABSTRACT FROM AUTHOR]

The stability of singly charged C60+ fullerene ions, produced by electron impact ionization of C60, has been studied as a function of the electron energy and the time elapsed from ionization in a Nier-type ion source/double-focusing, sector-field mass spectrometer system. A huge kinetic shift of more than 34 eV (dependent on the observation time) was observed for the dissociation process C60+→C58++C2. The ionization efficiency curves for C58+, C56+, and C54+ fragment ions have been recorded with an energy resolution of approximately 0.5 eV. This allowed us to construct a time-resolved breakdown graph of the decaying C60+ fullerene ion. Two different methods, i.e., the finite heat bath model of Klots and the Rice–Ramsperger–Kassel–Marcus (RRKM) expression, have been used to calculate the decay rates and the breakdown graph of the C60+ ion, and the results of the calculation have been compared with the experimentally obtained breakdown graph. The best fit leads to a dissociation energy (C58+-C2) of 7.1±0.4 eV. This value is somewhat larger than the dissociation energies predicted earlier from the kinetic energy release distributions, and possible reasons for this difference are discussed in this paper. Using a quasithermochemical cycle the dissociation energy (C58-C2) of the neutral C60 molecule has been estimated to be 7.64±0.4 eV. [ABSTRACT FROM AUTHOR]

Kr clusters produced in a supersonic nozzle expansion have been studied by electron impact ionization mass spectrometry. Mass resolved spectra (with n up to 180) show two homologous series consisting of Kr+n and Kr2+n ions. The distribution of Kr+n ions shows distinct magic number effects, the observed abundance anomalies being very similar to the ones observed in Ar and Xe. This confirms the superior stability of closed-shell and -subshell icosahedral structures. Moreover, we have found evidence for the occurrence of Kr3+n and Kr4+n ions. It was possible to determine appearance sizes of these multiply charged cluster ions (yielding n2=69, n3=156, and n4=264), and to study the electron energy dependence of singly and doubly charged cluster ions (yielding a linear threshold law). These results are discussed in view of various theoretical considerations and previous results where available. [ABSTRACT FROM AUTHOR]

Electron impact ionization of C60 and C70 was studied using a molecular/electron beam ion source in combination with a two sector field mass spectrometer operated in the ion beam deflection mode. Relative partial ionization cross sections for the production of singly and multiply charged parent ions (up to charge state z=4) and fragment ions (down to C+44 in the case of C60 and down to C2+50 in the case of C70) were determined from threshold up to 1000 eV electron energy. Absolute partial and total ionization cross sections are obtained using a novel approach for the absolute calibration involving an intercomparison of the cation with the anion yield. The results obtained reveal not only an anomalous large parent ion cross section as compared to other ionization channels [e.g., σ(C+60/C60) is more than a factor of 30 larger than σ(C+58/C60)] but also anomalies for the production of multiply charged parent and fragment ions. For instance, the maximum cross section for the formation of C2+60 amounts to 30% of the maximum C+60 cross section and that of C2+70 to about 50% of C+70. Moreover, for all fragment ions, the formation of the doubly charged fragment ions has a larger cross section than that of the respective singly charged fragment ion. These peculiar features of the kinetics of electron impact ionization of C60 and C70 are related to the specific electronic and geometric structures of these fullerenes. The present absolute cross-section data for the summed up partial cross sections are in good agreement with a recent semiclassical calculation from our laboratory. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]