Charge transfer in the Cl-CO cluster induced by core ionization

Ab initio calculations of core-ionization spectra of the anion-molecule Cl-CO cluster are performed. Particular attention is paid to the investigation of charge-transfer screening processes accompanying core ionization of the CO molecule in the cluster. The charge-transfer processes are very efficient and favored by the presence of a low-lying unoccupied pi* orbital in CO capable of accepting an electron from Cl-. The O1s(-1) and C1s(-1) core-ionization spectra are calculated and compared. Both reveal a breakdown of the quasiparticle picture of core ionization caused by the charge-transfer processes. Remarkable differences between these two spectra are found which manifest themselves in distinct intensity distributions in the prominent low-energy spectral bands. The underlying reason for these differences is elucidated and linked with the preference of the pi* orbital to localize mainly on carbon. Core-ionization spectra of anion-molecule clusters are very sensitive to the type of the molecule involved as the comparative analysis of the O1s(-1) core-ionization spectra of the Cl-CO and Cl-H(2)O clusters show.     

Photofragmentation study of hexamethyldisiloxane following core ionization and direct double ionization

X-ray photoelectron spectroscopy and Auger spectroscopy studies of gas-phase hexamethyldisiloxane (HMDSO) are presented. The photodissociation of this molecule is studied using various experimental coincidence techniques. We compare the fragmentation pathways observed after core ionization followed by Auger decay and after valence double photoionization of the molecule. A strongly selective production of the doubly charged tetramethyldisiloxane ion is observed in the low binding-energy regions. Theoretical calculations are carried out to tentatively explain the stability of the produced dication.     

Competitive ionization processes of anthracene excited with a femtosecond pulse in the multi-photon ionization regime

To clarify the ionization mechanism of large molecules under multi-photon ionization conditions, photo-electron spectroscopic studies on anthracene have been performed with electron imaging technique. Electron kinetic energy distributions below a few eV reveal that three kinds of ionization channels coexist, viz., vertical ionization, ionization from Rydberg states, and thermionic hot electron emission. Their relative yield is determined by the characteristic of the laser pulse. The duration in particular influences the ratio between the first two processes, while for higher intensities the last process dominates. Our results provide strong evidence that internal conversion plays an important role for the ionization of the molecule.     

Vibrational excitation following the Sls core ionization of CS

Ab initio non-relativistic spin-unrestricted Hartree-Fock calculations are performed on CS and its Sls core-ionized state. Geometrical relaxation is investigated. As in the corresponding first-row CO, relaxation greatly influences the vibrational band profile. The one-particle vibrational picture leads to bond-length shortening whereas relaxation and direct calculations indicate a slight bond-length increase, leading to a predicted small vibrational broadening.     

Negative shake-up energy in core ionization

In recent years unusual multipeak structures have been detected in the core electron ionization spectra of certain highly polar aromatic molecules. In this work the ionization spectrum of para-nitroaniline, NO₂C₆H₄NH₂, in the Nls energy region is calculated by ab initio methods taking configuration interaction between the single hole-hole configurations and manifold of two-hole-one-particle excited configurations into account. The splitting of the Nls(NO₂) line observed in the ESCA spectrum of para-nitroaniline is reproduced by the calculation. The calculation shows that in the presence of the core hole the lowest π → π* excitation energy is negative. This negative shake-up energy is responsible for the multi-peak structure of the Nls(NO₂) line. It is shown that the computational results can be simply rationalized by considering the charge distribution of the highest occupied and lowest unoccupied valence orbitals. The close analogy between the multi-peak phenomena in the K-shell ESCA spectra of nitroaniline (and related highly polar aromatic compounds) and molecules chemisorbed on metal surfaces is emphasized.     

A donor-acceptor molecular dyad showing multiple electronic energy-transfer processes in crystalline and amorphous states

Singlet-singlet, singlet-triplet, and triplet-triplet energy transfer takes place within single crystals and amorphous solid-state solutions of a molecular dyad comprising boron dipyrromethene and oligo-thiophene subunits. The crystal and sublimed thin-films are strongly fluorescent.     

Shake up in the core ionization of atomic zinc and cadmium

Shake up in atomic zinc and cadmium has been investigated by X-ray photoelectron spectroscopy using Al Kα (1487 eV) radiation. In zinc the 4s → 5s shake up associated with 2p ionization has been found to lie 12.5 eV from the main peak and to have a relative intensity of 7.3%. In cadmium the 5s → 6s shake up accompanying 3d ionization has an intensity of 4.5% and occurs at 11.8 eV from the main peak. The relative intensities reported in this work correspond to the sudden limit values as shown by calculations using equations developed by Thomas in 1984.     

Vibronic coupling and symmetry breaking in core electron ionization

It is shown that for highly symmetric molecules the ionization of a core electron leads quite generally to a lowering of the symmetry. The breaking of the symmetry is a consequence of the vibronic coupling between nearly degenerate core orbitals of different symmetry. The vibronic coupling leads to strong excitation of non-totally symmetric vibrational modes in addition to the usually observed excitation of totally symmetric modes. As an example, the vibrational structure of the Ols line of the CO₂ molecule is computed on the one-particle level.     

Molecular processes in an aqueous cluster

A cluster of 216 water molecules in a vacuum is simulated (molecular-dynamic trajectory is 40 ps); the average kinetic energy of water molecule movements corresponds to 300 K. The HLC model of data representation in time intervals of 0.082 ps is used to investigate the molecular scale processes. Volumetric and surface movements of water molecules are distinguished by the rank distributions based on the molecular movement character. The method for calculating volumetric and surface self-diffusion coefficients of molecules in a cluster is proposed. The cooperative character of movements of water molecules in a 5.4 Å-thick surface layer of the cluster is revealed.     

Indirect processes in the electron-impact ionization of Li-like ions

Detailed measurements of electron-impact ionization cross sections have been made in the vicinity of the excitation-autoionization thresholds of Li-like B²⁺, C³⁺, N⁴⁺, O⁵⁺ and F⁶⁺ ions. With an energy spread of 0.4 to 1% of the electron energy and statistical uncertainties as low as 0.1% we could clearly resolve thresholds for excitation of individual terms in 1s2s2l configurations. Numerous resonance features were found which are due to dielectronic capture of the incident electron with subsequent two-electron emission. In particular, dielectronic capture processes involving Δn=2 excitations of a 1s electron provide the dominant resonance contributions to the measured cross sections. Rydberg series 1s2snln′l′ of resonances withn=3 (Δn=2) andn=4 (Δn=3) are resolved up to principle quantum numbersn′ equal to 6 or 7.     

Electrochemical processes in electrospray ionization mass spectrometry

Editorial Comment Last month we presented, as a Special Feature, a set of five articles that constituted a Commentary on the fundamentals and mechanism of electrospray ionization (ESI). These articles produced some lively discussion among the authors on the role of electrochemistry in ESI. Six authors participated in a detailed exchange of views on this topic, the final results of which constitute this month's Special Feature. We particularly hope that younger scientists will find value in this month's Special Feature, not only for the science that it teaches but also what it reveals about the processes by which scientific conclusions are drawn. To a degree, the contributions part the curtains on these processes and show science in action. We sincerely thank the contributors to this discussion. The give and take of intellectual debate is not always easy, and to a remarkable extent this set of authors has maintained good humor and friendships, even when disagreeing strongly on substance. Graham Cooks and Richard Caprioli Copyright 2000 John Wiley & Sons, Ltd.     

Relaxation effects accompanying core ionization in diatomic molecules

The influence of electronic relaxation on the bond-length charges following inner-shell ionisation in diatomic molecules is studied. CO and N₂ are treated explicitly. Transition operators are used for predicting renormalized vibrational coupling constants and slopes of relaxation energy surfaces.     

Communication: the formation of helium cluster cations following the ionization of helium nanodroplets: influence of droplet size and dopant

The He(n)(+)/He(2)(+) (n ≥ 3) signal ratios in the mass spectra derived from electron impact ionization of pure helium nanodroplets are shown to increase with droplet size, reaching an asymptotic limit at an average droplet size of approximately 50,000 helium atoms. This is explained in terms of a charge hopping model, where on average the positive charge is able to penetrate more deeply into the liquid helium as the droplet size increases. The deeper the point where the charge localizes to form He(2)(+), the greater the likelihood of collisions with the surrounding helium as the ion begins to leave the droplet, thus increasing the probability that helium will be ejected in the form of He(n)(+) (n ≥ 3) cluster ions rather than He(2)(+). The addition of a dopant alters the He(n)(+)/He(2)(+) ratio for small helium droplets, an observation attributed to the potential energy gradient created by the cation-dopant interaction and its effect in drawing the positive charge towards the dopant in the interior of the droplet.     

Effect of internal and external conditions on ionization processes in the FAPA ambient desorption/ionization source

Ambient desorption/ionization (ADI) sources coupled to mass spectrometry (MS) offer outstanding analytical features: direct analysis of real samples without sample pretreatment, combined with the selectivity and sensitivity of MS. Since ADI sources typically work in the open atmosphere, ambient conditions can affect the desorption and ionization processes. Here, the effects of internal source parameters and ambient humidity on the ionization processes of the flowing atmospheric pressure afterglow (FAPA) source are investigated. The interaction of reagent ions with a range of analytes is studied in terms of sensitivity and based upon the processes that occur in the ionization reactions. The results show that internal parameters which lead to higher gas temperatures afforded higher sensitivities, although fragmentation is also affected. In the case of humidity, only extremely dry conditions led to higher sensitivities, while fragmentation remained unaffected.     

Competitive ionization of tetraphenylporphyrin in a laser-generated metal ion plasma

The ionization of tetraphenylporphyrin (TPP) in a laser-desorbed metal ion plasma is examined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Competitive reaction pathways observed to generate abundant molecular ion species include electron detachment, cation attachment, charge exchange, metallation, and transmetallation in the positive ion mode and electron capture, metallation, and transmetallation in the negative ion mode. In general, cation attachment reactions dominate positive ion spectra below the laser irradiance threshold for plasma ignition, although the metallation product from [TPP]⁺ reaction with the metal atom, M, is observed. Negative ion products are not observed in the FT-ICR spectrum when a plasma is not formed. Under plasma ignition conditions, positive ion spectra include [TPP]⁺ formed by charge exchange with M⁺, which is also present in the spectrum. Negative ion spectra are dominated by [TPP]^?; which is formed by attachment to thermal electrons generated in the plasma. Metallation reactions involving TPP and the metal substrate are examined. Positive ion metallation products are observed both in the absence of a plasma through reaction of [TPP]⁺ with M and by a second pathway under plasma ignition conditions through reaction of TPP with M⁺. In negative ion mode, metallation is only observed under plasma ignition conditions through reaction of [TPP]^? with M. Observation of metallated products is found to be consistent with formation of stable metal oxidation states in the metallated porphyrin.