Seung-Jin Han

Origin of ferromagnetism in Fe- and Cu-codoped ZnO

Fe- and Cu-codoped ZnO was previously reported as a room-temperature dilute magnetic semiconductor. We have investigated the origin of the ferromagnetism in Zn0.95−xFe0.05CuxO using the zero-field Fe57 nuclear magnetic resonance and neutron diffraction. These measurements reveal that some Fe ions of Zn0.95−xFe0.05CuxO form a secondary phase, ZnFe2O4. Detailed comparison of nuclear magnetic resonance spectra of Zn0.95−xFe0.05CuxO, bulk ZnFe2O4 with normal spinel structure, and nanocrystalline ZnFe2O4 with inverted spinel structure shows that the secondary phase possesses an inverted spinel structure and is ferrimagnetic at room temperature, while normal zinc ferrite is nonmagnetic. The ferromagnetism in Fe- and Cu-codoped ZnO stems from the secondary phase, while the majority of Fe ions substituted into the ZnO lattice appears to remain magnetically inert.

Discovery of CO2 precursor in the reaction of CO and O on Pt(111)

Using the technique of Cs+ reactive ion scattering (Cs+ RIS), we show that reaction of CO and O on a Pt(111) surface produces a CO2 precursor species that remains stable on the surface up to 300 K. The CO2 species desorbs with an activation energy of 46±3 kJ mol−1, whereas the direct CO2 gas formation via Langmuir–Hinshelwood (LH) reaction between CO and O occurs with an activation energy of 75±4 kJ mol−1. The observation reveals a new pathway for CO oxidation reaction, which is different from the direct LH pathway.

Space-charge effects on Fourier transform ion cyclotron resonance signals: Experimental observations and three-dimensional trajectory simulations

Space-charge effects were studied by monitoring Fourier transform ion cyclotron resonance spectra while scanning the laser wavelength near the origin of a two-photon resonant 3s ← n Rydberg transition of acetaldehyde. The rotational contour of the origin band permits the experimental control of space-charge density. Both the frequency shift and the inhomogeneous line broadening were observed as a function of space-charge density. Three-dimensional ion trajectories in the presence of Coulomb interactions between ions were simulated under the quadratic and exact trapping potentials. The simulated Fourier transform ion cyclotron resonance spectra were obtained from the image-charge signals induced by a uniform field of chirp or impulse excitation. Comparisons of experiments with three-dimensional simulations reveal that the inhomogeneous line broadening observed in experiments is most likely due to both large-amplitude oscillations of ions and Coulomb interactions between different m/q ions.

Time-resolved photodissociation of p-bromotoluene ion as a probe of ion internal energy

Abstract The unimolecular dissociation of the p-bromotoluene radical cation was studied by using Fourier transform ion cyclotron resonance spectrometry. The parent ions were prepared by charge-transfer reactions of toluene-d8 ions with p-bromotoluene. The toluene-d8 radical cations were produced by two-photon ionization at 266 nm. The photodissociation was accomplished by 532 nm. The p-bromotoluene ion dissociates to C7H7+ with a loss of Br via one- and two-photon processes. The structures of the C7H7+ products were identified by ion-molecule reactions. The dissociation rates were measured by time-resolved photo-dissociation (TRPD) spectroscopy. The C7H7+ products from one-photon dissociation (1PD) were reactive toward toluene-d8 and yielded CD3C6D4CH2+, indicating the benzyl ion structure. No unreactive tropylium ions were detected from 1PD. The rate constant of 1PD obtained at the internal energy of 2.43 eV was combined with the photoelectron-photoion coincidence data reported in the internal energy range 2.75–3.33 eV to estimate the activation parameters for the lowest barrier process that led to the benzyl ion exclusively. The two-photon dissociation (2PD) resulted in both the benzyl and tropylium ions with the branching ratio, [benzyl+]/[tropylium+] = 92/8. It appears that the tropylium ion results from the isomerization of the p-tolyl ion produced by 2PD via a direct C-Br cleavage. The TRPD spectroscopy was used as a thermometric probe of the average internal energy of the parent ions produced by electron impact. The rates of 1PD were compared as a function of cooling time and pressure to examine the energy cooling processes. The effect of radiative heating of an ICR cell by hot filament on the dissociation rate was also examined.

Application of sustained off-resonance irradiation : The beat frequency measurement and radial separation of mass-selected ions

Newer applications of sustained off-resonance irradiation (SORI) in Fourier-transform ion cyclotron resonance (FT-ICR) spectrometry are described. SORI induced circular gyrations of ions with their cyclotron radii varying sinusoidally at the corresponding beat frequency. The beat is observed in the temporal variation of FT-ICR peak height as a function of the duration of the SORI burst, and it allows the direct measurement of the beat frequency. The effective cyclotron frequency is determined from the measured beat frequency and the applied burst frequency. The frequency uncertainty is greatly reduced by nearly 3 orders of magnitude compared with the uncertainty in FT-ICR peak frequency that fluctuates with the rf burst duration. In addition, the oscillating cyclotron radii of the irradiated ions allow the radial separation of mass-selected ions so that the mass-selective photodissociation can be carried out with m-bromotoluene ions.

Vacuum ultraviolet photoionization and photodissociation of ferrocene

Abstract Photoionization and photodissociation of ferrocene [Fe(C5H5)2] is examined by using vacuum ultraviolet (VUV) photons from a synchrotron radiation source and a time-of-flight (TOF) photoionization mass spectrometer. VUV absorption by ferrocene results in Fe(C5H5)2·+, FeC5H5+, FeC3H3+, Fe·+ and C10Hx·+ (x = 8 − 10). The dependency of the product distribution on photon energy indicates sequential elimination of C5H5· ligands as a major dissociation channel, but concerted elimination of two C5H5· ligands also takes place to a lesser degree. Through analysis of TOF peak shape, it is found that Fe(C5H5)2·+ molecular ion dissociates into FeC5H5+ and C5H5· via two channels. One is nonstatistical dissociation with a fast rate, and the other is slow unimolecular decay that becomes more discernible at low photon energy. The rate of unimolecular decay, exemplified by a value of k = 2.4 ± 1.0 × 106 s−1 at photon energy of 14.76 eV, is well in accord with the RRKM rate.

Nonlinear oscillations in ion cyclotron resonance

Abstract Nonlinear equations of motion of an ion in a quartic electrostatic potential are solved using a perturbation method. A valid region of the quartic potential in an cubic ion trap is determined from the numerical comparison with the exact potentials. The quartic potential introduces the coupled cubic nonlinear forces to the equations of motion. Cubic nonlinearities that couple the axial and radial motions yield amplitude-dependent frequency shifts. Nonlinear solutions are derived to first order. The ion trajectory within the valid region of the quartic potential reproduces the essential features of the exact numerical results. The amplitudes of nonlinear oscillations are derived and their mass dependences are compared. The Fourier transform ion cyclotron resonance spectra are compared for the linear and nonlinear oscillations. The collisional damping is included as a perturbation in the nonlinear equations of motion. Collisions damp the cyclotron and axial oscillations but amplify the magnetron motion. The amplitude-dependent frequencies of nonlinear oscillations become time dependent due to the collisional damping of oscillation amplitudes. The collisional line broadenings of the nonlinear oscillation trajectories are compared with those of the linear oscillation trajectories.

Ionic-to-metallic layer transition in Cs adsorption on Si(111)-(7×7): Charge-state selective detection of the adsorbates by Cs+ reactive ion scattering

We examined the adsorption of Cs on a Si(111)-(7×7) surface by reactive ion scattering (RIS) of hyperthermal Cs+ beams. The RIS from a Cs-adsorbed surface gives rise to Cs2+, representing pickup of a surface Cs by a Cs+ projectile. The Cs2+ intensity is proportional to the surface coverage of Cs at a high substrate temperature (473 K), while it varies anomalously with the coverage at low temperatures (130–170 K). This observation indicates that RIS selectively detects metallic Cs on the surface, but discriminates ionic Cs. The transition from an ionic to metallic Cs adlayer is driven by the thermal diffusion of Cs and their clustering process.

Fourier transform ion cyclotron resonance detection of multiphoton ionization spectroscopy

Fourier transform ion cyclotron resonance (FT-ICR) detection was tested for resonanceenhanced multiphoton ionization (REMPI) spectroscopy. The (2+1) REMPI spectra of acetaldehyde were obtained in the wavelength range 364–354 nm via a two-photon resonant 3s ← n Rydberg transition. The space-charge effects on the REMPI spectra were examined in the vicinity of the 000 transition. The trapping efficiency measurement shows that all the ions produced from REMPI dissociation processes are arrested in the ion cyclotron resonance cell even in the presence of space-charge interactions. Axial kinetic energy release distributions of ions were extracted from the trapping efficiency data obtained under a new space-charge-free condition. FT-ICR peak heights were measured as a function of pressure at different laser powers, magnetic field strengths, and ion excitation methods to test for the detection linearity. The FT-ICR detection responds linearly to the number of ions in a low pressure limit. The product branching ratio was measured by using various ion excitation methods and was compared with the previous quadrupole mass spectrometric study. FT-ICR detection yields the mass-selected REMPI spectra and the product branching ratio in the absence of kinetic shifts.