Penning spectroscopy and structure of acetylene oligomers in He nanodroplets.

Penning ionization of (CH3)4C and (CH3)3CCl upon collision with He*(2 3S) metastable atoms were studied by angular and collision-energy resolved Penning ionization electron spectroscopy. The observed Penning ionization electron spectra (PIES) revealed peculiar features for the totally symmetric ionized state of the target molecules which can be ascribed to ionization from a molecular orbital having carbon 2s atomic orbital characters; (1) the relative PIES intensity is extremely enhanced, (2) the intensity distribution of the vibrational fine structure in PIES is clearly different from that observed for ultraviolet photoelectron spectra, (3) the angular distribution of Penning electrons is highly isotropic, and (4) the collision energy dependence of the partial Penning ionization cross section is slightly increasing and rather flattened with respect to the increase of the collision energy. These features were found to be due to the formation of an excimer-like state involving C2s hole characters in the target molecule which induces Auger-like transitions selectively from orbitals having the C2s characters. Effects of attractive interaction potentials were observed for (CH3)3CCl on the ionized state ascribed to the Cl nonbonding orbital.

Penning ionization electron spectroscopy of the CO molecule by He*(2 1 S) metastable atoms is studied. Vibrational populations are reported for the three energetically accessible states of the CO + ion (X 2 Σ + , A 2 Π and B 2 Σ + ). An obvious discrepancy is observed between the excited vibrational levels of the X 2 Σ + state and the calculated Franck-Condon factors. This result is attributed to an excitation transfer process via Rydberg vibrational progression converging to the CO + (D 2 Π) ionic state in agreement with the recent observations by fluorescence of neutral dissociate states. The presence of electrons coming from the well-known σ shape resonance is not excluded.

Penning ionization of propionitrile (CH3CH2C⋮N), acrylonitrile (CH2CHC⋮N), and 3-butenenitrile (CH2CHCH2C⋮N) upon collision with He*(23S) metastable atoms has been studied by two-dimensional Penning ionization electron spectroscopy in which Penning electron intensity is measured as a function of both collision energy and electron energy. We have observed a strong negative collision energy dependence of the partial ionization cross sections for ionization from nonbonding and πCN orbitals of all samples studied, which indicates that the interaction potentials between He*(23S) and target molecules are strongly attractive around the cyano group. For π-conjugated acrylonitrile, strong attractive interaction was shown for the πCC orbital region, while the attractive interaction for the πCC orbital region became weak for nonconjugated 3-butenenitrile. The observed results are consistent with calculated model potential curves. The electron density difference for charge transfer was analyzed for the lone pair orbital region where the molecule acts as an electron donor, while the molecule acts as an electron acceptor for the conjugated π orbital region.

Two-dimensional Penning ionization electron spectroscopic study on outer characteristics of molecules

Using a crossed-beams set-up, we have studied the electron energy spectra due to Penning and associative ionization in thermal energy collisions of state-selected metastable Ar*(4s 3P2, 3P0) and Kr*(5s 3P0) atoms with ground-state Hg atoms at an electron energy resolution of 30 meV. In all three cases the energy spectra exhibit a prominent peak located close to the energy difference between the metastable excitation energies and the Hg ionization energy; they show a sharp drop to lower and an extended tail towards higher electron energy with shapes which differ for the three systems. The energy-integrated ionization cross section for Ar*(3P0) exceeds that for Ar*(3P2) by about 30%. Excitation transfer to autoionizing Hg** states, resulting in a sharp additional structure, is only observed—as a weak channel—in Ar*(3P0) + Hg collisions. For the three investigated systems calculations of the electron energy spectra have been carried out within the local optical potential approximation. As input we use ab initio potential curves for the ionic exit channels as well as for the complex entrance channel potentials. Using these potentials (involving a scaled width for Kr* + Hg), impressive agreement with the experimental electron spectra is observed. In contrast, earlier empirical model potentials for the entrance channel are found to be inadequate. The calculated ionization rate coefficients at T = 300 K amount to (3.72, 5.26) × 10−10 cm3 s−1 for Ar*(3P2, 3P0) + Hg, respectively.

Study of outermost surface lavers of organic solids by penning ionization electron spectroscopy

Abstract

Photoelectron spectroscopy measures the energies for emission of electrons from molecules or materials through application of the photoelectric effect. For neutral molecules this is a direct measure of the ionization energies. The ionizations are explicitly defined in terms of transitions between the ground state of a molecule and accessible states of the molecular positive ion, but in a first order approximation, each ionization of a molecule can be described as removal of an electron from an individual orbital. The ionization energies can then be considered as measures of orbital energies, and shifts can be interpreted in terms of orbital stabilizations or destabilizations due to changes in electron distributions and bonding. The intensities and profiles of the ionization bands in the photoelectron spectra provide additional information on the characters and bonding natures of the orbitals. All orbitals can be probed, from valence orbitals such as lone pairs and bonds to orbitals in the cores of heavy atoms. Photoelectron spectroscopy is a large field of study that has many subfields and new techniques continue to be introduced. These different subfields all have different instrumental and sample constraints, but taken as a whole photoelectron spectroscopic experiments can be performed on almost any system of interest in inorganic chemistry. This article summarizes the basic physical principles of photoelectron emission caused by one or more photons, the relationship of photoelectron spectra to the electronic structure of neutral and anionic molecules, and the additional considerations for photoionization of bulk samples. Auger electron spectroscopy (AES) and Penning ionization electron spectroscopy (PIES), techniques which are closely related to photoelectron spectroscopy, are also discussed. Case studies are given for applications of photoelectron spectroscopy to study of bioinorganic model complexes, including iron‐sulfur and molybdenum‐sulfur bonding and covalency, electronic structure of porphyrins and metalloporphyrins, negative‐ion photoelectron spectroscopy of phthalocyanines, and the use of the technique to measure inner‐sphere reorganization energies.

We study theoretically nonequilibrium Landau-Zener-Stueckelberg (LZS) dynamics in a driven double quantum dot (DQD) including dephasing and, importantly, energy relaxation due to environmental fluctuations. We derive effective nonequilibrium Bloch equations. These allow us to identify clear signatures for LZS oscillations observed but not recognized as such and to identify the full environmental fluctuation spectra acting on a DQD in experiments [Petersson, Petta, Lu, and Gossard, Phys. Rev. Lett. 105, 246804 (2010)]. We find that the dominant dephasing and relaxation channels vary while driving. In detail, super-Ohmic fluctuations, typically due to phonons, are the main relaxation channel for a detuned DQD while dephasing is due to slow noise. In contrast, at zero detuning Ohmic fluctuations dominate dephasing and relaxation.

A single sheet of a clothlike macromolecule (atomic cloth; 4 Å thick) comprising the columns of polydiacetylene and polyacetylene chains alternately crosslinked to the rows of alkyl chains was prepared by the photopolymerization of 1,15,17,31-dotriacontatetrayne molecules laid flat in a monolayer. This process was monitored by Penning ionization electron spectroscopy.

We investigate the magnetic properties of the multiferroic quantum-spin\nsystem LiCu$_2$O$_2$ by electron spin resonance (ESR) measurements at $X$- and\n$Q$-band frequencies in a wide temperature range $(T_{\\rm N1} \\leq T \\leq\n300$\\,K). The observed anisotropies of the $g$ tensor and the ESR linewidth in\nuntwinned single crystals result from the crystal-electric field and from local\nexchange geometries acting on the magnetic Cu$^{2+}$ ions in the zigzag-ladder\nlike structure of LiCu$_2$O$_2$. Supported by a microscopic analysis of the\nexchange paths involved, we show that both the symmetric anisotropic exchange\ninteraction and the antisymmetric Dzyaloshinskii-Moriya interaction provide the\ndominant spin-spin relaxation channels in this material.\n

The problem of how single central spins interact with a nuclear spin bath is essential for understanding decoherence and relaxation in many quantum systems, yet is highly nontrivial owing to the many-body couplings involved. Different models yield widely varying time scales and dynamical responses (exponential, power-law, gaussian, etc.). Here we detect the small random fluctuations of central spins in thermal equilibrium [holes in singly charged (In,Ga)As quantum dots] to reveal the time scales and functional form of bath-induced spin relaxation. This spin noise indicates long (400 ns) spin correlation times at a zero magnetic field that increase to ∼5 μs as dominant hole-nuclear relaxation channels are suppressed with small (100 G) applied fields. Concomitantly, the noise line shape evolves from Lorentzian to power law, indicating a crossover from exponential to slow [∼1/log(t)] dynamics.

Combining first-principles calculations and sum frequency generation spectroscopy, we elucidate the microscopic details in the relaxation of the stretching vibration of hydrogen adsorbed on Ge(100). The dominant decay channels involve energy transfer from the stretching to the hydrogen bending modes, with the remaining energy difference being transferred to or from substrate phonons. The coupling between stretching and bending modes is treated from first principles using the calculated multidimensional adiabatic potential energy surface, while the coupling to phonons is treated in perturbation theory. For a surface solely saturated with light hydrogen, we calculate a vibrational lifetime of 1.56 ns at 400 K, in good agreement with experiment, and find a similar temperature dependence of the lifetime in both experiment and theory. The calculations show that the stretching energy dissipates to a vibrational state involving four bending quanta of hydrogen, concurrently absorbing a thermally excited surface phonon related to the Ge dimer rocking mode. For a Ge surface saturated with a mixture of H and D, our experiments find that the relaxation rate of the H stretching vibration is markedly increased when compared to a surface saturated with H only. Experimentally, a single decay is observed although H and D atoms will statistically pair on the surface dimers. The vibrational lifetime of the Ge-H stretching mode is up to six times shorter in the presence of adsorbed D atoms. The calculated relaxation rates are consistent with the experimentally observed trend. The theoretical analysis shows that the breaking of symmetry within the Ge surface dimer due to coadsorption of D opens up further relaxation channels that involve absorption or emission of a substrate phonon at various energies. Moreover, the calculations predict an even shorter vibrational lifetime of the Ge-D stretch mode due to efficient coupling to the Ge dimer rocking mode.

The collisional relaxation of highly vibrationally excited NO2 has been studied for a variety of collision partners (He, Ar, CO, N2, O2, N2O, NO2, CO2, SF6, and toluene) by time-resolved Fourier transform infrared emission spectroscopy. The average energy 〈E〉 of the vibrationally excited NO2 molecules during collisional quenching was obtained from the IR spectra by modeling the ν3 and ν1+ν3 bands, using the known harmonic frequencies and anharmonicity constants. The average amount of energy lost per collision 〈ΔE〉 was determined from the 〈E〉 versus time data. The results show that there is a dramatic increase in the amount of energy transferred for all bath gases at NO2 energies above 10 000–12 000 cm−1, which is near the origin of the NO2 Ã2B2/B̃2B1 states. This threshold in the energy-transfer rate occurs because of strong vibronic coupling between the X̃2A1 and Ã2B2/B̃2B1 electronic states. The increase in vibration-to-vibration (V-V) energy transfer can be understood within the context of the transition dipole coupling model. Vibronic coupling in NO2 produces extensive broadband emission in the IR and near-IR, which enhances the V-V energy-transfer rate by relaxing the resonance conditions in dipole coupling. The V-V energy-transfer probability was calculated using the dipole coupling model, where the transition dipole moment of excited NO2 was directly extracted from the IR emission spectra. These calculations successfully reproduced the observed threshold in the V-V energy transfer probability. The transition dipole coupling model was also used to estimate the relative contribution of V-V versus vibration-to-translation, rotation (V-T,R) energy transfer for NO2 deactivation. The calculations showed that V-T,R energy transfer is the major relaxation channel for triatomic or smaller collision partners. For larger species like SF6, however, V-V energy transfer is the dominant channel. Vibronic coupling may cause an increase in the V-T,R energy-transfer rate by allowing electronic potential related terms, possibly the electronic transition dipole moment, to contribute to the matrix elements responsible for V-T,R energy transfer.

Observation of structural change in organic monolayer film by penning ionization electron spectroscopy: fe-phthalocyanine on graphite