Dissociation pathways of doubly and triply charged gold clusters

Collision induced dissociation is applied to study the fragmentation channels of multiply charged gold clusters, Au N2⁺, size N= 7 –35, and Au> _N3+}, N = 19–35, stored in an ion cyclotron resonance (Penning) trap. The main dissociation pathways are neutral monomer evaporation, Au> _NZ+to Au> _N-1Z+} + Au, for the larger and fission into a charged trimer plus the remaining cluster, Au> _NZ+to{}Au> _N-3(Z-1)+} +{}Au> ₃₊}, for the smaller clusters. In the intermediate cluster size region an odd–even alternation of the two competing decay pathways is observed. In addition, for some specific cluster sizes there are indications of neutral dimer evaporation, Au> _NZ+to{}Au> _N-2Z++Au> ₂}, and of extremely asymmetric fission of the form Au> _NZ+to Au> _N-1(Z-1)+} + Au⁺. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electron momentum distributions and binding energies for the valence orbitals of hydrogen bromide and hydrogen iodide

Electron binding energy spectra and momentum distributions have been obtained for the valence orbitals of HBr and HI using noncoplanar symmetric electron coincidence spectroscopy at 1200 eV. The weakly bonding inner valence ns orbitals, which have not been observed previously, have their spectroscopic (pole) strength severely split among a number of ion states. For HBr the strength of the “main” inner valence (ns) transition is 0.42±0.03 whereas for HI it is 0.37±0.04, in close agreement with the value observed for the valence s orbitals of the corresponding isoelectronic inert-gas atoms. The spectroscopic strength for the two outermost orbitals is found to be close to unity, in agreement with many-body Green's function calculations. The measured momentum distributions are compared with several spherically averaged MO momentum distributions, as well as (for HBr) with a Green's function calculation of the generalized overlap amplitude (GOA). The GOA momentum distributions are in excellent agreement with the HBr data, both in shape and relative magnitude. Not all of the MO momentum distributions are in reasonable agreement with the data. Comparison is also made with the calculated momentum distributions for Kr, Br, Xe and I.     

Nuclear spin-lattice relaxation in the triply rotating frame and ultraslow molecular motions in solids

A method for measuring nuclear magnetic spin-lattice relaxation in solids in the effective field H_e3 acting in the triply rotating frame (TRF) is described. The method advances the previously described techniques whereby nuclear magnetic resonance and relaxation in the rotating (RF) and doubly rotating frames (DRF) are measured directly. In the present work, the RF and DRF are employed for suppressing the secular part of nuclear dipole-dipole (DD) interactions in the first two orders. As a result, the higher-order DD interactions (four- and five-particle ones) were separated, and their contribution to the nuclear spin-lattice relaxation in the TRF was studied experimentally. The experiments were carried out on protons in polycrystalline benzene. With the introduced technique, an overall spin-lattice relaxation decay in the TRF was recorded continuously during a single radio-frequency pulse with a length not exceeding 1 s. The contribution of multiproton nonsecular DD interactions to the proton spin-lattice relaxation in the TRF was observed selectively as a pronounced local minimum in the temperature dependence of the relaxation timeT _1ϱϱϱ. This contribution corresponds to ultraslow motion of benzene molecules with a rate about γH_e3 ≈ 2π · (10¹-10³) s^-1 and is determined quantitatively by specific correlation functions corresponding to the multiparticle nonsecular DD interactions of protons. The prospects of using this method for studying ultraslow atomic and molecular dynamics in solids are discussed.     

Dissociation energies of molecular hydrogen and the hydrogen molecular ion

We have obtained improved values for the dissociation energies of molecular hydrogen and its ion by using a high-resolution pulse-amplified laser to probe the second dissociation limit. The onset of the vibrational continuum is observed by state-selective detection of the atomic products of dissociation, and several auxiliary measurements link the results to the ground state. The dissociation energies are accurate to 0.010-0.026 cm(-1), improving previous measurements by a factor of 3-7. Agreement with ab initio calculations is good for H2, D2, and their ions, but not for HD and HD+.     

Superfluid molecular hydrogen

In order to produce a supercooled liquid phase of molecular hydrogen that may possibly change at a sufficiently low temperature to a superfluid state, it is suggested to reduce the temperature of its equilibrium coexistence with the solid phase by means of developing different pressures in these phases through the use of linear mechanical pressure on the solid phase or of external electric field. The thermodynamic functions of hydrogen are calculated in both the stable and metastable regions; its phase diagram and the region of possible transition to a superfluid state are also found. The values of excess pressure on the solid phase and of external electric field intensity are estimated, which are necessary for the stabilization of this state.     

Electron-electron separation in molecular hydrogen

Calculations of the mean distance and mean inverse distance between the electrons in the hydrogen molecule show that a Heitler-London wave function exhibits more correlation and a molecular orbital function less correlation than a covalent-ionic wave function. Introduction of an atomic screening constant decreases the mean distance between the electrons.     

Isotope effect in collision between heliumatom and hydrogen bromide molecule

The anisotropic potential developed in our previous research and the close-coupling method are applied to the HBr-3He （4He, 5He, 6He, 7He） system, and the partial cross sections （PCSs） at the incident energy of 60meV are calculated. Based on the calculations, the influences of the isotope helium atom on PCSs are discussed in detail. The results show that the excitation PCSs converge faster than the elastic PCSs for the collision energy and the systems considered here. Also the excitation PCSs converge more rapidly for the high-excited states. The tail effect is present only in elastic scattering and low-exclted states but not in high-excited states. With the increase of reduced mass of the collision system, the converging speed of the elastic and excitation PCSs slows down, and the tail effect goes up.     

Spontaneous radiative dissociation in molecular hydrogen

The spontaneous radiative dissociations of the discrete vibrational levels of the B¹Σ⁺_u electronic states of H₂, HD and D₂ of the C¹Π_u electronic state of H₂ into the vibrational continuum of the ground X¹Σ⁺_g state are calculated as a function of the emission wavelength. The fluorescent spectra of HD in the Lyman system and of H₂ in the Werner system resulting from an excitation source uniform in wavelength are predicted. The vibrational radiative lifetimes are tabulated as are the fractions of radiative decays that lead to dissociation. The effects of centrifugal distortion are discussed briefly. An appendix describes a sum rule used to check the numerical accuracy of the calculations.     

氦同位素被溴化氢散射的角分布

基于作者构造的He-HBr体系的各向异性势,采用密耦方法计算了碰撞能量分别为60和100 meV时3He、4He、6He和7He被HBr分子散射的角分布,详细讨论了氦同位素对散射角分布的影响.结果表明:在相同碰撞能量时,随着同位素氯原子质量的增加,总微分截面在0°时的角分布逐渐增大,同一级衍射振荡极小值位置逐渐向小散射角方向移动;弹性与总非弹性截面交界角逐渐减小,总非弹性截面逐渐增加.碰撞能量越低,入射同位素He原子的质量越大,彩虹现象越明显.     

Comparison of CID, ETD and metastable atom-activated dissociation (MAD) of doubly and triply charged phosphorylated tau peptides

The fragmentation behavior of the 2+ and 3+ charge states of eleven different phosphorylated tau peptides was studied using collision-induced dissociation (CID), electron transfer dissociation (ETD) and metastable atom-activated dissociation (MAD). The synthetic peptides studied contain up to two known phosphorylation sites on serine or threonine residues, at least two basic residues, and between four and eight potential sites of phosphorylation. CID produced mainly b-/y-type ions with abundant neutral losses of the phosphorylation modification. ETD produced c-/z-type ions in highest abundance but also showed numerous y-type ions at a frequency about 50% that of the z-type ions. The major peaks observed in the ETD spectra correspond to the charge-reduced product ions and small neutral losses from the charge-reduced peaks. ETD of the 2+ charge state of each peptide generally produced fewer backbone cleavages than the 3+ charge state, consistent with previous reports. Regardless of charge state, MAD achieved more extensive backbone cleavage than CID or ETD, while retaining the modification(s) in most cases. In all but one case, unambiguous modification site determination was achieved with MAD. MAD produced 15-20% better sequence coverage than CID and ETD for both the 2+ and 3+ charge states and very different fragmentation products indicating that the mechanism of fragmentation in MAD is unique and complementary to CID and ETD.     

Nanoscale molecular superfluidity of hydrogen

We present a microscopic quantum theoretical analysis of the nanoscale superfluid properties of solvating clusters of para-H2 around the linear OCS molecule. Path-integral calculations with N=17 para-H2 molecules, constituting a full solvation shell, show the appearance of a significant superfluid response to rotation around the molecular axis at T=0.15 K. This low-temperature superfluid response is highly anisotropic and drops sharply as the temperature increases to T approximately 0.3 K. These calculations provide definitive theoretical evidence that an anisotropic superfluid state exists for molecular hydrogen in this microscopic solvation layer.     

Compton profile of molecular hydrogen

The Compton profile of molecular hydrogen has been determined at an incident photon energy of 20 ke V based on the third generation synchrotron radiation, and the statistical accuracy of 0.2% is achieved at p z = 0. Different theoretical methods, i.e., the density functional method, and the Hartree–Fock method, were used to calculate the Compton profiles of hydrogen with different basis sets, and the theoretical calculations are in agreement with the experimental observation in the whole p z region. Compared with the HF calculation, the DFT-B3 LYP ones are in better agreement with the present experiment, which indicates the electron correlation effect is very important to describe the wavefunction in the ground state of hydrogen.     

Conduction mechanism in a molecular hydrogen contact

We present first principles calculations for the conductance of a hydrogen molecule bridging a pair of Pt electrodes. The transmission function has a wide plateau with T approximately 1 which extends across the Fermi level and indicates the existence of a single, robust conductance channel with nearly perfect transmission. Through a detailed Wannier function analysis we show that the H(2) bonding state is not involved in the transport and that the plateau forms due to strong hybridization between the H(2) antibonding state and states on the adjacent Pt atoms. The Wannier functions furthermore allow us to derive a resonant-level model for the system with all parameters determined from the fully self-consistent Kohn-Sham Hamiltonian.