Analytical photoion spectroscopy applied to dissociative single and double photoionization of diatomic molecules (H2, N2, O2, CO,and NO)

This work reports the principle, advantage, and limitations of analytical photoion spectroscopy which has been applied to dissociative photoionization processes for diatomic molecules such as H₂, N₂, CO, and NO. Characteristic features observed in the differential photoion spectra are summarized with a focus on (pre)dissociation of(i) multielectron excitation states commonly observed in the inner valence regions,(ii) shape resonances, and(iii) doubly charged parent ions. Possible origins for negative peaks in the differential spectra are discussed. This spectroscopy is applied to the reported photoion branching ratios for D₂ (and H₂ at high energies). The main findings are as follows: (1) The direct dissociation of theX ²Σ _g ⁺ (1sσ _g ) state of D ₂ ⁺ , the two-electron excited state¹Σ _u ⁺ (2pσ _u 2sσ _g ) of D₂, and the²Σ _u ⁺ (2pσ _u ) state of D ₂ ⁺ appear clearly in the differential spectrum, as previously observed for H₂. (2) Decay of H ₂ ⁺ (D ₂ ⁺ ) to H⁺ (D⁺) above 38 eV is due to the direct dissociation of highly excited states of H ₂ ⁺ (D ₂ ⁺ ) such as the²Σ _g ⁺ (2sσ _g ) and high-lying Rydberg states converging on H ₂ ²⁺ (D ₂ ²⁺ ). (3) In the ionization continuum of H ₂ ²⁺ (D ₂ ²⁺ ) peculiar dissociation pathways are observed. The differential photoion spectra for O₂ derived from the reported photoion branching ratios are also presented. The (pre)dissociation of theb ⁴Σ _g ^? ,B ²Σ _g ^? , III²Π _u ,²Σ _u ^? , and^2,4Σ _g ^? states of O ₂ ⁺ appears as the corresponding positive values in the spectra in accord with previous observations. Some other dissociation pathways possibly contributing to the spectra are discussed including dissociative double ionization.     

Ab initio calculation of the vertical excitation energies of small helium cluster ions

Quantum chemical ab initio calculations have been performed for the vertical excitation energies and oscillator strengths of all low-lying electronically excited states of small helium cluster ions, He _n ⁺ ,n=2, ..., 7. The geometrical structures of the ions were fixed at the equilibrium geometries of the respective ground states, for He ₄ ⁺ and He ₅ ⁺ also one alternative structure was considered. The low-lying excited states can be classified into two categories: the electronic transition can occur either within the central He ₂ ⁺ or He ₃ ⁺ unit or from the peripheral weakly bound He atoms to this unit. The latter transitions are very weak (f≈0.001), closely spaced, with vertical excitation energies of about 5.7 eV. The He ₂ ⁺ and He ₃ ⁺ units have strong transitions at 9.93 and 5.55 eV, respectively; these transitions are only slightly blue-shifted if He ₂ ⁺ or He ₃ ⁺ are placed as “chromophores” into the centre of a larger He _n ⁺ cluster. The large difference in the vertical excitation energy of the strong transition should enable an experimental decision of the question whether the cluster ions have He ₂ ⁺ or He ₃ ⁺ cores.     

Ab initio study of small He cluster ions He n + ,n=2, 3, 4, 5, and low-lying Rydberg states of He4

SCF and CEPA calculations are applied to study the structure of small He cluster ions, He _n ⁺ ,n=2, 3, 4, 5 and some low-lying Rydberg states of He₄. The effect of electron correlation upon the equilibrium structures and binding energies is discussed. He ₃ ⁺ has a linear symmetric equilibrium geometry with a bond length of 2.35a ₀ and a binding energyD _e =0.165 eV with respect to He ₂ ⁺ +He (experimentally:D ₀=0.17 eV which corresponds toD _e ≈0.20 eV). He ₄ ⁺ is a very floppy molecular ion with several energetically very similar geometrical configurations. Our CEPA calculations yield a T-shaped form with a He ₃ ⁺ centre (R _e = 2.35a ₀) and one inductively bound He atom (4.39a ₀ from the central He atom of He ₃ ⁺ ) as equilibrium structure. Its binding energy with respect to He ₃ ⁺ +He is 0.031 eV. A linear symmetric configuration consisting of a He ₂ ⁺ centre with a bond length of 2.10a ₀ and two inductively bound He atoms (4.20a ₀ from the centre of He ₂ ⁺ ) is only 0.02–0.03 eV higher in energy. We expect that in larger He cluster ions structures with He ₂ ⁺ and He ₃ ⁺ centres andn?2 orn?3 inductively bound He atoms have nearly the same energies. In He₄ a low-lying metastable Rydberg state (³ Π symmetry for linear He ₄ ^* ,³ B ₁ for the T-shaped form) exists which is slightly stronger bound with respect to He ₃ ^* +He than the corresponding ion.     

Applying ZEKE-spectroscopy to clusters

In this paper we describe the application of a special form of electron photodetachment spectroscopy, zero electron kinetic energy (ZEKE) spectroscopy, to the study of cold metal cluster anions. From this study we have obtained vibrational and thus structural information of several neutral and charged clusters, e.g. Au ₂ ^?/0 , Ag ₃ ^?/0 , Sn ₂ ^?/0 and Al ₃ ^?/0 . In addition, in separate measurements of the relative photodetachment cross section of Au ₂ ^? , we have discovered sharp resonances which are due to electronic autodetachment from a short lived excited state.     

MB 8 2− (M = Be, Mg, Ca, Sr, and Ba): Planar octacoordinate alkaline earth metal atoms enclosed by boron rings

Complexes involving planar octacoordinate alkaline earth metal atoms in the centers of eight-membered boron rings have been investigated by two density functional theory (DFT) methods. BeB ₈ ^2? with D _8h symmetry is predicted to be stable, both geometrically and electronically, since a good match is achieved between the size of the central beryllium atom and the eight-membered boron ring. By contrast, the other alkaline earth metal atoms cannot be stabilized in the center of a planar eight-membered boron ring because of their large radii. By following the out-of-plane imaginary vibrational frequency, pyramidal C _8v MgB ₈ ^2? , CaB ₈ ^2? , SrB ₈ ^2? , and BaB ₈ ^2? structures are obtained. The presence of delocalized π and σ valence molecular orbitals in D _8h BeB ₈ ^2? gives rise to aromaticity, which is reflected by the value of the nucleus-independent chemical shift. The D _8h BeB ₈ ^2? structure is confirmed to be the global minimum on the potential energy surface.     

Chemisorption and chemical reactions on size selected clusters

Low energy ion beam techniques have been used to perform a detailed study of the reactions of Al ₂₅ ⁺ and Si ₂₅ ⁺ with a range of simple molecules (D₂, CH₄, O₂, C₂H₄, CO and N₂). The reactions were studied over a center of mass collision energy range from 0.2eV up to 7eV. Activation barriers for chemisorption onto the clusters were deduced from the experimental results. The activation barriers for chemisorption on Al ₂₅ ⁺ and Si ₂₅ ⁺ are generally similar and show a qualitative correlation with the electronic properties of the reactant molecule. However, the products of the chemical reactions of Al ₂₅ ⁺ and Si ₂₅ ⁺ which result from cluster fragmentation are quite different. Si ₂₅ ⁺ shows a tendency to undergo fission as observed in a number of recent studies of the dissociation of the bare clusters.     

Quantized losses from charged nitrogen clusters

Metastable decay of (N₂) _n ⁺ , formed in a supersonic jet and ionized by electron impact, has been analyzed forn≤50. The probability for decay of (N₂) _n ⁺ into (N₂) _n?x ⁺ , plotted versusx, exhibits pronounced oscillations. The “period” of these oscillations increases with increasing precursor sizen, but converges to an average value of approximately 4.7 beyondn=25.     

Fragmentation of doubly charged noble metal clusters

The dissociation patterns of doubly charged noble metal clusters (M) _n ⁺⁺ to two singly charged clusters, (M) _m ^Emphasis>1/+ and (M) _m ^Emphasis>2/+ have been investigated using a double focusing mass spectrometer. They are compared with the dissociation patterns from singly charged clusters. The dissociation probabilities to (M) ₃ ⁺ and (M) ₉ ⁺ were large and the odd-even alternations were observed in both patterns.     

Spectroscopic and electrochemical properties of mixed-ligand cycloplatinated complexes based on 2-(2-thienyl)pyridine and 2-phenylpyridine with 1,10-phenantroline and its 1,4-diazine derivatives

A comparative study of dipyrido-and dibenzo-substituted 1,4-diazines {dipyrido[f,h]quinoxaline (dpq), dipyrido[a,c]phenazine (dppz), 6,7-dicyanodipyrido[f,h]quinoxaline (dicnq), dibenzo[f,h]quinoxaline, dibenzo[a,c]phenazine, 6,7-dicyanodibenzo[f,h]-quinoxaline}, o-phenantroline (phen), and also of the complexes [Pt(N∧C)(N∧N)]⁺[(N∧C)^? are deproronated forms of 2-phenylpyridine and 2-(2-thienyl)pyridine; (N∧N) is ethylenediamine, phen, dpq, dppz, dicnq] was carried out by the methods of ¹H NMR, electronic absorption, and emission spectroscopy and by cyclic voltammetry. It was found that in frozen solutions of [Pt(N∧C)·(N∧N)]⁺ complexes the photoexcitation energy decay from two lowest in energy electronic excited states has isolated character and is localized on {Pt(N∧C)} and {Pt(N∧N)} metal-complex fragments: (d-π _N∧C ^* ) and (d-π _phen ^* ) [(N∧N) = phen, dpq, dicnq)] or (d-π _N∧C ^* ) and (π-π _diaz ^* ) [(N∧N) = dppz]. Thermal quenching of the luminescence from the (d-π _phen ^* ) and (π-π _diaz ^* ) states gives rise to luminescence of the complexes in liquid solutions at 293 K only from the (d-π _N∧C ^* ) state.     

Influence of the organic layer thickness in (Metal-Assisted) secondary ion mass spectrometry using Ga+ and C 60 + projectiles

This article investigates the influence of the organic film thickness on the characteristic and molecular ion yields of polystyrene (PS), in combination with two different substrates (Si, Au) or gold condensation (MetA-SIMS), and for atomic (Ga⁺) and polyatomic (C ₆₀ ⁺ ) projectile bombardment. PS oligomer (m/z ～ 2000 Da) layers were prepared with various thicknesses ranging from 1 up to 45 nm on both substrates. Pristine samples on Si were also metallized by evaporating gold with three different thicknesses (0.5, 2, and 6 nm). Secondary ion mass spectrometry was performed using 12 keV atomic Ga⁺ and C ₆₀ ⁺ projectiles. The results show that upon Ga⁺ bombardment, the yield of the fingerprint fragment C₇H ₇ ⁺ increases as the PS coverage increases and reaches its maximum for a thickness that corresponds to a complete monolayer (～3.5 nm). Beyond the maximum, the yields decrease strongly and become constant for layers thicker than 12 nm. In contrast, upon C ₆₀ ⁺ bombardment, the C₇H ₇ ⁺ yields increase up to the monolayer coverage and they remain constant for higher thicknesses. A strong yield enhancement is confirmed upon Ga⁺ analysis of gold-metallized layers but yields decrease continuously with the gold coverage for C ₆₀ ⁺ bombardment. Upon Ga⁺ bombardment, the maximum PS fingerprint ion yields are obtained using a monolayer spin-coated on gold, whereas for C ₆₀ ⁺ , the best results are obtained with at least one monolayer, irrespective of the substrate and without any other treatment. The different behaviors are tentatively explained by arguments involving the different energy deposition mechanisms of both projectiles.     

Multi-reference configuration interaction calculations on the systems Xe 2 + and Xe 3 +

Potential curves for the ground (²Σ _u ⁺ ) and the three lowest excited states of the Xe ₂ ⁺ dimer ion (²Π _g ,²Π _u ,²Σ _g ⁺ ) have been calculated using pseudopotentials in MRD-CI (multi-reference single anddouble excitationconfigurationinteraction) calculations. Spin-orbit interaction — leading to the six states 1.(1/2) _u , 1.(3/2) _g , 1.(3/2) _u , 1.(1/2) _g , 2.(1/2) _u , 2.(1/2) _g — has been taken into account using a semiempirical technique [1]. Subsequently, starting with a relaxed Xe ₂ ⁺ ion in its ground state, the potential energy surface for the system Xe-Xe ₂ ⁺ was studied. We found that the collinear approach of the Xe atom leads to the most stable geometry. This is a linear symmetric molecule with bond lengths of 6.38 bohr. In the bestT-shaped structure, the Xe atom is 7.83 bohr away from the midpoint of the Xe ₂ ⁺ (r=6.1 bohr) dimer. The calculated binding energy of 0.25 eV for the equilibrium structure of the Xe ₃ ⁺ molecule (i.e. the linear symmetric geometry), is in very good agreement with experimental results of 0.27 ± 0.02 eV [2].     

Zur Elektronenstruktur von metallorganischen Komplexen der f-elemente

The temperature dependence of the magnetic susceptibility of Uranocene ((C₈H₈)₂U(IV)) between 1.25 and 298 K has been measured for the first time and the results correlated with a systematic study of the crystal field splitting. Independent estimates of the three crystal field parameters B ₂ ⁰ , B ₄ ⁰ and B ₆ ⁰ were obtained by adopting the purely electrostatic approach, the angularoverlap-model and the MHW-MO-method. Subsequently the crystal field splitting pattern was calculated by a simultaneous diagonalisation of the complete f ²-matrix. Assuming rigorous D _8h-crystal field symmetry, a crystal field splitting pattern involving a singlet ground state and a low lying first excited doublet state (ΔE=17cm^?1, ¦ΔJ₂¦ = 1) gives the best agreement with both the MO-results and the experimental findings. The experimental l/χ-vs-T-curve is reproduced to a good approximation by a calculation employing the optimal parameter set: B ₂ ⁰ = ?5610, B ₄ ⁰ = ?1426,B ₆ ⁰ = ?730cm^?1. A crystal field of slightly lowered symmetry having a significantly split doublet ground state and hence positive B ₂ ⁰ -values cannot be completely ruled out.     

Molecular orbital calculations on transition metal complexes. Part IX

Potential energy curves for the ground and some low energy excited states of a number of complexes with a 3d ⁵ electronic configuration have been computed from INDO type SCF MO calculations. The results agree extremely well with the known ground states of the complex ions MnF ₆ ^4? , FeF ₆ ^3? , CoF ₆ ^2? , and Fe(CN) ₆ ^3? , in particular the crossover from high to low spin being obtained for changes in both central metal ion oxidation state and ligand. The calculated contraction in metal ligand distance on passing from the high spin to the low spin state is ～ 0.05 Å for each complex in very good agreement with the value indicated by pressure dependent magnetic measurements. Computed electronic transition energies involving bothd-d type and charge-transfer excitations compare favourably with observed spectroscopic values.     

Small silicon oxide clusters: chains and rings

We study several silicon oxide cluster series with different Si:O stoichiometries using photoelectron spectroscopy (PES) of size-selected anions: (SiO) _n ^? (n=3–5), (SiO₂) _n ^? (n =1–4), and Si(SiO₂) _n ^? (n = 2,3). The (SiO)_n clusters are shown to be closed-shell molecules and the HOMOLUMO gaps are observed from the PES spectra to decrease for larger n. These clusters are shown to have ring sturctures. Si₃O₄ is known to have a D_2d structure with two perpendicular Si₂O₂ rhombuses.The PES spectrum of Si₄O ₆ ^? is very similar to that of Si₃O ₄ ^? . It is concluded that Si₄O₆ has a similar structure with a chain of three Si₂O₂ rhombuses. The (SiO₂)_n clusters all exhibit high electron affinities and only one band is observed at 4.66 eV photon energy. These clusters are shown to have similar chain structures containing Si₂O₂ rhombuses, but the two terminal Si atoms are bonded to an extra 0 atom each. The possibility of using these clusters to provide structural models for oxygen-deficient defects in bulk silicon oxides is also discussed.     

Shell correction study of fission of doubly charged silver clusters

Fission of doubly charged silver clusters is investigated by the method of shell corrections. The following fission events are considered: Ag ₂₂ ²⁺ → Ag _n ⁺ + Ag _{22 ?n} ⁺ , (n=11, 10, 9, 8); Ag ₂₁ ²⁺ → Ag _n ⁺ + Ag _{21 ?n} ⁺ , (n=10, 9, 8, 7); Ag ₁₈ ²⁺ → Ag _n ⁺ + Ag _{18 ?n} ⁺ , (n=9, 8, 7, 6). It is found that the shell correction energy is comparable to or larger than the deformation energy of the liquid drop. Threshold energies for the fission events are calculated and compared with the experimental abundance spectra obtained by Katakuse et al. (1990). Correspondence between the calculated threshold energies with the shell corrections and the experimental abundance is very good, showing products from lower threshold fission channels yield more abundance. The threshold energies without the shell corrections are almost constant irrespective of the fission channels and cannot explain the experimental abundance. Abundance of some products are too small to be accounted for only by the threshold energies. The low abundance of those products may be explained by the presence of competing fission channels that have similar minimal energy paths. It is found in fission of Ag ₁₈ ²⁺ that the shell correction overwhelms the Coulomb energy and the fission channel to Ag₈ + Ag ₁₀ ²⁺ is preferred over the fission channel to Ag ₈ ⁺ + Ag ₁₀ ⁺ .     

Pressure and isotope effects on the proton jump of the hydroxide ion at 25°C

The limiting molar conductances Λ⁰ of potassium deuteroxide KOD in D₂O and potassium hydroxide KOH in H₂O were determined at 25°C as a function of pressure to disclose the difference in the proton-jump mechanism between an OH^? (OD^?) and a H₃O⁺ (D₃O⁺) ion. The excess conductance of the OD^? ion in D₂O λ _E ^O (OD ^-), as estimated by the equation $$\lambda _E^O (OD^ - ) = \Lambda ^O (KOD/D_2 O) - \Lambda ^O (KCl/D_2 O)$$ increases a little with pressure as well as the excess conductance of the OH^? ion in H₂O $$\lambda _E^O (OH^ - ) = \Lambda ^O (KOH/H_2 O) - \Lambda ^O (KCl/H_2 O)$$ However, their rates of increase with pressure are much smaller than those of the excess deuteron and proton conductances, λ _E ^O (D ⁺) and λ _E ^O (H ⁺). With respect to the isotope effect on the excess conductance, λ _E ^O (OH ^-)/λ _E ^O (D ⁺) decreases with presure as in the case of λ _E ^O (H ⁺)/λ _E ^O (D ⁺), but the value of λ _E ^O (OH ^-)/λ _E ^O (OD ^-) itself is much larger than that of λ _E ^O (H ⁺)/λ _E ^O (D ⁺) at each pressure. These results are ascribed to the difference in the pre-rotation of water molecules, which is brought about by the difference in the intial orientation of the rotating water molecule adjacent to the OH^? (OD^?) or the H₃O⁺ (D₃O⁺) ion.     

Photodissociation of pyrene dimer radical cation during the pulse radiolysis–laser flash photolysis combined method

Photodissociation of pyrene (Py) dimer radical cation (Py ₂ ^?+ ) giving pyrene radical cation (Py^?+) and Py and subsequent regeneration of Py ₂ ^?+ by association of Py^?+ and Py were directly observed during the pulse radiolysis–laser flash photolysis combined method at room temperature. When Py ₂ ^?+ was excited at the local excitation band with the 532-nm laser flash, the rapid growth and decay of monomeric Py^?+ were observed at 460 nm. The dissociation of Py ₂ ^?+ proceeded via a one-photon process to give the ground-state Py^?+(D₀) and Py in the quantum yield (Φ_diss) of (2.9 ± 0.9) × 10^?3. It was shown that Py^?+ decayed with a time constant of several tens of nanoseconds, indicating that the association of Py^?+ with Py regenerating Py ₂ ^?+ proceeds at a diffusion-controlled rate. The photodissociation proceeded from the lowest excited state of Py ₂ ^?+ , even when Py ₂ ^?+ was excited to the higher excited state. The difference between the Φ_diss value of Py ₂ ^?+ and that previously reported for naphthalene dimer radical cation (Np ₂ ^?+ ) is discussed.     

Kinetic energy release of clusters after multiphoton ionisation with time of flight and electrostatic analyser techniques

A new technique is presented which allows direct observation of initial kinetic energies in multiphoton ionisation-fragmentation processes of molecules and clusters and provides an unambiguous determination of metastable decay channels. Results are presented for the unimolecular loss of a monomer from clusters (C₆H₆) ₈ ⁺ to (C₆H₆) ₁₂ ⁺ and for the reaction C₆H ₆ ⁺ →C₄H ₄ ⁺ +C₂H₂. We also observe a significant amount of probably collision induced fragmentation processes (C₆H₆) _n ⁺ →(C₆H₆) _n?x ⁺ + (C₆H₆) _x withx much larger than 1.