bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct spectroscopic determination of the Hg2 bond length and an analysis of the 2540 Å band
Authors: van Zee Roger D. , Blankespoor Stephen C. , Zwier Timothy S.
Published: July 2002

The ground state bond length of the gas phase Hg2 was determined by taking high resolution spectra of an isotopically selective vibronic band of the jet-cooled molecule via the X̃(O+g)→D̃(1u) transition. The rotational structure allows a determination of the ground state equilibrium internuclear separation of Re=3.63±0.04 Å, about 0.4 Å greater than the previous accepted value from gas phase viscosity measurements. Combining this with the previously determined change in bond length between the X̃ and D̃ states (ΔRe=1.1 Å) gives a D̃-state bond length of only 2.5 Å. A careful analysis of the 2540 Å X̃(O+g)→F̃(O+u) band of the dimer, which has resisted previous analyses, shows that the band is in fact made up of a long progression of sequence bands which are virtually superimposed. Furthermore, the v″=0–v′=1 transition is about 300 times less intense than the 0–0 transition. Franck–Condon analysis leads us to conclude that the O+u potential energy curve is virtually identical to that of the ground state, with internuclear separations in the two states which differ by less than 0.02 Å.
Exclusive production of Ba(6s6p 3P2) in the collisional deactivation of Ba(6s6p 1P1) by the rare gases
Authors: Breckenridge W. H. , Merrow Clifton N.
Published: July 2002

The collisional deactivation of Ba(6s6p 1P1) by He, Ne, Ar, Kr, and Xe results in the exclusive production of Ba(6s6p 3P2). The lower-lying Ba(6s6p 3P1,0), Ba(6s5d 1D2), and Ba(6s5d 3D3,2,1) states are produced in negligible yield, within experimental error. From the known rate of radiative production of the Ba(6s5d 3D2) and Ba(6s5d 1D2) states from Ba(6s6p 1P1), absolute cross sections from the production of Ba(6s6p 3P2) in the collisional deactivation of Ba(6s6p 1P1) by the rare gases were determined and found to lie in the 0.4–4.0 Å2 range. A general mechanism is proposed in which there are avoided crossings between slightly attractive Ba⋅RG(1Π1) molecular potential curves correlating with Ba(6s6p 1P1) and repulsive Ba⋅RG(3Σ1) curves correlating with Ba(6s6p 3P2).
Statistical theory of electron densities
Authors: Pratt Lawrence R. , Hoffman Gary G. , Harris Robert A.
Published: July 2002

An optimized Thomas–Fermi theory is proposed which retains the simplicity of the original theory and is a suitable reference theory for Monte Carlo density functional treatments of condensed materials. The key ingredient of the optimized theory is a neighborhood sampled potential which contains effects of the inhomogeneities in the one-electron potential. In contrast to the traditional Thomas–Fermi approach, the optimized theory predicts a finite electron density in the vicinity of a nucleus. Consideration of the example of an ideal electron gas subject to a central Coulomb field indicates that implementation of the approach is straightforward. The optimized theory is found to fail completely when a classically forbidden region is approached. However, these circumstances are not of primary interest for calculations of interatomic forces. It is shown how the energy functional of the density may be constructed by integration of a generalized Hellmann–Feynman relation. This generalized Hellmann–Feynman relation proves to be equivalent to the variational principle of density functional quantum mechanics, and, therefore, the present density theory can be viewed as a variational consequence of the constructed energy functional.
Experimental studies of the dynamics of nitrogen van der Waals cluster scattering from metal surfaces
Authors: Holland R. J. , Xu G. Q. , Levkoff J. , Robertson A. , Bernasek S. L.
Published: July 2002

Experimental measurements of the dynamics of the collisions between N2 van der Waals clusters and metallic surfaces are discussed. Large N2 van der Waals clusters are formed by high pressure nozzle expansions of pure N2 and N2 seeded in helium. Angular distributions and rotational energy distributions of the fragments of these clusters resulting from impact with well characterized silver and iron surfaces were obtained using electron beam induced fluorescence. Angular distributions were characterized by tangent shifted scattering lobes, and rotational energies which are largest at the surface normal. The dependence of the angular distributions on N2 fraction in the seeded beam, expansion backing pressure, and surface structure, composition, and temperatures were investigated.
Dependence of metal cluster reaction kinetics on charge state. II. Chemisorption of hydrogen by neutral and positively charged iron clusters
Authors: Zakin M. R. , Brickman R. O. , Cox D. M. , Kaldor A.
Published: July 2002

The kinetics of D2 chemisorption by gas-phase neutral (Fex ) and positively charged (Fe+x) iron clusters has been studied under identical reactor conditions. Similar to the neutrals, the reaction rate for the ions displays a nonmonotonic dependence on x, varying by more than four orders of magnitude between x = 1 and x = 31. The presence of the positive charge is found to have a substantial influence on reaction rate for the majority of clusters in the 1–31 atom size range. Clusters containing 4–6 and >17 atoms experience an enhancement in rate while in contrast those containing 3 and 9–14 atoms experience a decrease in rate. Further, studies of H2 chemisorption onto Fe+x (x = 4–22) indicate an enhanced, cluster size-dependent isotope effect for the cluster ions which is quite similar to that previously observed for Fex. The effect of the positive charge on D2 chemisorption reactivity is explained within the framework of the frontier orbital model of activated chemisorption by invoking an ‘‘effective’’ activation barrier for Fe+x which incorporates the electrostatic interactions arising from the nonzero charge.
The role of rotational tunneling in the metastable decay of rare gas cluster ions
Authors: Ferguson E. E. , Albertoni C. R. , Kuhn R. , Chen Z. Y. , Keesee R. G. , Castleman A. W.
Published: July 2002

The decay of (Ar+3)*→Ar+2+Ar is found to be metastable on a time scale longer than 40 μs. Theoretical considerations are presented for these slow metastable dissociation rates reported in this and other studies. From model calculations it is inferred that tunneling lifetimes from 10−10 to 10 s may be observed.
Size-selective dehydrogenation of benzene by gas-phase niobium cluster ions (Nb+x)
Authors: Zakin M. R. , Brickman R. O. , Cox D. M. , Kaldor A.
Published: July 2002

The gas-phase reaction of positively charged niobium cluster ions Nb+x with normal and perdeuterobenzene has been studied using a fast-flow reactor and direct ion sampling techniques. Chemisorption of benzene onto the ion is facile, with the rate being essentially independent of x. However, both the resulting product distributions, consisting of species NbxC6H+m and NbxC6D+m with m=0,2,4,6, and the relative product yields not only display a dramatic dependence on cluster size, but are also remarkably similar to those previously observed for neutral Nbx using gentle, low-fluence, single-photon 6.42 eV photoionization. Direct observation of products with m≤6 indicates that both chemical dehydrogenation of benzene and desorption of hydrogen from the cluster ions must occur in the fast-flow reactor. Further, the relative yield of dehydrogenated products is found to increase substantially with increase in cluster ‘‘temperature.’’ Complete dehydrogenation of benzene to m=0 is also found to occur for niobium cluster oxides NbxO+ containing 5–14 Nb atoms. The present results are discussed in terms of the mechanism of the dehydrogenation process, as well as with respect to their relevance in determining ‘‘true’’ parent product distributions formed in reactions of the corresponding neutral clusters.
Fourier transform ion cyclotron resonance studies of H2 chemisorption on niobium cluster cations
Authors: Elkind J. L. , Weiss F. D. , Alford J. M. , Laaksonen R. T. , Smalley R. E.
Published: July 2002

Reaction rates and saturation values were determined for H2 dissociative chemisorption on positive niobium cluster ions in an FT-ICR apparatus. Clusters with 8,10,12, and 16 atoms were found to be particularly unreactive, in remarkable agreement with the reactivity patterns observed previously for neutral niobium clusters. Saturation coverage for most clusters was found to occur near a hydrogen/niobium ratio of 1.3, although some clusters (8–12,16, and 19) reached effectively inert compositions at considerably lower coverages. Several examples were found of clusters having two isomeric forms with different reactivities. One form of Nb+19 was found to readily react with H2, whereas a second form representing one-third of the original sample of 19 atom clusters was completely inert to H2 chemisorption under the same FT-ICR conditions. The geometrical shape of these niobium clusters must therefore have a critical effect on reactivity.
Neon matrix ESR investigation of 69,71GaAs+ generated by the photoionization of laser vaporized GaAs(s)
Authors: Knight Lon B. , Petty J. T.
Published: July 2002

The first spectroscopic results are reported for the 69,71GaAs+ cation radical generated by photoionizing GaAs (g) produced by the pulsed laser vaporization of GaAs (s). The GaAs+ cation was trapped in neon matrices at 4 K for ESR investigations which show the ground electronic state to be 4Σ− in agreement with previously reported theoretical calculations. Components of the gallium A tensors are 69Ga; A⊥=184.2(2), A∥=270 (30) MHz, and for 71Ga A⊥=234.4(2), A∥=345(35) MHz. The unresolved As hfs was estimated to be less than 8 MHz for the perpendicular direction and the molecular g⊥ value as 1.9978(2) with g∥ presumably∼2.00. These nuclear hyperfine parameters are used to obtain a description of the valence electronic structure for GaAs+ which can be compared with recent MCSCF/CI theoretical results.
Electronic and vibrational structure of transition metal trimers: Photoelectron spectra of Ni−3, Pd−3, and Pt−3
Authors: Ervin Kent M. , Ho Joe , Lineberger W. C.
Published: July 2002

The transition metal trimer anions Ni−3, Pd−3, and Pt−3 are prepared in a flowing afterglow ion source with a cold cathode dc discharge. The low-lying electronic states of the neutral trimers are probed by 488 nm negative ion photoelectron spectroscopy at an electron kinetic energy resolution of 5–12 meV. Each trimer exhibits multiple low-lying electronic states. Vibrational progressions are observed in tripalladium and triplatinum. The adiabatic electron affinities are found to be EA(Ni3)=1.41±0.05 eV, EA(Pd3)≲1.5±0.1 eV, and EA (Pt3)=1.87±0.02 eV.
Production of vibrationally state-selected O+2 via newly discovered 4s–3d and 5s–4d Rydberg states of O2
Authors: Park Haiyoon , Miller Paul J. , Chupka William A. , Colson Steven D.
Published: July 2002

The 4s–3d and 5s–4d gerade Rydberg states of O2 have been observed for the first time by (2+1) multiphoton ionization (MPI) spectroscopy. At least five vibrational progressions (v=0–3) are seen in the 4s–3d states. Photoelectron spectroscopy has demonstrated that vibrationally state-selected O+2 can be produced in good yields (>80%) via (2+1) MPI of at least one of the newly observed 4s–3d, v′=0–3 progressions.
Stability and properties of C4 isomers
Authors: Bernholdt David E. , Magers David H. , Bartlett Rodney J.
Published: July 2002

The relative electronic energies of the 1Ag rhombus and the 3Σ−g linear isomers of C4 have been computed using a 5s3p1d basis at various levels of coupled-cluster and many-body perturbation theory. At the highest level performed, CCSD+T(CCSD), the two isomers are essentially isoenergetic. Themodynamic results indicate, however, that the linear isomer will be significantly more abundant in the plasma in which C4 is formed. Vibrational frequencies are presented and compared with experimental results; some of the prior assignments are questioned. The isotropic ESR hyperfine splitting parameters for the linear triplet state have been computed and are also compared with experimental results. Finally, the ionization potential and electron affinity for both structures are presented.
Postattachment interactions in K(n
d)–SF6 collisions at intermediate n
Authors: Zheng Z. , Smith K. A. , Dunning F. B.
Published: July 2002

Rate constants for Rydberg atom destruction and for free SF−6 ion formation in K(nd)–SF6 collisions at intermediate values of n are reported. The data show that the rate constants for collisional destruction are consistent with those expected on the basis of Rydberg electron attachment to SF6, but that only a fraction of such events lead to an observable SF−6 ion. This fraction can, however, be significantly increased by application of an external electric field.
Stability of face-centered cubic and icosahedral Lennard-Jones clusters
Authors: van de Waal Benjamin W.
Published: July 2002

The binding energies of solid Lennard-Jones clusters of various sizes and shapes have been calculated and compared. A crossover from the icosahedral to the bulk fcc structure is expected to take place in the 3000 atoms size region, in agreement with experiment.
Transformation of ice VIII to amorphous ice by ‘‘melting’’ at low temperature
Authors: Klug D. D. , Handa Y. P. , Tse J. S. , Whalley Edward
Published: July 2002

The melting curve of ice VIII near 25 kbar, which has a positive slope, has been estimated from various thermodynamic data and extrapolated approximately to 60 K at zero pressure. When ice VIII is heated from 77 K and ambient pressure it should, therefore, ‘‘melt,’’ presumably below the glass transition. It has been shown to do so, and transforms to low-density amorphous ice when heated to about 125 K at ambient pressure. Ice I, whose melting curve has a negative slope, is already known to transform to a high-density amorphous ice at 77 K and 10 kbar, and so this and the present transformation are symmetrical equivalents.
The use of weak magnetic fields in collisions of polarized atoms
Authors: Meijer H. A. J. , Zeegers Th. , Pelgrim T. J. C. , Heideman H. G. M. , Morgenstern R.
Published: July 2002

We have experimentally and theoretically investigated the behavior of excited Na atoms under the combined influence of resonant laser light and a weak magnetic field. The use of weak magnetic fields gives another possibility to vary the polarization of the atoms, additional to variation of the polarization of the exciting laser. We calculated in detail the influence of weak magnetic fields on the density matrix describing the excited atoms, using generalized rate equations. We checked the results of our model experimentally, at the Na(3 2 S1/2, Fl=2) →(3 2 P3/2, Fu =3) transition, excited with circularly polarized light. We determined the polarization of the excited atoms, by measuring the fluorescence light at various angles, as a function of both magnetic field strength and laser irradiance. Model and experiment agreed very well. We applied our model to the associative ionization of the excited Na atoms. We measured the ion signal as a function of magnetic field. Analyzing the results, we could determine a specific associative ionization cross section, which is not accessible in our experimental geometry without the use of magnetic fields. Thereby we showed that the application of magnetic fields in polarization dependence studies can provide us with additional information about the process.
Ultraviolet photodissociation and thermochemistry of CH2BrCH2I, CF2BrCF2I, and CF2ICF2I
Authors: Nathanson Gilbert M. , Minton Timothy K. , Shane Stacey F. , Lee Yuan T.
Published: July 2002

Using photofragment translational spectroscopy, we have monitored the dissociation of CH2BrCH2I at 248, 266, and 308 nm, and CF2BrCF2I and CF2ICF2I at 308 nm. The primary fragments are I(2P3/2) and I(2P1/2) and the corresponding haloethyl radicals. The I(2P3/2) contribution decreases upon fluorination, but it is dominant for CH2BrCH2I at 308 nm. The electronic absorption dipole lies roughly along the C–I bond axis in every case. Stable CF2CF2Br and CF2CF2I radicals can be readily generated through photodissociation of the parent compounds, while stable CH2CH2Br could not be unambiguously observed. Upper limits to the reaction enthalpy at 0 K for CF2ICF2Br(I)→C2F4+I+Br(I) are 75±1(59±1) kcal/mol. The TOF spectra and related data suggest that there is a barrier to decomposition for CF2CF2I→C2F4+I that exceeds the C–I bond energy in the radical.
Ar+3 photodissociation and its mechanisms
Authors: Chen Z. Y. , Albertoni C. R. , Hasegawa M. , Kuhn R. , Castleman A. W.
Published: July 2002

The photodissociation spectrum of Ar+3 between 520 and 620 nm is reported. A broadband peaking near 520 nm is observed with a cross section of ∼1.8×10−16 cm2 at the peak, in agreement with the findings of Levinger et al. [J. Chem. Phys. 89, 71 (1988)]. However, in the present work, a shoulder whose prominence is highly temperature dependent is observed between 545 and 555 nm. The new results are discussed in terms of various electronic transitions and pathways to dissociation. The photodissociation mechanisms are studied by translational energy analysis of photofragmentation. Three photodissociation mechanisms that involve two electronic energy surfaces are proposed which account for the experimental findings.
Electron-spin-resonance characterization of nonlinear C4 trapped in solid argon
Authors: Cheung H. M. , Graham W. R. M.
Published: July 2002

The electron-spin-resonance spectrum of the C4 molecule has been observed by trapping the products of the vacuum ultraviolet photolysis of the diacetylene (C4H2) and 1,3-butadiene (C4H6) molecules in Ar at 12 K. Using highly dilute samples with argon to parent molecule ratios ranging from 2000:1 to 50 000:1, results in absorption lines with substantially narrower linewidths than those observed previously for C4 produced by the evaporation of graphite. The sharper spectra reveal splitting of the perpendicular lines into separate x and y components, indicating nonlinear character for C4 trapped in solid argon. The possibility that the splitting originates with multiple trapping sites or hyperfine interaction with a hydrogen nucleus is eliminated on the basis of the results of extensive annealing experiments and the lack of any effect when the parent molecules are deuterated. Various carbon-13 substitutions in the butadiene parent yield hyperfine splitting components for the outer (α) and inner (β) pairs of equivalent carbon nuclei. Measured magnetic constants are gxx=2.0016(5), gyy=2.0041(5), ‖D‖=0.2237(2) cm−1, ‖E‖=2.52(2)×10−4 cm−1, ‖Axx(13Cα)‖=29.7(6) MHz, ‖Ayy(13Cα)‖=29.7(6) MHz, ‖Axx(13Cβ)‖=6.2(6) MHz, and ‖Ayy(13Cβ)‖=7.2(6) MHz. It is concluded that C4 formed by trapping the photolysis products of C4H2 and C4H6 in solid argon is slightly bent.
Very low energy electron attachment to SF6 clusters
Authors: Desfrancois C. , Khelifa N. , Schermann J. P.
Published: July 2002

Electron attachment to SF6 clusters has been studied in the 6–110 meV range with a crossed pulsed beam experiment using laser excited Rydberg atoms. The width of the very low energy resonance varies with the cluster size.
Multiphoton ionization of O2 X 3Σ−g, a 1Δg, and b 1Σ+g via the two-photon resonant n
sσg, n
dσg, and n
dπg Rydberg levels
Authors: Ogorzalek Loo R. , Marinelli W. J. , Houston P. L. , Arepalli S. , Wiesenfeld J. R. , Field R. W.
Published: July 2002

Multiphoton ionization spectra have been obtained and analyzed for excitation in the 215–360 nm region from the X 3Σ−g, a 1Δg, and b 1Σ+g states of O2. The 0–0 band of the C 1Πg state is reported for the first time. Measurements of other vibrational bands terminating in the C 3Πg and d 1Πg states are in good agreement with determinations by other groups. Several vibrational levels (v′=0–5) of the 3dπg Rydberg complex have been assigned on the basis of (1) an analysis of the spin–orbit couplings between the (Λ,S) basis-set states, (2) spectral simulation, and (3) the behavior of the states when the excitation radiation is changed from linear to circular polarization.
Electron bombardment fragmentation of size selected NH3 clusters
Authors: Buck U. , Lauenstein Ch.
Published: July 2002

Ammonia clusters that are generated in a supersonic expansion with He are size selected by a scattering process with He atoms. By measurements of angular dependent mass spectra and time-of-flight distributions at all fragment masses, the complete fragmentation pattern for electron impact ionization of clusters from the dimer to the pentamer are obtained for electron energies of 70 eV. The dimer fragmentation which is dominated by the NH+4 ion (0.58) can be explained completely by fast ion–molecule reactions of the monomer fragments with the partner molecule. For the larger clusters up to n=11 the NH+4 ion is still the largest fragment channel. The other protonated ions NH+4(NH3)x start to contribute to the fragmentation with increasing cluster size but never exceed the contribution at mass 18 amu.
Magic numbers through chemistry: Evidence for icosahedral structure of hydrogenated cobalt clusters
Authors: Klots T. D. , Winter B. J. , Parks E. K. , Riley S. J.
Published: July 2002

The equilibrium adsorption of H2O on hydrogenated cobalt clusters is used to estimate the cluster–H2O binding energy. Magic numbers, corresponding to clusters with enhanced binding energy, are found for clusters having one more metal atom than closed shells or subshells of the third Mackay icosahedron. It is argued that a lone metal atom outside a closed (sub)shell should have enhanced ability to bind H2O, and that such chemical probes can be used to provide useful information about metal cluster structure.
Dynamics of CO chemisorption on a metal cluster
Authors: Adams John E.
Published: July 2002

The understanding of the dynamics of collisions between molecules and small metal clusters is an important component of a general understanding of practical heterogeneous catalytic processes. We report here quasiclassical trajectory studies of CO colliding with a five-atom metal cluster, with our primary focus being on the modeling of collisions with a copper cluster. Since the forces in these systems are still largely unknown, we have considered a number of simple potential-energy parametrizations so as to determine the range of behavior which might be expected. In general, CO is found to add readily to the metal cluster, forming a complex which is stable on the time scale of the trajectories. The effects of variations in the cluster geometry and extensions to systems having higher molecule-cluster binding energies are also considered here, as is the question of whether or not there is a significant distortion of the metal cluster geometry upon addition of CO. This last topic is conveniently approached via a combination of trajectory and Monte Carlo simulated annealing techniques.
Two-photon double resonance spectroscopy of bacteriorhodopsin. Assignment of the electronic and dipolar properties of the low-lying 1A*−g-like and 1B*+u-like π, π* states
Authors: Birge Robert R. , Zhang Chian-Fan
Published: July 2002

The electronic and dipolar properties of the all-trans retinyl polyene in light-adapted bacteriorhodopsin are examined by using two-photon double resonance spectroscopy to assign the Franck–Condon maxima, the absolute two-photon absorptivities and the change in dipole moments upon excitation of the low-lying ‘‘forbidden’’ 1A*−g -like and ‘‘allowed’’ 1B*+u -like π, π* excited singlet states. The second-order hyperpolarizability is also determined. The two-photon double resonance spectrum, collected with laser excitation from 820–1200 nm in 10 nm steps, displays two maxima, an intense band at ∼18 000 cm−1 assigned to the 1B*+u -like π, π* excited singlet state and a weaker shoulder at ∼21 000 cm−1 assigned to the 1A*−g -like π, π* excited singlet state. Thus, the 1A*−g -like state is 3500±500 cm−1 above the 1B*+u -like state, which is indicative of a protonated Schiff base chromophore.

A log-normal fit of the two-photon spectrum indicates that the maximum two-photon absorptivity of the 1B*+u -like state is 290±50 GM whereas the maximum two-photon absorptivity of the 1A*−g -like state is less than half this value, 120±90 GM. The ‘‘1B*+u ’’ state exhibits an absorptivity that is dominated by initial and final state contributions to the two-photon tensor, and this observation allows an accurate assignment of the change in dipole moment upon excitation yielding Δμso=13.5±0.8 D. A similar analysis of the ‘‘1A*−g’’ state predicts that the change in dipole moment upon excitation into the latter state is slightly smaller (Δμso=9.1±4.8 D). We demonstrate that the second-order hyperpolarizability of a molecule can be determined directly from the two-photon absorptivity of the low-lying charge transfer state and other spectroscopic parameters, all but one of which can be determined directly from experiment. Our analysis of light adapted bacteriorhodopsin indicates that β=βxxx+(1/3)[βxyy +2βyyx+βxzz+2βzzx] =(2250±240)×10−30cm5/esu for a laser wavelength of 1.06μ assuming that the homogeneous linewidth is 250 cm−1. Preliminary analyses of the two-photon data indicate that the chromophore in bacteriorhodopsin is a protonated Schiff base chromophore in a very ionic, and possibly charged, binding site. The two-photon data are not consistent with charged species near the β-ionylidene ring, but are consistent with polar species near the β-ionylidene ring. Direct hydrogen bonding of a negative counterion with the imine proton is not supported by the two-photon data.
Rotationally resolved ultraviolet spectrum of the benzene–Ar complex by mass-selected resonance-enhanced two-photon ionization
Authors: Weber Th. , von Bargen A. , Riedle E. , Neusser H. J.
Published: July 2002

High resolution laser excitation was combined with the technique of mass-selected two-photon ionization via a resonant intermediate state to measure rotationally resolved UV spectra of benzene–Ar van der Waals clusters. When the second laser pulse in the two color experiment is delayed by 7 ns no line broadening due to the second ionizing absorption step is observed. Spectra of three vibronic bands in the S1 ←S0 transition of benzene (h6)–Ar and benzene (d6)–Ar were measured yielding a line spectrum with a linewidth of 130 MHz. Resolution is sufficient to demonstrate that no asymmetry splitting of the rotational lines occurs and the spectrum is to a high precision that of a symmetric rotor. A detailed analysis of the rotational structure yields an accurate set of rotational constants. We find that the Ar is located on the C6 rotational axis. Its distance from the benzene ring plane is 3.582 Å in the electronic ground state and decreases by 59±3 mÅ in the electronically excited state due to the increased polarizability of the benzene molecule after electronic excitation.
Determination of carbon cluster ionization potentials via charge transfer reactions
Authors: Bach Stephan B. H. , Eyler John R.
Published: July 2002

Ionization potentials (IPs) for carbon clusters containing 6–24 atoms have been determined from charge transfer reactions of carbon cluster ions with compounds of known ionization potential in a Fourier transform ion cyclotron resonance mass spectrometer. Cluster IPs generally decrease with increasing cluster size, but the IPs for clusters containing 4n+3 atoms (n=1–5) are found to be ∼0.5 eV lower than those of neighboring clusters. The relationship between cluster IP and structure is discussed.
Gas-phase methanol solvation of Cs+ : Vibrational spectroscopy and Monte Carlo simulation
Authors: Draves Jeffrey A. , Luthey-Schulten Zaida , Liu Wen-Long , Lisy James M.
Published: July 2002

The solvation of the cesium ion by methanol has been investigated by gas-phase vibrational spectroscopy and Monte Carlo simulations of small ion clusters: Cs(CH3OH)+N, N=4–25. The solvated ions, generated by thermionic emission and a molecular-beam source, have considerable amounts of internal energy. After excessive energy is dissipated by evaporation, quasistable cluster ions are mass-selected for vibrational predissociation spectroscopy using a line-tunable cw-CO2 laser. Analysis of the vibrational spectra indicates that the first solvation shell about the cesium ion consists of ten methanol molecules. Larger Cs(CH3OH)+N (N>18) appear to have small clusters of methanol bound to the surface of a solvated ion. Monte Carlo simulations using pairwise interaction potentials at 200, 250, and 300 K have been performed on Cs(CH3OH)+N, N=6–16 and 25. The results from the simulations are consistent with the observed solvent shell size and suggest a significant role for hydrogen bonding in the larger solvated ions (N≥10). Once the first solvation shell is filled, the size of the solvent shell appears to be independent of the number of additional solvent molecules. Gas-phase solvated ions appear to be extremely useful models for dilute electrolyte solutions.
An intense, simple carbon cluster source
Authors: Campbell E. E. B. , Ulmer G. , Hasselberger B. , Busmann H. -G. , Hertel I. V.
Published: July 2002

An intense source of positive, negative, and neutral carbon clusters using excimer laser ablation of polyimide is described. The detection probability for large masses using a standard channel plate configuration is shown to increase exponentially with ion velocity. The large clusters are formed via aggregation of atomic carbon or small carbon molecules. The small ‘‘clusters’’ may be molecular fragments from the polymer which rearrange on leaving the surface to form stable structures.
Hard-sphere cluster-ion structures
Authors: Lethbridge P. G. , Del Mistro G. , Stace A. J.
Published: July 2002

A series of hard-sphere cluster-ion structures have been constructed, based on the notion of a central stable dimer ion core. Starting from a structure representative of icosahedral Ar+19, two separate packing sequences are considered; edge packing and face packing, and clusters containing up to 310 atoms have been generated. It is shown that many of the symmetric structures derived from the two packing sequences coincide with the ‘‘magic’’ numbers observed both in the mass spectra and fragmentation patterns of argon and krypton cluster ions. A slice through the cluster shows that the central dimer ion core is part of a linear chain of atoms, which offer the possibility for further charge delocalization as suggested in recent molecular orbital calculations.
Energetics, structure and photodissociation dynamics of the cluster Ar⋅N+2
Authors: Kim Hyun-Sook , Bowers Michael T.
Published: July 2002

A mass selected ion beam of ArN+2 clusters is brought to a spatial focus and crossed with the polarized output of an argon ion laser. Photofragment ions are mass and energy analyzed using an electrostatic analyzer and detected using single ion counting methods. Photoproducts observed over the photon energy range of 2.1 to 3.5 eV are Ar+/N2 and N+2/Ar with the former favored by about a factor of 3. Analysis of the data indicate the upper state is purely repulsive leading to strongly translationally and vibrationally excited products. The absolute cross section has an onset at about 600 nm and smoothly increases to 357 nm. In order to reasonably interpret the data it is suggested the higher energy Ar+(2P3/2)/N2 asymptote diabatically correlates to the ground state of ArN+2 and the lower energy N+2(X 2∑)/Ar asymptote diabatically correlates to the repulsive state accessed by the photon. Detailed dynamics in the region where the curves cross are responsible for the observed product distribution. Application of an impulsive model indicates the ground state of ArN+2 is linear. Finally, equilibrium measurements of the reaction (N2)+2+Ar↔ArN+2+N2 indicate ΔH00=−1.0±0.3 kcal/mol and ΔS0=1.5±0.5 cal k−1 mol−1 in agreement with the result of Teng and Conway which had been disputed in the literature.
Single-event, energy-resolved, Auger–electron, multiple-ion coincidence mass spectroscopy
Authors: Hanson D. M. , Ma C. I. , Lee K. , Lapiano-Smith D. , Kim D. Y.
Published: July 2002

An experimental method is described that is directed at monitoring the creation and decay of core hole states in molecules from state-to-state. Data in the form of a coincidence map are presented and analyzed for the case of nitrous oxide.
Microscopic studies of collective spectra of quantum liquid clusters
Authors: Rama Krishna M. V. , Whaley K. B.
Published: July 2002

Theoretical methods are developed for determining the collective vibrational excitation spectra of quantum clusters, and applied to clusters of 4He. A quantum liquid drop model gives excitation energies in terms of two-point ground state density correlations, which are evaluated from microscopic calculations of the ground state wave functions. An alternative approach based on a Bijl–Feynman ansatz for the excited states also yields excitation energies in terms of ground state correlations, without the imposition of a sharp liquid surface. These methods are applied to the calculation of the excitation spectra of 4HeN , N=20, 70, and 240 clusters. The relationship between the collective spectra of these clusters and that of bulk fluid He II, and the significance of an observed roton minimum in the spectrum for N≥70 are discussed.
Low energy (0–10 eV) electron attachment to CF3Cl clusters: Formation of product ions and analysis of excess translational energy
Authors: Kühn Alexander , Illenberger Eugen
Published: July 2002

Electron attachment to CF3Cl clusters has been studied mass spectrometrically in a beam experiment. In addition to the fragments known from dissociative attachment to the isolated compound (F−, Cl−, etc.), a veriety of larger complexes such as M−n, Mn⋅Cl−, Mn⋅F−, n≥1 (M=CF3Cl) could be observed. The resonance profiles of the ion yield curves suggest that the initial step of electron capture proceeds via the formation of a temporary CF3Cl− ion within the target aggregate followed by different decomposition reactions. Among the various products, the parent radical anion (CF3Cl−) is generated in its relaxed configuration not accessible in electron capture by the isolated molecule. A time-of-flight (TOF) analysis of the Cl− products reveals two decay channels, one associated with high excess translational energy and a second releasing the Cl− ion with only thermal energy.
A theoretical study of I2 vibrational motion after excitation with an ultrashort pulse
Authors: Metiu Horia , Engel Volker
Published: July 2002

We calculate the population created by a short pump pulse exciting the I2 molecule to the bound region of the B state, followed by excitation with a short probe pulse to the state E (or F). The nuclear state produced by the pump oscillates in the well of the B state and the probe is absorbed to populate the E (or F) state only when the wave function passes through the Franck–Condon region of the B→E (or B→F) transition. Because of this, the population on the E (or F) state oscillates with the delay time between the pump and the probe. The calculations agree with the experiment in the case when the probe excites the E state. When the F state is excited the theory predicts a doublet structure which is not observed; moreover, in some cases the experiment and theory differ at the shortest delay times. We discuss the dependence of the LIF signal on the pulse width and the initial state, the long time behavior of the LIF signal, and illustrate the role of the population transients on the B states at early times, during or immediately after the pump pulse acted on the molecule.
Spectral shifts and structural classes in microsolutions of rare gas clusters containing a molecular chromophore
Authors: Perera Lalith , Amar François G.
Published: July 2002

We present the results of a series of molecular dynamics (MD) trajectory calculations of the structural, spectral, and dynamical properties of model clusters consisting of one impurity particle interacting with a pure host cluster. Our initial focus is on SF6Arn and SiF4Arn clusters in order to compare MD predictions of structure with the results of the IR studies of these systems performed by the Scoles group. We then show, more generally, how the preferred structural class (matrix or surface) of the heterogeneous cluster system depends on the interaction potential between guest and host molecules. The temperature dependence of the mobility of the impurity within the cluster is also investigated. Finally, the way in which our results can be adapted to interpret and predict solvation behavior for a wide variety of heterogeneous cluster systems is discussed.
Classical model of femtosecond time-resolved absorption spectra of dissociating molecules
Authors: Walkup R. E. , Misewich J. A. , Glownia J. H. , Sorokin P. P.
Published: July 2002

We present a simple classical model for understanding time-resolved absorption spectra of molecules that are in the process of dissociating. The model applies to absorption spectra that are obtained by measuring the spectral power density of an ultrafast, continuum probe pulse after transmission through the sample. We show that the classical model can yield results in good agreement with quantum-mechanical wave packet propagation calculations. In a close analogy with collisional line broadening, the time-resolved absorption spectra are shown to have an impact region near the separated-atom transition frequency and a far-wing region. The impact region is due to radiation emitted after the molecule has separated into atomic fragments, and the far-wing region is due to radiation emitted during the time of strong molecular interaction. The spectrum in the impact region depends upon an effective phase shift for a ‘‘partial’’ collision, which begins at the time that the probe pulse sweeps through the molecular transition frequency. For narrow wave packets, this phase shift can be directly measured, and the molecular transition frequency can be recovered as a function of time along the path of dissociation. For very broad wave packets, the time-resolved absorption spectra approach a statistical limit, in which the absorption line shape becomes an image in frequency space of the probability density in configuration space at the time of excitation by the probe pulse. In all cases, the frequency-integrated absorption is proportional to the net population of molecules that are excited by the probe pulse. In principle, this result can be used to obtain the strength of the transition dipole moment as a function of internuclear separation. We also consider fluorescence induced by a short optical probe pulse, as in the experiments of Zewail and co-workers. Fluorescence measurements are shown to be fundamentally different from measurements of the transmitted spectral power density: fluorescence depends upon the net population excited by the probe pulse, whereas the transmitted spectral power density depends upon interference between the incident probe field and the polarization field. Thus these two experimental techniques are sensitive to different aspects of the dissociation process.
Study of the 4 1Σ+g ‘‘shelf’’ state of Na2 by optical–optical double resonance spectroscopy
Authors: Wang He , Whang Thou-Jen , Lyyra A. Marjatta , Li Li , Stwalley William C.
Published: July 2002

Due to an avoided crossing, the 4 1Σ+g state of Na2 has an unusual potential energy curve with a ‘‘shelf’’ on its outer wall. By using optical–optical double resonance techniques, vibrational levels from v=0 to 130 have been observed and the shelf was found around v=48–60. From the experimental data we have successfully constructed a Rydberg–Klein–Rees (RKR) potential curve, which can reproduce measured data with a standard deviation of 0.064 cm−1. A set of Dunham coefficients was obtained for the lower part v=0–45 of that state. Several interesting effects related to the shelf, such as the unusual behavior of Bv and Dv, of the vibrational wave functions and the semiclassical vibrational periods, and of the rotational effective potential, are discussed.
Solvent and temperature dependence of the lowest excited singlet state lifetime of all-t
r
a
n
s-7′,7′-dicyano-7′-apo-β-carotene
Authors: O’Neil Michael P. , Wasielewski Michael R. , Khaled Mazen M. , Kispert Lowell D.
Published: July 2002

The lifetime of the lowest excited singlet state of a highly polarizable polyene, all-trans-7′,7′-dicyano-7′-apo-β-carotene, 1, was measured as a function of solvent dielectric constant at 295 K and temperature from 7.4–295 K. The polyene was excited in separate experiments with 610 nm, 2 ps, and 450 nm, 4 ps laser pulses. The broad 400–650 nm absorption band of the polyene bleaches immediately upon excitation, while a strong absorption centered between 680–705 nm appears. Both the recovery of the ground state absorption band and the disappearance of the 680–705 nm excited state absorption band exhibit the same single exponential kinetics that depend on both solvent dielectric constant and on temperature. As the dielectric constant of the solvent increases from 1.9 to 36, the lifetime of the lowest excited singlet state of 1 decreases from 11.7 ps to 1.9 ps. As the temperature is lowered the lifetime of the lowest excited singlet state of 1 approaches that observed in low dielectric constant media. The behavior of 1 is ascribed to a large change in dipole moment between the ground and lowest excited singlet state. Interaction of the excited state dipole with solvent dipoles results in enhanced charge transfer character within the excited state, which in turn enhances nonradiative decay of the excited state. The excited state dipole moments of the ground, first, and second excited states of both 1 and β-8′-apocarotenal, 2 were calculated using the AM1 molecular orbital method. The calculations predict that 1 has a 5.1 Debye difference in dipole moment between the ground and first excited state.
Inelastic processes in e–HCl collisions in the energy range including the dissociation threshold
Authors: Fabrikant I. I. , Kalin S. A. , Kazansky A. K.
Published: July 2002

Inelastic processes in low-energy collisions of electrons with HCl molecules are considered. We present results of calculations of vibrational excitation cross sections from the ground state and vibrationally excited states, dissociative attachment cross sections, and electron-impact dissociation cross sections near threshold. Our approach is based on the resonance R-matrix theory with the quasiclassical treatment of the nuclear motion. We use a rigorous procedure for the inclusion of the vibrational continuum. This allows for calculations of the cross sections in the energy range including dissociation threshold. The results for the dissociation cross sections are analyzed from the point of view of the threshold law for the three-particle break-up process.
Dissociation energies for carbon cluster ions (C+2–15): A system where photodissociation is misleading
Authors: Sowa Marianne B. , Hintz Paul A. , Anderson Scott L.
Published: July 2002

This letter reports a collision-induced dissociation study of the stabilities and fragmentation patterns for carbon cluster ions C+n (n = 2–15). For clusters of six or more atoms, the primary fragmentation channel is loss of C3 neutral. Fragmentation thresholds are analyzed to yield dissociation energies, which are found to be substantially higher than the upper limits estimated from photodissociation experiments of Geusic et al. [Z. Phys. D 3, 309 (1986)], but in good agreement with theoretical estimates. The stabilities oscillate strongly, with some evidence of a change in behavior as size increases above 9 atoms. This is the size where the most stable structure is believed to change from linear to cyclic. Photodissociation measurements were also done, and we find that the clusters dissociate efficiently at photon energies far below the CID thresholds. Possible reasons for the anomalous photodissociation results are discussed.
Photoionization of jet-cooled HI with coherent vacuum ultraviolet radiation: Evidence for Hund’s case (e)
Authors: Mank A. , Drescher M. , Huth-Fehre T. , Böwering N. , Heinzmann U. , Lefebvre-Brion H.
Published: July 2002

In the range of 83 500–89 200 cm−1, the photoionization spectrum of HI is strongly influenced by autoionization of Rydberg series converging to the higher lying 2Π1/2 component of the spin–orbit split 2Π ionic ground state. The photoelectron yield spectrum, exhibiting the various autoionization processes, has been measured on a rotationally cooled sample (Trot∼13 K), with a spectral resolution of the exciting vacuum ultraviolet radiation of 0.5 cm−1. Using a multichannel quantum-defect theory treatment explicitly including rotation, we calculate spin–orbit and rotational autoionization fine structure in the spectrum. In order to find a qualitative agreement of theory and experiment, the Rydberg states of HI have to be described in terms of a transition in angular momentum coupling cases, going from Hund’s case (c) to case (e).
Structure, dynamics, and the electronic absorption of benzene–argon clusters
Authors: Fried Laurence E. , Mukamel Shaul
Published: July 2002

We present a new method for calculating cluster absorption spectra using classical molecular dynamics and simulated annealing techniques. We then apply this method to benzene–Ar clusters. Cluster absorption spectra are shown to be dominated by an inhomogeneous distribution of isomer absorptions. The absorption spectrum of each isomer, however, results from the interplay of structure, fluctuations, and dynamics. We find that accompanying the solid to liquid transition, there is a spectroscopic transition from a periodic to a decaying autocorrelation function of the electronic energy gap. Benzene–Ar clusters are found to undergo transitions from a solid to a 2D liquid to a 3D liquid as the number of Ar atoms is increased from 1 to 21 at 20 K.
Third generation of conducting polymers: Spectroelectrochemical investigations on viologen functionalized poly (3-alkylthiophenes)
Authors: Sariciftci N. S. , Mehring M. , Gaudl K. U. , Bäuerle P. , Neugebauer H. , Neckel A.
Published: July 2002

Results of in situ electron spin resonance, optical absorption, and Fourier transform infrared spectroscopic studies during electrochemical doping and undoping processes of viologen functionalized poly (3-alkylthiophenes) are reported. Effects of the functional group on the electronic properties of the polythiophene backbone in its conducting state are observed resulting in localization of the charge carriers. Radicals on the viologen side group show hyperfine structured electron spin resonance signals indicating a rapid motional averaging of anisotropic electron-nuclear couplings. Experimental results are discussed by comparison with other substituted polythiophenes in terms of conformational effects of the acceptor group on the conducting polymer backbone. A structural model with counterion complexation between polythiophene backbone and the viologen functional unit is presented.
Observations of Coulomb explosion in doubly charged atomic and molecular clusters
Authors: Gotts N. G. , Lethbridge P. G. , Stace A. J.
Published: July 2002

Coulomb explosion has been promoted in a range of doubly charged atomic and molecular clusters. In these new experiments, mass selected clusters of Ar2+n, (CO2)2+n, (H2O)2+n, (H2O)nH2+2, (CH3CN)nH2+2, and (C6H6)2+n have been subjected to collisional activation with a background gas. For species close to the Coulomb cutoff, each collision removes sufficient atoms or molecules (approximately six) as to render the clusters unstable. As a result, charge separation occurs and part (≂30%) of the Coulomb repulsion energy is released in the form of center of mass kinetic energy in the fragments. The remaining Coulomb energy appears as internal excitation in the fragments and subsequently leads to extensive evaporation. It is shown that the latter process is continuing even 10−6 s after Coulomb explosion. All the molecular systems studied show evidence of asymmetric charge separation, with some singly charged fragments containing up to 65% of the initial cluster mass. A detailed quantitative analysis of the results is made difficult by the very broad range of fragment ion sizes.