Mass spectrometry and photoelectron spectroscopy of tetracene cluster anions, (tetracene)n- (n = 1-100): evidence for the highly localized nature of polarization in a cluster analogue of oligoacene crystals

Photoelectron spectroscopy of tetracene cluster anions, (tetracene)n- (n = 1-100), reveals the coexistence of two types of isomers, designated as isomers I and II-1 (n = 10-50) or isomers I and II-2 (n > 60), in a wide size range. The vertical detachment energies (VDEs) of isomer I increase persistently due to polarization and structural relaxation effects, where a monomeric anion core is encompassed with geometrically reorganized neutral molecules. Conversely, a characteristic ion distribution in the mass spectrum of (tetracene)n-ensues from the two-dimensional (2D) herringbone-type ordering of isomer II-1, whose VDEs remain constant at 1.80 eV for n >/= 14. Also, isomer II-2, presumably adopting multilayered structural motifs, exhibits invariable VDEs of 2.0 eV, a manifestation of significant charge screening effects in these isomers. The invariable nature of the VDEs of isomers II-1 and II-2 unambiguously demonstrates a largely localized nature of polarization induced by the excess charge residing in microscopic crystal-like environments. Surprisingly, only 14 tetracene molecules within a 2D herringbone-type layer including an excess charge can provide the charge stabilization energy corresponding to approximately 80% of that of the crystal, and the rest of the energy is provided by polarization of neutral molecules in adjacent layers.     

Comprehensive photoelectron spectroscopic study of anionic clusters of anthracene and its alkyl derivatives: electronic structures bridging molecules to bulk

The evolution of the electronic structure of molecular aggregates is investigated using anion photoelectron (PE) spectroscopy for anionic clusters of anthracene (Ac) and its alkyl derivatives: 1-methylanthracene (1MA), 2-methylanthracene (2MA), 9-methylanthracene (9MA), 9,10-dimethylanthracene (DMA), and 2-tert-butylanthracene (2TBA). For their monomer anions (n=1), electron affinities are confined to the range from 0.47 to 0.59 eV and are well reproduced by density functional theory calculations, showing the isoelectronic character of these molecules. For cluster anions (n=2-100) of Ac and 2MA, two types of isomers I and II coexist over a wide size range: isomers I and II-1 (4< or =n<30) or isomers I and II-2 (n> or = approximately 40 for Ac and n> or = approximately 55 for 2MA). However, for the other alkyl-substituted Ac cluster anions (i.e., 1MA, 9MA, DMA, and 2TBA), only isomer I is exclusively formed, and neither isomer II-1 nor II-2 is observed. The vertical detachment energies (VDEs) of isomer I in all the anionic clusters depend almost linearly on n(-1/3). In contrast, the VDEs of isomers II-1 (n> or =14) and II-2 (n=40-100), appeared only in Ac and 2MA cluster anions, remain constant with n and are approximately 0.5 eV lower than those of isomer I. The PE spectra revealed the characteristics of each isomer: isomer I possesses a monomeric anion core that is gradually embedded into the interior of the cluster with increasing n. On the other hand, isomers II-1 and II-2 possess a multimeric (perhaps tetrameric) anion core, but they differ in the number of layers from which they are made up; monolayer (isomer II-1) and multilayers (isomer II-2) of a two-dimensionally ordered, finite herringbone-type structure, in which electron attachment produces only little geometrical rearrangement. Moreover, the agreement of the constant VDEs of isomer II-2 with the bulk data demonstrates the largely localized nature of the electronic polarization around the excess charge in a crystal-like environment, where about 50 molecules provide a charge stabilization energy comparable to the bulk.     

Photoelectron spectroscopy of cluster anions of naphthalene and related aromatic hydrocarbons

The electronic structures and structural morphologies of naphthalene cluster anions, (naphthalene)(n)(-) (n=3-150), and its related aromatic cluster anions, (acenaphthene)(n)(-) (n=4-100) and (azulene)(n)(-) (n=1-100), are studied using anion photoelectron spectroscopy. For (naphthalene)(n) (-) clusters, two isomers coexist over a wide size range: isomers I and II-1 (28 < or = n < or =60) or isomers I and II-2 (n > or = ~60). Their contributions to the photoelectron spectra can be separated using an anion beam hole-burning technique. In contrast, such an isomer coexistence is not observed for (acenaphthene)(n) (-) and (azulene)(n) (-) clusters, where isomer I is exclusively formed throughout the whole size range. The vertical detachment energies (VDEs) of isomer I (7 < or = n < or = 100) in all the anionic clusters depend linearly on n(-13) and their size-dependent energetics are quite similar to one another. On the other hand, the VDEs of isomers II-1 and II-2 produced in (naphthalene)(n)(-) clusters with n > or = approximately 30 remain constant at 0.84 and 0.99 eV, respectively, 0.4-0.6 eV lower than those of isomer I. Based upon the ion source condition dependence and the hole-burning photoelectron spectra experiments for each isomer, the energetics and characteristics of isomers I, II-1, and II-2 are discussed: isomer I is an internalized anion state accompanied by a large change in its cluster geometry after electron attachment, while isomers II-1 and II-2 are crystal-like states with little structural relaxation. The nonappearance of isomers II-1 and II-2 for (acenaphthene)(n)(-) and (azulene)(n)(-) and a comparison with other aromatic cluster anions indicate that a highly anisotropic and symmetric pi-conjugated molecular framework, such as found in the linear oligoacenes, is an essential factor for the formation of the crystal-like ordered forms (isomers II-1 and II-2). On the other hand, lowering the molecular symmetry makes their production unfavorable.     

Photoelectron imaging of tetrahydrofuran cluster anions (THF)(n) (-) (1≤n≤100)

Anionic tetrahydrofuran clusters (THF)(n) (-) (1≤n≤100) are studied with photoelectron imaging as gas-phase precursors for electrons solvated in THF. Photoelectron spectra of clusters up to n=5 show two peaks, one of which is attributed to a solvated open chain radical anion and the other to the closed THF ring. At n=6, the spectra change shape abruptly, which become more characteristic of (THF)(n) (-) clusters containing solvated electrons. From n=6-100, the vertical detachment energies (VDEs) of these solvated electron clusters increase from 1.96 to 2.71 eV, scaling linearly with n(-1/3). For fully deuterated (THF-d8)(n) (-) clusters, the apparent transition to a solvated electron cluster is delayed to n=11. Extrapolation of the VDEs to infinite cluster size yields a value of 3.10 eV for the bulk photoelectric threshold. The relatively large VDEs at onset and small stabilization with increasing cluster size compared to other solvated electron clusters may reflect the tendency of the bulk solvent to form preexisting voids that can readily solvate a free electron.     

导数-固体基质室温燐光法同时测定痕量对位三联苯和间位三联苯

本文提出间三联苯和对三联苯的二阶导数-固体基质室温燐光法(d²SS-RTP)。本法在λ_ax=288nm,用448nm处正峰和460nm处负峰的峰峰高度值定量测定间三联苯,对三联苯不干扰测定.线性范围0.46～46ng.检出限为0.1ng/斑点.对三联苯用526nm处正峰和548nm处负峰的峰峰高度值测定.间三联苯不干扰.线性范围0.46～46ng.检出限0.07ng/斑点.     

Structural evolution and stabilities of neutral and anionic clusters of lead sulfide: joint anion photoelectron and computational studies

The geometric and electronic structures of both neutral and negatively charged lead sulfide clusters, (PbS)(n)/(PbS)(n)(-) (n = 2-10) were investigated in a combined anion photoelectron spectroscopy and computational study. Photoelectron spectra provided vertical detachment energies (VDEs) for the cluster anions and estimates of electron affinities (EA) for their neutral cluster counterparts, revealing a pattern of alternating EA and VDE values in which even n clusters exhibited lower EA and VDE values than odd n clusters up until n = 8. Computations found neutral lead sulfide clusters with even n to be thermodynamically more stable than their immediate (odd n) neighbors, with a consistent pattern also being found in their HOMO-LUMO gaps. Analysis of neutral cluster dissociation energies found the Pb(4)S(4) cube to be the preferred product of the queried fragmentation processes, consistent with our finding that the lead sulfide tetramer exhibits enhanced stability; it is a magic number species. Beyond n = 10, computational studies showed that neutral (PbS)(n) clusters in the size range, n = 11-15, prefer two-dimensional stacking of face-sharing lead sulfide cubical units, where lead and sulfur atoms possess a maximum of five-fold coordination. The preference for six-fold coordination, which is observed in the bulk, was not observed at these cluster sizes. Taken together, the results show a preference for the formation of slightly distorted, fused cuboids among small lead sulfide clusters.     

Structures and dissociation channels of protonated mixed clusters around a small magic number: infrared spectroscopy of ((CH3)3N)n-H(+)-H2O (n = 1-3)

The magic number behavior of ((CH(3))(3)N)(n)-H(+)-H(2)O clusters at n = 3 is investigated by applying infrared spectroscopy to the clusters of n = 1-3. Structures of these clusters are determined in conjunction with density functional theory calculations. Dissociation channels upon infrared excitation are also measured, and their correlation with the cluster structures is examined. It is demonstrated that the magic number cluster has a closed-shell structure, in which the water moiety is surrounded by three (CH(3))(3)N molecules. The ion core (protonated site) of the clusters is found to be (CH(3))(3)NH(+) for n = 1-3, but coexistence of an isomer of the H(3)O(+) ion core cannot be ruled out for n = 3. Large rearrangement of the cluster structures of n = 2 and 3 before dissociation, which has been suggested in the mass spectrometric studies, is confirmed on the basis of the structure determination by infrared spectroscopy.     

Structures and stability of lithium monosilicide clusters SiLin (n = 4–16): What is the maximum number,magic number,and core number for lithium coordination to silicon?

In the coordination, hypervalent and cluster chemistry, three important characteristic properties are the maximum coordination number, magic number, and core coordination number. Yet, few studies have considered these three numbers at the same time for an ML(n) cluster with n larger than 8. In this article, we systematically studied the three properties of SiLi(n) (n = 4-16) clusters at the B3LYP/6-31G(d), B3LYP/6-311++G(2d), and CCSD(T)/6-311++G(3df)//B3LYP/6-311++G(2d) (for energy only) levels. Various isomeric forms with different symmetries were calculated. For each SiLi(n) (n = 4-9), silicon cohesive energy (cE) from SiLi(n) --> Si + Li(n) reaction, vertical ionization potential (vIP), and vertical electron affinity (vEA) were obtained for the lowest-energy isomer. We found that the maximum Li-coordination number of Si is 9, which is the largest number among the known MLi(n) clusters. All cE, vIP, and vEA values predicted that 6 is the magic Li-coordination number of Si. For small SiLi(n) (n < or = 6) clusters, Li atoms favor direct coordination to Si, whereas for larger SiLi(n) (n > or = 7) clusters, there is a core cluster that is surrounded by excessive Li atoms. The core Li-coordination number is 6 for SiLi(n) (n = 7,8), 7 for SiLi(n) (n = 9,10), 8 for SiLi(n) (n = 11-15) and 9 for SiLi(n) (n > or = 16). Through the calculations, we verified the relationship between the structure and stability of SiLi(n) with the maximum coordination number, magic number, and core coordination number.     

Origin of the magic numbers of water clusters with an excess electron

Electron-bound water clusters [e(-)(H(2)O)(n)] show very strong peaks in mass spectra for n=2, 6, 7, and (11), which are called magic numbers. The origin of the magic numbers has been an enigma for the last two decades. Although the magic numbers have often been conjectured to arise from the intrinsic properties of electron-bound water clusters, we attributed them not to their intrinsic properties but to the particularly weak stability of the corresponding neutral water clusters (H(2)O)(n=2,6,7, and (11)). As the cluster size increases; this nonsmooth characteristic feature in stability of neutral water clusters is contrasted to the smooth increase in stability of e(-)-water clusters. As the magic number clusters have significant positive adiabatic electron affinities, their abundant distributions in atmosphere could play a significant role in atmospheric thermodynamics.     

Structural rearrangements and magic numbers in reactions between pyridine-containing water clusters and ammonia

Molecular cluster ions H(+)(H(2)O)(n), H(+)(pyridine)(H(2)O)(n), H(+)(pyridine)(2)(H(2)O)(n), and H(+)(NH(3))(pyridine)(H(2)O)(n) (n = 16-27) and their reactions with ammonia have been studied experimentally using a quadrupole-time-of-flight mass spectrometer. Abundance spectra, evaporation spectra, and reaction branching ratios display magic numbers for H(+)(NH(3))(pyridine)(H(2)O)(n) and H(+)(NH(3))(pyridine)(2)(H(2)O)(n) at n = 18, 20, and 27. The reactions between H(+)(pyridine)(m)(H(2)O)(n) and ammonia all seem to involve intracluster proton transfer to ammonia, thus giving clusters of high stability as evident from the loss of several water molecules from the reacting cluster. The pattern of the observed magic numbers suggest that H(+)(NH(3))(pyridine)(H(2)O)(n) have structures consisting of a NH(4)(+)(H(2)O)(n) core with the pyridine molecule hydrogen-bonded to the surface of the core. This is consistent with the results of high-level ab initio calculations of small protonated pyridine/ammonia/water clusters.     

Exploring the rich energy landscape of sulfate-water clusters SO4(2-) (H2O)(n=3-7): an electronic structure approach

We present a reinvestigation of sulfate-water clusters SO4(2-) (H2O)(n=3-7), which involves several new aspects. Using a joint molecular mechanics/first principles approach, we perform exhaustive searches for stable cluster geometries, showing that the sulfate-water landscape is much richer than anticipated previously. We check the compatibility of the new structures with experiment by comparing vertical detachment energies (VDEs) calculated at the B3LYP/6-311++G** level of theory and determine the energetic ordering of the isomers at the RI-MP2/aug-cc-pVTZ level. Our results are bench-marked carefully against reference energies of estimated CCSD(T)/aug-cc-VTZ quality and VDEs of CCSD(T)/aug-cc-pVDZ quality. Furthermore, we calculate anharmonic vibrational corrections for up to the n = 6 clusters, which are shown to be significant for isomer energy ordering. We use energy decomposition analysis (EDA) based on the absolutely localized fragment (ALMO) expansion to gain chemical insight into the binding motifs.     

Theoretical characterization of four distinct isomer types in hydrated-electron clusters, and proposed assignments for photoelectron spectra of water cluster anions

Water cluster anions, (H(2)O)(N)(-), are examined using mixed quantum/classical molecular dynamics based on a one-electron pseudopotential model that incorporates many-body polarization and predicts vertical electron detachment energies (VDEs) with an accuracy of ~0.1 eV. By varying the initial conditions under which the clusters are formed, we are able to identify four distinct isomer types that exhibit different size-dependent VDEs. On the basis of a strong correlation between the electron's radius of gyration and its optical absorption maximum, and extrapolating to the bulk limit (N → ∞), our analysis supports the assignment of the "isomer Ib" data series, observed in photoelectron spectra of very cold clusters, as arising from cavity-bound (H(2)O)(N)(-) cluster isomers. The "isomer I" data reported in warmer experiments are assigned to surface-bound isomers in smaller clusters, transitioning to partially embedded isomers in larger clusters. The partially embedded isomers are characterized by a partially formed solvent cavity at the cluster surface, and they are spectroscopically quite similar to internalized cavity isomers. These assignments are consistent with various experimental data, and our theoretical characterization of these isomers sheds new light on a long-standing assignment problem.     

Photoelectron spectroscopy of the [glycine x (H2O)(1,2)]- clusters: sequential hydration shifts and observation of isomers

The electron binding energies of the small hydrated amino acid anions, [glycine x (H2O)(1,2)]-, are determined using photoelectron spectroscopy. The vertical electron detachment energies (VDEs) are found to increase by approximately 0.12 eV with each additional water molecule such that the higher electron binding isomer of the dihydrate is rather robust, with a VDE value of 0.33 eV. A weak binding isomer of the dihydrate is also recovered, however, with a VDE value (0.14 eV) lower than that of the monohydrate. Unlike the situation in the smaller (n < or = 13) water cluster anions, the [Gly x (H2O)(n > or = 6)]- clusters are observed to photodissociate via water monomer evaporation upon photoexcitation in the O-H stretching region. We discuss this observation in the context of the mechanism responsible for the previously observed [S. Xu, M. Nilles, and K. H. Bowen, Jr., J. Chem. Phys. 119, 10696 (2003)] sudden onset in the cluster formation at [Gly x (H2O)5]-.     

Photoelectron spectroscopic study of the hydrated nucleoside anions: Uridine(-)(H(2)O)(n=0-2), cytidine(-)(H(2)O)(n=0-2), and thymidine(-)(H(2)O)(n=0,1)

The hydrated nucleoside anions, uridine(-)(H(2)O)(n=0-2), cytidine(-)(H(2)O)(n=0-2), and thymidine(-)(H(2)O)(n=0,1), have been prepared in beams and studied by anion photoelectron spectroscopy in order to investigate the effects of a microhydrated environment on parent nucleoside anions. Vertical detachment energies (VDEs) were measured for all eight anions, and from these, estimates were made for five sequential anion hydration energies. Excellent agreement was found between our measured VDE value for thymidine(-)(H(2)O)(1) and its calculated value in the companion article by S. Kim and H. F. Schaefer III.     

Ultracold water cluster anions

Attachment of free electrons to water clusters embedded in helium droplets leads to water-cluster anions (H2O)n(-) and (D2O)n(-) of size n > or = 2. Small water-cluster anions bind to up to 10 helium atoms, providing compelling evidence for the low temperature of these complexes, but the most abundant species are bare cluster anions. In contrast to previous experiments on bare water clusters, which showed very pronounced magic and anti-magic anion sizes below n = 12, the presently observed size distributions vary much more smoothly, and all sizes are easily observed. Noticeable differences are also observed in the stoichiometry of fragment anions formed upon dissociative electron attachment and the energy dependence of their yield. Spectroscopic characterization of these ultracold water-cluster anions promises to unravel the relevance of metastable configurations in experiments and the nature of the still controversial bonding sites for the excess electron in small water-cluster anions.     

Proton mobility and stability of water clusters containing the bisulfate anion, HSO4(-)(H2O)n

Bisulfate water clusters, HSO(4)(-)(H(2)O)(n), have been studied both experimentally by a quadrupole time-of-flight mass spectrometer and by quantum chemical calculations. For the cluster distributions studied, there are some possible "magic number" peaks, although the increase in abundance compared to their neighbours is small. Experiments with size-selected clusters with n = 0-25, reacting with D(2)O at a center-of-mass energy of 0.1 eV, were performed, and it was observed that the rate of hydrogen/deuterium exchange is lower for the smallest clusters (n < 8) than for the larger (n > 11), with a transition taking place in the range n = 8-11. We propose that the protonic defect of the bisulfate ion remains rather stationary unless the degree of hydration reaches a given level. In addition, it was observed that H/D scrambling becomes close to statistically randomized for the larger clusters. Insight into this size dependency was obtained by B3LYP/6-311++G(2d,2p) calculations for HSO(4)(-)(H(2)O)(n) with n = 0-10. In agreement with experimental observations, these calculations suggest pronounced effectiveness of a 'see-saw mechanism' for pendular proton transfer with increasing HSO(4)(-)(H(2)O)(n) cluster size.     

Experimental and theoretical investigation on binary semiconductor clusters of Bi/Si, Bi/Ge, and Bi/Sn

Bi(m)M(n)- (M = Si, Ge, Sn) binary cluster anions are generated by using laser ablation on mixtures of Bi and M (M = Si, Ge, Sn) samples and studied by reflectron time-of-flight mass spectrometer (RTOF-MS) in the gas phase. Some magic number clusters are present in the mass spectra which indicate that they are in stable structures. For small anions (m + n < or = 6), their structures are investigated with the DFT method and the energetically lowest lying structures are obtained. For the binary anionic clusters with the same composition containing Si, Ge, and Sn, they share similar geometric and electronic structure in the small size except that BiSi3-, BiSi5-, Bi2Si2-, Bi2Si3-, and Bi4Sn2- are different for the lowest energetic structures, and the ground states for all the anions are in their lowest spin states. The calculated VDE (vertical detachment energy) and binding energy confirm the obviously magic number cluster of BiM4- (M = Si, Ge, Sn), which agrees with the experimental results.     

Solvation-induced cluster anion core switching from NNO2(-)(N2O)n-1 to O(-)(N2O)n

We report a photoelectron imaging study of the [O(N(2)O)(n)](-), 0or=4 (and up to at least n=9) signatures of an O(-) core are predominantly observed. Photofragmentation studies at 355 nm support these results.     

Configuration dependent critical nuclei in the self assembly of magic clusters

Evidence for the formation of various 2-D structures possessing different numbers of Co-Si magic clusters (size approximately 10.0 +/- 0.5 A), configurations and lifetimes are studied in real time on a Si(111)-(7 x 7) surface at elevated temperature in the STM. Observations of individual cluster diffusion, attachment and detachment dynamics resolve unequivocally the question of self assembly over surface reconstruction. The smallest stable structure consisting of seven individual Co-Si magic clusters arranged in a hexagonal closed packed formation (i = 7) is found to retain sufficient cohesive energy to avoid dissociation. A configuration dependent critical 2-D nuclei (i* = 6) is determined to exist in facilitating the self assembly dynamics.     

Electron and nuclear dynamics of molecular clusters in ultraintense laser fields. III. Coulomb explosion of deuterium clusters

In this paper we present a theoretical and computational study of the energetics and temporal dynamics of Coulomb explosion of molecular clusters of deuterium (D2)n/2 (n = 480 - 7.6 x 10(4), cluster radius R0 = 13.1 - 70 A) in ultraintense laser fields (laser peak intensity I = 10(15) - 10(20)W cm(-2)). The energetics of Coulomb explosion was inferred from the dependence of the maximal energy EM and the average energy Eav of the product D+ ions on the laser intensity, the laser pulse shape, the cluster radius, and the laser frequency. Electron dynamics of outer cluster ionization and nuclear dynamics of Coulomb explosion were investigated by molecular dynamics simulations. Several distinct laser pulse shape envelopes, involving a rectangular field, a Gaussian field, and a truncated Gaussian field, were employed to determine the validity range of the cluster vertical ionization (CVI) approximation. The CVI predicts that Eav, EM proportional to R0(2) and that the energy distribution is P(E) proportional to E1/2. For a rectangular laser pulse the CVI conditions are satisfied when complete outer ionization is obtained, with the outer ionization time toi being shorter than both the pulse width and the cluster radius doubling time tau2. By increasing toi, due to the increase of R0 or the decrease of I, we have shown that the deviation of Eav from the corresponding CVI value (Eav(CVI)) is (Eav(CVI) - Eav)/Eav(CVI) approximately (toi/2.91tau2)2. The Gaussian pulses trigger outer ionization induced by adiabatic following of the laser field and of the cluster size, providing a pseudo-CVI behavior at sufficiently large laser fields. The energetics manifest the existence of a finite range of CVI size dependence, with the validity range for the applicability of the CVI being R0 < or = (R0)I, with (R0)I representing an intensity dependent boundary radius. Relating electron dynamics of outer ionization to nuclear dynamics for Coulomb explosion induced by a Gaussian pulse, the boundary radius (R0)I and the corresponding ion average energy (Eav)I were inferred from simulations and described in terms of an electrostatic model. Two independent estimates of (R0)I, which involve the cluster size where the CVI relation breaks down and the cluster size for the attainment of complete outer ionization, are in good agreement with each other, as well as with the electrostatic model for cluster barrier suppression. The relation (Eav)I proportional to (R0)I(2) provides the validity range of the pseudo-CVI domain for the cluster sizes and laser intensities, where the energetics of D+ ions produced by Coulomb explosion of (D)n clusters is optimized. The currently available experimental data [Madison et al., Phys. Plasmas 11, 1 (2004)] for the energetics of Coulomb explosion of (D)n clusters (Eav = 5 - 7 keV at I = 2 x 10(18) W cm(-2)), together with our simulation data, lead to the estimates of R0 = 51 - 60 A, which exceed the experimental estimate of R0 = 45 A. The predicted anisotropy of the D+ ion energies in the Coulomb explosion at I = 10(18) W cm(-2) is in accord with experiment. We also explored the laser frequency dependence of the energetics of Coulomb explosion in the range nu = 0.1 - 2.1 fs(-1) (lambda = 3000 - 140 nm), which can be rationalized in terms of the electrostatic model.