Infrared ion dip and ultraviolet spectroscopy of 4-phenyl imidazole, its tautomer, 5-phenyl imidazole, and its multiply hydrated clusters

Mass-resolved resonant two photon ionisation (R2PI) and infrared ion dip spectra have been recorded for 4-phenylimidazole (4PI) and its singly and multiply hydrated clusters 4PI(H₂O)_{n = 0 - 4}, under supersonic expansion conditions. In the case of 4PI(H₂O)_0,1, it has also been possible to record infrared spectra in both the ground (S₀) and excited (S₁) states. Combining the experimental data with the results of ab initio calculations has led to the structural assignment of each cluster. In each case, the water molecules bind primarily to the NH site of the imidazole ring. Clusters with n≥ 2 incorporate linear water chains, in which the proton donating terminus bridges either to the π-electron system (n = 2) or to the >N: atom site (n = 3, 4) on the imidazole ring. Despite the creation of a “water wire”, connecting the donor and acceptor sites of imidazole, there is no evidence of proton transfer in either the ground or excited state. Received 20 December 2001 Published online 13 September 2002     

l mixing and dissociation of Rydberg molecules accompanying slow collisions with inert-gas atoms

Processes involving a change of orbital angular momentum l and the dissociation of Rydberg molecules when they collide with inert-gas atoms are considered, using a method based on analyzing the terms of the interacting systems. The proposed method makes it possible to take into account the perturbations in a large group of Rydberg states that weakly penetrate into the ion core and to go beyond the limits of the two-level approximations [R. F. Stebbings and F. B. Dunning, eds., Rydberg States of Atoms and Molecules (Cambridge University Press, Cambridge, 1983; Mir, Moscow, 1985)] widely used in the theory of l mixing with the participation of Rydberg atoms. Using the Na**(nd)+Xe reaction as an example, it is demonstrated that this method gives good results in a wide range of variation of the principal quantum number n of the Rydberg electron (n≈10–40). Features of the l mixing and dissociation of Rydberg molecules are studied for the H₂**+Xe system. It is shown that collisional dissociation occurs by the formation of an intermediate Rydberg state of the molecules, followed by self-decay into a dissociative continuum. Zh. éksp. Teor. Fiz. 114, 100–113 (July 1998)     

Photoinduced dissociation reactions of silver fluoride cluster ions

Photoinduced dissociation in the ultraviolet region has been investigated for Ag _nF _n-1 ⁺ cluster ions. Photodissociation spectrum of Ag₂F⁺ in the energy of 3.8–5.6 eV exhibits several sharp bands corresponding to the transition to electronically excited states. In this dissociation, only the Ag₂ ⁺ ion was observed as a fragment ion. Theoretical calculation indicates that the parent Ag₂F⁺ ion has a linear Ag-F-Ag equilibrium geometries in the ground and excited states. Since conformational changes by excitation of bending vibration are necessary for the fragmentation of an F atom, this indicates that production of Ag₂ ⁺ from Ag₂F⁺ is a result of internal conversion and following conformational changes.     

Electron multiplicity in slow collisions of Ar ions with C

Electron capture by Ar⁸⁺ in collisions with C₆₀ fullerene has been investigated using coincident measurements of the number n of ejected electrons, the mass and charge of multicharged Cr+ ₆₀ recoil ions and their fragments Ci+ m and the final charge state of outgoing projectiles Ar(8-s)+ (). The number of captured electrons r is the sum of the numbers of stabilized and emitted electrons: r = n + s. The ratio n / s decreases by a factor three with s increasing from 1 to 7 showing that the multiply excited states populated by capture of a large number of electrons are rather stable against auto-ionisation. Each kinetic energy spectrum of Ar⁺ and Ar²⁺ projectiles is composed of two peaks which we attribute to collisions “inside” and “outside” the C₆₀ cage. The measured energy shift of the projectile keV is consistent with the corresponding energy loss keV in a carbon foil with an equivalent thickness. Inside collisions are characterized by a strong dissociation of recoil ions into light monocharged fragments and by a high multiplicity of ejected electrons. Received: 25 March 1998 / Received in final form and Accepted: 9 June 1998     

Energy dependence of the decay pathways of optically excited small gold clusters

The pathway competition between neutral monomer and neutral dimer evaporation from optically excited odd-size gold cluster ions Au⁺ _n, n=7–15, has been investigated as a function of cluster size and excitation energy. Gold cluster ions of these sizes are the only ones to show observable pathway competition while all other sizes exclusively evaporate either neutral monomers or neutral dimers. The investigation has been performed by photoexcitation of stored size-selected gold cluster ions with a single 10-ns laser pulse. Subsequent time-resolved observation of the delayed dissociation allows us to quantitatively determine the relative fragment yields of the respective decay channels as a function of excitation energy. Contrary to theoretical expectations, the dimer-to-monomer branching ratio of evaporated particles is found to decrease monotonously with increasing excitation energy for all cluster sizes under investigation. Possible explanations for this behaviour are discussed. Received: 9 July 2001 / Revised version: 24 September 2001 / Published online: 15 October 2001     

New approaches to stored cluster ions

Ion traps are wall-less containers which allow the extended storage of selected species. During the storage various interaction steps may be repeatedly applied. To this end no further hardware has to be added - in contrast to beam experiments. In this progress report two examples of recent developments are presented: the experiments have been performed with metal clusters stored in a Penning (ion cyclotron resonance) trap. A new experimental scheme has been developed which allows precision measurements of the dissociation energies of polyatomic species. It has been triggered by investigations on the delayed photodissociation of stored metal clusters. However, the technique is also readily available for application to a broad variety of different species and it is not even restricted to trapping experiments. The second development is more closely connected with ion storage in Penning traps: by application of an electron bath singly charged anionic clusters can be converted into multiply charged species. Subsequently, they are charge selected and investigated with respect to their reaction upon excitation. In particular, preliminary results indicate that dianionic metal clusters emit two electrons upon photoexcitation whereas the singly charged species show dissociation.     

The transition from direct to delayed ionisation of C 60

The timescale for the coupling of electronic and vibrational excitation in isolated fullerenes is determined by recording positive ion time-of-flight mass spectra on excitation with ultrashort laser pulses at 790 nm of the same fluence but different pulse durations. The coupling leads to the onset of a delayed ionisation “tail” on the parent fullerene ion peak. This occurs for a pulse duration of 500-1000 fs, depending on laser fluence. Received 20 October 2000     

The influence of the anion vibrational temperature on the fs dynamics in a NeNePo experiment

A negative-neutral-positive (NeNePo) femtosecond charge-reversal experiment studying the temperature-dependent relaxation dynamics of linear Ag₃ is described. A wavepacket is prepared on the potential-energy surface of the electronic ground state of the neutral trimer by photodetachment from an anion ensemble held at different, well-defined temperatures between 20 K and 350 K. The wavepacket dynamics is probed by resonant two-photon ionization. The cooled octopole ion trap developed to prepare the cold anions is described. The relaxation dynamics of the initially linear Ag₃ from a saddle point of the potential-energy surface into the triangular equilibrium configuration shows a significant dependence on the anion temperature, which is rationalized in terms of a simple model. We demonstrate that a low anion vibrational temperature results in the generation of “narrower” wavepackets on the neutral potential-energy surface. This allows us to probe coherent effects more sensitively. Received: 10 January 2000 / Published online: 24 July 2000     

Vibronic state specific predissociation rates from excited electronic states

It is shown that predissociation can be perceived as a primary process due to the continuum part of a Morse oscillator potential. In the model proposed here internal conversion to the ground state is thus not necessarily the primary process of a consecutive dissociation but may be a simultaneous decay. As a consequence, dissociation rates should show strong variations from specific (ro-) vibrational states of the first excited electronic states that are similar to those known from “pure” internal conversion rates. This behaviour is demonstrated by calculating predissociation rates for the process. Especially the out-of-plane modes seem to play an extraordinary role in the excess energy behaviour of the predissociation rate. At lower excess energies, rates from single vibronic levels with out-of-plane mode characteristics may show an increase by several orders of magnitude. Received: 13 November 1998     

Collectivity in the optical response. of small metal clusters

The question of whether the linear absorption spectra of metal clusters can be interpreted as density oscillations (collective “plasmons”) or can only be understood as transitions between distinct molecular states is still a matter of debate for clusters with only a few electrons. We calculate the photo-absorption spectra of Na₂ and Na₅ ⁺ comparing two different methods: quantum fluid dynamics and time-dependent density functional theory. The changes in the electronic structure associated with particular excitations are visualized in “snapshots” via transition densities. Our analysis shows that even for the smallest clusters, the observed excitations can be interpreted as intuitively understandable density oscillations. For Na₅ ⁺, the importance of self-interaction corrections to the adiabatic local density approximation is demonstrated. Received: 1 July 2001 / Published online: 10 October 2001     

Classical dynamical simulation of spontaneous alloying

“Spontaneous alloying” observed by Yasuda, Mori et al. for metallic small clusters is simulated using classical Hamiltonian dynamics. Very rapid alloying occurs homogeneously and cooperatively starting from the solid phase of the cluster if the heat of solution is negative and the size of cluster is less than a critical size. Analysis of 2D models reveals that the alloying rate obeys an Arrhenius-type law, which predicts the alloying time much less than second at room temperature. Evidences manifesting that the spontaneous alloying proceeds in the solid phase without melting are also presented. The simulation reproduces the essential features of the experiments. Received: 2 March 1998 / Revised: 21 May 1998 / Accepted: 28 May 1998     

Molecular picture of excited states and fragmentation paths of the Na 5 F 4 cluster

The stability against fragmentation and possible relaxation of the lowest excited states of the Na₅F₄ cluster (representative of cubic non stoechiometric clusters with an excess sodium atom, also called sodium-tail) is investigated by means of one-electron pseudopotential calculations with particular reference to photoabsorption processes from the ground state. Whereas the equilibrium configuration of the ground state has C_3v symmetry, the doubly degenerate 1²E excited state is affected by a conical intersection and a Jahn-Teller effect associated with the rotation of the sodium tail around the C₃-axis. This yields a “Mexican hat" topology for the lowest sheet with three equivalent C_s minima. Alternatively the 2²A₁ state has a minimum retaining the C_3v symmetry. The dissociation paths of the cluster along the C₃-axis into respectively Na₄F_{4 +} Na and Na₄F_{3 +} NaF are also investigated. Among the former paths, the excited states are found adiabatically stable with respect to the products. However in the A₁ symmetry, fragmentation into NaF exhibits an interesting avoided crossing between configurations correlated respectively with Na₄F₃ ^{+ +} NaF^- and Na₄F_{3 +} NaF. Such interaction, similar to the well-known charge exchange processes in elementary molecules might induce non adiabatic predissociation of the 2²A₁ state. This mechanism is invoked to explain the differences between R2PI and depletion spectra, correlated with the dissociation or relaxation of the excited states. Received 24 March 2000 and Received in final form 11 July 2000     

Mesoscopic and macroscopic dipole clusters: Structure and phase transitions

A two dimensional (2D) classical system of dipole particles confined by a quadratic potential is studied. This system can be used as a model for rare electrons in semiconductor structures near a metal electrode, indirect excitons in coupled quantum dots etc. For clusters of N ≤ 80 particles ground state configurations and appropriate eigenfrequencies and eigenvectors for the normal modes are found. Monte-Carlo and molecular dynamic methods are used to study the order-disorder transition (the “melting” of clusters). In mesoscopic clusters (N < 37) there is a hierarchy of transitions: at lower temperatures an intershell orientational disordering of pairs of shells takes place; at higher temperatures the intershell diffusion sets in and the shell structure disappears. In “macroscopic” clusters (N > 37) an orientational “melting” of only the outer shell is possible. The most stable clusters (having both maximal lowest nonzero eigenfrequencies and maximal temperatures of total melting) are those of completed crystal shells which are concentric groups of nodes of 2D hexagonal lattice with a number of nodes placed in the center of them. The picture of disordering in clusters is compared with that in an infinite 2D dipole system. The study of the radial diffusion constant, the structure factor, the local minima distribution and other quantities shows that the melting temperature is a nonmonotonic function of the number of particles in the system. The dynamical equilibrium between “solid-like” and “orientationally disordered” forms of clusters is considered.     

Nuclear dissociation after the O 1s $\rightarrow (^4\Sigma_\text{u}^-)$3sσ excitation in O$_2$ molecules

We investigate the dissociation dynamics of core-excited $\mathrm{O}_2$ molecules using a high-resolution energy-resolved electron-ion coincidence experimental setup. The excited cationic states with two valence holes and one Rydberg electron are created after spectator Auger decay induced by $\mathrm{O}$ 1s $\rightarrow (^4\Sigma_{\rm u}^-)3{\rm s}\sigma$ core excitation in $\mathrm{O}_2$. From the energy correlation between the kinetic energy of the Auger electron and the ion kinetic energy release, we distinguish several dissociation channels. Rather complex dissociation channels of the spectator Auger final states are disclosed, which can be explained by the increased number of the crossing point due to the existence of Rydberg electron. The quantum system will evolve into different dissociation limits at each crossing point between the potential energy curves.     

Structure and dynamics of cationic van-der-Waals clusters

Ar_nHCl⁺ van-der-Waals clusters for n = 1–13 are investigated with the “minimal diatomics-in-molecules (DIM) model” using ab-initio input data obtained from multi-reference configuration-interaction calculations plus subsequent projection onto valence-bond wavefunctions. The results for the complexes with n = 1–3 are checked against ab-initio calculations at the coupled-cluster (CCSD) level with the same one-electron atomic basis set as for the input data generation (aug-cc-pVTZ from Dunning). In addition to the electronic ground state, the first excited state for the triatomic complex (n = 1) is also studied. The results from the DIM model are shown to be in fair agreement with those from advanced conventional ab-initio calculations, although there are differences in detail. The comparison justifies the extension of the DIM approach to n > 3. Systematic analysis of the local minima of the multi-dimensional potential-energy surfaces (PESs), carried out with the combined method described in part I (Monte-Carlo sampling plus subsequent steepest-descent optimization), reveals simple building-up regularities for the most stable structures (i.e. those corresponding to the global PES minimum) at each n: apart from always having a nearly linear (Ar–H–Cl)⁺ fragment as core, the aggregates show little or no symmetry. Secondary local minima are also determined and their structures interpreted. The PESs for the low-lying excited states reveal a much more complicated topography compared to the Ar_nH⁺ clusters allowing a variety of photo-processes. The energy level sequence of the first five excited electronic states and the stability of the clusters in these states is studied as a function of the cluster size n.     

Vibronic charge-transfer excitons: Possible nature of the unusual properties of virtual perovskitelike ferroelectrics

A vibronic charge-transfer exciton, which is a pair of Jahn-Teller electron and hole polarons, is considered as a possible cause of the appearance of the Müller phase in the virtual ferroelectric SrTiO₃ and the “green” luminescence in the virtual ferroelectric KTaO₃. The two “green” luminescence bands can be associated with emission from two states of a typical intrinsic defect, viz., a vibronic charge-transfer exciton trapped by an oxygen vacancy and an isolated vibronic charge-transfer exciton. In both cases the “green” luminescence corresponds to the recombination of the electron and the hole in the vibronic charge-transfer exciton, which is accompanied by the emission of light. The properties of the Müller phase can be attributed to mixing of the normal state and states of the vibronic charge-transfer exciton phase when they interact with polarization in the soft SrTiO₃ matrix under the conditions of a pseudo-Jahn-Teller (pseudo-JT) effect on a soft TO mode of the displacement type. In this case the vibronic charge-transfer exciton phase forming the low-lying excited states has “order-disorder” degrees of freedom and exists at temperatures significantly below the point of the order-disorder ferroelectric transition in SrTiO₃ at T=T Q≈37 K. The corresponding lowering of the symmetry of the vibronic charge-transfer exciton phase to polar symmetry leads to the possibility of a long-period incommensurate phase in such excited states, which arises as a result of the appearance of a Lifshitz invariant. The valence-band state making the largest contribution of the pseudo-JT effect corresponds to a wave vector equal to the critical wave vector of the incommensurate vibronic charge-transfer exciton phase. When the temperature is lowered, the pseudo-JT distortion increases down to ∼T _Q and subsequently saturates in accordance with the saturation of the dielectric constant. The basic assumption in the model is that the temperature T=T _Q corresponds to the narrow temperature range for the transition from an intermediate to a strong pseudo-JT effect under the conditions for the realization of polarization tunneling states. The appearance of a significant admixture of states of the modulated ferroelectric vibronic charge-transfer exciton phase to the ground state under the conditions for the realization of polarization tunneling states at low temperatures provides an explanation for the principal properties of the Müller phase. Fiz. Tverd. Tela (St. Petersburg) 40, 907–909 (May 1998)     

Photoionization thresholds and structures of third group metals clustered with ammonia

A photoionization study of the Me(NH₃) clusters formed in the reaction of photoablated third group metal vapor with gaseous ammonia is reported. The photoionization spectra exhibit some features due to vibrational excitation of ionic clusters and to transitions to neutral Rydberg states leading to autoionization. DFT quantum chemical calculations are performed on the Me(NH₃). The cluster geometries are fully optimized imposing the C_3v symmetry. The calculated values of the IPs are in agreement with those experimentally determined. Received: 16 February 1998 / Revised and Accepted: 7 May 1998     

Experimental evidence for molecular ultrafast dissociation in O<Subscript>2</Subscript> clusters

Resonant Auger spectra of O₂ clusters excited at the O1s edge are reported. After excitation to the repulsive 1s^-13σ^* state, the resulting resonant Auger spectrum displays features that remain constant in kinetic energy as the photon energy is detuned. The shift between known atomic fragment features and these features is consistent with that observed for atoms and clusters in singly charged states in direct photoemission. These findings are strong evidence for the existence of molecular ultrafast dissociation processes within the clusters or on their surface.     

Stability and fragmentation of multiply ionized clusters

The stability of neutral, singly and multiply ionized silicon clusters, (N = 2-7, M = 0, , , ), has been investigated using an ab initio density functional method. We show that the fragmentation effect significantly affects the structure of mass-spectra of multiply ionized silicon clusters. For clusters, the clusters with a large fragmentation energy are found to correspond to the high peaks at N = 4 and 6 in mass-spectra. For clusters, a peak at N = 5 in mass-spectra has been predicted to be especially high. Received: 9 June 1997 / Revised: 8 January 1998 / Accepted: 25 February 1998     

Spectroscopic observation of helium-ion- and hydrogen-catalyzed hydrino transitions

Four predictions of Mills’ Grand Unified Theory of Classical Physics (GUTCP) regarding atomic hydrogen undergoing a catalytic reaction with certain atomized elements and ions which resonantly, nonradiatively accept integer multiples of the potential energy of atomic hydrogen, m · 27.2 eV wherein m is an integer, have been confirmed experimentally. Specifically, a catalyst comprises a chemical or physical process with an enthalpy change equal to an integer multiple m of the potential energy of atomic hydrogen, 27.2 eV. For He⁺ m = 2, due to its ionization reaction to He²⁺, and two H atoms formed from H₂ by collision with a third, hot H can also act as a catalyst with m = 2 for this third H. The product is H(1/p), fractional Rydberg states of atomic hydrogen called “hydrino atoms” wherein n = 1/2, 1/3, 1/4, …, 1/p(p≤137 is an integer) replaces the well-known parameter n = integer in the Rydberg equation for hydrogen excited states. The predictions for the hydrino reaction of (1) pumping of the catalyst excited states, (2) characteristic EUV continuum radiation, (3) fast H, and (4) hydrino products were observed in multiple catalyst-hydrogen plasma systems.