Infrared Photodissociation Spectroscopy of C4 N- , C6 N- and C8 N.

The gas-phase vibrational spectroscopy of cold C2n N- (n=2-4) anions is investigated in the CC and CN multiple bond stretching region (1700-2250 cm-1 ) by means of infrared photodissociation (IRPD) spectroscopy in a cryogenically cooled ion trap of the corresponding messenger-tagged complexes. The IRPD spectra are assigned to N-terminated linear structures with triplet ground states (3 Σ- ) based on a comparison with harmonic vibrational frequencies and intensities from density functional theory computations. In contrast to the polyacetylenic C2n+1 N- anions, the linear C-C chains investigated here exhibit cumulenic character, which is most pronounced in C4 N- and decreases with chain length. Additional intense transitions are observed for C6 N- above 3000 cm-1 and are attributed to overtone and combination bands involving the CC stretching modes, based on anharmonic computations. The influence of a D2 tag on the vibrational features of C2n N- anions is shown to be small.

Laser-induced plasma cloud interaction and ice multiplication under cirrus cloud conditions.

Potential impacts of lightning-induced plasma on cloud ice formation and precipitation have been a subject of debate for decades. Here, we report on the interaction of laser-generated plasma channels with water and ice clouds observed in a large cloud simulation chamber. Under the conditions of a typical storm cloud, in which ice and supercooled water coexist, no direct influence of the plasma channels on ice formation or precipitation processes could be detected. Under conditions typical for thin cirrus ice clouds, however, the plasma channels induced a surprisingly strong effect of ice multiplication. Within a few minutes, the laser action led to a strong enhancement of the total ice particle number density in the chamber by up to a factor of 100, even though only a 10(-9) fraction of the chamber volume was exposed to the plasma channels. The newly formed ice particles quickly reduced the water vapor pressure to ice saturation, thereby increasing the cloud optical thickness by up to three orders of magnitude. A model relying on the complete vaporization of ice particles in the laser filament and the condensation of the resulting water vapor on plasma ions reproduces our experimental findings. This surprising effect might open new perspectives for remote sensing of water vapor and ice in the upper troposphere.

Supercontinuum pulse shaping in the few-cycle regime.

The synthesis of nearly arbitrary supercontinuum pulse forms is demonstrated with sub-pulse structures that maintain a temporal resolution in the few-cycle regime. Spectral broadening of the 35 fs input pulses to supercontinuum bandwidths is attained in a controlled two-stage sequential filamentation in air at atmospheric pressure, facilitating a homogeneous power density over the full spectral envelope in the visible to near infrared spectral range. Only standard optics and a liquid crystal spatial light modulator (LC-SLM) are employed for achieving pulse compression to the sub 5 fs regime with pulse energies of up to 60 µJ and a peak power of 12 GW. This constitutes the starting point for further pulse form synthesis via phase modulation within the sampling limit of the pulse shaper. Transient grating frequency-resolved optical gating (TG-FROG) allows for the characterization of pulse forms that extend over several hundred femtoseconds with few-cycle substructures.

On the role of thermal activation in selective photochemistry: mechanistic insight into the oxidation of propene on the V4O11- cluster.

An experimental methodology for a mechanistic analysis of gas phase chemical reactions is presented in the context of structure-reactivity relationships of metal oxide clusters relevant to photocatalysis. The spectroscopic approach is demonstrated with the investigation of the photoinduced oxygenation of propene on the V(4)O(11)(-) cluster, where the thermal activation and subsequent photoreaction are deduced with the information from (i) the temperature dependency of the aggregation kinetics in the propene-seeded helium atmosphere of an ion-trap reactor; (ii) the fluence dependency in the yield of different product channels of the photoreaction and (iii) the intensity dependency in the fragmentation of neutral reaction products that are probed via in situ multi-photon ionization. For the thermal reaction, selective hydrogen abstraction from the allylic position of propene accompanied by the linkage to the cluster at the dioxo moiety is postulated as the mechanism in the aggregation of propene on the V(4)O(11)(-) cluster. In accordance with an insightful neutralization-reionization study (Schröder et al., J. Mass. Spectrom., 2010, 301, 84), the subsequent photoinduced reaction is defined by an allylic oxidation in the formation of acrolein from the initial allyloxy radical photoproduct. The relevance of the observed selectivity is discussed in view of the electronic structure and bond motifs offered by high valence oxide systems such as the V(4)O(11)(-) cluster.

Time-resolved femtosecond photoelectron spectroscopy by field-induced surface hopping.

We present the extension of our field-induced surface hopping method for the description of the photoionization process and the simulation of time-resolved photoelectron spectra (TRPES). This is based on the combination of nonadiabatic molecular dynamics "on the fly" in the framework of TDDFT generalized for open shell systems under the influence of laser fields with the approximate quantum mechanical description of the photoionization process. Since arbitrary pulse shapes can be employed, this method can be also combined with the optimal control theory in order to steer the photoionization or to shape the outgoing electronic wavepackets. We illustrate our method for the simulation of TRPES on the prototype system of Ag(3), which involves excitation from the equilibrium triangular geometry, as well as excitation from the linear transition state, where in both cases nonadiabatic relaxation takes place in a complex manifold of electronic states. Our approach represents a generally applicable method for the prediction of time-resolved photoelectron spectra and their analysis in systems with complex electronic structure as well as many nuclear degrees freedom. This theoretical development should serve to stimulate new ultrafast experiments.

PCF-based cavity enhanced spectroscopic sensors for simultaneous multicomponent trace gas analysis.

A multiwavelength, multicomponent CRDS gas sensor operating on the basis of a compact photonic crystal fibre supercontinuum light source has been constructed. It features a simple design encompassing one radiation source, one cavity and one detection unit (a spectrograph with a fitted ICCD camera) that are common for all wavelengths. Multicomponent detection capability of the device is demonstrated by simultaneous measurements of the absorption spectra of molecular oxygen (spin-forbidden b-X branch) and water vapor (polyads 4v, 4v + δ) in ambient atmospheric air. Issues related to multimodal cavity excitation, as well as to obtaining the best signal-to-noise ratio are discussed together with methods for their practical resolution based on operating the cavity in a "quasi continuum" mode and setting long camera gate widths, respectively. A comprehensive review of multiwavelength CRDS techniques is also given.

Cavity ring-down absorption spectrography based on filament-generated supercontinuum light.

We performed simultaneous, multispectral CRDS measurements that for the first time use the Supercontinuum light source. We called this approach Supercontinuum Cavity Ring-Down Spectrography (SC CRDSpectrography) and successfully applied it to measuring the absorption spectrum of NO2 gas at a concentration of 2 ppm. The extrapolated sensitivity of our setup was much greater, about 5 ppb. The ppb sensitivity level is comparable to this obtainable with single wavelength dye-lasers based CRDS systems. It is, therefore, feasible to construct extremely broadband and sensitive CRDS devices basing on the SC CRDSpectrography scheme.

Binding energies of O2 and CO to small gold, silver, and binary silver-gold cluster anions from temperature dependent reaction kinetics measurements.

A detailed analysis of experimentally obtained temperature-dependent gas-phase kinetic data for the oxygen and carbon monoxide adsorption on small anionic gold (Au(n)(-), n = 1-3), silver (Ag(n)(-), n = 1-5), and binary silver-gold (Ag(n)Au(m)(-), n + m = 2, 3) clusters is presented. The Lindemann energy transfer model in conjunction with statistical unimolecular reaction rate theory is employed to determine the bond dissociation energies E(0) of the observed metal cluster complexes with O(2) and CO. The accuracy limits of the obtained binding energies are evaluated by applying different transition-state models. The assumptions involved in the data evaluation procedure as well as possible sources of error are discussed. The thus-derived binding energies of O(2) to pure silver and binary silver-gold cluster anions are generally in excellent agreement with previously reported theoretical values. In marked contrast, the binding energies of O(2) and CO to Au(2)(-) and Au(3)(-) determined via temperature-dependent reaction kinetics are consistently lower than the theoretically predicted values.

A complete reactant-product analysis of the oxygen transfer reaction in [V4O11 x C3H6]-: a cluster complex for modeling surface activation and reactivity.

The anionic V4O11 cluster is presented as a gas-phase system of low dimensionality for modeling surface activation of molecular oxygen and the reactivity toward unsaturated hydrocarbons. Together with the charged cluster aggregates and fragments taking part in the reaction, neutral reactant and product species are monitored via multiphoton ionization for the first time within the instrumentation of tandem mass spectrometry and ion trap reactors. This novel approach allows for a comprehensive analysis of the photoinduced oxygen transfer reaction to propene within the defined aggregate complex [V4O11 x C3H6]- that simulates coadsorption and activation under fully controlled conditions.

Gas phase vibrational spectroscopy of mass-selected vanadium oxide anions.

The vibrational spectra of vanadium oxide anions ranging from V(2)O(6)(-) to V(8)O(20)(-) are studied in the region from 555 to 1670 cm(-1) by infrared multiple photon photodissociation (IRMPD) spectroscopy. The cluster structures are assigned and structural trends identified by comparison of the experimental IRMPD spectra with simulated linear IR absorption spectra derived from density functional calculations, aided by energy calculations at higher levels of theory. Overall, the IR absorption of the V(m)O(n)(-) clusters can be grouped in three spectral regions. The transitions of (i) superoxo, (ii) vanadyl and (iii) V-O-V and V-O single bond modes are found at approximately 1100 cm(-1), 1020 to 870 cm(-1), and 950 to 580 cm(-1), respectively. A structural transition from open structures, including at least one vanadium atom forming two vanadyl bonds, to caged structures, with only one vanadyl bond per vanadium atom, is observed in-between tri- and tetravanadium oxide anions. Both the closed shell (V(2)O(5))(2,3)VO(3)(-) and open shell (V(2)O(5))(2-4)(-) anions prefer cage-like structures. The (V(2)O(5))(3,4)(-) anions have symmetry-broken minimum energy structures (C(s)) connected by low-energy transition structures of C(2v) symmetry. These double well potentials for V-O-V modes lead to IR transitions substantially red-shifted from their harmonic values. For the oxygen rich clusters, the IRMPD spectra prove the presence of a superoxo group in V(2)O(7)(-), but the absence of the expected peroxo group in V(4)O(11)(-). For V(4)O(11)(-), use of a genetic algorithm was necessary for finding a non-intuitive energy minimum structure with sufficient agreement between experiment and theory.

Electric events synchronized with laser filaments in thunderclouds.

We investigated the possibility to trigger real-scale lightning using ionized filaments generated by ultrashort laser pulses in the atmosphere. Under conditions of high electric field during two thunderstorms, we observed a statistically significant number of electric events synchronized with the laser pulses, at the location of the filaments. This observation suggests that corona discharges may have been triggered by filaments.

Poor man's source for sub 7 fs: a simple route to ultrashort laser pulses and their full characterization.

A practicable and economic method for the generation and full characterization of laser pulses ranging down to sub 7 fs duration with energies spanning the full microJ domain is presented. The method utilizes a self-induced and self-guiding filamentation of titanium-sapphire based, amplified pulses in air for spectral broadening, a standard chirp mirror compression scheme and transient grating frequency resolved optical gating for determining the spectral phase over the full visible to near infrared range. In this manner, few-cycle laser pulses with a high quality in the spatial beam profile have been generated in an robust arrangement with a minimal amount of standard optical components for their full characterization. The optical scheme demonstrates an uncomplicated, versatile access to this regime of pulsed laser radiation accompanied by a comprehensive analysis.

Argon physisorption as structural probe for endohedrally doped silicon clusters.

We report on an element-dependent critical size for argon physisorption at 80 K on transition-metal-doped silicon clusters. Argon does not attach to elemental silicon clusters but only to surface-located transition-metal atoms. Thus physisorption provides structural information. Specifically, the minimal cluster size for the formation of endohedral singly metal-doped silicon cages has been determined. The observed critical size for doubly doped silicon clusters indicates that larger caged molecules can be formed, eventually leading to the growth of metal-doped silicon nanorods.

Gas-phase infrared photodissociation spectroscopy of tetravanadiumoxo and oxo-methoxo cluster anions.

The infrared spectra of the binary vanadium oxide cluster anions V(4)O(9)(-) and V(4)O(10)(-) and of the related methoxo clusters V(4)O(9)(OCH(3))(-) and V(4)O(8)(OCH(3))(2)(-) are recorded in the gas phase by photodissociation of the mass-selected ions using an infrared laser. For the oxide clusters V(4)O(9)(-) and V(4)O(10)(-), the bands of the terminal vanadyl oxygen atoms, nu(V-O(t)), and of the bridging oxygen atoms, nu(V-O(b)-V), are identified clearly. The clusters in which one or two of the oxo groups are replaced by methoxo ligands show additional absorptions which are assigned to the C-O stretch, nu(C-O). Density functional calculations are used as a complement for the experimental studies and the interpretation of the infrared spectra. The results depend in an unusual way on the functional employed (BLYP versus B3LYP), which is due to the presence of both V-O(CH(3)) single and V=O double bonds as terminal bonds and to the strong multireference character of the latter.

Isotope selective photoionization of NaK by optimal control: theory and experiment.

We present a joint theoretical and experimental study of the maximization of the isotopomer ratio (23)Na(39)K(23)Na(41)K using tailored phase-only as well as amplitude and phase modulated femtosecond laser fields obtained in the framework of optimal control theory and closed loop learning (CLL) technique. A good agreement between theoretically and experimentally optimized pulse shapes is achieved which allows to assign the optimized processes directly to the pulse shapes obtained by the experimental isotopomer selective CLL approach. By analyzing the dynamics induced by the optimized pulses we show that the mechanism involving the dephasing of the wave packets between the isotopomers (23)Na (39)K and (23)Na (41)K on the first excited state is responsible for high isotope selective ionization. Amplitude and phase modulated pulses, moreover, allow to establish the connection between the spectral components of the pulse and corresponding occupied vibronic states. It will be also shown that the leading features of the theoretically shaped pulses are independent from the initial conditions. Since the underlying processes can be assigned to the individual features of the shaped pulses, we show that optimal control can be used as a tool for analysis.

Infrared spectroscopy of hydrated sulfate dianions.

We report the first infrared spectra of multiply-charged anions in the gas phase. The spectra of SO(4) (2-)(H(2)O)(n), with n=3-24, show four main bands assigned to two vibrations of the dianionic core, the water bending mode, and solvent libration. The triply degenerate SO(4) (2-) antisymmetric stretch vibration probes the local solvent symmetry, while the solvent librational band is sensitive to the hydrogen bonding network. The spectra and accompanying electronic structure calculations indicate a highly symmetric structure for the n=6 cluster and closure of the first solvation shell at n=12.

Ultrafast dynamics in atomic clusters: analysis and control.

We present a study of dynamics and ultrafast observables in the frame of pump-probe negative-to-neutral-to-positive ion (NeNePo) spectroscopy illustrated by the examples of bimetallic trimers Ag2Au-/Ag2Au/Ag2Au+ and silver oxides Ag3O2-/Ag3O2/Ag3O2+ in the context of cluster reactivity. First principle multistate adiabatic dynamics allows us to determine time scales of different ultrafast processes and conditions under which these processes can be experimentally observed. Furthermore, we present a strategy for optimal pump-dump control in complex systems based on the ab initio Wigner distribution approach and apply it to tailor laser fields for selective control of the isomerization process in Na3F2. The shapes of pulses can be assigned to underlying processes, and therefore control can be used as a tool for analysis.

Femtosecond dynamics of proteoheparan sulfate (HS-PG) after UV excitation--a readout for arteriosclerotic nanoplaque formation?

Ultrashort UV laser pulses were used to excite tryptophan residues of heparan sulfate proteoglycan (HS-PG) in blood substitute Krebs solution. Tryptophan fluorescence is sensitive to the environment, so its shift and decay indicate the conformation and solvation state of the protein. We monitored stimulated emission and excited-state absorption by probing with delayed white-light femtosecond pulses. Comparison with bare tryptophan revealed transient absorption features which are characteristic for HS-PG. Furthermore, the effect of adding calcium salt was investigated. Differences in the spectra from solutions with and without calcium developed during several minutes, which points to changes in protein conformation, but could only be measured in the sub-ps regime. These results provide a first step to a better understanding of the molecular formation of nanoplaques in blood vessels. The goal of this work is to open a way towards biosensing of the initial stages in atherogenesis allowing for a risk assessment in cardiovascular disease.