Ultrafast fragmentation and ionisation dynamics of ammonia clusters

The formation of protonated and unprotonated ammonia cluster ions is studied by femtosecond two colour two photon pump-probe techniques applied to (NH₃) _n and (ND₃) _n clusters withn up to 8. The fourth harmonic (～ 200 nm, 6.2 eV, 160 fs) of a Ti: Sapphire laser pulse is used to excite the clusters in a state corresponding to theà state of NH₃ while the third harmonic (267 nm, 4.65 eV) is used for the subsequent ionisation step. Employing a combination of the optical Bloch equations for the excitation process and rate equations for the cluster dynamics we calibrate the zero time delay and carefully analyse the time dependence of the pump-probe signal. Several distinct intermediate steps in the time evolution can be distinguished, having characteristic time constants ranging from 40 fs to over 100 ps. They are discussed in a consistent scheme for the excitation, ionisation and protonation dynamics, accounting also for characteristic differences observed between deuterated and undeuterated species. A particularly remarkable time dependence of the homogeneous (NH₃) ₂ ⁺ cluster ion signal is interpreted as a fingerprint of internally protonated neutral precursors of the type NH₃NH₂NH₄.     

Ultrafast dynamics of pyrrolidinium cation ionic liquids

We have investigated the ultrafast molecular dynamics of five pyrrolidinium cation room temperature ionic liquids using femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The ionic liquids studied are N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P14+/NTf2-), N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P1EOE+/NTf2-), N-ethoxyethyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide P1EOE+/NTf2-), N-ethoxyethyl-N-methylpyrrolidinium bromide P1EOE+, and N-ethoxyethyl-N-methylpyrrolidinium dicyanoamide P1EOE+/DCA-). For comparing dynamics among the five ionic liquids, we categorize the ionic liquids into two groups. One group of liquids comprises the three pyrrolidinium cations P14+, P1EOM+, and P1EOE+ paired with the NTf2- anion. The other group of liquids consists of the P1EOE+ cation paired with each of the three anions NTf2-, Br-, and DCA-. The overdamped relaxation for time scales longer than 2 ps has been fit by a triexponential function for each of the five pyrrolidinium ionic liquids. The fast ( approximately 2 ps) and intermediate (approximately 20 ps) relaxation time constants vary little among these five ionic liquids. However, the slow relaxation time constant correlates with the viscosity. Thus, the Kerr spectra in the range from 0 to 750 cm(-1) are quite similar for the group of three pyrrolidinium ionic liquids paired with the NTf2- anion. The intermolecular vibrational line shapes between 0 and 150 cm(-1) are fit to a multimode Brownian oscillator model; adequate fits required at least three modes to be included in the line shape.     

Isomerization dynamics and thermodynamics of ionic argon clusters

The dynamics and thermodynamics of small Ar(n) (+) clusters, n=3, 6, and 9, are investigated using molecular dynamics (MD) and exchange Monte Carlo (MC) simulations. A diatomic-in-molecule Hamiltonian provides an accurate model for the electronic ground state potential energy surface. The microcanonical caloric curves calculated from MD and MC methods are shown to agree with each other, provided that the rigorous conservation of angular momentum is accounted for in the phase space density of the MC simulations. The previously proposed projective partition of the kinetic energy is used to assist MD simulations in interpreting the cluster dynamics in terms of inertial, internal, and external modes. The thermal behavior is correlated with the nature of the charged core in the cluster by computing a dedicated charge localization order parameter. We also perform systematic quenches to establish a connection with the various isomers. We find that the Ar(3) (+) cluster is very stable in its linear ground state geometry up to about 300 K, and then isomerizes to a T-shaped isomer in which a quasineutral atom lies around a charged dimer. In Ar(6) (+) and Ar(9) (+), the covalent trimer core is solvated by neutral atoms, and the weakly bound solvent shell melts at much lower energies, occasionally leading to a tetramer or pentamer core with weakly charged extremities. At high energies the core itself becomes metastable and the cluster transforms into Ar(2) (+) solvated by a fluid of neutral argon atoms.     

Ultrafast photo-excitation dynamics in isolated, neutral water clusters

Using the efficient nonlinear conversion scheme which was recently developed in our group [M. Beutler, M. Ghotbi, F. Noack, and I. V. Hertel, Opt. Lett. 134, 1491 (2010); M. Ghotbi, M. Beutler, and F. Noack, ibid 35, 3492 (2010)] to provide intense sub-50 fs vacuum ultraviolet laser pulses we have performed the first real time study of ultrafast, photo-induced dynamics in the electronically excited A?-state of water clusters (H(2)O)(n) and (D(2)O)(n) , n=2-10. Three relevant time scales, 1.8-2.5, 10-30, and 50-150 fs, can be distinguished which-guided by the available theoretical results-are attributed to H (D)-ejection, OH (OD) dissociation, and a nonadiabatic transition through a conical intersection, respectively. While a direct quantitative comparison is only very preliminary, the present results provide a crucial test for future modeling of excited state dynamics in water clusters, and should help to unravel some of the many still unresolved puzzles about water.     

Fragmentation dynamics of size-selected pyrrole clusters prepared by electron impact ionization: forming a solvated dimer ion core

Structure and dynamics of size-selected charged pyrrole clusters have been studied by means of molecular beam scattering experiments and ab initio calculations. Small neutral Pyn clusters were produced in Py/He mixture expansions, and the scattering experiment with a secondary beam of He-atoms was exploited to select the neutral clusters of different sizes. The complete size-selected fragmentation patterns for the neutral dimer to the tetramer after an electron impact ionization at 70 eV were obtained from the measurements of the angular and velocity distributions at different fragment masses. All the investigated cluster sizes decay mainly to the monomer ions Py+1 (from 60 to 80% of the corresponding neutral size) and to the dimer ion Py+2 (20-30%). The trimer ions Py+3 are generated to less than 10% from the neutral trimer and tetramer. To explain the observed results, we have calculated the structures and energetics of pyrrole clusters up to the trimer for the neutral and the ionic state using DFT and PMP2 methods. The ab initio calculations show that ionized pyrrole clusters are formed with a dimeric core that is solvated by neutral pyrrole molecules. In addition, the ground and ionic state of Py-Ar complexes were calculated at CCSD(T) level with extended basis in relevance to the mixed clusters produced in supersonic expansions of Py seeded in Ar. The calculated dissociation energies of the Py-Ar and (Py-Ar)+ complexes indicate that Ar atoms are able to rapidly evaporate after ionization. The combined analysis of the fragmentation probabilities, and calculations allowed us to estimate the distribution of energy deposited in the clusters after the electron impact, which peaks above 1 eV and has a tail up to 5 eV.     

Ultrafast dynamics in ammonia clusters: analysis of protonated and unprotonated cluster ion signals

The dynamics of ammonia clusters excited to the à state with 160 fs laser pulses of 6.2 eV was studied by pump-probe experiments with a low probe photon energy of 3.1 eV. Protonated as well as unprotonated cluster ion signals have been observed. The time evolution of both species is characteristic of the intermediate rearrangement and fragmentation processes. The observations strongly support a previously developed kinetic model for this dynamics with the signal at long delay times>6 ps reflecting the species involved in the absorption dissociation ionization (ADI) mechanism. Strong evidence is found for the formation of an internally ‘quasi protonated’ excited state and of ammoniated NH₄ radicals.     

Ultrafast solvation dynamics of coumarin 153 in imidazolium-based ionic liquids

The dynamic Stokes shift of coumarin 153 has been measured in two room-temperature ionic liquids, 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-propyl-3-methylimidazolium tetrafluoroborate, using the fluorescence up-conversion technique with a 230 fs instrumental response function. A component of about 10-15% of the total solvation shift is found to take place on an ultrafast time scale < 10 ps. The amplitude of this component is substantially less than assumed previously by other authors. The origin of the difference in findings could be partly due to chromophore-internal conformational changes on the ultrafast time scale, superimposed to solvation-relaxation, or due to conformational changes of the chromophore ground state in polar and apolar environments. First three-pulse photon-echo peak-shift experiments on indocyanine green in room-temperature ionic liquids and in ethanol indicate a difference in the inertial component of the early solvent relaxation of <100 fs.     

Ultrafast dynamics in noble metal clusters: The role of internal vibrational redistribution

We present a theoretical study of the ultrafast dynamics in noble metal clusters interacting with molecular oxygen which is of fundamental importance for the understanding and design of cluster-based heterogenous nanocatalysts. We demonstrate that intrinsic dynamical properties can significantly promote the reactivity of small noble metal clusters towards O₂. This concept is illustrated by performing collision simulations between and clusters and O₂ in the framework of the ab initio molecular dynamics (MD) using density functional theory (DFT). We show that different nature and efficiency of the internal vibrational energy redistribution (IVR) during the collisions with O₂ are responsible for considerably different sticking probabilities of O₂ to silver and gold clusters, respectively. In the case of , resonant IVR between the cluster and the O₂ subunit activates the O–O bond and promotes the subsequent oxidation reaction. In contrast, in the case of fast dissipative IVR on the time scale of 1 ps leads to a higher sticking probability for O₂ but the O–O bond is very rapidly deactivated and cannot participate in further oxidation processes. These findings allow us to introduce the nature of IVR as a criterion for promoting the reactivity of noble metal clusters. Such different behaviour of silver and gold clusters colliding with O₂ originates from difference in relativistic effects which are considerably more pronounced in the case of gold clusters causing more directional rigid bonding in contrast to silver clusters with more s-metallic floppy character. Moreover, we demonstrate that breaking of O–O bond can be induced in by a selective excitation of the O–O bond with an ultrashort pulse in the infrared spectral range. This opens the perspective to control the action of nanocatalysts by employing shaped laser pulses and thus bridges the fields of femtochemistry and cluster nanocatalysis.     

Rotational dynamics of CO solvated in small He clusters: a quantum Monte Carlo study

The rotational dynamics of CO single molecules solvated in small He clusters (CO @ HeN) has been studied using reptation quantum Monte Carlo simulations for cluster sizes up to N = 30. Our results are in good agreement with the rotovibrational features of the infrared spectrum recently determined for this system and provide a deep insight into the relation between the structure of the cluster and its dynamics. Simulations for large N also provide a prediction of the effective moment of inertia of CO in the He nanodroplet regime, which has not been measured so far.     

Ultrafast dynamics of room temperature ionic liquids after ultraviolet femtosecond excitation

The photochemistry and relaxation dynamics of four room-temperature ionic liquids (RTILs) after ultraviolet (UV) photolysis were investigated by femtosecond pump-probe absorption spectroscopy. A pulse duration-limited rise of the induced absorption in halide-containing RTILs at various probe wavelengths was attributed to the generation of solvated electrons. With continuous irradiation (static conditions), di- and trihalide ion formation became apparent especially below 1000 nm. The formation of trihalide ions was further confirmed by steady-state UV absorption spectroscopy. All RTILs showed a rich photochemistry after UV photolysis leading to the build-up of various long-lived intermediate products as evidenced from the observation that ionic liquids turn yellow upon continuous irradiation. On the other hand, exposing RTILs to the excitation pulse for a short time (rapid-scan method) significantly suppressed the formation of halides. The results suggest that the development of flow-cell systems for highly viscous ionic liquids is urgently needed to quantitatively investigate their ultrafast dynamics.     

A molecular dynamics investigation of rare-gas solvated cation-benzene clusters using a new model potential

The main static and dynamic properties of some ionic heteroclusters, involving K+, C6H6, and Ar, have been investigated. A new representation of the intermolecular potential energy, which takes into account both electrostatic and non-electrostatic contributions to the overall noncovalent interaction, was used. Dynamical calculations were performed for a microcanonical ensemble. Particular attention was paid to the opening of the isomerization and dissociation processes for K+-C6H6-Ar(n) and to the formation of some of its fragments at increasing temperatures of the cluster considered.     

A microscopic view of substitution reactions solvated by ionic liquids

The solvation effect of the ionic liquid 1-N-butyl-3-methylimidazolium hexafluorophosphate on nucleophilic substitution reactions of halides toward the aliphatic carbon of methyl p-nitrobenzenesulfonate (pNBS) was investigated by computer simulations. The calculations were performed by using a hybrid quantum-mechanical/molecular-mechanical (QM/MM) methodology. A semiempirical Hamiltonian was first parametrized on the basis of comparison with ab initio calculations for Cl(-) and Br(-) reaction with pNBS at gas phase. In condensed phase, free energy profiles were obtained for both reactions. The calculated reaction barriers are in agreement with experiment. The structure of species solvated by the ionic liquid was followed along the reaction progress from the reagents, through the transition state, to the final products. The simulations indicate that this substitution reaction in the ionic liquid is slower than in nonpolar molecular solvents proper to significant stabilization of the halide anion by the ionic liquid in comparison with the transition state with delocalized charge. Solute-solvent interactions in the first solvation shell contain several hydrogen bonds that are formed or broken in response to charge density variation along the reaction coordinate. The detailed structural analysis can be used to rationalize the design of new ionic liquids with tailored solvation properties.     

Protonated adenine: Tautomers,solvated clusters,and dissociation mechanisms

Ab initio and density functional theory calculations at the B3-MP2 and CCSD(T)/6-311 + G(3df,2p) levels of theory are reported that address the protonation of adenine in the gas phase, water clusters, and bulk aqueous solution. The calculations point to N-1-protonated adenine (1+) as the thermodynamically most stable cationic tautomer in the gas phase, water clusters, and bulk solution. This strongly indicates that electrospray ionization of adenine solutions produces tautomer 1+ with a specificity calculated as 97-90% in the 298-473 K temperature range. The mechanisms for elimination of hydrogen atoms and ammonia from 1+ have also been studied computationally. Ion 1+ is calculated to undergo fast migrations of protons among positions N-1, C-2, N-3, N-10, N-7, and C-8 that result in an exchange of five hydrogens before loss of a hydrogen atom forming adenine cation radical at 415 kJ mol(-1) dissociation threshold energy. The elimination of ammonia is found to be substantially endothermic requiring 376-380 kJ mol(-1) at the dissociation threshold and depending on the dissociation pathway. The overall dissociation is slowed by the involvement of ion-molecule complexes along the dissociation pathways. The competing isomerization of 1+ proceeds by a sequence of ring opening, internal rotations, imine flipping, ring closures, and proton migrations to effectively exchange the N-1 and N-10 atoms in 1+, so that either can be eliminated as ammonia. This mechanism explains the previous N-1/N-10 exchange upon collision-induced dissociation of protonated adenine.     

Ultrafast solvation dynamics in room temperature ionic liquids observed by three-pulse photon echo peak shift measurements

Three-pulse photon echo peak shift (3PEPS) measurement was applied to the investigation of the primary part (<100 ps) of the solvation dynamics in a series of imidazolium ionic liquids (IL) with an organic dye, oxazine 4 (Ox4), utilized as a probe. The ultrafast solvent response in the range of ≤300 fs exhibited dependence on the square root of the anion mass, indicating its relation with the inertial motion of anion. The inertial response of ILs with chloride anion was the fastest among other ILs with heavier and larger anions. Because Ox4 is a cationic dye, it holds a stronger interaction with the anion of IL, thus the ultrafast part of the solvation is strongly affected by the inertial motion of anions. The second solvation component in the range of ≤3.5 ps had better correlation with the reduced mass and the size of both ions included, indicating the beginning of a more global solvation process.     

Mechanism and dynamics of azobenzene photoisomerization

The excited-state dynamics of trans-azobenzene were investigated by femtosecond time-resolved photoelectron spectroscopy and ab initio molecular dynamics. Two near-degenerate pipi* excited states, S2 and S3,4, were identified in a region hitherto associated with only one excited state. These results help to explain contradictory reports about the photoisomerization mechanism and the wavelength dependence of the quantum yield. A new model for the isomerization mechanism is proposed.     

First observation of clusters for solvated tropylium ions

We have first observed clusters for solvated tropylium ions (Tr+(ROH)n) which were isolated from ROH-CH3CN (1:1 by vol.; R = Me, Et, and Prn) solutions by using a specially designed mass spectrometer and found the clear-cut essential features concerning the solvation structure around Tr+.     

Photoelectron spectroscopy of negative ions solvated in clusters

We have measured the photoelectron spectra of Cl^?, Br^? and I^? complexed with water molecule and of I^?·(CO₂) _n clusters wheren=1?7. The significance of these measurements to the understanding of solvation phenomena is discussed.     

Photo-detrapping of solvated electrons in an ionic liquid

We studied the dynamics of photo-detrapped solvated electrons in the ionic liquid trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI) using laser flash photolysis. The solvated electrons were produced by the electron photodetachment from iodide via a 248 nm KrF excimer laser. The solvated electron decayed by first-order kinetics with a lifetime of about 240 ns. The spectrum of the solvated electron in the ionic liquid TMPA-TFSI is very broad with a peak around 1100 nm. After the 248 nm pulse, a 532 nm pulse was used to subsequently detrap the solvated electrons. After the detrapping pulse, quasi-permanent bleaching was observed. The relative magnitude of the bleaching in the solvated electron absorbance was measured from 500 to 1000 nm. The amount of bleaching depends on the probe wavelength. The fraction of bleached absorbance was larger at 500 nm than that at 1000 nm, suggesting that there are at least two species that absorb 532 nm light. We discuss the present results from viewpoint of the heterogeneity of ionic liquids.     

Comparison of dansylated aminopropyl controlled pore glass solvated by molecular and ionic liquids

We compare how (i) four ionic liquids (ILs) (1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][Tf2N]), 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C4mpy][Tf2N]), and trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide ([P(C6)3C14][Tf2N])) and (ii) two conventional molecular liquids (methanol and 1-octanol) solvate/wet luminescent organic moieties that are covalently attached to the surface of silica controlled pore glass (CPG). A series of aminopropyl CPG particles that have been covalently tagged with the solvatochromic fluorescent probe group dansyl were used in this study. The results demonstrate that ILs solvate/wet the silica surface differently in comparison to molecular liquids (MLs). Specifically, when comparing ILs and MLs that appear to solvate the free probe, dansylpropylsulfonamide (DPSA), equally in solution, we find that ILs do not solvate/wet the silica surfaces as well as the corresponding MLs. The cation component in these ILs is the significant factor in how the ILs solvate/wet silica surfaces. Solvation/wetting of surface-bound species at a silica surface depends on the cation size. Chlorosilane end-capping of the surface silanol and amine residues attenuates the cation's affects.     

Characterization of dynamics and reactivities of solvated ions by ab initio simulations

Based on a systematic investigation of trajectories of ab initio quantum mechanical/molecular mechanical simulations of numerous cations in water a standardized procedure for the evaluation of mean ligand residence times is proposed. For the characterization of reactivity and structure-breaking/structure-forming properties of the ions a measure is derived from the mean residence times calculated with different time limits. It is shown that ab initio simulations can provide much insight into ultrafast dynamics that are presently not easily accessible by experiment.