Michal Fárník

Acidic protons before take-off: A comparative jet Fourier transform infrared study of small HCl- and HBr-solvent complexes

A comprehensive study of the proton vibrational dynamics in isolated (HX)m⋅(M)n (X=Cl, Br, and M=H2O, D2O, CH3OH) clusters is presented. Mid-IR (1900–3900 cm−1) spectra are measured in pulsed, seeded slit nozzle expansions. A close similarity among the HCl and HBr systems is revealed and exploited for cluster assignment. Distinct absorptions by small HX and mixed HX–M clusters are superimposed on broadbands, partly due to electrolytically dissociated HX in larger mixed clusters. Assignments of bands corresponding to mixed dimers and trimers based on their X–H and O–H stretching fundamentals are provided. The broad Cl–H stretching band profile of the 1:2 complex is indicative of sub-picosecond intracluster vibrational energy redistribution. The assignments are compared to matrix isolation experiments, when available. Systematic quantum chemistry calculations are performed for HCl–water complexes with up to three molecules and their isotopomers. Hydrogen bond induced frequency shifts of Cl–H stretching vibr...

Dynamics of chemical reactions of doubly-charged ions. CF2D+ formation in collisions of CF22+ and D2

Abstract The dynamics of the formation of the chemical rearrangement product CF 2 D + and the charge transfer product CF 2 + in collisions of the dication CF 2 2+ with D 2 was investigated in a crossed-beam scattering experiment at a collision energy of 0.6 eV (c.m.). The scattering diagrams obtained show that both products are formed in impulsive processes in which Coulomb repulsion between two singly charged products, CF 2 D + + D + and CF 2 + + D 2 + , respectively, plays a dominant role. The energy partitioning of the total energy available in these highly exoergic processes is about half in the relative translation and about half in the internal energy (electronic and/or vibrational).

Ion-molecule reactions in 4He droplets: Flying nano-cryo-reactors

Ion-molecule reactions are studied inside large (approximately equal to 10(4) atoms) very cold (0.37 K) superfluid (4)He droplets by mass spectrometric detection of the product ions. He+ ions initially formed inside the droplets by electron impact ionization undergo charge transfer with either embedded D(2), N(2), or CH(4). For D(2) this charge transfer process was studied in detail by varying the pickup pressure. For either N(2) or CH(4) the reagent ions were formed by this charge transfer and the reaction pathways of the secondary reactions N(2) (+)+D(2), CH(4) (+)+D(2), and CH(3) (+)+D(2) each with an additionally embedded D(2) molecule were also determined from the pickup pressure dependencies. In several cases, notably He.N(2) (+) and CH(3)D(2) (+) reaction intermediates are observed. The analysis is facilitated by the tendency for molecular ion products to appear without (or with only very few) attached He atoms whereas the atomic ion products usually appear in the mass spectra with several attached He atoms, e.g., He(m).D+ ions with up to m=19.

Dynamics of chemical and charge transfer reactions of molecular dications: beam scattering and total cross section data on CF2D+ (CF2H+), CF2+, and CF+ formations in CF22+ + D2(H2) collisions

Abstract Dynamics of formation of the chemical rearrangement product CF 2 D + , the charge transfer product CF 2 + , and the dissociative products CF + and CFD + in collisions of the molecular dication CF 2 ++ with D 2 was investigated in crossed beam scattering experiments over the collision energy range 0.3–1.0 eV (center of mass). The scattering data show that coulomb repulsion between two singly charged products, CF 2 + + D + and CF 2 + + D 2 + , plays a dominant role in the nondissociative processes. A large fraction of the energy available (about 6 eV in the chemical reaction, about 4 eV in the charge transfer) goes into relative translational energy of the products. Relative total cross sections for formation of the nondissociative and dissociative products in collision of CF 2 ++ with D 2 and H 2 were determined over the collision energy range of 0.2–3.6 eV. The shape of the relative velocity dependence of the cross section for CF 2 + formation can be described by a simple model based on the Landau-Zener formalism. The data suggest that the dissociative product CF + is formed prevailingly in a subsequent dissociation of the charge transfer product CF 2 + . A potential surface model is described which accounts for competition of various processes in dication–neutral collisions.

Generation and orientation of organoxenon molecule H-Xe-CCH in the gas phase

We report on the first observation of the organoxenon HXeCCH molecule in the gas phase. This molecule has been prepared in a molecular beam experiment by 193 nm photolysis of an acetylene molecule on Xe(n) clusters (n approximately 390). Subsequently the molecule has been oriented via the pseudo-first-order Stark effect in a strong electric field of the polarized laser light combined with the weak electrostatic field in the extraction region of a time-of-flight spectrometer. The experimental evidence for the oriented molecule has been provided by measurements of its photodissociation. For comparison, photolysis of C(2)H(2) on Ar(n) clusters (n approximately 280) has been measured. Here the analogous rare gas molecule HArCCH could not be generated. The interpretation of our experimental findings has been supported by ab initio calculations. In addition, the experiment together with the calculations reveals information on the photochemistry of the HXeCCH molecule. The 193 nm radiation excites the molecule predominantly into the 2 (1)Sigma(+) state, which cannot dissociate the Xe-H bond directly, but the system evolves along the Xe-C coordinate to a conical intersection of a slightly nonlinear configuration with the dissociative 1 (1)Pi state, which then dissociates the Xe-H bond.

Experimental and theoretical study of the pyrrole cluster photochemistry: Closing the πσ* dissociation pathway by complexation

Photolysis of size selected pyrrole clusters has been investigated and compared to the photolysis of an isolated pyrrole molecule. Experimentally, size distributions of different mean cluster sizes (n¯=3 and n¯⪢5) have been prepared in supersonic expansions and the clusters were photolyzed at 243 and 193nm. The kinetic energy distributions of the H photofragments have been measured. The distributions exhibit a bimodal character with fast and slow H-fragment peaks similar to the spectra of the bare molecule. However, with increasing cluster size the slow component gains intensity with respect to the fast one. A similar effect is observed with increasing the excitation energy from 243to193nm. Theoretical calculations at the CASSCF/CASPT2 level have been performed for bare and complexed pyrroles (pyrrole is complexed with an argon atom and with another pyrrole unit). Combination of theoretical and experimental approaches leads to the conclusion that the direct dissociative pathway along the πσ* potential ene...

Oriented xenon hydride molecules in the gas phase

The production of the xenon hydride molecules HXeX with X = I and Cl in the gas phase is reviewed. These molecules are generated by the photolysis of the hydrogen halide HI and HCl molecules on the surface of large xenon Xe n clusters. Molecular dynamics simulations show that the flexible H atoms react with the heavy XeX moiety and form the desired molecules with nearly no rotational motion. They are observed by photodissociation with subsequent detection of the kinetic energy of the H atom fragment. During the generating process, the cluster starts to evaporate and the hydride molecule is left essentially free. For further discrimination against the H atom fragments from HX, the HXeX molecules are oriented in a combined pulsed laser field and a weak electrostatic field. The three topics which represent the background of our experiments are briefly reviewed: the nature and generation of rare gas hydrides, the alignment and orientation of molecules in electric fields, and the photodissociation of selected molecules in rare gas clusters. The conditions for detecting them in the gas phase are discussed. This is the trade off between the stability, which requires high electron affinity, and the conditions for orientation, which necessitate large polarizability anisotropies and dipole moments. Finally the prospects of detecting other classes of molecules are discussed. Contents page 1. Introduction 584 2. Background information 586  2.1. Rare gas hydride compounds 586  2.2. Orientation of molecules 588  2.3. Photodissociation of molecules in clusters 591 3. Experimental 594 4. Formation and orientation of the molecules 597 5. Results 599  5.1. Results for HXel 604  5.2. Results for HXeCl 604  5.3. Other molecules 607 6. Summary and prospects 609 Acknowledgments 610 References 611

Uptake of atmospheric molecules by ice nanoparticles: Pickup cross sections

Uptake of several atmospheric molecules on free ice nanoparticles was investigated. Typical examples were chosen: water, methane, NO(x) species (NO, NO(2)), hydrogen halides (HCl, HBr), and volatile organic compounds (CH(3)OH, CH(3)CH(2)OH). The cross sections for pickup of these molecules on ice nanoparticles (H(2)O)(N) with the mean size of N≈260 (diameter ~2.3 nm) were measured in a molecular beam experiment. These cross sections were determined from the cluster beam velocity decrease due to the momentum transfer during the pickup process. For water molecules molecular dynamics simulations were performed to learn the details of the pickup process. The experimental results for water are in good agreement with the simulations. The pickup cross sections of ice particles of several nanometers in diameter can be more than 3 times larger than the geometrical cross sections of these particles. This can have significant consequences in modelling of atmospheric ice nanoparticles, e.g., their growth.

Photodissociation of hydrogen halide molecules on free ice nanoparticles.

Photodissociation of water clusters doped with HX(X=Br,Cl), molecules has been studied in a molecular beam experiment. The HX(H2O)n clusters are dissociated with 193 nm laser pulses, and the H fragments are ionized at 243.07 nm and their time-of-flight distributions are measured. Experiments with deuterated species DBr(H2O)n and HBr(D2O)n suggest that the photodissociation signal originates from the presence of the HX molecule on the water cluster, but does not come directly from a photolysis of the HX molecule. The H fragment is proposed to originate from the hydronium molecule H3O. Possible mechanisms of the H3O production are discussed. Experimental evidence suggests that acidic dissociation takes place in the cluster, but the H3O+ ion remains rather immobile.

Emergence of Charge-Transfer-to-Solvent Band in the Absorption Spectra of Hydrogen Halides on Ice Nanoparticles : Spectroscopic Evidence for Acidic Dissociation

Extensive ab initio calculations complemented by a photodissociation experiment at 193 nm elucidate the nature of hydrogen halide molecules bound on free ice nanoparticles. Electronic absorption spectra of small water clusters (up to 5 water molecules) and water clusters doped with hydrogen fluoride, hydrogen chloride and hydrogen bromide were calculated. The spectra were modeled at the time-dependent density functional (TDDFT) level of theory with the BHandHLYP functional using the reflection principle. We observe the emergence of a charge-transfer-to-solvent (CTTS) band in the absorption spectra upon the acidic dissociation of the hydrogen halides. The CTTS band provides a spectroscopically observable feature for the acidic dissociation. The calculated spectra were compared with our new experimental photodissociation data for larger water clusters doped with HCl and HBr. We conclude that HCl and HBr dissociate to a large extent on the surface of ice nanoparticles at temperatures near 120 K and photoactive products are formed. The acidic dissociation of HX leads to an enhancement by about 4 orders of magnitude of the HCl photolysis rate in the 200-300 nm region, which is potentially relevant for the halogen budget in the atmosphere.