Study of processes in afterglow plasma. Recombination of D3+ ions with electrons.

For this thesis, recombination of molecular ion D3+ was studied. This ion was chosen for the consideration to obtain a clearer understanding of the complex H3+ and D3+ phenomenon. The H3+ and D3+ ions were the subjects of a number of studies because of their important roles in the media kinetics of astrophysical and laboratory interest. However, despite enormous efforts the results of studies determining the rate of recombination of H3+ and D3+ ions with electrons give values that vary by at least one order of magnitude. We realized limitations and advantages of methods used in previous studies and built a new afterglow experiment - Advanced Integrated Stationary Afterglow (AISA). Our new apparatus has a large discharge vessel in order to reduce diffusion losses. Comparatively large flow of very pure He was used to dilute flow of impurities and so to suppress reactive losses. In this sense AISA utilises advantages of both the stationary and the flowing afterglow techniques. In this thesis description of the apparatus, used diagnostics and data analysis are given and additional attention was paid to ion-molecular reactions, diffusion losses and to the influence of recombining impurity ions in the plasma.Available from STL Prague, CZ / NTK - National Technical LibrarySIGLECZCzech Republi

Atmospheric processes on ice nanoparticles in molecular beams

This review summarizes some recent experiments with ice nanoparticles (large water clusters) in molecular beams and outlines their atmospheric relevance: (1) Investigation of mixed water–nitric acid particles by means of the electron ionization and sodium doping combined with photoionization revealed the prominent role of HNO3 molecule as the condensation nuclei. (2) The uptake of atmospheric molecules by water ice nanoparticles has been studied, and the pickup cross sections for some molecules exceed significantly the geometrical sizes of the ice nanoparticles. (3) Photodissociation of hydrogen halides on water ice particles has been shown to proceed via excitation of acidically dissociated ion pair and subsequent biradical generation and H3O dissociation. The photodissociation of CF2Cl2 molecule in clusters is also mentioned. Possible atmospheric consequences of all these results are briefly discussed

Vibrationally mediated photodissociation dynamics of pyrrole

We investigate photodissociation of vibrationally excited pyrrole molecules in a velocity map imaging experiment with IR excitation of N–H bond stretching vibration v1 = 1, νIR= 3532 cm−1, and UV photodissociation at λUV= 243 nm. In the IR+UV experiment, the H-fragment signal is enhanced with respect to the 243 nm UV-only photodissociation due to a more favorable Franck-Condon factor for the vibrationally excited molecule. In the measured H-fragment kinetic energy distribution, the maximum of the fast peak in the IR+UV experiment is shifted by 0.23 eV compared to the UV-only photodissociation which corresponds to 53 % of the vibrational energy deposited into the fragment kinetic energy. We compare our results with an isoenergetic UV-only photodissociation at λUV= 224 nm. About 72 % of the total available energy, is released into the fragment kinetic energy in the IR+UV experiment, while it is only 61 % in the UV-only photodissociation. This can be substantiated by the coupling of the N–H bond stretching vibration into the kinetic energy of the departing H-fragment. We also probe the time-dependent dynamics by a nanosecond pump-probe experiment. The IR excitation enhances the N–H bond dissociation even when the UV photodissociation is delayed by 150 ns. This enhancement increases also the yield of the fast fragments at the position of the peak corresponding to the IR+UV excitation, i.e. even 150 ns after the IR vibrational excitation, the same amount of the IR excitation energy can be converted into the H-fragment velocity as immediately after the excitation

Clustering of Uracil Molecules on Ice Nanoparticles

We generate a molecular beam of ice nanoparticles (H₂O)_<i>N</i>, <i>N̅</i> ≈ 130–220, which picks up several individual gas phase uracil (U) or 5-bromouracil (BrU) molecules. The mass spectra of the doped nanoparticles prove that the uracil and bromouracil molecules coagulate to clusters on the ice nanoparticles. Calculations of U and BrU monomers and dimers on the ice nanoparticles provide theoretical support for the cluster formation. The (U)_<i>m</i>H⁺ and (BrU)_<i>m</i>H⁺ intensity dependencies on <i>m</i> extracted from the mass spectra suggest a smaller tendency of BrU to coagulate compared to U, which is substantiated by a lower mobility of bromouracil on the ice surface. The hydrated U_<i>m</i>·(H₂O)_<i>n</i>H⁺ series are also reported and discussed. On the basis of comparison with the previous experiments, we suggest that the observed propensity for aggregation on ice nanoparticles is a more general trend for biomolecules forming strong hydrogen bonds. This, together with their mobility, leads to their coagulation on ice nanoparticles which is an important aspect for astrochemistry

Lack of Aggregation of Molecules on Ice Nanoparticles

Multiple molecules adsorbed on the surface of nanosized ice particles can either remain isolated or form aggregates, depending on their mobility. Such (non)­aggregation may subsequently drive the outcome of chemical reactions that play an important role in atmospheric chemistry or astrochemistry. We present a molecular beam experiment in which the controlled number of guest molecules is deposited on the water and argon nanoparticles in a pickup chamber and their aggregation is studied mass spectrometrically. The studied molecules (HCl, CH₃Cl, CH₃CH₂CH₂Cl, C₆H₅Cl, CH₄, and C₆H₆) form large aggregates on argon nanoparticles. On the other hand, no aggregation is observed on ice nanoparticles. Molecular simulations confirm the experimental results; they reveal a high degree of aggregation on the argon nanoparticles and show that the molecules remain mostly isolated on the water ice surface. This finding will influence the efficiency of ice grain-mediated synthesis (e.g., in outer space) and is also important for the cluster science community because it shows some limitations of pickup experiments on water clusters

Clustering and Photochemistry of Freon CF₂Cl₂ on Argon and Ice Nanoparticles

The photochemistry of CF₂Cl₂ molecules deposited on argon and ice nanoparticles was investigated. The clusters were characterized via electron ionization mass spectrometry, and the photochemistry was revealed by the Cl fragment velocity map imaging after the CF₂Cl₂ photodissociation at 193 nm. The complex molecular beam experiment was complemented by ab initio calculations. The (CF₂Cl₂)_<i>n</i> clusters were generated in a coexpansion with Ar buffer gas. The photodissociation of molecules in the (CF₂Cl₂)_<i>n</i> clusters yields predominantly Cl fragments with zero kinetic energy: caging. The CF₂Cl₂ molecules deposited on large argon clusters in a pickup experiment are highly mobile and coagulate to form the (CF₂Cl₂)_<i>n</i> clusters on Ar_<i>N</i>. The photodissociation of the CF₂Cl₂ molecules and clusters on Ar_<i>N</i> leads to the caging of the Cl fragment. On the other hand, the CF₂Cl₂ molecules adsorbed on the (H₂O)_<i>N</i> ice nanoparticles do not form clusters, and no Cl fragments are observed from their photodissociation. Since the CF₂Cl₂ molecule was clearly adsorbed on (H₂O)_<i>N</i>, the missing Cl signal is interpreted in terms of surface orientation, possibly via the so-called halogen bond and/or embedding of the CF₂Cl₂ molecule on the disordered surface of the ice nanoparticles

Nucleation of Mixed Nitric Acid–Water Ice Nanoparticles in Molecular Beams that Starts with a HNO₃ Molecule

Mixed (HNO₃)_<i>m</i>(H₂O)_<i>n</i> clusters generated in supersonic expansion of nitric acid vapor are investigated in two different experiments, (1) time-of-flight mass spectrometry after electron ionization and (2) Na doping and photoionization. This combination of complementary methods reveals that only clusters containing at least one acid molecule are generated, that is, the acid molecule serves as the nucleation center in the expansion. The experiments also suggest that at least four water molecules are needed for HNO₃ acidic dissociation. The clusters are undoubtedly generated, as proved by electron ionization; however, they are not detected by the Na doping due to a fast charge-transfer reaction between the Na atom and HNO₃. This points to limitations of the Na doping recently advocated as a general method for atmospheric aerosol detection. On the other hand, the combination of the two methods introduces a tool for detecting molecules with sizable electron affinity in clusters