Irregular shapes of water clusters generated in supersonic expansions.

We report cross sections for pickup of guest molecules on neutral argon and water clusters with the mean sizes in the range from N = 50 to 600. The experiments are supported by molecular dynamics simulations and analytical models based on the interaction potentials. The cross sections for argon clusters are consistent with their assumed spherical shape and follow approximately the theoretically justified N(1/3) dependence. On the other hand, the cross sections of water clusters depart from this dependence and are considerably larger starting from N ≥ 300. We interpret this increase of cross section by the occurrence of highly irregular shapes of water clusters produced in the supersonic expansion of water vapor under the conditions of the large cluster generation.

Imaging of hydrogen halides photochemistry on argon and ice nanoparticles.

The photodissociation dynamics of HX (X = Cl, Br) molecules deposited on large ArN and (H2O)N, N̄ ≈ 10(2)-10(3), clusters is investigated at 193 nm using velocity map imaging of H and Cl photofragments. In addition, time-of-flight mass spectrometry after electron ionization complemented by pickup cross section measurements provide information about the composition and structure of the clusters. The hydrogen halides coagulate efficiently to generate smaller (HX)n clusters on ArN upon multiple pickup conditions. This implies a high mobility of HX molecules on argon. On the other hand, the molecules remain isolated on (H2O)N. The photodissociation on ArN leads to strong H-fragment caging manifested by the fragment intensity peaking sharply at zero kinetic energy. Some of the Cl-fragments from HCl photodissociation on ArN are also caged, while some of the fragments escape the cluster directly without losing their kinetic energy. The images of H-fragments from HX on (H2O)N also exhibit a strong central intensity, however, with a different kinetic energy distribution which originates from different processes: the HX acidic dissociation followed by H3O neutral hydronium radical formation after the UV excitation, and the slow H-fragments stem from subsequent decay of the H3O. The corresponding Cl-cofragment from the photoexcitation of the HCl·(H2O)N is trapped in the ice nanoparticle.

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.

Cluster cross sections from pickup measurements: are the established methods consistent?

Pickup of several molecules, H(2)O, HBr, and CH(3)OH, and Ar atoms on free Ar(N) clusters has been investigated in a molecular beam experiment. The pickup cross sections of the clusters with known mean sizes, Ñ≈ 150 and 260 were measured by two independent methods: (i) the cluster beam velocity decrease due to the momentum transfer of the picked up molecules to the clusters, and (ii) Poisson distribution of a selected cluster fragment ion as a function of the pickup pressure. In addition, the pickup cross sections were calculated using molecular dynamics and Monte Carlo simulations. The simulations support the results of the velocity measurements. On the other hand, the Poisson distributions yield significantly smaller cross sections, inconsistent with the known Ar(N) cluster sizes. These results are discussed in terms of: (i) an incomplete coagulation of guest molecules on the argon clusters when two or more molecules are picked up; and (ii) the fragmentation pattern of the embedded molecules and their clusters upon ionization on the Ar cluster. We conclude that the Poisson distribution method has to be cautiously examined, if conclusions should be drawn about the cluster cross section, or the mean cluster size Ñ, and the number of picked up molecules.

Velocity map imaging of HBr photodissociation in large rare gas clusters.

We have implemented the velocity map imaging technique to study clustering in the pulsed supersonic expansions of hydrogen bromide in helium, argon, and xenon. The expansions are characterized by direct imaging of the beam velocity distributions. We have investigated the cluster generation by means of UV photodissociation and photoionization of HBr molecules. Two distinct features appear in the hydrogen atom photofragment images in the clustering regime: (i) photofragments with near zero kinetic energies and (ii) "hot" photofragments originating from vibrationally excited HBr molecules. The origin of both features is attributed to the fragment caging by the cluster. We discuss the nature of the formed clusters based on the change of the photofragment images with the expansion parameters and on the photoionization mass spectra and conclude that single HBr molecule encompassed with rare gas "snowball" is consistent with the experimental observations.

A remeasurement of the products for electron recombination of N2H+ using a new technique: no significant NH+N production.

A remeasurement of the product distribution from dissociative electron-ion recombination (DR) of N2H+ has been made using a new technique. The technique employs electron impact to ionize the neutral products prior to detection by a quadrupole mass analyzer. Two experimental approaches, both using pulsed gas techniques, isolate and quantify the DR products. In one approach, an electron-attaching gas is pulsed into a flowing afterglow to transiently quench DR. Results from this approach give an upper limit of 5% for the NH+N product channel. In the second approach, the reagent gas N2 is pulsed. The absolute percentages of products were monitored versus initial N2 concentration. Results from this approach also give an upper limit of 5% for NH+N production. This establishes that N2+H is the dominant channel, being at least between 95 and 100%, and that there is no significant NH production contrary to a recent storage ring measurement that yielded 64% NH+N and 36% N2+H. Possible reasons for this dramatic difference are discussed.

Recombination of D3+ ions in the afterglow of a He-Ar- D2 plasma.

We report measurements of the rate coefficient alpha for recombination of D3+ with electrons in a He-Ar-D2 plasma. The observed rate coefficient is dependent on the deuterium number density indicating that third-body assisted recombination is efficient and significantly contributes to recombination. When the deuterium number density is decreased to 7x10(10) cm(-3), the rate coefficient also decreases to alpha approximately 6x10(-9) cm3 x s(-1). These data indicate that the binary dissociative recombination of D3+ is very slow with alphaDR<6x10(-9) cm3 x s(-1). The observation of a deionization process proceeding via formation of D5+ is also reported.