W.J. van der Zande

Dissociative recombination and excitation of N2+: Cross sections and product branching ratios

The absolute dissociative recombination and absolute dissociative excitation rate coefficients and cross sections have been determined for N2+ and electrons for collision energies between 10 meV and 30 eV. The ion storage ring CRYRING has been used in combination with an imaging technique with a position-and-time-sensitive detector. Information is retrieved on the ion beam vibrational state populations and on the product branching in the dissociative recombination process at 0 eV collisions. A hollow cathode ion source has been used to lower the vibrational excitation in the ion beam; a more traditional hot-cathode ion source was used as well. The most important findings are the following. The rate coefficient for an N2+ ion beam (46%, v=0, 27% v=1) versus electron temperature (K) is α(Te)=1.75(±0.09)×10−7(Te/300)−0.30 cm3 s−1. The dissociative recombination rate is found to be weakly dependent on the N2+ vibrational level. At 0 eV collision energy, the v=0 product branching is found to be 0.37(8):0.11(6)...

Simple models for the calculation of photoionization and electron impact ionization cross sections of polyatomic molecules

Abstract In this paper two semi-empirical models are presented. By the one model the electron impact ionization cross sections of polyatomic molecules can be estimated for electron energies in the range from 15 to 150 eV. Using the other model the photoionization cross section of polyatomic molecules is determined at photon energies of 11.8 and 16.7 eV. In the case of electron impact, the model reproduces the molecular ionization cross sections within 20% at electron energies above 50 eV. The photoionization cross sections calculated with the model deviate not more than 20% from the experimentally determined values for all polyatomic molecules tested.

VIBRATIONAL LADDER CLIMBING IN NO BY (SUB) PICOSECOND FREQUENCY-CHIRPED INFRARED LASER PULSES

Abstract The anharmonic vibrational ladder of nitric oxide (NO) is climbed by irradiating the molecule with chirped intense (sub)picosecond infrared (IR) pulses ( I max =1.0×10 10 W/cm 2 ). The chirp of the broadband IR ( hν =1850 cm −1 , Δ hν =50 cm −1 ) pulse from a free-electron laser is controlled by a pulse shaper. Transfer up to the excited vibrational state X 2 Π 1/2 (v=3) of NO shows a strong enhancement when the frequency chirp of the IR pulses follows the consecutive anharmonic vibrational transitions.

Laboratory studies of scattering matrices for randomly oriented particles: potentials, problems, and perspectives

A number of issues relevant to laboratory studies of scattering matrices as functions of the scattering angle for randomly oriented particles in the visible part of the spectrum are discussed. The usefulness of experiments is compared with that of numerical computations, in particular, for ensembles of natural nonspherical particles with broad ranges of sizes and shapes. It is argued that measurements of the entire scattering matrix have considerable advantages over measurements of only the intensity and polarization of the scattered light for incident unpolarized light. Results of special test experiments are presented which show that our experimental results for scattering matrices are not significantly contaminated by multiple scattering and that the orientation of the particles can be adequately described as random. Some ways are pointed out to overcome the lack of measurements for very small and very large scattering angles. A possibility to reduce the amount of material needed in the experiments is indicated. Finally, characterizations of the particles in terms of sizes, shapes and refractive indices are discussed.

Experimental and computational study of light scattering by irregular particles with extreme refractive indices: hematite and rutile

We present measurements of the complete scattering matrix as a function of the scattering angle of randomly oriented irregular hematite and rutile particles. The measurements were made at a wavelength of 632.8 nm in the scattering angle range from 5-174 degrees. Apart from their astronomical interest, these two samples are extremely interesting from a theoretical point of view, because they both have high real parts of the refractive index ( about 3.0 for the hematite and 2.73 for the rutile). In addition, the hematite sample has a high imaginary part of the refractive index k, with values between 10(-1) and 10(-2), whereas rutile is a non-absorbing material (k approximate to 0) at the studied wavelength. The scattering patterns of these mineral particles are quite similar to each other but show remarkable differences when compared to the results obtained for irregular mineral particles with moderate real parts of the refractive index. The measured results for both samples were compared with results of Mie calculations for projected surface equivalent spheres and T-matrix calculations for various spheroidal and cylindrical shapes. Both the experimental and theoretical results presented in this work seem to indicate that the scattering behavior of irregular mineral particles that have a high real part of the refractive index is not very dependent on the shape of the particles. In this case, Mie theory may give reasonable results despite the irregular shapes of the particles.

A new type of electrostatic ion trap for storage of fast ion beams

A new technique for trapping of fast (keV) ion beams is presented. The trap, which is electrostatic, works on a principle similar to that of optical resonators. The main advantages of the trap are the possibility to trap fast beams without need of deceleration, the well-defined beam direction, the easy access to the trapped beam by various probes, and the simple requirement in terms of external beam injection. Results of preliminary experiments related to the radiative cooling of molecular ions are also reported.

Charge exchange of O2+ with Cs: spectroscopy and predissociation pathways for the Πg Rydberg states of O2

Abstract Electron capture by keV O 2 + (X 2 Π g ) ions from Cs atoms yields predominantly the 1 Π g and 3 Π g Rydberg states of O 2 which subsequently predissociate. Through the use oftranslational spectroscopy on the neutral product atoms, we have located a number of vibrational levels of these states and determined the dissociation channels. Furthermore we have observed competition between diabatic and adiabatic behaviour in the dissociating channels.

Application of a time-resolved event counting technique in velocity map imaging

We illustrate the use of a three-dimensional (x,y,t) charge-coupled-device (CCD) camera detection system in an ion imaging experiment. The time measurement is based on the decay characteristics of the phosphor screen, which is recorded in two successive images by a double exposure CCD camera. The strength of the method is illustrated in a velocity map imaging experiment on iodine molecules that are ionized and dissociated by intense femtosecond laser pulses. Singly and doubly charged iodine fragments are detected and their (x,y) coordinates and arrival time are recorded in an event counting routine. We estimate the time resolution of the system to be 1.3 ns. We show that the fragment velocity distribution derived from the (x,y,t) data is similar and in some conditions more accurate than the distribution obtained by a mathematical inversion of the (x,y) data only. This principle of detection can be used in all situations in which inversion methods are impossible, for example, when the particle distribution does not have an axis of symmetry.

Fine-structure population distributions of O(3Pj) in the H + O2 reaction and the photolysis of NO2

Abstract The relative populations of the oxygen-atom fine-structure levels 3 P j have been measured for the bimolecular reaction H+O 2 →OH+O and the unimolecular reaction NO 2 + hv →NO+O using two-photon laser-induced fluorescence. In this method the 3s 3 S j″ ←3p 3 P j′ fluorescence intensity at 845 nm is recorded following the two-photon transition 3p 3 P j′ ← 2p 3 P j at 226 nm. In the H+O 2 reaction, the fine-structure distribution is approximately in the ratio of the statistical degeneracies (5:3:1) at a collision energy of 1.6 eV, whereas at a collision energy of 2.4 eV the population favors the fine-structure levels with lower energies. For NO 2 photolysis, the O( 3 P j ) distribution is described by a temperature of 200 K at 355 nm, whereas at 226 nm the O( 3 P j ) distribution is inverted, i.e. the population distribution favors fine-structure levels with higher energies. Those observations are compared with predictions from a model in which the kinetic energy of the recoiling products is responsible for fine-structure changing transitions.

Experimental light scattering by fluffy aggregates of magnesiosilica, ferrosilica, and alumina cosmic dust analogs

Context. Fluffy aggregates are generally assumed to be important constituents of circumstellar and interplanetary environments as well as to be present among the solid debris ejected from active comets. Aims. We experimentally study light scattering properties of several fluffy aggregate samples. These cosmic dust analog aggregates are composed of coagulated magnesiosilica grains, ferrosilica grains, and alumina grains. The samples contain aggregates with different porosities. The individual grains have diameters of the order of a few tens of nanometers; the aggregates have diameters up to several micrometers. Methods. The samples were produced in a Condensation Flow Apparatus. Their light scattering properties were measured with the Amsterdam Light Scattering Facility at a wavelength of 632.8 nm. Results. We measured two scattering matrix elements as functions of the scattering angle, namely F11(θ) (phase function) and −F12(θ)/F11(θ) (degree of linear polarization for incident unpolarized light) for seven different samples of aggregates in random orientations in an aerosol jet. The samples consisted of fluffy aggregates with cosmic dust analog compositions. We provide detailed information about their production and nature. In addition, for four of these samples we measured F22(θ)/F11(θ). We covered an angle range of 5 ◦ to 174 ◦ ,i n small steps of 1 ◦ in the range from 5 ◦ to 10 ◦ and 170 ◦ to 174 ◦ and in steps of 5 ◦ for the rest of the angle range. Conclusions. The results for the analog samples show an extremely high −F12(θ)/F11(θ), with maxima between about 60% to almost 100%. This Rayleigh-like behavior has been demonstrated before for fluffy aggregates and suggests that the small-sized grains in the aggregates are the main cause. Measured results for phase functions are more scarce. The phase functions we measured show shapes that are similar to those of compact micron-sized particles, suggesting that it is the overall size of the aggregates that determines their shape. The modest negative branch of −F12(θ)/F11(θ) found for all seven samples seems to be mainly governed by aggregate structure. Thus, the unique combination of accurately measured phase functions and polarization functions over a fine mesh of scattering angles for cosmic dust analog aggregates enables the exploitation of the data as powerful diagnostic tools to constrain the different physical properties of dust in e.g. circumstellar clouds and in comet ejecta.