Arn clusters generated in a supersonic nozzle beam are investigated by exploiting their different kinematical behavior in a scattering experiment. In a crossed molecular beam experiment the Arn clusters are scattered from He atoms at E=69.7 meV. By measuring the angular and velocity distributions of the scattered clusters, the different species can be separated from each other. In this way fragmentation probabilities of small Arn clusters up to n=6 by electron impact ionization are measured. The influence of collisional dissociation of the dimer is accounted for by classical trajectory calculations. The dimer fragmentation is found to be f21=0.40±0.05, while the trimer totally fragments to dimers ( f32=0.7±0.3) and monomers ( f31=0.3). All clusters with n from 4 to 6 appear with a probability of 0.95 on the dimer ion mass.

The formation of Av n clusters in a supersonic expansion and their fragmentation by electron impact ionization is measured by analyzing the kinematically different behavior in a scattering experiment. The fragmentation of dimers and trimers is appreciable but depends only slightly on the electron energy. PACS numbers; 36.40.+d, 35.20.Wg Since the discovery of cluster formation in supersonic expansions 1 a new area of research has emerged to study the properties and practical applications of these aggregates of atoms and molecules between the gas and the solid phase. The clusters are usually detected by electron impact or photoionization. One of the unsolved problems of this detection mechanism is the unknown fragmentation of the clusters during the ionization process which makes a reliable determination of the neutral cluster distribution in the beam very difficult. There is clear experimental evidence of heavy fragmentation of small clusters studied by means of pure electron impact ionization 2 or in combinations with mass-flux gauge 3 or infrared laser-bolometric techniques, 4 by photoionization, 5 by Penning and heavy-particle impact ionization, 6 and by spectroscopic investigations. 7 Neither effort succeeded in obtaining complete information on the fragmentation process. In this Letter we present an alternative method of getting information on the cluster distribution which exploits the kinematically different behavior in a Ar n velocity FIG. 1. Newton diagram for the scattering of Ar^ clusters from He for the velocities v (Ar") =570 m/s and v(He) =1790 m/s. The different circles denote the positions for final velocities of elastically scattered particles. Note that the velocity arrows are not to scale near the zero point.scattering experiment and which is therefore independent of the detection mechanism. The first reliable information on the fragmentation probability of small Ar clusters is obtained in this wayThe principle of the experiment is explained by looking at the kinematic (Newton) diagram of the scattering of Ar" clusters from He shown in Fig. 1. Because the Ar n clusters are formed in a supersonic nozzle expansion they have nearly the same velocity but different masses. Therefore, the elastically scattered Ar" clusters appear according to their different centers of mass at different laboratory scattering angles and different final velocities as marked by the circles in Fig. 1. Because the clusters are heavier than He, there are always two contributions at one laboratory angle, a slow and a fast peak. In addition, each cluster is kinematically constrained to scatter within a certain laboratory angular range, which can be used to distinguish between the lowest two species. 8 However, a combined angular and velocity analysis is necessary in order to separate also the larger clusters from each other.The experiment has been performed in a crossed molecular beam apparatus which has been described in detail elsewhere. 9,10 The schematic diagram of the apparatus is shown in Fig. 2. The primary clust...

Angular distributions of state-selected NO and O products in the photoinitiated unimolecular decomposition of jet-cooled NO 2 have been measured by using both the photofragment ion imaging technique with velocity map imaging and ion time-of-flight translational spectroscopy. The recoil anisotropy parameter of the photofragments, ␤, depends strongly on the rotational angular momentum of the photoproduct. O( 3 P jϭ2,0 ) angular distributions are recorded at photolysis wavelengths 371.7, 354.7, and 338.9 nm. At these wavelengths, respectively, vibrational levels v ϭ0, vϭ0,1 and vϭ0 -2 of NO are generated. In addition, ␤ values for NO(vϭ2) in specific high rotational levels are determined at ϳ338 nm. The experimental observations are rationalized with a classical model that takes into account the transverse recoil component mandated by angular momentum conservation. The model is general and applicable in cases where fragment angular momentum is large, i.e., a classical treatment is justified. It is applied here both to the experimental NO 2 results, and results of quantum calculations of the vibrational predissociation of the Ne-ICl van der Waals complex. It is concluded that deviations from the limiting ␤ values should be prominent in fast, barrierless unimolecular decomposition, and in certain dissociation processes where a large fraction of the available energy is deposited in rotational excitation of the diatom. The application of the model to NO 2 dissociation suggests that the nuclear dynamics leading to dissociation involves a decrease in bending angle at short internuclear separations followed by a stretching motion. This interpretation is in accord with recent theoretical calculations.

The spectroscopic characterization of the electronic ground and excited state interactions of NO with diOE erent rare gases is reviewed. In its electronic ground state, the NO molecule continues to serve as one of the most important benchmark systems for investigating van der Waals interactions involving open shell molecules. Combining high resolution infrared excitation with resonance enhanced multiphoton ionization (REMPI) detection, bound intermolecular bend± stretch levels have been detected for the complexes NO± Ar and NO± Ne for the ® rst time.In parallel with these new experimental advances, new high-level ab-initio potential energy surfaces have been calculated very recently for several NO± Rg (Rgˆrare gas) systems. Predictions for the bound states based on these potentials are in excellent agreement with the high resolution infrared spectra for NO± Ar and NO± Ne, demonstrating the great accuracy achieved with current ab initio methods. The large number of excited electronic states of NO provides ideal opportunities for studying molecular interactions in diOE erent excited states. The low lying Rydberg states are especially interesting because the Rydberg electron can reside in an orbital whose size is comparable with or larger than the van der Waals radius of the complexed atom. The existing set of experimental data on Rydberg states with nˆ3…lˆ0; 1; 2 † and nˆ4…lˆ0 † are reviewed critically. The excited states have been investigated employing mainly REMPI spectroscopy. The data provide detailed information about the bound levels of the associated intermolecular surfaces. Most of the work has concentrated on the complexes with Ar and Ne. Additional data for the statesà A 2 § andC C 2 ¦ are available for complexes with Kr and Xe.

In an extension of recent work [Y. Kim, K. Patton, J. Fleniken, and H. Meyer, Chem. Phys. Lett. 318, 522 (2000)], overtone pumping followed by resonantly enhanced multiphoton ionization (REMPI) detection is used to record spectra corresponding to excitation of the lowest five bend–stretch vibrational levels of the NO(X 2Π,v=2)–Ar complex. High-quality ab initio potential energy surfaces, coupled-electron-pair (CEPA) and coupled cluster single double triple [CCSD(T)], are used to predict the positions of these same five states, but in the NO(X 2Π,v=0)–Ar complex. The vibrational wave functions and basis set expansion coefficients, determined within the adiabatic bender model, are then used to simulate the observed spectrum for excitation of the NO(X 2Π, v=2)–Ar complex. The overall position and rotational substructure matches the experiment extremely well, particularly when the simulation is based on the presumably more accurate CCSD(T) potential energy surfaces.