Scattering Analysis of Cluster Beams: Formation and Fragmentation of SmallArnClusters

Abstract

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...

Study on the Structure and Vibrational Dynamics of Functional Molecules and Molecular Clusters by Double Resonance Vibrational Spectroscopy

“…A good agreement is seen between the calculated and observed IR spectra of Aw2-I. Thus, we conclude that band ¡¤ is due to the cyclic-form L-PheÍ(H 2 O) 2 , and the bands in Figure 12c are assigned as follows; the band at 3002 cm Õ1 is the H-bonded carboxyl OH stretch. Those at 3330, and 3474 cm Õ1 are the H-bonded OH stretches of two waters in the H-bond ring, and the bands at 3680 and 3717 cm Õ1 are OH stretches free from the H-bond.…”

“…The IRÍUV double-resonance spectrum of band c (Figure 18c) shows four OH stretching bands, indicating band c to be DB18C6Í(H 2 O) 2 . By comparing the IR spectra of band a with that of c, we assign the IR bands at 3562 and 3623 cm Õ1 in Figure 18c to the symmetric and antisymmetric OH stretching vibrations of a bidentate water, which are redshifted by 18 and 25 cm Õ1 from those of band a (Figure 18a), respectively.…”

“…The cluster in "bath (2)" dissociates with the rate constant k 3 (=1/¸V P ). The time profiles of the OH level, the bath (1) and the bath (2) The transient UV spectrum involves both the transitions of bath (1) and (2), and their relative intensity is different at the different frequency so that the profile of the transient UV spectrum should be frequency dependent. The pumpÍprobe signals observed in the lower frequency region showed faster decay than that observed at higher frequency.…”

“…Huisken and co-workers 1 first reported an IR photodissociation spectroscopic study of small-size water clusters by using a novel sizeselection technique involving molecular beam scattering which was developed by Buck and Meyer. 2 Direct high-resolution IR absorption spectroscopic measurement for water clusters has been extensively studied by Saykally's group. 3 Another powerful method for obtaining the vibrational spectra of neutral clusters is the double resonance method.…”

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

“…Generally, due to the strong fragmentation upon ionization of the weakly bound species, mass spectrometry is of little help here. [26][27][28] For the small clusters N 10 the scattering method of Buck and Meyer 26,27 can be exploited to measure the neutral cluster sizes. For the large and medium sizes (N ∼10-10…”