Electron diffraction of CS2 nanoclusters embedded in superfluid helium droplets

Abstract

We report experimental results from electron diffraction of CS2 nanoclusters embedded in superfluid helium droplets. From detailed measurements of the sizes of doped droplets, we can model the doping statistics under different experimental conditions, thereby obtaining the range of cluster sizes of CS2. Using a least squares fitting procedure, we can then determine the structures and contributions of dimers, trimers, and tetramers embedded in small droplets. While dimers prefer a stable gas phase structure, trimers and tetramers seem to forgo the highly symmetric gas phase structures and prefer compact cuts from the crystalline structure of CS2. In larger droplets containing more than 12 CS2 monomers, the diffraction profile is consistent with a three-dimensional nanostructure of bulk CS2. This work demonstrates the feasibility of electron diffraction for in situ monitoring of nanocluster formation in superfluid helium droplets.

Electron diffraction as a structure tool for charged and neutral nanoclusters formed in superfluid helium droplets

“…50 However, when the droplet size is small, and the size reduction after each pickup event is substantial, the pickup statistics have to be modeled in detail. 51,52 In this latter case, there is an upper size limit for the dopant cluster due to the small droplet size; once the number of collisions between a droplet and the dopant molecules exceeds the limit, the cooling capacity of the droplet can no longer accommodate a new dopant molecule, the droplet is destroyed, and it can no longer reach the diffraction region. In reality, a droplet has to retain thousands of atoms to maintain its translational momentum to reach the diffraction region, 53,54 hence the actual upper limit for doped clusters should be even lower than the estimate from the heat capacity of the dopant and the droplet.…”

“…Active background subtraction, termed “toggle” mode operation, is typically used to remove background contributions from a variety of sources. 51,55 For example, in diffraction experiments of neutral clusters embedded in droplets, the sample pulse valve and the electron gun operate at 10 Hz, but the droplet pulse valve (DPV) operates at 5 Hz. The diffraction pattern obtained when the DPV is on is named the “signal” image, while the image recorded in the very next electron pulse when the DPV is off is named the “background” image.…”

“…We have recorded diffraction profiles of molecular clusters formed inside helium droplets, both from neutral and ionized species. 47,51,55,64,69 Limited by the doping method, the nanoclusters are polydisperse, which offers a window to observe the structure evolution with cluster sizes. For neutral clusters, so far all of our samples approach bulk crystal structures when more than four monomers are present.…”

“…For neutral clusters, so far all of our samples approach bulk crystal structures when more than four monomers are present. 47,51,69 When the global minimum structure in the gas phase deviates from a cut of the crystal structure, some small clusters adopt the structures in the gas phase. 47,51,55,64 However, the situation is not universal, and for example, in the case of iodine dimers, 69 a halogen bond is already formed, analogous to the most compact dimeric cut from bulk solids.…”

“…In the case of CS 2 , 51 the situation is very different from I 2 ; high resolution gas phase experiments using supersonic slit jets have determined the structures of small CS 2 clusters. The dimer forms a cross with a D 2d symmetry, 74 the trimer forms a pinwheel with a D 3 symmetry, 75 and the tetramer forms a barrel with a D 2d symmetry, 76 in general agreement with theoretical calculations.…”