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The Journal of Chemical Physics

Evidence for a bound HeH2 halo molecule by diffraction from a transmission grating

Anton Kalinin, Oleg Kornilov, Lev Yu. Rusin, Jan Peter Toennies

Published Jun 23, 2004• 15 citations
Publisher

Abstract

The HeH2 van der Waals complex has been identified in a molecular beam produced by a cryogenic (T0=24.7 K) free jet expansion of a 1% H2 mixture in 99% 4He gas. The weakly bound HeH2 complexes in the beam are identified via their first order diffraction angles after passing through a 100 nm period transmission grating. An electron impact mass spectrometer analysis of the diffraction patterns is used to discriminate against ion fragments of the constituent gas clusters.

DOI
10.1063/1.1768935

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The HeH2 van der Waals complex has been identified in a molecular beam produced by a cryogenic (T0=24.7 K) free jet expansion of a 1% H2 mixture in 99% 4He gas. The weakly bound HeH2 complexes in the beam are identified via their first order diffraction angles after passing through a 100 nm period transmission grating. An electron impact mass spectrometer analysis of the diffraction patterns is used to discriminate against ion fragments of the constituent gas clusters.
Small weakly bound boson-fermion 4Hem3Hen^{4}\mathrm{He}_{m}^{3}\mathrm{He}_{n}4Hem3​Hen​ clusters formed in a free jet expansion are identified using nondestructive transmission grating diffraction. The observations confirm the existence of more than 11 very tenuous complexes including the three-body halo molecule 4He23He^{4}\mathrm{He}_{2}^{3}\mathrm{He}4He23​He and the pseudo-Borromean complex 4He23He2^{4}\mathrm{He}_{2}^{3}\mathrm{He}_{2}4He23​He2​. Effective cluster formation temperatures, extracted from a sudden freeze model for cluster growth using theoretical binding energies, increase smoothly with cluster size, thereby confirming the calculations with the possible exception of 4He23He2^{4}\mathrm{He}_{2}^{3}\mathrm{He}_{2}4He23​He2​.
A molecular beam consisting of small helium clusters is diffracted from a 100 nm period transmission grating. The relative dimer intensities have been measured out to the 7th order and are used to determine the reduction of the effective slit width resulting from the finite size of the dimer. From a theoretical analysis of the data which also takes into account the van der Waals interaction with the grating bars, the bond length (mean internuclear distance) and the binding energy are found to be $〈r〉\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}52\ifmmode\pm\else\textpm\fi{}4\AA{}$ and $|{E}_{b}|\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.1+0.3/\ensuremath{-}0.2$ mK.
The size distributions of neutral 4He clusters in cryogenic jet beams, analyzed by diffraction from a 100 nm period transmission grating, reveal magic numbers at N=10-11, 14, 22, 26-27, and 44 atoms. Whereas magic numbers in nuclei and clusters are attributed to enhanced stabilities, this is not expected for quantum fluid He clusters on the basis of numerous calculations. These magic numbers occur at threshold sizes for which the quantized excitations calculated with the diffusion Monte Carlo method are stabilized, thereby providing the first experimental confirmation for the energy levels of 4He clusters.
Molecular beams of rare gas atoms and D2{\mathrm{D}}_{2}D2​ have been diffracted from 100-nm-period SiNx{\mathrm{SiN}}_{x}SiNx​ transmission gratings. The relative intensities of the diffraction peaks out to the eighth order depend on the diffracting particle and are interpreted in terms of effective slit widths. These differences have been analyzed by a new theory which accounts for the long-range van der Waals ${\ensuremath{-}C}_{3}/{l}^{3}$ interaction of the particles with the walls of the grating bars. The values of the C3{C}_{3}C3​ constant for two different gratings are in good agreement and the results exhibit the expected linear dependence on the dipole polarizability.
The formation of dimers, trimers, and tetramers in a free jet cryogenic expansion of He4 atoms has been studied by diffraction from a nanostructure transmission grating. The final average velocities, speed ratios and ambient temperatures of the expansions for source temperatures of 30, 12, and 6 K and source pressures between 0.1 and 80 bar were determined from time-of-flight measurements of the He atoms. The final mole fractions of the He2, He3, and He4 clusters in the beam were determined from the intensities of the corresponding first-order diffraction peaks for the same range of source conditions. For each source temperature, the final mole fractions of these small clusters first rise, pass through a maximum and then decrease with increasing source pressure. The processes leading to the formation of these clusters are simulated with a kinetic model that allows for density and temperature changes in the expanding beam. The best-fit three-body recombination rate constant for dimer formation increases by over three orders of magnitude as the thermal energy decreases from 1 K to 1 mK, in qualitative agreement with recent theories.
We study the coherent inelastic diffraction of very weakly bound two-body clusters from a material transmission grating. We show that internal transitions of the clusters can lead to new separate peaks in the diffraction pattern, whose angular positions determine the excitation energies. Using a quantum mechanical approach to few-body scattering theory, we determine the relative peak intensities for the diffraction of the van der Waals dimers (D 2 ) 2 and H 2 D 2 . Based on the results for these realistic examples, we discuss the possible applications and experimental challenges of this coherent inelastic diffraction technique.