Frontiers in Spectroscopy Lectures
Spectroscopy without Photons: Diffraction of
Weakly Bound Complexes from Nano-Gratings
Max-Planck-Institut für Strömungsforschung, Bunsenstrasse 10,
D-37073
Recent nanotechnology advances have made it possible to produce free standing transmission gratings with periods of 100 nm and slits of about 50 nm width [1]. The diffraction of molecular beams of atoms, molecules and small clusters from such gratings provide beautiful textbook examples of matter wave phenomena [2]. Since only the wave nature of the particles is involved the method is essentially non-destructive. This experimental technique has been used to identify small very weakly bound van der Waals complexes such as He2, He3, He4, 4He23He2, 4He43He3, HeH2, (H2)2, (H2)3, etc., some of which had not previously been known to form stable complexes. With this technique it has also been possible to determine the long range van der Waals interaction of the small He clusters, other rare gas atoms and D2 with a solid surface (the bar walls) from measurements of the relative diffraction peak intensities out to high orders [3]. From a comparison of the diffraction peak intensities of He2 dimers with He atoms the average bond distance of the He dimer could be determined to be about 50 Å and from this the binding energy is found to be only 1.3·10-3 K (10.8·10-6 Kcal/mol) [4]. Thus the dimer is the largest and most weakly bound ground state naturally occurring molecule. Recently the large bond distance could be confirmed by measurements of the He2 cross sections in scattering from Kr atoms [5].
[1] He atom diffraction from nanostructure transmission gratings: The role of imperfections
R.E. Grisenti, W. Schöllkopf, J.P. Toennies, J.R. Manson, T.A. Savas and H.I. Smith
Phys. Rev. A 61, 033608(15) (2000)
[2] Nondestructive mass selection of small van der Waals clusters
W. Schöllkopf and J.P. Toennies
Science 266, 1345 (1994)
[3] Determination of atom-surface van der Waals potentials from transmission-grating diffraction intensities
R.E. Grisenti, W. Schöllkopf, J.P. Toennies, G.C. Hegerfeldt and T. Köhler
Phys. Rev. Lett. 83, 1755 (1999)
[4] Determination of the bond length and binding energy of the helium dimer by diffraction from a transmission grating
R.E. Grisenti, W. Schöllkopf, J.P. Toennies, G.C. Hegerfeldt, T. Köhler and M. Stoll
Phys. Rev. Lett. 85, 2284 (2000)
See also Physical Review Focus story of 6. September 2000
[5] The "eclipse" effect in the scattering of weakly bound helium clusters
A. Kalinin, O. Kornilov, L. Rusin, J.P. Toennies and G. Vladimirov
submitted
The problem of homogeneous nucleation has challenged both theoreticians and experimentalists since Gibb's theory of 1876. The diffraction technique has been used to study the number size distributions of small He clusters (N £ 4) as a function of source pressures and temperatures. The fit to a kinetic model provides information on three-body recombination rates which are found to be an order of magnitude larger than expected on the basis of a simple classical theory [1]. By increasing the angular resolution the diffraction technique has been extended to measurements of size distributions up to about N = 60 particles. The distributions reveal unexpected magic numbers which could be explained by a new mechanism involving the liquid drop excitations of the quantum fluid clusters [2]. Similar experiments with (pH2)-clusters show only a weak magic number at N = 13. The growth of small H2 clusters inside jet expansions has also been studied (albeit using photons!) with high spatial and spectral resolution Raman spectroscopy [3].
In the last part of the lecture a Mach-Zehnder universal matter-wave interferometer based on three identical transmission gratings will be described. This device opens up a wide range of intriguing new experiments in molecular physics. Transmission Fresnel zone plates have also been used to focus an atomic beam onto a 1 micron diameter spot [4], so that an essential part of a uniquely surface sensitive atom microscope is now available.
[1] The formation of dimers and trimers in free jet 4He cryogenic expansions
L.W. Bruch, W. Schöllkopf and J.P. Toennies
J. Chem. Phys. 117, 1544 (2002)
[2] Diffraction of neutral helium clusters: Evidence for "magic numbers"
R. Brühl, R. Guardiola, A. Kalinin, O. Kornilov, J. Navarro, T. Savas and J.P. Toennies
submitted
[3] Raman spectroscopy of small para-H2 clusters formed in cryogenic free jets
G. Tejeda, J.M. Fernández, S. Montero, D. Blume and J.P. Toennies
submitted
[4] Towards realization of a de Broglie microscope: Helium atom focussing using Fresnel zone plates
R.B. Doak, R.E. Grisenti, S. Rehbein, G. Schmahl, J.P. Toennies and Ch. Wöll
Phys. Rev. Lett. 83, 4229 (1999)