TEAMS / IHS

Thermal Energy Atomic and Molecular Scattering

Dr. Daniel Farías

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The understanding of fundamental physics and chemistry of surfaces requires a solid knowledge of the atomic forces binding atoms into and onto solid surfaces.The measurement of the interatomic coupling constants at the surface provides an accurate description of different surface properties and processes (surface relaxation and reconstruction, surface diffusion, adsorption and desorption of atoms, phase transitions, momentum and energy transfer to the surface,...).

Due to the low incident energies used (10-300 meV) the incident atoms and molecules probe the topmost surface layer of the substrate. Moreover, such atomic and molecular beams may be used equally to investigate metals, semiconductors or insulator surfaces. In particular, Helium Atom Scattering (HAS) is the ideal tool to investigate weakly bounded systems, like physisorbed molecules and the self-organization of organic molecules, but also to study growth at surfaces, due to the giant cross section for scattering from defects or adsorbates.

Helium Atom Scattering is the ideal tool to probe the topmost surface layer: While electrons can penetrate 3-5 layers below the surface, helium atoms are scattered by a surface of constant total electron density, whereby the classical turning points are about 3-4 Å away from the surface atom cores.

For light particles (especially He) at low energies (10-300 meV), elastic scattering is the dominant process, and as the de Broglie wavelengths are of the order of several tenths to a few Angstroms, diffraction effects occur. Measurements of diffraction spectra allow not only the determination of the size and orientation of the surface unit cells but also, by means of analysing diffraction intensities, yield the surface corrugations which very often provide direct pictures of the geometrical arrangement of the surface atoms. Producing short incident particle beam pulses and applying time-of-flight (TOF) techniques, the dispersion of surface phonons can be determined with high resolution.

Activities

Probing gas-surface potential energy surfaces with diffraction of hydrogen molecules

An understanding of the dissociative chemisorption of molecules on solid surfaces is fundamental in order to get a detailed picture of the basic steps involved in many surface chemical reactions. The possibility of carrying out high-dimensional dynamical calculations, has renewed the interest in diffraction studies with H2 / D2 molecular beams.

These calculations have shown that information regarding the dissociative chemisorption potential can be obtained not only by measuring the fraction of molecules that stick to the surface but also, and perhaps more precisely, by analyzing diffraction of molecules over a wide range of incident energies and with an angular resolution high enough to allow investigation of rotationally inelastic transitions. The diffraction of H2 and D2 molecular beams from surfaces is in principle quite similar to diffraction of He [1], the only major difference being the possibility of rotational-state transitions in the case of molecular scattering. At low surface temperatures, this occurs mainly via an inelastic process, in which the incident molecules convert part of their translational energy into rotational energy.

This leads to the appearance of additional diffraction peaks in the angular distributions, which are called rotationally inelastic diffraction (RID) peaks. We have recently investigated the diffraction and rotational transitions of D2 scattered from NiAl(110) at incident energies between 88 and 157 meV. The measurements were done along the [110] azimuth and using a set up which allows recording of diffraction patterns at a fixed angle of incidence. The absolute 0-2 transition probability was found to increase from 10 to 20 % in the energy range investigated, whereas the one corresponding to the 2-0 transition remained constant at 10 %. An important conclusion of our work is that the behaviour exhibited by these two transitions as a function of incident energy is independent of angle of incidence.

Growth of thin films and superlattices

A new approach to electronics, called spintronics or magnetoelectronics is now emerging, whose purpose is to use the electron spin, rather than its charge, to carry information. This field is becoming one of the most rapidly growing areas in electronics, and the recent commercialisation of devices based on the giant magnetoresistance effect (GMR) for magnetic information storage is a perfect example of its potential.

One of the main problems when dealing with novel, artificial materials, is to develop methods that allow us to prepare them in a controlled way, with a high degree of structural and morphological perfection. Besides, for practical applications it is often necessary to reach some minimum dimensions or amount of material. This requirement frequently collides with the tendency of the employed materials to adopt their equilibrium structures. In our lab, we are currently investigating several alternatives to grow epitaxial metallic superlattices containing bimetallic films. One of these techniques is the coevaporation of two components , assisted by a surfactant layer.

We have recently demonstrated that the structural quality of Co and Fe samples grown on Cu(111) can be greatly improved by this method. Our work was focused in the characterization of Fe-Cu and Co-Cu superlattices prepared by the technique of codeposition, i.e. by adding a second material (Cu in our case) in the growth process. The quality of the films grown was monitored by TEAS (Thermal Energy Atom Scattering). A major advantage of TEAS, as compared to other techniques, is its high sensitivity to step distributions in the topmost surface layer, which is due to the giant cross-section of He atoms for diffuse scattering. As a consequence, TEAS is the ideal tool for determining the concentration of defects at the surface of a growing film.

However, the thermal-energy, neutral He atoms used in a TEAS experiment cannot penetrate below the surface, and therefore these measurements do not offer any information regarding the stacking sequence within the growing film. To characterize this aspect of the problem, one needs to use a penetrating probe, such as electrons.

We have measured LEED I/V (intensity vs. energy) curves in-situ for different thicknesses and compositions in order to determine the crystalline structure of the deposits.These results were complemented by a modeling effort based on the BFS method for alloys, meant to elucidate the main characteristics of the early growth stages of Co and Fe films on Cu(111). This is a very powerful method for determining structural properties of bulk alloys, and has recently been extensively applied to the study of surface alloys.

In spite of the bulk immiscibility of Co and Cu, BFS results indicate an affinity of Cu for Co and Fe, resulting in features that can be explained in terms of the competition of strain and chemical effects. Also, we performed Monte Carlo simulations aimed to determine the structure of superlattices formed by codeposition of Cu-Co and Cu-Fe films. This work has been done in collaboration with Prof. J.J. de Miguel and Prof. A.L. Vázquez de Parga.

Study of self-assembling of organic molecules at surfaces with HAS

The structures formed through self-assembling of molecules are controlled by the energetic balance between the molecule-molecule interaction, the molecule-surface interaction and the thermal energy available, which controls the appearance of long-range order. This energetic balance is usually very delicate and difficult to predict theoretically.

A good example is given by the c(4x2) phase formed by SCH3 on Au(111), whose structure is not clear yet in spite of being one of the most studied self-assembling systems. One of the main goals of our work is to learn more about the assembling process at a molecular level. To do that, we will determine the structure of this and similar phases by exact calculations of He diffraction, using the method proposed by Manolopoulos et al. [J. Chem. Soc. Farad. Trans. 86, 1641 (1990)] in which the calculation time scales like N (number of diffraction channels) instead of N3 (usual method).

As a first step, we are currently analysing He-diffraction data for the c(4x2)-SCH3 structure formed on Au(111) (the measurements were performed in the group of Prof. G. Scoles at the Princeton University, Dep. of Chemistry). The next step will consist in analysing phases formed by thiols ending in OH groups, since it is known that changing the terminal group changes the tilt angle of the adsorbed molecules and the formed structures. We are currently building an Organic Molecular Beam Epitaxy (OMBE) system for depositing organic molecules on surfaces in UHV.

We plan to study the growth of thiols and flat molecules (PVBA and PEBA) on Cu(111) and Ru(0001). These systems represent the cases of weak and strong substrate-molecule interaction respectively, and are expected to help us to understand the role played by the different interactions in the self-assembling process.

Setups

The Atomic and Molecular Beam Diffraction Apparatus

The high-resolution HAS apparatus is one of the seven UHV chambers at the Laboratory of Surface Science at the Universidad Autónoma de Madrid (LASUAM), headed by Prof. R. Miranda. It has been transferred in September 2002 from the Freie Universitaet Berlin, where it has been running for many years in the group of Prof. K.H. Rieder.

It is equipped with standard LEED/Auger and ion gun systems to characterize and clean the surface, in addition to a quadrupole mass spectrometer with an axial-beam ion source for recording thermal desorption spectra.

The sample is mounted on a manipulator (Fig. 1) that allows azimuthal rotation of the sample, as well as heating to 1200 K and cooling to 60 K.

Atomic and molecular beams are generated by supersonic expansion through a 1 mm diameter platinum tube in which a hole of approximately 8x10-2 mm diameter was spark eroded in the side. The tube is clamped between massive copper supports, and can be resistively heated to 800 K and cooled to 100 K by contact with a liquid nitrogen reservoir.

After expansion, the beam is collimated by a 0.5 mm diameter skimmer and traverses two differential pumping stages before entering the sample chamber. The beam is mechanically chopped with a magnetically coupled rotary motion feedtrough in the third stage to allow phase sensitive detection.

This set-up allows rotation of the detector about two axes independently of the incident angle. It can measure diffraction peaks for a fixed angle of incidence, making comparison with calculations easier. Another advantage of this set-up is that it allows direct measurement of the incident beam intensity, making it possible very accurate determination of absolute diffraction probabilities. The base pressure in the chamber is typically 3x10-11 mbar, reaching 5x10-10 mbar with the He or D2 beam on. This pressure increase gives rise to a continuous background in the scattering chamber which limits the signal-to-noise ratio; the signal is recovered from the background by means of a lock-in system.

The Atomic and Molecular Beam Time Of Flight Apparatus

The Time Of Flight (TOF) apparatus is one of the seven UHV chambers at the Laboratory of Surface Science at the Universidad Autónoma de Madrid (LASUAM), headed by Prof. R. Miranda. It has been transferred in September 2004 from the Max-Planck-Institut in Goettingen, where it has been running for many years in the group of Prof. J.P. Toennies.

The beam is generated by an adiabatic expansion from a high pressure (100 bar) through a cylindrical orifice with a diameter of 10 microns. Attached to the source cell, a closed cycle helium refrigerator permits its temperature to be varied continuously from 80 to 300K, which corresponds to beam energies in the range 20-65 meV (in case of He).

The center part of the expanded beam is extracted by a conical skimmer (fig. 1), and then passes through two vacuum chambers. Inside one of them, a rotating apperture (fig. 3) is used to create well-defined in time beam packets, which then, enter in the UHV main chamber.

The sample is mounted on a manipulator with six degrees of freedom. The sample holder is provided with an electron bombardment heater and a liquid nitrogen cooling device, which allows crystal temperatures of 90-1600K.

In order to enlarge the time of flight spread between elastic and inelastic events, several differential pumping stages are installed along the flight path. Under chopped beam conditions, pressure is reduce from 10e-3 mbar (source) to 10e-11 mbar (detector)

The main chamber is equiped with a sputter ion gun, LEED and a quadrupole mass spectrometer, for sample preparation and characterization.

The scattered atoms are detected by a homemade magnetic sector field mass spectrometer, located 1.6 m. from the target at a fixed angle of incidence of Ti+Tf =106.8 degrees (respect to incidence). The detector consists of an electron bombardment ionizer, which ionizes the neutral incident particle. The residual ions are extracted and accelerated towards the electron muliplier. A magnetic field is used to select the desired mass for the ions. The detector design is optimized for use with light particles. Intensity from 20 counts/s to more than 10e6 counts/s can be measured. The relative energy resolution (as determined by the velocity spread of the beam molecules, the chopper opening time, axial detector lenght and kinematics) is about 2 % (~ 0.5 meV)

This set-up is designed to perform inelastic measurements. When the beam is scattered from the surface, part of its energy and momentum can be transferred to the surface to create or annihilate one or more phonons. The diffracted packet can, therefore, gain or loose energy due to the interaction with the surface. The time difference between the elastic and inelastic scattered particles are measured and translated into energy transfers.