Helium beams

In recent years there is a growing interest in the study of vibrational properties of both clean and adsorbate covered surfaces of metals. For several years two complementary experimental methods have been used to measure the dispersion relations of surface phonons on different crystal faces. These are the scattering of thermal helium beams" and the high-resolution electron-energy-loss-spectroscopy. ... 

It would be of great interest to experimentally verify these new results of phonon modes, MSB s and relaxations by suitable methods, such as electron-energy-loss-spectroscopy or thermal helium beam scattering. 

Fig. 4.12. Atomic-beam diffraction. A nearly monochromatic beam of helium, generated by a nozzle, falls on the solid surface with an angle of incidence. The diffracted beam is collected at an outgoing angle. The angular distribution of the diffracted helium beam contains the information about the topography and structure of the...

On surfaces of some d band metals, the 4= states dominated the surface Fermi-level LDOS. Therefore, the corrugation of charge density near the Fermi level is much higher than that of free-electron metals. This fact has been verified by helium-beam diffraction experiments and theoretical calculations (Drakova, Doyen, and Trentini, 1985). If the tip state is also a d state, the corrugation amplitude can be two orders of magnitude greater than the predictions of the 4-wave tip theory, Eq. (1.27) (Tersoff and Hamann, 1985). The maximum enhancement factor, when both the surface and the tip have d- states, can be calculated from the last row of Table 6.2. For Pt(lll), the lattice constant is 2.79 A, and b = 2.60 A . The value of the work function is c() w 4 cV, and k 1.02 A . From Eq. (6.54), y 3.31 A . The enhancement factor is... 

Figure 14 Equilibrium charge fractions of hydrogen beam (left panel) and helium beam (right panel) in H2O calculated by the theory of Allison [215].

The value of 100 cal mole chosen for the adsorption energy of helium is consistent with the fact that helium has no dipole or quadrapole and is only slightly polarizable. Thus, it will minimally interact with any surface. Based upon reflections of a helium beam from LiF and NaCl cleaved surfaces, de Boer estimated the adsorption energy to be less than 100cal mole i. 

Another important property of the specularly scattered fraction of atoms is their great sensitivity to surface disorder. On scattering from a well ordered surface, nearly 15% of the scattered helium atoms appear in the specular helium beam. This fraction decreases to 1 to 5% when the surface is disordered. Thus measurements of the fraction of specularly scattered helium can provide information on the degree of atomic disorder in the solid surface. 

Recent measurements utilizing the crossed-beam technique have been performed as follows.37 A metastable helium beam is formed by electron-impact excitation of a thermal helium beam effusing from a multichannel array. The optical quenching method12 described earlier is applied to obtain results for He(2 5 ) and He(23S) separately. The target gas beam is... 

Scattering studies with metastable atoms are in many cases easier (and less expensive) than experiments with ground-state atoms, The discussion that follows is mainly concerned with helium, as most of the information is available for this atom. Figure 2 shows a skeletal setup of the experiment. A helium beam from a supersonic nozzle source is excited by electron impact to its two metastable states. The singlet state can be quenched by the 2g radiation from a helium-gas discharge lamp ... 

Figure 3. Newton diagrams for the two sets of measurements shown in Figs. 4 and 5. On left, He beam source is cooled to liquid-nitrogen temperature and ground-state helium beam is at room temperature, whereas beam temperatures are interchanged on right. This gives same center-of-mass kinetic energy but different laboratory energies for scattered atoms.

Figure 7. Time-of-flight distributions of metastable helium beams. Singlet distributions are generally narrower than triplet ones. The 540-meV distribution was obtained with plasma-jet source (see Section III.A.6).

The second peak, at larger flight times, which has exactly the velocity of the unexcited helium beam, is caused by a subsequent resonant energy transfer ... 

From specular reflection experiments with molecular beams of hydrogen or helium (4) it may be concluded that cleavage surfaces of LiF or NaCl are very smooth surfaces indeed, the inequalities being caused only by the temperature movement and amounting to the order of magnitude of 10 8 cm. Diffraction spectra obtained with helium beams (5), indicate moreover (6) that the distance between two fluorine ions in the outer layer of LiF is exactly the same as in the crystal. 

Finally, we wish to mention that, at Harvard University, another laser spectroscopy experiment was carried out in a discharge cell, and not in a clean environment as the metastable helium beam of the other experiments. The uncertainty of this unpublished measurement is 8 kHz [11], probably due to systematic effects from collisions in the helium cell [13]. 

We detected the saturated fluorescence emitted by a beam of 23S metastable atoms as they cross at right angle the slave laser light. A 1015 atoms/s.sterad flux of metastable helium atoms was produced by electronic collisions in a DC discharge of a helium atomic beam, similar to that described in [15]. To improve the precision of the linecenter determination, we increased the signal-to-noise ratio S/N by means of standard frequency modulation the third harmonic demodulated lineshape is shown in Fig. 4. The function expected for a Lorentzian spectrum was fit and linecenters were calculated with an uncertainty ranging between 10 kHz and 20 kHz, that is consistent with the observed S/N, mainly limited by the stability of the reference frequency and of the metastable helium beam. The reproducibility was two or three times worse than the uncertainty,... 

Both spin components of the lowest rotational transition were observed, and an improved value of the rotational constant obtained. The technique was also applied to study the rotational spectrum of the CN radical, produced when methyl cyanide was added to the helium beam. In this case the rotational transition exhibited splitting due to the spin-rotation and nuclear hyperfine interactions. The results were essentially the same as those described in chapter 10, obtained from a conventional microwave absorption experiment. 

Since about 10 years ago (thermal) helium beams have been used for the diagnostics of fusion boundary plasmas as they can penetrate relatively far because of the high ionization potential of the atoms (nearly 25eV) [61,62]. From the line ratios of the triplet and singlet lines one can derive local electron temperatures and densities (Fig. 6.18) provided the population rates and their equipartition times are known and allow the application of a steady state model [63], The corresponding rates have been improved during the last few years, and although it is now a well-established technique, there are still open questions and scope for future developments. 

G. Comsa, G.H. Comsa, and J.K. Fremery. Peculiarities of the Helium-Beam Scattering on Metal Surfaces. Z. Naturforsch. 29A 189 (1974). 

This study used helium scattering to locate energy levels of bound states of He/LiF((X)l). If the angle and kinetic energy of the helium beam are varied, dips in the surface reflectivity will occur whenever the atom s kinetic energy perpendicular to the surface equals one of the discrete bound-particle energies of the surface-potential well. In the case in question, four bound levels were observed this information could compare to several different theoretical models. 

The incident helium beam energies in this instrument currently can be varied from about 15 to 60 meV by suitably cooling the nozzle source. These values correspond to incident wavevectors of — 5 to 11 A , or de Broglie wavelengths (27t/A ,) of about 0.6 to 1.3 A, and provide a good match with the energies and momenta of the surface phonons for determining the surface phonon dispersion. 

An interesting application of the theory was made by Devonshire (73) in an attempt to explain the anomalous diffraction of helium beams at crystal surfaces observed by Frisch and Stem (74). It was found that at suitable angles of incidence impinging helium atoms need not be reflected from the surface, but could move along it in a mobile state for some distance before being emitted. It could also be shown that... 

Poelsema B, Mechtersheimer G, Cotnsa G The interaction of hydrogen with platinum(s)-9 (111) X (111) studied with helium beam diffraction. SurfSci 111 519-544, 1981. 

---