Deflection of atoms

Compared to ground states then, the energy shifts produced by the van der Waals interaction for high n-states have two distinguishing characteristics the shifts scale as n " so the deflection of atoms can be observed at much larger distances and secondly the shifts can lead to observable splittings. 

Fig. 9.11 Deflection of atoms in a collimated atomic beam using a multiple-path geometry. The molecular beam travels into the z-direction (a) view into the z-direction (b) view into the y-direction. On the dashed return paths the laser beam does not intersect the atomic beam...

Another application is the deflection of atoms by photon recoil. For sufficiently good beam collimation, the deflection from single photons can be detected. The distribution of the transverse-velocity components contains information about the statistics of photon absorption [1207]. Such experiments have successfully demonstrated the antibunching characteristics of photon absorption [1208]. The photon statistic is directly manifest in the momentum distribution of the deflected atoms [1209]. Optical collimation by radial recoil can considerably decrease the divergence of atomic beams and thus the beam intensity. This allows experiments in crossed beams that could not be performed before because of a lack of intensity. 

Nebenzahl, A. Szdke, Deflection of atomic betims by resonance radiation using stimulated emission. Appl. Phys. Lett. 25, 327 (1974)... 

Fig. 1. Schematic diagram of a triple resonance atomic beam apparatus. Between source S (a hot oven) and detector D, magnets A and B produce inhomogeneous deflecting fields, and act as polarizer and analyzer magnet C produces a homogeneous field in which magnetic resonance transitions occur at loops A, B and C. Resonance is detected by the deflection of atoms away from the detector D, if the gradients in magnets A and B are in opposite directions.

Ffe. 2.2 Basic t) pes of the normal vibrational modes. Arrows show directions of deflections of atoms, plus and minus signs indicate deflections of atoms above and below the plane... 

In 1903, Rutherford and associates were finally able to deflect the a-rays by electric and magnetic fields, showing that these are positively charged. Measurement of the charge-to-mass ratio indicated that a-rays were of atomic dimensions. In 1908 definitive experiments showed a-rays to be doubly chaiged helium atoms, ie, helium nuclei. 

Yet another modification is atomic force microscopy (ATM), in which a fine tip attached to a tiny flexible beam is scanned across the surface. The atom at the end of the tip experiences a force that pulls it toward or pushes it away from the atoms on the surface. The deflection of the beam, which shows the shape of the surface, can he monitored by using light from a laser. 

Figure 4.29. Experimental set-up for atomic force microscopy. The sample is mounted on a piezo electric scanner and can be positioned with a precision better than 0.01 nm in thex, y, and z directions. The tip is mounted on a flexible arm, the cantilever. When the tip is attracted or repelled by the sample, the deflection of the cantilever/tip assembly is...

Figure 3.2 The essential elements of an atomic force microscope. The sample is moved beneath a tip mounted on a cantilever a laser beam reflected off the back of the tip and on to a photodiode amplifies deflections of the cantilever.

Thus, each atom experiences a force which is proportional to the z-component of its electronic angular momentum. By measuring the deflection of the beam Fz can be calculated. 

Figure 7.14 Experimental set-up for atomic force microscopy. The sample is mounted on a piezoelectric scanner and can be positioned with a precision better than 0.01 nm in the x, y, and z direction. The tip is mounted on a flexible arm the cantilever. When the tip is attracted or repelled by the sample, the deflection of the cantilever/tip assembly is measured as follows. A laser beam is focussed at the end of the cantilever and reflected to two photodiodes, numbered 1 and 2. If the tip bends towards the surface, photodiode 2 receives more light than 1, and the difference in intensity between 1 and 2 is a measure of the deflection of the cantilever and thus of the force between the sample and the tip. With four photodiodes, one can also measure the sideways deflection of the tip, for example at an edge on the sample surface.

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