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So how does a scanning tunnelling microscope work?
And, of course, we are not 'seeing' anything because we see with light and light just goes round atoms because its wavelength is too long for it to show anything. We could only see pictorial representations of how it interacts with electrons or, possibly, em waves of much shorter wavelength.
Don't understand why a single atom can't scatter visible light.
It's to do with the wavelength and wave-particle duality.
Quote from: lightarrow on 13/04/2008 22:07:23Don't understand why a single atom can't scatter visible light.Molecular scattering of light definitely occurs in the atmosphere (the blue sky isn't just because of dust), according to my old Professor Ditchburn's book, 'Light'. But that doesn't mean that you can actually see an image.
Scattering must involve some energy transfer. If an object is very small, all things being equal, then it will not interact with much of the incident radiation.
If the atom happens to be resonant at the incident frequency then it can absorb a photon and then re radiate it. The radiated wave will be omnidirectional (or at least cover a very wide angle).With or without the resonant condition, you can look at things classically and consider how a simple electric dipole interacts with a radio wave. When the dipole is a half wavelength in length it is 'tuned' and will resonate strongly with the wave. A lot of power will be absorbed and re radiated (all around), giving strong scattering. (The thin wire actually behaves as if it has a large area as far as absorbing energy is concerned). As the dipole is shortened it is 'off resonance' and interacts less and less with the incident wave- producing less scattering. It presents less and less of a scattering cross section.An atom will have a size which is a tiny fraction of optical wavelengths so it will scatter very little power. Rayleigh scattering of light in the atmosphere demonstrates this effect; the amount of scattering is roughtly proportional to 1/λ at these wavelengths.You can use extra components to tune (resonate) an antenna, artificially and you can increase its scattering cross section so that a very short dipole can interact with long wavelengths. Medium frequency receivers use a resonant antenna (ferrite rod) to extract significant energy from a radio broadcast although they may be a 1/5000 λ in size. This is a link to the quantum level interaction of atoms with waves.Diffraction: In order to see an image, the object must produce a diffraction pattern which is recognisable. You have to 'see' a difference in brightness at different angles and, if the diffraction pattern is broader than the object then you just won't locate an image in any particular place - it will be 'totally fuzzy' (to use a technical term). The image is dominated by the fuzziness of the edges and there is no significant central area which can be identified as 'the atom'.
It's to do with the wavelength and wave-particle duality. Because light behaves as an electromagnetic wave (or a probability wave if thinking of it as a particle), objects much smaller than the wavelength have little effect on the wave. It is only when the object's size gets to be similar to that of the wavelength that the wave gets significantly affected. Optical microscopes (with visible light) can't resolve images of objects below about 1000nm. They look rather fuzzy. This is because the wavelength of visible light is between 400nm and 700nm. You can still get photographic images by using ultra-violet light which extends down to about 10nm, but of course the eye can't see this directly. Semiconductor devices, with fine patterns, are still made using deep UV light and photoresists that react to the UV (in a similar way to how photographic film reacts chemically to visible light). To resolve images below these dimensions needs shorter wavelengths such as X-rays, but these are difficult to manage and translate into images. It is not easy to focus X-rays for example, although X-ray lithography is used in some cases. For even finer resolution electrons are used and they can be controlled with electric fields and magnets. They can also behave as a wave and have similar limitations but the effective wavelength is much shorter, allowing much finer image resolution.
When IBM published their famous iconic image in 1990 showing IBM spelled out in atoms of xenon, the atoms resembled small grey footballs.But what are we actually looking at when seeing those footballs, and how was that image produced?...So how does a scanning tunnelling microscope work?
If it could be possible to enlighten an atom (let's say as big as 10-10 m) and see it through a 1 mm lens using visible light (500 nm) at 10 atoms' distance, the resolution would be 5*10-13 m, that is you could see 500 parts of that atom.
Quote from: lightarrow on 14/04/2008 18:37:04If it could be possible to enlighten an atom (let's say as big as 10-10 m) and see it through a 1 mm lens using visible light (500 nm) at 10 atoms' distance, the resolution would be 5*10-13 m, that is you could see 500 parts of that atom.I suspect that you're using Fresnel diffraction theory (i.e. the resolution is ~λf/a, where f is the focal length, and a is the lens diameter). However, you're far too close to the lens to use Fresnel diffraction, which requires that the angle that your lens subtends as you look at it from the focal point be small. You'll need to do a full electromagnetic treatment of how the field interacts with the lens in order to calculate what you'll see at such a short distance. My guess is that any image will be swamped out by evanescent field components (which quickly decay as you move further from the lens). I don't know what kind of information you can extract from an image like that, but it certainly won't be a traditional "image" of the atom.Now, if you want to use Fresnel diffraction theory, you need to be roughly paraxial. For simplicity you could assume that your lens diameter and lens-to-image distance are approximately the same. In that case, the smallest feature you can image is ~λ, the wavelength of the light.
Everything correct, but, again, I don't understand why should theoretically be impossible to have an atom's image.
As someone who wrote software to process images from an STM way back in ... 1988-1990, allow me to explain. In principle, you have a very fine probe "needle" tip (typically made of tungsten) and a conducting surface to observe. The needle tip is typically mounted on a 3-axis piezo transducer, so applying a high voltage to the piezo moves the tip around on a very fine scale. You may have a second, larger, piezo transducer and/or mechanical means to get coarse "tip-approach" control...
What amazes me about it is that you can get such a fine "needle" tip and move it with such precision.How fine must the tip be to move just one atom only?