interaction of laser with matter

Discussion in 'Chemistry' started by star_2121, Apr 20, 2009.

  1. star_2121 Banned Banned

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    An atom can absorb a photon by raising an electron from the ground state to an excited state. A
    short time later, spontaneous emission takes place, and the excited atom returns to its ground
    state by emitting a photon which has energy equal to the energy level difference between the
    excited and ground states. The direction and phase of these spontaneously emitted photons are
    completely random, which renders them useless if one wants a directed beam of light.
    Albert Einstein proposed an idea he called stimulated emission of radiation, which is the basis for
    all laser light emission (laser is the acronym for Light Amplification by Stimulated Emission of
    Radiation). He reasoned that if a photon with the same frequency as the spontaneously emitted
    photon scattered from an atom that was in its excited state, the atom could release a photon
    identical to the first in frequency, direction, phase and polarization. The two photons are thus,
    coherent.
    The atoms inside a laser tube have to be brought to the necessary excited state independently of
    the photons. This process of raising atoms to a higher energy level is known as "pumping".
    Lasers can be pumped in several different ways. The helium-neon lasers you will use, work by
    applying a high voltage across the laser tube. This causes the helium inside the tube to absorb
    energy. After gaining energy, a helium atom eventually collides with a neon atom which absorbs
    the energy that was contained in the helium atom. The neon atom then spontaneously emits a
    photon which produces red light. This light corresponds to the strongest and most visible of the
    wavelengths of light your helium-neon laser emits.
    For a continuously emitting laser, though, a majority of the atoms must be pumped to an excited
    state, that is, there must be a population inversion. This characteristic is maintained by pumping
    atoms at the same rate energy is lost. Also, two mirrors are used to amplify the beam. One
    mirror is almost totally reflective while the other is about 99% reflective. These mirrors are
    placed at opposite ends of the laser tube. This is done so that the photons emitted along the axis
    of the tube can then reflect back and forth through it, so that they may have a much higher chance
    of colliding with an excited atom than light traveling in other directions. This increases the 51
    number of photons stimulated along the axis of the tube on each pass. About 1% of the light
    passes the aperture on each reflection and forms the coherent laser beam.
    Laser light is highly directional, monochromatic and very bright. It is directional because only the
    light traveling parallel to the long axis of the laser tube is amplified by stimulated emission,
    multiple times. The monochromaticity of a helium-neon laser emission occurs because of the
    single transition energy yielding visible photons. The high intensity is due to the large number of
    sources emitting in coherence.
     
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  3. Xylene Valued Senior Member

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    The main problem with laser as a weapon, IMO, is that the light beam spreads out as it travels, and therefore becomes less intense. In order to avoid this, I'd argue that you'd want to use a group of lasers which can be focused in unison onto one particular point at any chosen distance. This would compensate for the beam's propensity to spread out and become ineffective. The only problem to worry about then would be obvious drawback that lasers bounce off mirrored surfaces.
     
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  5. Oli Heute der Enteteich... Registered Senior Member

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    That would be why the beam can (would) be focussed to a specific range.
    In an atmosphere thermal blooming is a major problem, a US/ German study published some years ago on weaponising an FEL (Free Electron laser) gave power losses of 30% per mile of atmosphere traversed at sea-level.
    And a mirror isn't that effective since a major part of laser damage is caused by thermal shock amongst other things.
    One early study of SDI orbital platforms showed that an ICBM warhead killer laser required somewhere around 1 tonne of liquid nitrogen per second simply to cool the focussing mirrors, (from memory).
    And those mirrors would have been designed specifically to reflect the laser - unlike any potential target.
     
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  7. PieAreSquared Woo is resistant to reason Registered Senior Member

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  8. James R Just this guy, you know? Staff Member

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    Do you have a question, star_2121? It looks like you've just copied a description out of a textbook. Why?
     
  9. Trippy ALEA IACTA EST Staff Member

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