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Metal colors and unoccupied levels

  1. Sep 27, 2007 #1
    Hi all,

    It is my understanding that the color of a material results from the inability (or lesser ability) to absorb in that particular wavelength. Reading Atkins' "Physical Chemistry" (8th ed.), I run into a statement I couldn't quite get. On page 730 (chap. 20), he states the following:

    So far it makes perfect sense. But then comes trouble:

    If copper has fewer unoccupied energy levels that can be excited with violet, blue and green light, wouldn't that make it absorb less light of those colors and therefore exhibit a blue-ish color? And as far as I know (and as unambiguously stated in the end of the above quote), copper is red-ish.

    Any light on this would be most appreciated. Thanks in advance.
     
  2. jcsd
  3. Sep 28, 2007 #2

    Gokul43201

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    Reflectivity of a metal in the visible-UV range is related to the plasmon dispersion of that metal (rather than to the single electron states). A plasmon is a vibrational mode of the entire free-electron gas living in the background of a periodic, positively charged lattice. This electron gas has a natural frequency of oscillation, known as the plasma frequency. For most metals, the plasma frequency lies somewhere in the far UV region.

    When the frequency of the incident light is smaller than the plasma frequency, the electron gas responds to the oscillating E-field and screens it out. So, for frequencies below the plasma frequency, the light can not be transmitted through the metal and hence, is almost entirely reflected back. When light has a frequency higher than the plasma frequency, the electron gas can not respond fast enough to screen out the oscillating E-field inside the metal. As a result, such high frequencies are almost entirely transmitted through the metal (and hence, most metals are transparent to UV).

    Now, for most metals, the plasma frequency is far into the UV regime, making the metal reflact all visible wavelengths almost equally. This is why most metals look a lustrous silvery-white. Some metals, like gold and copper, have a lower plasma frequency (closer to violet/near UV regime), making them better reflectors of lower frequency light than of high frequency light. As a result, the reflection spectrum of such metals is weighted more at the lower frequencies, giving them a reddish or yellowish color.
     
    Last edited: Sep 28, 2007
  4. Sep 29, 2007 #3
    Wow! That was heavy but extremely enlightening.

    Thanks a million for sharing.
     
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