The SPR absorption spectrum of a nano-rod is seen to have two peaks with the longer wavelength peak corresponding to the absorption by the longitudinal plasmons and the other to the transverse plasmons. When the intensities are compared.. longitudinal plasmon peaks are almost always more intense than transverse ones. Why?

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    $\begingroup$ this clearly belongs to physics SE $\endgroup$
    – permeakra
    Feb 14, 2018 at 13:56
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    $\begingroup$ The topic is taught as the part of a course that addresses nano sciences. Nano sciences being inter disciplinary and my being a chemistry graduate sure does make this relevant to the chemistry SE too. $\endgroup$ Feb 14, 2018 at 14:01
  • $\begingroup$ @permeakra I disagree, I think this belongs equally well on chemistry; the language of physicist in nano is different from that of chemists. $\endgroup$ Feb 15, 2018 at 18:01

1 Answer 1


The intensity of a plasmon resonance is related to the magnitude of the transition dipole moment. Longitudinal resonances have a larger transition dipole moment compared to transverse resonances, and so have more intense absorption peaks. You can rationalize this by imagining the change in dipole moment from a longitudinal versus transverse electron density oscillation.

  • $\begingroup$ I did think about that. But the catch is , the ratio of the intensities remain unchanged, even when the lengths of the nano rods are increased , keeping their diameters the same. The intensity seems to be dependent on something more in addition to the magnitude of dipole moment change.. $\endgroup$ Feb 15, 2018 at 13:26

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