Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Light's interaction with particles

  1. May 27, 2005 #1
    From what i have read, electron energy is not the only thing split into energy levels - also vibrational, rotational and translational energy is quantized. But which part of the atom absorbs the electromagnetic waves which cause these energy levels to increase? Is it still the electrons as with electrons energy levels, and if so how do these electrons affect the atom as a whole as they do?

    Thanks in advance. :wink:
  2. jcsd
  3. May 27, 2005 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The kinetic energy of an atom comes mostly from the nucleus, which is way heavier than the electrons...but really, typical vibrational modes in a molecule like H2O are separated in energy equivalent to those of microwave photons, whose wavelength is a gazillion times bigger than an atom. Discard the classical picture; it does not work at the microscopic level. Besides, electronic levels are themselves a result of both the electrons and the protons in the nucleus (and are not the property of the elctrons alone).

    The energy of the photon goes into the atom as a whole.
    Last edited: May 27, 2005
  4. May 28, 2005 #3
    Does it not depend on the frequency??? Microwaves are so large they affect the molecule as a whole (and make it rotate), Infrared radiation affects the electrons more (because the bonds vibrate due to electron contraction due to increase and decrease in energy) and UV and visible light affect the ionisation energy and have the capability to release an electron from the hold of the nucleus. Gamma rays affect the nucleus.

    I may be wrong but Bohr's model uses a hydogren atom and different regions of electron excitment depend on the frequency of the radiation the electron absorbs. Just because it is a molecule should not make too much difference to what absorbs the energy. Because of the wavelength there is some disagreement (microwaves for one) but anything higher than this (infrared) must surely be electron based. Bonds are due to electrons and so to make then vibrate it must be the electrons that are affected (most if not completely).

    Please correct me. :smile: I do not want to believe something that is incorrect. I also apologise if what I have said in this post sounds harsh.

    The Bob (2004 ©)
  5. May 28, 2005 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The Bob,

    I'm going to be a little harsh on you and point out some of the errors in your argument. I hope this will help elucidate what makes for a sound scientific argument and what doesn't. I intend no ill-will.

    What is electron contraction ? There is no such thing that I'm aware of. Also, what do you mean by "bonds vibrate" ? A bond is merely a spatial configuration of electron density. To make a bond vibrate, you must vibrate the things that are responsible for the bonds. Some of the modes of "vibration" of this bond would necessarily involve the motion of the nucleii relative to each other. In an isolated diatomic molecule, the primary mode of vibration involves changing the internuclear spacing.

    Do not say that they "affect the ionisation energy". They do not. The ionisation energy remains practically unchanged/unaffected.

    The energy of an isolated atom can be crudely written down as the sum of three terms : (1) the KE of the nucleus, (2) the KE of the electrons, and (3) the PE from the electrostatic interaction between the electrons and the nucleus (for now, we neglect the interactions between electrons themselves). When a photon ionises an electron, it changes (2) and (3)...so how is one justified in saying that only electrons are affected ?

    How ? What happens to the nucleus when you shine gamma rays on an atom ?

    What are "regions of electron excitement" ? You are right if you mean that different electronic transitions involve different frequencies. But even the term "electronic transition" refers to transitions between levels that are created by the electron's interaction with the nucleus.

    You have not shown why it is logically consistent to extend previous ideas from a hydrogen-like atom to say, a large polysaccharide.

    "Disagreement" over what ?

    "Electron based" ? You are once again using imprecise terminology. Didn't you just say something about gamma rays and the nucleus ?

    But are they due to electrons alone ? Would I expect identical structures from isoelelctronic molecules/radicals ?

    Where is the justification for the part within brackets - "most if not completely" ?
    Last edited: May 28, 2005
  6. May 29, 2005 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    Yes, the result of an interaction between a photon and an atom or molecule does depend on frequency, however, it is primarily the electrons which interact with photons. The exceptions would be in the extremes of energy.

    A molecule can behave as a dipole (on atom having more affinity of the electron(s) than the other), and the molecule itself will respond to EM.

    As Gokul pointed out, an atomic (molecular) bond is nothing more than an artifact (effect) of the electron spatial distribution, and each bond, which is characteristic of the atoms (elementally speaking) responds to a particular energy of EM.

    At the other extreme are gamma rays. Gamma rays are by convention formed by nuclear and sub-atomic particle phenomenon (e.g. decay, disintegration or annhiliation). Once formed, gamma rays will interact primarily with electrons, which are after all, more spatial distributed than the nucleons. Gamma-rays will ionize atoms (photoelectric effect) or scatter electrons (Compton scattering). Above a threshold of ~1.0221 MeV, a gamma ray may interact with an atomic nucleus resulting in the production of a positron-electron (pair production). Gamma-rays of an energy above 1.67 MeV may interact with Be-9 to produce a neutron (photo-neutron interaction). This is the basis of the Sb-Be photon neutron source which is used as a startup source of neutrons in new nuclear reactors, which have an entire core of fresh fuel.

    X-rays by convention are those photons formed by electrons dropping into the K or L shells of atoms, which is coincident with a K or L shell electron being 'knocked out' in the first place.

    A good source of information on radiation, both photon and particle, is the Radiological Health Handbook. My copy from 1970 was published by US Dept. of Health, Education and Welfare (Bureau of Radiological Health). It may have been revised in 1992.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook