Heat radiation.....

[Cheman]

When you heat up an object by radiation (ie - hit it with infrared photons) it gets hotter. Therefore, its atoms/ molecules must be vibrating faster. However, I have also been told that when an electron absorbs a photon it moves to a new evergy level and then releases the photon again as it returns to its ground state. So, when do these two different phenominum occur? ie - what must the photon in the 1st senario strike, etc

[mathman]

It depends on the frequency of the photons. Lower frequency photons interact with molecules (like microwave ovens). At higher frequencies (light, near infrared, ultraviolet, and higher) electrons get excited. Einstein's Nobel prize was for analyzing how this works - photoelectric effect.

[Nenad]

ya, it all depends on the energy of the incoming photons. As you know, E = hf, or E = hc/(wavelength). The actual function for photoelectric effect is hc/(wavelength) = Wo + Ek. Where Wo is the work function of the metal or object. This work function represents the amount of energy needed to eject an electron out of orbit. But if the energy of the photon is too small (<Wo) then the electron will not be ejected but it will only absorb some of the energy, oscilate, and increase temperature. This is what seperates the two scenarios. Does that answer your question.

[Antimatter]

There are generally 4 methods in which an incoming photon can be absorbed or lose energy: Photo-electric effect, Compton effect, Pair production [high energy incoming photon (E>1.022MeV for e- e+)] , Photo-nuclear [(gamma,n); (gamma,p); ...] reactions [high energy incoming photon (E>threshold energy for reaction with given isotope, MeV range)] . From the infra-red part of your question, I would like to add that no matter if it's IR, visible, UV, your photon will normally be absorbed and 'heat up' the radiated body (except like gamma rays that risk passing your body unharmed due to large penetrative power). At normal temperatures, a body irradiates a spectrum in the IR range indeed (varries with Planck spectrum vs temperature), this is to cool down. So, with Compton-, and at a little more energy of your incoming photon, Photo-electric effect, you get secundary electrons, which can transfer energy to the atoms of your body by elastic scattering, causing your body to heat up.