Page 268 - Mike Murphy's Blog Posts

light – we lose this amount each time we fire a laser pulse into the skin! (Note: not all Fresnel Reflections occur in the stratum corneum – they simply begin there).
The next layer is the top of the epidermis – the stratum granulosum layer (we will ignore the lucidum since this is only found on the palms and the soles). We will undoubtedly encounter something here – an atom or a molecule. There are plenty about, so it’s unlikely we will miss them.
Incidentally, it will have taken between around 0.3 and 3 nanoseconds to have reached this point from the laser (depending on which laser construction you are using – handpiece or articulated arm).
So, we finally ‘meet’ an atom (or molecule). A couple of things might happen:
1. We might just pass by with no interaction at all; we will continue our flight;
• We might encounter an electron in an outer shell of the atom. Now, what happens next is determined by the ‘absorption’ or ‘scattering’ probability of this atom. These probabilities are dependent on both the atom and the wavelength of the light. If the absorption probability is high, then the electron will interact with the photon and ‘steal’ its energy. This is where the photon’s journey ends. It has ceased to be. It is now an ex-photon!
• When the electron has absorbed the photon’s energy, its own energy increases. This causes it to ‘jump’ to a higher orbit around the atom’s nucleus (according to the ‘Bohr Model of the Atom’). But, electrons are funny buggers – they don’t like heights!! So, soon after jumping up, it will drop back down to its original orbit.
• At this point, one of two things may happen:
a. If the conditions are right, a new photon will be created using the energy which the electron had gained from the old photon. If this is an ‘elastic’ situation, then the new photon will have exactly the same energy, and hence wavelength, as the old photon. (Energy and wavelength are linked by Planck’s Equation).
• If a different set of conditions exist, then a new photon will not be created. Instead, the energy released by the electron dropping back down, will be released into the local environment as vibrational energy which may be thought of as thermal (heat) energy. If many photons are absorbed by some material, this vibrational energy will be felt as heat as the temperature rises.
• If the original photon’s energy is lost as heat, then that photon is dead!
• However, if a new photon has been created, then we say that a ‘scattering’ event has occurred. Scattering is not ‘photons bouncing off atoms’ – it is, in fact, an absorption event followed by the creation of a new photon.
Mike’s Blog Posts 268