Biophotonics_Concepts_to_Applications

To have a phenomenological understanding of the tissue photoablation process,
consider two atoms that are bound by a common electron in a molecule. If the
energy of an incident photon is less than the atomic bonding energy, absorption of
the photon excites the common electron to a higher molecular vibrational level. In
the case when the photon energy is higher than the atomic bonding energy, then the
electrons are raised to non-bonding orbitals, as is shown in Figs.6.6and6.25. From
this excited state condition either the molecule willfluoresce (that is, the electron
drops back to a lower level and gives off a photon) or the two previously bonded
atoms disassociate (separate) at the next immediate molecular vibration. This
photodisassociation leads to a rapid expansion of the irradiated volume, which
basically creates small explosions of vaporized tissue and ejection of the tissue from
the surface.
For photodisassociation to take place, the energy of the incident photon must be
greater than the bond energy. The photoablation irradiation values are in the 10^7 –
108 W/cm^2 range with pulse durations on the order of nanoseconds. The bond
disassociation photoablation mode typically involves UV lasers, because high

Molecular dissociation

UV photon absorption

Molecular vibrational

energy levels

Morse potential

Interatomic distance (arbitrary units)

Energy (arbitrary units)

Fluorescence

Fig. 6.25 Photoablation can
lead to eitherfluorescence or
molecular dissociation

Plasma

threshold

Ablation threshold E 0 = Eab

0

Ablation depth z

ab

ln (E 0 )

zab=^1

μa

lnE^0

Eab

E 0 = Eplasma

Fig. 6.26 Ablation depth as
a function of incident
irradiation in photoablation

184 6 Light-Tissue Interactions