Photoexcitation-induced desorption occurs through three processes: (1) photoexcitation, (2) bond cleavage, and (3) desorption.

① Photoexcitation
When an electron in a σ orbital (bonding orbital) absorbs light with a wavelength corresponding to the energy difference between the σ and σ* orbitals, it is excited to the σ* orbital (antibonding orbital).
By using high-intensity vacuum ultraviolet light, the sample can be efficiently excited even in thin-film samples with short optical paths.

② Bond Cleavage
When one electron occupies the σ orbital and another occupies the σ* orbital, their combined energy is the sum of the energy levels of both orbitals. When the bond energy is negative, cleavage occurs in the σ bond.
③ Desorption
The cleavage of σ bonds generates various fragments. In some molecules, recombination occurs on the surface. If the generated fragments are not bound within the sample, desorption occurs.
Next, we will explain this using the potential curve (Figure 1).

Figure 1: Potential Curve

In the state where two electrons are in the bonding orbitals (green potential curve in Figure 1), they are distributed according to the Boltzmann distribution across each vibrational level.
When vacuum ultraviolet light with a wavelength that induces an electronic transition in this bond strikes the system, it transitions to a state where there is one electron in each of the bonding and antibonding orbitals (red potential curve in Figure 1).
Molecules with a potential energy of zero or greater undergo bond cleavage.


　Figure 2: Cleavage by Vacuum Ultraviolet Light

On the other hand, when multiple bonds or fragments are bound, the bond may not break completely, or even if it breaks, the fragments may not be able to escape.


Figure 3: Case where the bond breaks but fragmentation fails (1)


　Figure 4: Case 2: Bond is cleaved but fragment does not detach