The ablation threshold is determined by the thermal properties of the material i. e. heat capacity, thermal conductivity etc. Ablation will occur when the incident power density (fraction that is absorbed) is high enough to set up a thermal gradient in the material that is sufficiently high to ensure minimal; melting and complete subsequent vaporisation of the molten phase. It should be relatively simple to model this process provided that the absorption coefficient can be determined.

Thanks a lot. What I guess is that I need to estimate the volume to which the energy would have been imparted possibly related to spot size on the target/ material to be ablated

I recommend reading the following books:

(1) Martin von Allmen /Andreas Blatter: "Laser-Beam Interactions with Materials", Spinger-Verlag, 2nd Edition 1995

[Please refer to section 5.2.1 Beam Heating and Evaporation]

(2) Dieter Bäuerle: "Laser processing and Chemistry", 4th Edition 2011, Springer-Verlag; [see Part III Material Removal]

It should be kept in mind that surface defects and contamination can drastically reduce the ablation threshold. The above methods make assumptions about absorption and hardness. However, contaminants that can greatly increase absorption are often present on the surface. Also, presence of cracks on the surface also reduces its mechanical hardness, further lowering the failure threshold.