Hi Everyone, I am new to this forums. I am not sure where to post but general discussion is a good place. I am trying to figure out a problem: I have a sample X in gas phase. Say it has an absorption coefficient of 9.895e-18 cm2 molecule-1 @ 532nm. I have a laser pulsed source (say 50 Hz) giving me 532nm light. How does one calculate what energy one needs to excite 5% or any percentage of the sample? The measured power of the pulsed laser is related to the energy by: E=power/rep.rate and one can use this to calculate the number of photons N,by E=Nhc/lambda where lambda is the wavelength (in the example above it is 532nm). we can also calculate the fluence by measuring the spot size as we focus the laser beam into the sample. I can't figure out how people choose the energy/power from absorption cross section calculations? I am a novice, so need a little help please. Thank you. sciphys1