Hi All, Lets say I know the absorption cross-section of a gaseous species e.g. O2. I have a pulsed laser of a certain energy and of pulse duration, which will hopefully guide me with the number of photons and I focus that laser beam into a stream of O2 gas. Can I calculate the percentage of molecules I will excite? I have been scratching my head over this problem apart from others (which I will ask later, one at a time!). It will help a lot if you can help with the clarification. Note: I am learning about spectroscopy but this is not a homework question. Thank you for your help. Mark.