# A spectroscopy question

1. Jan 5, 2015

### sciphys1

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.

2. Jan 5, 2015

### Bystander

Einstein coefficient for absorption, okay.
Wavelength and "flux." Okay. Pulse duration.
Number excited. (Past tense.) Steady state percentage? Maybe.

3. Jan 8, 2015

### Quantum Defect

You need to know a lot of additional information to be able to do this. The absorption cross section is likely the integrated cross section over the entire spectrum. You need to know if the spectrum is diffuse or sharp -- i.e. are you able to excite the entire ensemble of molecules or only a small subset? What is the laser linewidth, compared with the linewidth of the spectrum you are exciting. If the spectrum is sharp, and you know what rovibrational state of oxygen you are exciting, what is the temperature?

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook