To calculate the Keldysh parameter, I need to use the optical frequency of a laser in atomic unit. Since for the time: 1 a.u. = 2.42×10-17 s, I would assume that for the frequency: 1 a.u. = 4.13×1016 s-1 which is juste the inverse of the time one. However, I found several sources such as: http://www.diss.fu-berlin.de/diss/s...ISS_derivate_000000001950/08_app_C.pdf?hosts= and http://web.ift.uib.no/AMOS/PHYS261/2008/LATEX-work/AtomicUnits/Lene_Raymond_Abdul.pdf where for the frequency: 1 a.u. = 6.58×1015 s-1 which correspond to: 4.13×1016 s-1 / (2π). So I would assume that by frequency, they mean angular frequency and then it would make sense to me. But maybe I overlooked something and that is why I am posting here. Thanks in advance for any help.