OK chem-wizards... I'm looking at a paper which states "....the integrated absorptivity of the stretching vibrations of a water molecule is 4.89 cm micro mol^-1" (I'm transcribing a 'mu' character as micro in the above. If mu doesn't stand for micro, then let me know!) OK, so some questions: 1) Is the paper quoting an integrated 'molar absorptivity'? 2) The integration is over the frequency axis, but is this frequency in wave-numbers? 3) Is this defn. for molar absorptivity correct? Molar Absorptivity,? = A/ c l ( where A= absorbance, c = sample concentration in moles/liter & l = length of light path through the cuvette in cm.) taken from http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/uvspec.htm 4) According to this Wikipedia article, absorbance is calculated as a base 10 logarithm of I/I0. Is that defn. universally used? http://en.wikipedia.org/wiki/Absorbance 5) I can't understand the units. If A=absorption and alpha = absorption coefficient and L=sample length, then A=alpha L, so that means that alpha is in units of inverse length. If e=absorptivity, and c is the concentration in mols per liter and e=A/cL=alpha/c, then e is in units of mols^-1 liters ^-1 cm^-1. Thus, I'd expect the integrated absorptivity to be in mols^-1 liters^ -1 cm ^-2, given that the integral is over wavenumbers which have units of cm^-1. 6) Basically what I want is to convert the number in the paper into a value for the absorption coefficient. Thanks in advance for any help!