  Current and future experiments need to know the stopping power of liquid
argon. It is used directly in calibration, where commonly the minimum-ionizing
portion of muon tracks is used as a standard candle. Similarly, muon range is
used as a measure of muon energy. More broadly, the stopping power figures into
the simulation of all charged particles, and so uncertainty propagates widely
throughout data analysis of all sorts. The main parameter that controls
stopping power is the mean excitation energy, or I-value. Direct experimental
information for argon's I-value come primarily from measurements of gaseous
argon, with a very limited amount of information from solid argon, and none
from liquid argon. A powerful source of indirect information is also available
from oscillator strength distribution calculations. We perform a new
calculation and find that from oscillator strength information alone, the
I-value of gaseous argon is $(187\pm 5)$\,eV. In combination with the direct
measurements and other calculations, we recommend $(187\pm 4)$\,eV for gaseous
argon. For liquid argon, we evaluate the difference in central value and
uncertainty incurred by the difference of phase and recommend $(197\pm 7)$\,eV.
All uncertainties are given to 68\% C.L.
