Measurement of relative extreme-wing absorption coefficients by excited-state degenerate four-wave mixing

The population of the 3P state in atomic sodium produced by absorption in the extreme blue wing of the D lines is measured by excited-state degenerate four-wave mixing (ESDFWM). The strength of this ESDFWM signal is shown to give a measure of the pressure-broadened extreme-wing absorption coefficient alpha ( omega ). Relative values of alpha ( omega ) for the rare gases He, Ne and Ar are measured and compared with values calculated from theoretical interatomic potentials and from extreme-wing emission data. Very good agreement is found demonstrating the ability of this nonlinear technique to make accurate measurements of small excited-state densities with excellent spatial and temporal resolution. Advantages of the method for other applications are briefly discussed.