Yields and mean free paths of photoelectrons from liquid hexamethyl phosphoric triamide

Monochromatic photons between 775 and 500 A were incident almost normally (15°) on the surface of liquid hexamethyl phosphoric triamide (HMPT). Incident and reflected beam strengths were measured with a double ion chamber filled with argon and the current leaving the cup of liquid was determined. After subtracting a portion of the current due to Ar+ drift into the cup, we obtained the photocurrent and, hence, the absolute photoemission yield. The yield falls from 9% at 16 eV to 5% at 25 eV. Such a falloff with increasing photon energy is expected from theory. The values are about double those calculated from theory using a three‐step model for the photoemission and using the Born approximation to calculate electron mean free paths. Conversely, if the experimental yields are used in the three‐step model, the electron mean free paths calculated from the data range from about 935 A at 16 eV to about 80 A at 24 eV and are about two to three times the theoretically predicted values.