Physical state effects on the mean excitation energy of water as determined from. cap alpha. -particle stopping-power measurements

Recent measurements of the stopping powers of liquid water and water vapor for 2.0- to 5.5-MeV ..cap alpha.. particles have been analyzed with Bethe-Bloch theory including a Barkas-effect correction term in order to extract mean excitation energies reflecting the two physical states of the target substance. The resulting value of the mean excitation energy of liquid water, 68 eV, exceeds that characteristic of water vapor by about 13%. Although the difference in values is consistent with expectation in both magnitude and direction, the mean excitation energy of liquid water lies some 10% below a recent theoretical prediction.