We have developed a photoionization method for complete conversion of a quantum-selected population to ionization, making possible sensitive and absolute measurement of the selected populations in a gas. Each photoionization involves the absorption of two photons (from a pulsed dye laser), one of which is resonant with an intermediate state. In this demonstration we measured the absolute number of $\mathrm{He}\left(2\mathrm{}{}_{}{}^1{}_{}{}^{}S\right)$ states per ion pair following interaction of pulses of 2-MeV protons with He.

• Received 5 May 1975

DOI:https://doi.org/10.1103/PhysRevLett.35.82