Photon detection based on pulsed, laser-enhanced ionization and photoionization of magnesium vapour: quantum efficiency versus ion yield. Invited lecture

Abstract

The concepts of quantum efficiency and ion yield pertinent to a photon detector consisting of an atmospheric pressure oxygen–argon–acetylene (or hydrogen) flame containing magnesium atomic vapour, and also some published experimental results obtained with this detector, are critically discussed. In these experiments, collision-induced ionization and direct photoionization of magnesium atoms are performed. In the former instance, one dye laser excites the atoms from the ground state into the 3s3p–¹P₁⁰ state and another laser, in temporal and spatial coincidence with the first, brings the excited atoms into the 3s5d–¹D₂ state, from which collisional ionization occurs. In the last instance, direct photoionization from the ¹P₁⁰ level is accomplished either with the excimer laser radiation at 308 nm or by tuning the second laser into an autoionizing resonance at 300.9 nm. It is shown that the total loss rate from the ¹P₁⁰ level can be calculated by time resolving the resonance fluorescence waveform, with and without the presence of the second laser, and that, by integrating this signal over the duration of the laser pulse, the fluorescence dip obtained can be related to the ion yield of the excitation–ionization scheme. It is also stressed that, because of the presence of a metastable level which can act as a trap for the excited atoms, owing to quenching collisions in the flame, the quantum efficiency of this detector will never exceed the ‘ion branching ratio’, i.e., the ratio between the ionization rate and the total loss rate from the excited level. By evaluating several experimental results obtained with different laser systems, it is shown that, in the collision-induced ionization mode, both fluorescence and ionization measurements are necessary to derive the quantum efficiency, whereas in the direct photoionization mode, fluorescence data suffice.