Photon detection based on pulsed laser-enhanced ionization and photoionization of magnesium vapour: experimental characterization

Abstract

A resonance ionization detector (RID) based on the two-step laser-enhanced ionization of Mg in a miniature air–acetylene flame is described. The detector utilizes the 285.213 nm resonance absorption of Mg as the signal transition, i.e., photons to be measured. Magnesium atoms excited by absorption of photons at 285.2 nm were further excited to higher lying energy states by absorption of photons at 571.2 or 435.2 nm, from which collisional ionization could occur, or by absorption of photons at 300.9 nm, which directly ionized the excited Mg atoms via an autoionizing level above the ionization continuum. The miniature flame used (about 2 mm in diameter) had very low noise characteristics and the minimum detectable number of photons (MDP) was limited, in some instances, by the noise of the transimpedance amplifier used. The lowest MDP, obtained for the excitation scheme 3s²¹S (285.2 nm)→3p ¹P⁰(435.2 nm)→6d ¹D, was experimentally determined to be 1 × 10³(7 × 10^–16 J). The quantum efficiency of this excitation scheme, defined as the number of electrons created per photon absorbed at the signal transition (285.2 nm), was found to be 0.75. The spectral response bandwidth of the detector in the signal transition was determined to be mostly Lorentzian in nature, owing to pressure broadening in the atmospheric pressure flame, with a stray light rejection ratio of approximately 1 × 10^–5 at 100 cm^–1 displacement from the absorption maximum of the RID at 285.213 nm. An application of the RID is given in the detection of weak Raman scatter of carbon tetrachloride, chloroform and dimethyl sulfoxide.