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Journal of Analytical Atomic Spectrometry

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

Nicol� Omenetto, Benjamin W. Smith, Paul B. Farnsworth, James D. Winefordner

Published Jan 1, 1992• 3 citations
Publisher

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.

DOI
10.1039/ja9920700089

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Journal of Physics B Atomic Molecular and Optical Physics•Dec 14, 1991•12 citations
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.
Polarization and intensity spectra have been obtained for excited-state magnesium (3s3p 1P1) - rare gas optical collisions. The magnesium was excited by a linearly polarized laser tuned to the 3s21s0 → 3s3p 1P1 atomic transition at 285 nm. A second linearly polarized laser was tuned near the 3s3p 1P1 → 3s5s 1S0 transition at 571 nm (experiment A) and near the 3s3p 1P1 → 3s4d 1D2 transition at 553 nm (experiment B). The polarization vector of the second laser was oriented either parallel or perpendicular to the polarization direction of the first laser. The absorption of nonresonant light from the second laser, possible only during a collision, was monitored by cascade fluorescence from the 3s21S0 ← 3s4p 1P1 transition at 203 nm. For each detuning of the second laser from resonance, the degree of linear polarization and the intensity were measured. For experiment A, argon and neon were used as rare gases. For experiment B, argon was the rare gas used. For experiment B, fluorescence from the 3s3p 1P1 ← 3s5s 1S0 transition, and the 3s3p3Pj ← 3s5s 3S1 transition at 333 nm were also measured in an attempt to observe the region of inelastic transitions in the molecular states. No inelastic branching was observed in this experiment.
We report what is to our knowledge the first experimental demonstration of detection of Raman photons by means of laser-enhanced ionization of magnesium atoms in an air-acetylene flame. A frequency-doubled dye laser was directed into a cell containing carbon tetrachloride. The resulting Stokes Raman photons were transferred into the flame, where they excited the magnesium atomic vapor into the 3p(1)P(1) state. The excited atoms were then pumped by another laser into the 5d(1)D(2) state, from which efficient collisional ionization occurred. We estimate a quantum efficiency of approximately 50% for the detector.
We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.
Laser-induced photoionization and fluorescence signals were simultaneously observed in atomic uranium using a single Nd:YAG-pumped dye laser. These signals were recorded in two specific cases. In the first case, the dye laser was resonant to the first-step transition (0–16900.38 cm−1). In the second case, the laser was near-resonant to the first-step transition with a slight detuning (0.15 cm−1) so that it became two-photon resonant at 33801.06 cm−1. The uranium atoms in the ground state were ionized by a single-color, three-photon photoionization technique resulting in the photoionization signal, and the fluorescence signal was simultaneously obtained from the first excited state involved in the photoionization process. The photoionization and the fluorescence signals in the above-mentioned cases were also estimated theoretically for several values of the photoionization cross section for the transition between the second excited state at 33801.06 cm−1 and the autoionization state at 50701.59 cm−1 using density matrix formalism. From the comparison of theoretically calculated ratios of fluorescence signals in the two specific cases with the experimentally obtained values, the photoionization cross section for the 33801.06–50701.59 cm−1 transition has been obtained, which is found to be (5±1)×10−16 cm2.
An anomalous resonant dip of two-photon ionisation with 3s3p 1 P as an intermediate state and a stimulated emission by the 3s4s 3 S-3s3p 3 P transition, at 518.37 nm following the pulsed laser excitation near the 3s 2 1 S-3s3p 1 P resonance at 285.21 nm in magnesium vapour are observed and studied. The mechanism of the ionisation dip is attributed to formation and predissociation of the A 1 Sigma + u Mg 2 dimer, which transfer the population from the laser-pumped 3s3p 1 P level to 3s3p 3 P levels. Re-excitation of the produced 3s3p 3 P atoms and the subsequent radiative processes are presumed to explain the generation of stimulated emission. A theoretical model is employed to interpret the experimental phenomena.
Multiphoton ionization (MPI) of barium atom using an excimer-pumped pulsed dye laser has been studied in the wavelength range 570-608 nm at a laser intensity approximately 10.0 GW cm -2 . Resonances in MPI were recorded by monitoring either the ions produced in the multiphoton process using a thermionic detector or fluorescence from the ions formed in their various excited states of Ba + . The observation of photoion and fluorescence spectra were made on Ba vapour in the heat-pipe oven in the moderate-density regime ( approximately 10 13 atoms/cm 3 ) so that the collisional effects are negligible. Fine-structure resolution of the final ionic state (6p 2 P 1/2,3/2 ), produced after four-photon autoionizing decay was obtained. The decay of the Ba atom after four-photon absorption preferentially populates the 6p 2 P 3/2 state relative to the 6p 2 P 1/2 state.