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Optics Letters

Experimental demonstration of a Raman scattering detector based on laser-enhanced ionization

Ben W. Smith, Paul B. Farnsworth, James D. Winefordner, Nicolo’ Omenetto

Published Jul 15, 1990• 17 citations
Publisher

Abstract

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.

DOI
10.1364/ol.15.000823

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•May 30, 2002•3 citations
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.
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.
A report is given of the first quantitative study of the spontaneous and stimulated electronic Raman scattering by a resonance doublet of the sodium atom excited resonantly at the D 1 and D 2 lines by a tunable dye laser. Theoretical and experimental analyses of the differential scattering cross section and polarization properties at the Stokes and anti-Stokes frequencies show that the observed scattering is due to a nearly resonant process involving the ground state 3 S 1/2 as the dominant intermediate state. The stimulated Stokes and anti-Stokes scattering lines, which are shifted from the D 1 and D 2 lines by an amount corresponding to the frequency separation between lowest-lying excited doublets 3 P 1/2 and 3 P 3/2 , are observed by exciting a sodium vapor cell with a passively mode-locked dye laser.
The manifestation of spontaneous Raman scattering upon the three-photon ionization of a samarium atom has been investigated. The dependence of the Sm+ yield on the laser frequency, along with the peaks due to the two-photon excitation of bound states from different levels of the 4f 66s 2 7 F ground term, exhibits a strong peak due to single-photon excitation of the 4f 55d6s 2 7 D°3 state from the 4f 65d6s 9 H 2 excited state. The 4f 65d6s 9 H 2 state was populated as a result of spontaneous Raman scattering. The large amplitude of the observed peak indicates a high efficiency of this process. The estimations performed indicate that the probability of resonant ionization through an excited state populated due to spontaneous Raman scattering can be comparable with the probability of three-photon ionization through the two-photon intermediate resonance. The necessary conditions for implementing this channel of resonant three-photon ionization are formulated.
Rubidium vapor (1011 - 1014 cm-3) was irradiated by a dye-laser beam (10 kW(DOT)cm-2 - 2 MW(DOT)cm-2, pulse duration approximately 30 nsec, linewidth <EQ 0.25 cm-1), which was tuned to the D1,2-resonance lines. The mutual competition of the two photon ionization, associative ionization and Penning ionization of the laser excited rubidium atoms was investigated. The data of laser intensities and a rubidium vapor concentrations, which results in domination of the each ionization channels were founded. On the basis of the received experimental results, the cross-section values for the multiphoton and collisional ionization channels of laser-excited atoms were determined.