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Laser Applications to Chemical, Security and Environmental Analysis

New Fluorescence Techniques for Detecting Noble Gases(*)

Tom J. Whitaker, Bret D. Cannon, Bruce A. Bushaw

Published Jan 1, 1987• 0 citations
Publisher

Abstract

Two new concepts for detecting noble gases are reported. Both techniques involve formation of the long-lived 1s5 metastable state of noble gases. The first technique utilizes the photon-burst method and should be capable of isotopically selective detection at extremely small relative abundances. The second concept incorporates a shelving technique that stores noble gas atoms in the metastable state and then pumps these atoms to a higher excited state that radiatively cascades to the ground state, emitting vacuum ultraviolet (VUV) photons. A significant advantage is that AlGaAs diode lasers can be used for the techniques rather than continuous wave cw dye lasers.

DOI
10.1364/laca.1987.ma9

Related papers

Towards Purely Laser Based Generation and Trapping of Metastable Krypton

M. Abou Dakka

Radioisotopes that break down at a constant rate over time through radioactive decay allow scientists to understand the development of geological and anthropological history by dating samples that contain such isotopes. While many techniques exist to measure the age of a sample, in this thesis we explore the possibility of enhancing the atom trap trace analysis (ATTA) method, which has made significant strides in the area of ultra-sensitive radioisotope detection, especially with the use of metastable noble gas radionuclides. A metastable atom is an atom in a stable, long-lived (>1 ms) excited state. Noble gases have the longest metastable lifetimes, which combined with their geochemical inertness, make them ideal candidates for geological and environmental radiometric techniques. The production of metastable noble gases for this purpose has traditionally relied on discharge sources, but this method suffers from important drawbacks such as high levels of crosscontamination and poor excitation efficiencies. In an effort to overcome these drawbacks, the method described in this work presents a new platform for implementing, characterising and comparing purely laser-based production of metastable noble gases. In the first part of this thesis, we present both state-of-the-art experimental results in the purely laser-based generation of metastable krypton atoms, and a physically realistic simulation with good agreement with our experimental results. Our calculation of the twophoton absorption cross section for krypton (to a metastable-generating excited state) using 215nm deep ultraviolet (DUV) laser radiation, obtained by fitting with theory, confirms that high metastable production efficiencies are possible with laser-based techniques. In addition to this parameter, the branching ratio and photoionisation rate from the excited state are calculated and agree with the literature. In the second part, we present the experimental progress to date for a brand new method of manipulating metastable noble gas atoms using a two-stage approach of metastable production then trapping via a magneto-optical trap (MOT). While the results are limited due to experimental difficulties, they are complemented by a physically realistic 3D simulation based on the Monte Carlo method. This simulation suggests that the production and trapping of metastable noble gases is feasible without the use of a discharge source, a Zeeman slower, and even a secondary excitation chamber. However, the loading rate for such a MOT is not sufficient for ATTA measurements without drastic changes in the experimental configuration. In the final part, we propose a next-generation ATTA system based on stimulated rapid adiabatic passage (STIRAP) to transfer noble gas atoms to their metastable state with near 100% efficiency. This technique for the generation of metastable noble gases would be purely laser-based and would compete with the loading rates of conventional production techniques, while avoiding their deleterious effects. We conclude by identifying limits of laser-based techniques and explore approaches to overcome them.

•Jan 1, 2019•0 citations

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We have investigated a new scheme for excitation of the 5s, J = 2 metastable level of Kr (5s[3/2] J = 2 ) which can be readily extended to other rare gases. In the scheme, an ultraviolet (UV) lamp is used to create a population of Kr atoms in the 5s[3/2] J = 1 level in a gas cell. The excited atoms are then pumped to the 5p[3/2] J = 2 level, using 819 nm light from a Ti:sapphire laser, from which they decay to the metastable state with a branching ratio of 77%. We made two striking observations: (1) the laser power required to saturate the second step decreases markedly as a function of gas cell pressure, and (2) the UV photon flux is converted with very high efficiency (≈10%) to metastable atom flux. A Monte Carlo study of the scattering of UV photons in the cell reproduces the trends observed. The understanding achieved points to the design of a higher flux source of metastable atoms.

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Experimental work on cold, trapped metastable noble gases is reviewed. The aspects which distinguish work with these atoms from the large body of work on cold, trapped atoms in general is emphasized. These aspects include detection techniques and collision processes unique to metastable atoms. Several experiments exploiting these unique features in fields including atom optics and statistical physics are described. Precision measurements on these atoms including fine structure splittings, isotope shifts, and atomic lifetimes are also discussed.

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Optically enhanced production of metastable xenon

Garrett Hickman, J. D. Franson, T. B. Pittman

Metastable states of noble gas atoms are typically produced by electrical discharge techniques or “all-optical” excitation methods. Here we combine electrical discharges with optical pumping to demonstrate “optically enhanced” production of metastable xenon (Xe*). We experimentally measure large increases in Xe* density with relatively small optical control field powers. This technique may have applications in systems where large metastable state densities are desirable.

arXiv:1607.07310•Optics Letters•Sep 13, 2016•13 citations
Two new concepts for detecting noble gases are reported. Both techniques involve formation of the long-lived 1s5 metastable state of noble gases. The first technique utilizes the photon-burst method and should be capable of isotopically selective detection at extremely small relative abundances. The second concept incorporates a shelving technique that stores noble gas atoms in the metastable state and then pumps these atoms to a higher excited state that radiatively cascades to the ground state, emitting vacuum ultraviolet (VUV) photons. A significant advantage is that AlGaAs diode lasers can be used for the techniques rather than continuous wave cw dye lasers.
Radioisotopes that break down at a constant rate over time through radioactive decay allow scientists to understand the development of geological and anthropological history by dating samples that contain such isotopes. While many techniques exist to measure the age of a sample, in this thesis we explore the possibility of enhancing the atom trap trace analysis (ATTA) method, which has made significant strides in the area of ultra-sensitive radioisotope detection, especially with the use of metastable noble gas radionuclides. A metastable atom is an atom in a stable, long-lived (>1 ms) excited state. Noble gases have the longest metastable lifetimes, which combined with their geochemical inertness, make them ideal candidates for geological and environmental radiometric techniques. The production of metastable noble gases for this purpose has traditionally relied on discharge sources, but this method suffers from important drawbacks such as high levels of crosscontamination and poor excitation efficiencies. In an effort to overcome these drawbacks, the method described in this work presents a new platform for implementing, characterising and comparing purely laser-based production of metastable noble gases. In the first part of this thesis, we present both state-of-the-art experimental results in the purely laser-based generation of metastable krypton atoms, and a physically realistic simulation with good agreement with our experimental results. Our calculation of the twophoton absorption cross section for krypton (to a metastable-generating excited state) using 215nm deep ultraviolet (DUV) laser radiation, obtained by fitting with theory, confirms that high metastable production efficiencies are possible with laser-based techniques. In addition to this parameter, the branching ratio and photoionisation rate from the excited state are calculated and agree with the literature. In the second part, we present the experimental progress to date for a brand new method of manipulating metastable noble gas atoms using a two-stage approach of metastable production then trapping via a magneto-optical trap (MOT). While the results are limited due to experimental difficulties, they are complemented by a physically realistic 3D simulation based on the Monte Carlo method. This simulation suggests that the production and trapping of metastable noble gases is feasible without the use of a discharge source, a Zeeman slower, and even a secondary excitation chamber. However, the loading rate for such a MOT is not sufficient for ATTA measurements without drastic changes in the experimental configuration. In the final part, we propose a next-generation ATTA system based on stimulated rapid adiabatic passage (STIRAP) to transfer noble gas atoms to their metastable state with near 100% efficiency. This technique for the generation of metastable noble gases would be purely laser-based and would compete with the loading rates of conventional production techniques, while avoiding their deleterious effects. We conclude by identifying limits of laser-based techniques and explore approaches to overcome them.
Other than for the three-dimensonal case of blackbody radiation, low-dimensional photon gases can exhibit Bose- Einstein condensation, as been experimentally demonstrated in the visible spectral regime first in 2010 using a dye-filled optical microcavity [1]. Here a short, wavelength sized spacing of cavity mirrors provides a low- frequency cutoff imprinting a spectrum of photon energies restricted to well above the thermal energy, and photons thermalize by repeated absorption and (re-)emission processes on the dye molecules to room temperature. As a prominent feature, a Bose-Einstein condensate of photons is as a coherent light source. This is of special interest in the vacuum-ultraviolet spectral regime (100 nm-200 nm, VUV), as it is difficult to realize lasers in this wavelength range. This is mainly due to the1/ω31/\omega^{3}1/ω3-scaling of excited state lifetimes, which requires increasingly high pump powers at short wavelengths to create population inversion in a laser medium.
Laser can be used to cool and manipulate neutron atoms via momentum exchange between photons and atoms. Laser cooling and trapping is well established on alkali, alkali earth, and noble gas elements. Noble gases, with stable physical and chemical properties, no reactions with other elements, have great potential uses in cold collision, spectroscopy measurement, photo-association, atom optics, etc. However, trapping noble gas atoms is generally more difficult because of the lack of suitable UV lasers. Some of these problems will be overcome with future advances in UV laser technology. Excited by electron collisions, noble gas can reach the long life metastable state for the use of laser decelerating, focusing and trapping. In this paper some main methods which are used to achieve cooling and trapping metastable noble gas atoms are discussed.
We have investigated a new scheme for excitation of the 5s, J = 2 metastable level of Kr (5s[3/2] J = 2 ) which can be readily extended to other rare gases. In the scheme, an ultraviolet (UV) lamp is used to create a population of Kr atoms in the 5s[3/2] J = 1 level in a gas cell. The excited atoms are then pumped to the 5p[3/2] J = 2 level, using 819 nm light from a Ti:sapphire laser, from which they decay to the metastable state with a branching ratio of 77%. We made two striking observations: (1) the laser power required to saturate the second step decreases markedly as a function of gas cell pressure, and (2) the UV photon flux is converted with very high efficiency (≈10%) to metastable atom flux. A Monte Carlo study of the scattering of UV photons in the cell reproduces the trends observed. The understanding achieved points to the design of a higher flux source of metastable atoms.
Experimental work on cold, trapped metastable noble gases is reviewed. The aspects which distinguish work with these atoms from the large body of work on cold, trapped atoms in general is emphasized. These aspects include detection techniques and collision processes unique to metastable atoms. Several experiments exploiting these unique features in fields including atom optics and statistical physics are described. Precision measurements on these atoms including fine structure splittings, isotope shifts, and atomic lifetimes are also discussed.