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Physics Today

Resonance electron spectroscopy detects single atoms

Gloria B. Lubkin

Published Sep 1, 1977• 2 citations
Publisher

Abstract

A group at Oak Ridge National Laboratory reports that by means of the new technique of resonance ionization spectroscopy they have observed single atoms of cesium. They say that one atom in an environment of 1019 atoms or molecules of another kind can be measured with their method, and they do not believe this to be a limit. In subsequent experiments, the group has observed density fluctuations of atoms, absolute photodissociation cross sections, time-resolved diffusion and chemical reaction of atoms, and the detection of cesium atoms from the spontaneous fission of californium nuclei.

DOI
10.1063/1.3037703

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A group at Oak Ridge National Laboratory reports that by means of the new technique of resonance ionization spectroscopy they have observed single atoms of cesium. They say that one atom in an environment of 1019 atoms or molecules of another kind can be measured with their method, and they do not believe this to be a limit. In subsequent experiments, the group has observed density fluctuations of atoms, absolute photodissociation cross sections, time-resolved diffusion and chemical reaction of atoms, and the detection of cesium atoms from the spontaneous fission of californium nuclei.
Resonance ionization spectroscopy, RIS, is a multistep photon absorption process in which the final state is the ionization continuum of an atom. The RIS process can be saturated with available pulsed lasers, so that one electron can be removed from each atom of the selected type. This method was first applied to the determination of the absolute number of He(21S2^{1}S21S) excited states produced when pulsed beams of protons interacted with helium gas. Laser schemes for RIS are classified into five basic types; with these, nearly all of the elements can be detected with commercially available lasers. A periodic table is included showing schemes applicable to all of the elements except He, Ne, F, and Ar. A compact theory of the RIS process is presented which delineates the conditions under which rate equations apply. Questions on the effects of collisional line broadening, laser coherence time, and nonresonant multiphoton ionization processes are discussed. The initial demonstration of one-atom detection of Cs is described. By using laser beams to saturate the RIS process and by using proportional counters as single-electron detectors, one-atom detection is made possible. With RIS, one-atom detection is highly selective, has the ultimate in sensitivity, and has excellent space and time resolution. Furthermore, a modification of the technique in which single electrons (or single ions) are detected with a channel electron multiplier permits single-atom detection in a vacuum. Resonance ionization spectroscopy has application to classical phenomena such as the diffusion of free atoms, the chemical reaction of free atoms with a gas, the measurement of concentration fluctuations in a dilute vapor, and a variety of other atomic fluctuation phenomena. The authors describe how RIS can be used for photophysics measurements such as far wing collisional line broadening, measurements of photoionization cross sections for excited states, and collisional redistribution among excited states. Modern applications include the detection of single atoms in individual ionization tracks, such as those created by the binary fission of a parent atom, and the extension of this technique to the detection of stable or unstable daughter atoms in time coincidence with the decay of a parent atom. These methods are beig developed for use in low-level counting to determine, for example, the flux of solar neutrinos on the earth.
A new measuring technique which can detect a single atom of a given kind, even in the presence of 1019{10}^{19}1019 or more atoms of another kind, with good space and time resolution, was utilized to study the density fluctuation of less than 100 cesium atoms in a small volume of space filled with inert gases. Repeated measurements of the absolute number of atoms in a defined volume at an arbitrary time were recorded for the first time in order to obtain a statistical distribution giving the fluctuation of the number of atoms around the mean value. Numerous other physics applications of the one-atom detector are briefly described.
Single magnetically trapped 87Rb^{87}\text{R}\text{b}87Rb atoms on an atom chip are detected with $67\ifmmode\pm\else\textpm\fi{}12\text{ }%$ efficiency by three photon ionization and subsequent ion detection in a channel electron multiplier. State selective detection of single atoms and high-resolution optical spectroscopy on trapped atom clouds are demonstrated. The temperature and particle density of a trapped atomic gas is monitored in situ and in real time with negligible atom loss due to ionization below 5%.
By severely reducing the number of solute molecules in the illuminated sample, the optical resonances of individual molecules can be resolved in a fluorescence excitation spectrum. Single molecule lines can be studied as a function of time and temperature: sudden jumps of their resonance frequencies are due to spectral diffusion processes. The signal from a single molecule displays specific correlations which makes time-resolved studies possible. Here, emphasis is put on photon-bunching arising from intersystem crossing (ISC). ISC rates are deduced from the observed decay rates of the correlation and are found to differ from molecule to molecule. A single molecule is a truly local probe of its environment by means of which fundamental studies of the matrix dynamics as well as nanophysics experiments may be undertaken.
We review our recent experimental results on the non-destructive quantum-state detection and spectroscopy of single trapped molecules. At the heart of our scheme, a single atomic ion is used to probe the state of a single molecular ion without destroying the molecule or even perturbing its quantum state. This method opens up perspectives for new research directions in precision spectroscopy, for the development of new frequency standards, for tests of fundamental physical concepts and for the precise study of chemical reactions and molecular collisions with full control over the molecular quantum state.
Chemical reactions typically proceed via stochastic encounters between reactants. Going beyond this paradigm, we combined exactly two atoms in a single, controlled reaction. The experimental apparatus traps two individual laser-cooled atoms [one sodium (Na) and one cesium (Cs)] in separate optical tweezers and then merges them into one optical dipole trap. Subsequently, photoassociation forms an excited-state NaCs molecule. The discovery of previously unseen resonances near the molecular dissociation threshold and measurement of collision rates are enabled by the tightly trapped ultracold sample of atoms. As laser-cooling and trapping capabilities are extended to more elements, the technique will enable the study of more diverse, and eventually more complex, molecules in an isolated environment, as well as synthesis of designer molecules for qubits.