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Spectroscopy Letters

Fluorescence Correlation Spectroscopy Applied to the Measurement of Diffusion Coefficients of Atoms

M. Á. Rebolledo

Published Jan 1, 1980• 1 citation
Publisher

Abstract

The application of fluorescence correlation spectroscopy to the measurement of diffusion coefficients of atoms is proposed. After a theoretical analysis of the problem the experimental viability of this technique is studied in the particular case of Tl. Finally applications to other experiments such as ground state level-crossing are proposed.

DOI
10.1080/00387018008064051

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Journal of Physics B Atomic and Molecular Physics•Dec 28, 1984•40 citations
The application of fluorescence correlation spectroscopy to the measurement of diffusion coefficients of atoms is proposed. After a theoretical analysis of the problem the experimental viability of this technique is studied in the particular case of Tl. Finally applications to other experiments such as ground state level-crossing are proposed.
This thesis describes the laser resonance fluorescence correlation technique for single-atom velocity measurement. Using this technique, we have detected individual sodium atoms diffusing through a laser beam in a slow helium flow. From the width of the fluorescence bursts detected, the transit time for the diffusing atom is determined. This is the first measurement of the motion of a single atom in a buffer gas. A probability analysis was developed which allowed us to estimate the average burst size of emitted fluorescence photons by an atom traversing the laser beam. All these results were in general agreement with the theoretical predictions. With improvements, we will be able to measure the velocity of a single-atom either in a flow or in a vacuum. By averaging over many sodium atoms, the diffusion coefficients of sodium atoms in helium and argon buffer gases were investigated using this technique. The measured diffusion coefficients were found to be in reasonable agreement with theoretical predictions and previous experimental results. To our knowledge, this is the first application of resonance fluorescence correlation technique to the measurement of diffusion coefficient of fast moving atoms in gases.
Using Fluorescence Correlation Spectroscopy (FCS), the motion of atoms have been measured in real time. Single sodium atoms have been detected with this method and their flow velocity crudely measured. By averaging over many atoms, very clear signals were obtained and velocities up to 85 m/sec were measured. These velocities were determined by measuring the transit time of atoms across two resonantly tuned laser beams. Diffusion constants, D, of sodium in helium, neon and argon buffer gases were also measured using FCS. The products, Dp, where p is the pressure of the buffer gas, for these three cases were determined to be 427 ± 7 Torr cm2/sec, 277 ± 10 Torr cm2/sec and 200 ± 9 Torr cm2/sec, respectively. These numbers were obtained from measuring the average duration of the fluorescence burst from individual sodium atoms diffusing through the laser beam. Also measured was the diffusion constant for the excited sodium atom (2P3/2) in a helium buffer gas at 200 Torr pressure, with a result of 598 ± 84 Torr cm2/sec.
We show that an enhanced intensity correlation shows up in the diffusion of photons multiply scattered by an atomic gas of large optical depth b . This enhancement occurs only for scattering atoms that are Zeeman degenerate and it leads to a deviation from the Rayleigh law. The fluctuations measured by their variance, display a resonance peak as a function of an applied magnetic field. The resonance width is proportional to the small factor 1/ b . We derive closed analytic expressions for all these physical quantities which are directly accessible experimentally.
Fluorescence correlation spectroscopy when fluorescent nanoemitters are diffusing in the vicinity of a dielectric mirror is studied both theoretically and experimentally. We demonstrate that two important effects occur when the confocal detection volume is located on the mirror’s surface. First, the count rate per emitter is significantly enhanced owing to control of spontaneous emission and enhancement of the excitation field. Second, interference fringes in the excitation beam give rise to a new characteristic time in the photocurrent’s autocorrelation function. This new time is found to be independent of the transverse excitation field’s beam waist and permits accurate measurement of diffusion coefficients without any a priori knowledge of the confocal volume geometry.
Using the concept of the time-dependent physical spectrum introduced by Eberly and Wodkiewicz (1977) the authors develop the theoretical basis of time-resolved correlation spectroscopy. The corresponding measuring device consists of two coupled photodetectors with frequency resolving equipment in front of each. A formula is presented for the time-resolved correlation spectrum depending on two time and two frequency arguments. It may be used to gain insight into the time correlation of photons with given frequencies. The authors apply the theory to the resonance fluorescence radiation of a two-level atom irradiated monochromatically. It is shown that in the case of a weak irradiation with large detuning the photons are emitted in a definite time order into the two sidebands of the fluorescence triplet. Under strong resonant irradiation the photons show up bunching (antibunching) behaviour if they belong to different (equal) sidebands. In addition to these results which are already known the authors present new results concerning intermediate situations.