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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Photon-Burst Correlation Spectroscopy

C. Y. She, W. M. Fairbank, Jr.

Published Oct 19, 1983• 1 citation
Publisher

Abstract

Using a single-clipped digital correlator, the burst of photons resulting from laser induced resonance fluorescence of a single atom as it passes through a laser beam can be recorded and its transit-time measured. By averaging many fluorescent atoms, the transport characteristics of these atoms in a gaseous system, such as diffusion, flow velocity, and chemical reaction rates can be conveniently determined by photon-burst correlation spectroscopy.

DOI
10.1117/12.936235

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Using a single-clipped digital correlator, the burst of photons resulting from laser induced resonance fluorescence of a single atom as it passes through a laser beam can be recorded and its transit-time measured. By averaging many fluorescent atoms, the transport characteristics of these atoms in a gaseous system, such as diffusion, flow velocity, and chemical reaction rates can be conveniently determined by photon-burst correlation spectroscopy.
Using a digital correlator, we record the burst of photons emitted by a single atom as it passes through a resonant laser beam. A high efficiency light collection system allows us to register many counts from each atom. The transit time of the atom across the laser beam is computed from the displayed autocorrelation function. By averaging over many atoms, we determine diffusion coefficients for ground state sodium in helium, neon and argon. We have also used this technique to measure rapid flow speeds of gases.
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Single atoms are detected and their motion measured for the first time to our knowledge by the fluorescence photon- burst method in the presence of large quantities of buffer gas. A single-clipped digital correlator records the photon burst in real time and displays the atom's transit time across the laser beam. A comparison is made of the special requirements for single-atom detection in vacuum and in a buffer gas. Finally, the probability distribution of the bursts from many atoms is measured. It further proves that the bursts observed on resonance are due to single atoms and pot simply to noise fluctuations.
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.