The sensitivity of the resonance-fluorescence method for the measurement of very-low sodium-vapor densities (<1010 atoms/cm3) has been studied. Light from a sodium spectral lamp is sent through a sodium-containing vessel, whose temperature can be varied. The vapor density has been measured in the region of 47–146 °C. At 47 °C, the vapor density is about 107 atoms/cm3. The measurements were performed with both a photon-counting device and a lock-in amplifier. The experimental results have been normalized to absolute density data given in the literature. Fitting our points to the equation logN = a + b/T yields the values a = 23.715 and b = −5257.7 K.
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.
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.
Sodium vapor, in the density range
1013 to 5 \ifmmode\times\else\texttimes\fi{}
1014 ${\mathrm{cm}}^{\ensuremath{-}3}$, was excited by a cw dye laser, tuned 20-150 GHz from either the
D1 or
D2 resonance line. We observed a three-peak scattered spectrum, consisting of the Rayleigh component at the laser frequency, and the two fluorescence components (direct and sensitized) at the atomic resonance-line frequencies. Corrections to the Rayleigh signals for anisotropy and polarization effects, and to the fluorescence signals for radiation trapping, were made in order to obtain the ratio of the sum of the total intensities of the two fluorescence components to that of the Rayleigh component. This ratio combined with a measurement of the line-wing absorption coefficient yields the sodium density and the
D-line self-broadening rate coefficients [${k}_{\mathrm{br}}=4.67\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$
cm3${\mathrm{s}}^{\ensuremath{-}1}$ (\ifmmode\pm\else\textpm\fi{}15%) for the
D2 line and ${k}_{\mathrm{br}}=3.07\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$
cm3${\mathrm{s}}^{\ensuremath{-}1}$ (\ifmmode\pm\else\textpm\fi{}15%) for the
D1 line]. Asymmetry in the self-broadened line wings due to fine-structure recoupling was observed. The measured intensity ratio of the
D lines, combined with pulsed measurements of the effective radiative decay rates in the presence of radiation trapping, yields the fine-structure collisional-mixing cross section [$\ensuremath{\sigma}(3{P}_{\frac{3}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}})=172$
A˚2(\ifmmode\pm\else\textpm\fi{}18%)] at $T\ensuremath{\cong}300\ifmmode^\circ\else\textdegree\fi{}$ C. Our results are compared to other experiments and to theory.
The intensity ratio D2/D1 for the sodium doublet has been measured for radiation from excited sodium atoms which result from the optical dissociation of sodium iodide vapor by ultraviolet light in the 2082→2500 A region. With shorter wavelength ultraviolet light as exciting source, the D2/D1 ratio is 0.98 and increases to above 1.5 with longer exciting wavelengths. Addition of foreign gases cause the observed D2/D1 ratio to increase to 1.89 for large foreign gas pressure. Cross sections for the transfer of excited sodium atoms from the 32P32 to the 32P½ state and vice versa due to collisions with foreign molecules have been measured as they depend on the relative velocity between excited sodium atom and foreign molecule. The Doppler broadening of the separate D lines emitted in the atomic fluorescence of sodium iodide has been estimated from absorption measurements. Both lines show equal broadening within the experimental error of the observations. Measurements were made with a photomultiplier tube. The sodium doublet was resolved with a reflection grating monochromator.
We have used the resonance radiation pressure from 40 mW of cw dye laser light propagating axially down a tube filled with sodium vapor to increase the sodium pressure (density) up to 50% over a length of 20 cm. The magnitude of the effect agrees well with measurements of the absorbed power. These observations were made with the laser tuned to the low-frequency side of the sodium D2 resonance line; for tuning to the high-frequency side, an additional unexplained effect leading to density decreases was observed.