The steady-state absorption of a beam of sodium atoms irradiated by a train of pulses from a mode-locked dye laser has been measured. The sodium atoms were prepared as two-level systems by optical pumping with a two-mode laser. Absorption spectra on the 589.0-nm D2 line were recorded over 20 GHz with a frequency-stabilized, tunable, mode-locked dye laser having a bandwidth of 3.7 GHz and a mode spacing of 82 MHz. The laser-pulse profile was determined from the spectral profile assuming transform-limited pulses and was incorporated in theoretical calculations of excitation spectra obtained by numerical integration of the optical Bloch equations. By comparison of measured absorption spectra and theoretical excitation spectra the validity of the weak-field solution as well as the predicted saturation behavior were fully verified by the experimental results. For the highest level of irradiation applied, the laser pulses were identified as π pulses.
The subject of this thesis is the construction of a
stabilized cw dye laser and its use with a sodium atomic beam to
gain information about the range of validity of certain atomic
radiation theories. By lengthening the
laser cavity, cooling the dye solution and adding many other
refinements the bandwidth of a single mode cw dye laser was reduced
to 3 MHz over a period of ten seconds. The laser was designed so
that by the insertion of three uncoated glass etalons single mode
tunable operation was achieved. The purpose of stabilizing the dye
laser was to be able to perform experiments on the hyperfine
structure of sodium at high enough resolution and intensity to see
any possible differences in the theoretical predictions of Quantum
Electrodynamics, the optical Bloch equations, and Neoclassical
Radiation Theory.
The traditional method of
eliminating Doppler shifting was used, that is a well-collimated
thermal atomic beam of sodium was constructed. By crossing the
atomic and laser beams at right angles the natural linewidth of 10
MHz was observable with only one or two MHz residual broadening.
Two types of experiments were carried out:
those studying the absorption spectrum, and those studying the
emission spectrum. In the first experiments Lorentzian lineshapes
were observed for all applied field strengths. This proved that if
the Lamb shift of Neoclassical Theory exists at all it is
small compared with 10 MHz for the sodium D lines. Further, the
sharply peaked spectrum predicted by Neoclassical Theory for a
certain field strength was not observed. The width of the
absorption at high field strengths was not in good agreement with
either theory. In the second type of experiment the observed
emission spectra were in qualitative, but not good quantitative,
agreement with Quantum Electrodynamics when the effects of nearby
levels were included. The results are in disagreement with a
two-level neoclassical treatment, and we have been unable thus far
to obtain better agreement in more sophisticated treatments.
Undoubtedly a better multi-level calculation is required in all
theories since our system is not really two-level. In addition, we
have found that a factor of 2 increase in the critical intensity
calibration would bring QED into reasonable agreement with most of
the experiments. However, we have been unable to establish the
origin of such a factor.
While these
results were not sufficient to finally and absolutely settle the
controversy regarding Quantum Electrodynamics and Neoclassical
Theory they did provide a great deal of new information. For the
first
time the dynamic Stark effect was observed near resonance in
the optical region with data that could be compared to theory.
Also, the measurements of the absorption spectrum were the most
complete and among the highest resolution measurements yet obtained
by direct optical techniques. And finally, a number of of optical
pumping phenomena were observed.
We have used two-photon absorption from counterpropagating laser beams of different frequencies to measure Stark shifts and splittings of the
62S,
72S,
82S,
52D, and
62D states in sodium. Beams from a fixed-frequency, single-mode
Ar+ laser and a tunable, single-frequency cw dye laser were focussed into a coarse sodium atomic beam and the fluorescence was monitored. The collimation of the atomic beam reduced the residual Doppler width inherent in using two different frequencies. Using the two lasers allowed two-photon transitions to be observed at frequencies unattainable with the dye laser alone, and enhanced the transition rates due to the near resonance of the dye laser with the strong Na $3^{2}S\ensuremath{\rightarrow}3^{2}P$ transition. Our experimental results for the scalar and tensor polarizabilities, ${\ensuremath{\alpha}}_{0}$ and ${\ensuremath{\alpha}}_{2}$, agree with calculations in the Bates and Damgaard Coulomb approximation within 2% for ${\ensuremath{\alpha}}_{0}$ and 9% for ${\ensuremath{\alpha}}_{2}$. As a by-product the hyperfine-interaction constants
a for the
S states and the fine-structure intervals $\ensuremath{\delta}\ensuremath{\nu}=\frac{({E}_{\frac{3}{2}}\ensuremath{-}{E}_{\frac{5}{2}})}{h}$ for the
D states were obtained.
We have performed experimental and theoretical studies of the deflection of a sodium atomic beam by a laser tuned to the 3
2S1/2,F=2--3
2P3/2,F=3 transition. The intensity profile of the deflected beam was measured at different laser power levels and for different laser polarizations. Calculations using a constant force derived from a two-level atom yield deflections which are too large. The results of more realistic simulation calculations agree very well with the measured deflection. Those calculations incorporate the atomic-beam spatial and velocity distributions as well as the laser frequency distribution and polarization state. Optical pumping, loss of atoms to the nonresonant F=1 ground state, and Doppler shifting of the atoms out of resonance are all considered, as also are the statistics and angular distributions of the spontaneous photons.
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.
We have performed a high-resolution study of the interaction of two monochromatic laser fields with a prepared three-level cascade system in an atomic beam of sodium. The pump laser was held fixed near resonance with the lower levels while the probe laser was tuned over the upper two levels. For weak fields we observed a probe line shape that agrees with theory and shows that the intermediate level does not contribute to the linewidth. Data taken with a strong pump field were in disagreement with theory, but this disagreement is removed when atomic recoil suffered during the interaction is included in the calculation.