Reduced absorption coefficients γBaBa were measured for the far-wing absorption of the self-broadened Ba6s2 1S0–5d6p 1P1 line, Ba(6s2 1S0) +Ba(6s2 1S0)+ℏω → Ba*(5d6p 1P1)+Ba(6s21 S0)and the pair absorption Ba(6s2 1S0)+Ba(6s2 1 S0) +ℏω→Ba*(6s6p 1P1 +Ba*(6s5d 1D2). The self-broadening shows the red-shaded asymmetric profile: γBaBa on the red wing is proportional to Δν−3/2, where Δν is the detuning from the line center. The pair absorption shows a broad asymmetric peak having a blue tail and a blue satellite. The estimated γBaBa, which is based on the quasistatic line-broadening theory and the long-range dipole–dipole interaction between the relevant atomic excited levels, shows reasonable agreement for the red wing of the self-absorption whereas it gives larger values by a factor of 3 for the blue tail of the main peak for the pair absorption.
Broadening and shift of spectral lines in dense Ba and Sr vapor have been studied below and above their first ionization limits. The latter are well-known autoionizing spectral lines that exhibit many different spectral features. However, at larger pressures of noble gases some previously unobserved or very weak spectral phenomena become visible and their studies might be also linked to energy conversion purposes. Originally, we used nanosecond laser which was scanned in a certain spectral region where the two-photon excitation of autoionization lines occurred. This pulsed laser was overlapped by a mode-locked femtosecond laser and a very sensitive thermionic detection was used to observe autoionizing spectral lines. We also presented our preliminary results with hot strontium vapor excited to the resonance level at 460.7 nm.
Three-photon ionization spectra of Ba were studied which result from two-photon resonances between the 6
s2 1S0 ground state and the 6snd J=2,6s(n+1)s
1S0 (16\ensuremath{\le}n40) Rydberg states and 5d7d
1D2,
3P0,2 perturber states. A tunable laser with field strength ${10}^{\mathrm{\ensuremath{-}}(4\mathrm{\ensuremath{-}}5)}$ a.u. was used to drive the excitation in a low-density atomic beam (n\ensuremath{\lesssim}3\ifmmode\times\else\texttimes\fi{}
108 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$). Resonant line shapes were systematically studied as a function of light polarization and intensity. Studies with linearly polarized light find all resonant line shapes manifest the same peak shift and asymmetry which is traced to the ac Stark shift of the ground state. The resonant peak height, shift, and linewidth scale linearly with light intensity which is accounted for with a rate-equation formulation.For the case of circular polarization, a distinct minimum of ionization occurs between each J=2 doublet of the 6snd series. One level (generally
3D2) is significantly broadened in comparison to the laser linewidth (0.1 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) with increasing intensity while its sister level remains narrow (\ensuremath{\sim}0.2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). Further, light shifts of the
3D2 states are generally \ensuremath{\gtrsim}1 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ to the red while shifts of
1D2 states are negligible. This behavior switches for the n=27 doublet. Lastly, the dynamic shifts are not quadratic with the field strength. These effects are attributed in part to cancellation due to channel interference and a two-photon Raman process which mixes each J=2 doublet. These processes act in conjunction with the ac Stark effect to produce the observed profile behavior. The shifts and coupling of the excited states are attributed to the influence of the 6pnd ${J}^{\ensuremath{\pi}}$${=3}^{\mathrm{\ensuremath{-}}}$ autoionization series and are shown to be sensitive to the singlet-triplet mixing coefficient \ensuremath{\beta} of the J=2 doublets. A perturbation analysis is introduced which accounts for the salient features of the data, although it does not include bandwidth and saturation effects. Calculations of light shifts of the 6snd J=2 levels based on this analysis require \ensuremath{\beta} be modified from previous assignments based on hfs measurements for n in the vicinity of the 5d7d
1D2 perturber.
The cross section for the transfer of excitation from the
23P0 level of helium to the
23P1 and
23P2 levels as a result of collisions with the ground-state helium atom (
11S0) has been measured with a conventional resonance-fluorescence technique. A
He4 source excited the $2^{3}S_{1}\ensuremath{-}2^{3}P_{0}$ transition in a
He3 absorption cell. The scattered light was examined with a high-resolution grating spectrometer and the ratio of the $2^{3}P_{0}\ensuremath{\rightarrow}2^{3}S_{1}$ to the $2^{3}P_{1,2}\ensuremath{\rightarrow}2^{3}S_{1}$ intensities was measured as a function of cell pressure from 0.05 to 10 Torr. The cross section for collision-induced transitions out of the
23P0 level is found to be (68\ifmmode\pm\else\textpm\fi{}3) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$
cm2 at room temperature.
We have measured the pressure broadening and shift of the excitation and emission lines of the 6
s2 1S0\ensuremath{\rightarrow}6s6p
1P1 transition of Ba atoms implanted in liquid and solid helium at 1.5 K. The observed spectra are quantitatively explained within the adiabatic line-broadening theory by assuming a bubble structure of the trapping sites for the Ba atoms. The evolution of the bubble shape and size in the optical excitation-deexcitation cycle is modeled theoretically.