Newell' has calculated the spherically symmetrical contribution 0~to the magnetic shielding constant for H2. Extrapolating his result to HD through considerations based on the relative internuclear distances for the two molecules combined with an average of Eqs. (82) and (83) yields the following average shielding constant for the HD molecule: HD (~) (ss/3m') HD (Q rr) HD (~hf) = (3.216&0.010)X 10 '-(0.594&0.030) X 10 ' = (2.622&0.032) X 10 '.
Sharp-line fluorescence and absorption in the region of 7935 A were observed in Cr 3+ -doped and undoped SrTiC>3. It is reasonably established that the fluorescence and absorption are due to the 2 E <-> *A 2 transition (R line) of the Cr 3+ ion. Polarization and absorption measurements indicate that the i?-line transition is magnetic dipole. Fluorescence from Mn 4+ -doped SrTi03 was seen at about 7230 A. At low temperatures, a temperature-dependent splitting of the i?-line transition of Cr 3+ is seen. It is shown that this is due to the lifting of the degeneracy of the 2 E state by the temperature-dependent tetragonal field present in SrTiOs below 103±2°K. These results are compared with previous measurements on rare-earth ions in SrTi0 3 . The possible origin of the tetragonal transition is discussed.
The sodium D resonance lines have been studied in saturated absorption with a repetitively pulsed tunable dye laser. The hyperfine splitting of the 3 S&g2 and 3 P&y2 states of Na~is resolved. Measurements with a delayed probe reveal a remanent hole burning in the velocity distributions of the two ground-state levels, caused by a velocity-selective optical pumping cycle. A time-resolved observation of the collisional-velocity thermalization in the presence of Ar buffer gas is reported.In the past the techniques of laser saturation spectroscopy' ' that eliminate Doppler broadening by spectral hole burning have essentially been restricted to either gas-laser transitions or molecular transitions in accidental coincidence.Although organic-dye lasers have been tuned to atomic resonance lines, " they generally did not provide the required spectral resolution and stability. The present paper reports on a study of the sodium resonance lines D, at 5896 A and D, 0 at 5890 A in saturated absorption using a repetitively pulsed tunable dye laser with an instrumental bandwidth of 7 MHz and a pulse length of 30 nsec. The use of a pulsed laser source is made possible by a recently introduced sensitive technique of saturation spectroscopy. " Two light beams of equal frequency are sent in opposite directions through the resonant atomic vapor, and the change in absorption for one of the beams, the probe, caused by the other, the bleaching or saturating beam, is recorded. A Nl TROGE N TUNABLE
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