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PHYSICAL REVIEW LETTERS 28 APRIL 1975 TABLE Io Calculated and experimental Stark shifts in MHz/(kV/cm) 2 . Level a Q (expt) oi 2 ( e *pt) oi Q (calc) a 2 (calc) 4:d 2 D 3/2 155.3(1.7) -38.5(7) 152.02 -35.64 4d 2 D 5/2 156.1(1.3) -53.2(5) 151.97 -51.06 5sS 1/2 5.2(3) 5.81expressed in terms of integrals of the radial wave functions as follows:«o(i) = -a(P + f*% a 2 (f) = f 5 (P + fF),where the notation is that of Ref. 6 and we have letP(|)=P(|)=P andP(|)=P(|)=P. It has been assumed that the spin-orbit interaction does not affect these radial integrals. Using the same assumption we find o: 0 (|) = a 0 (f) and a 2 (f) = ^a! 2 (f).The experimental values agree with these expressions to within 3.5% e One may solve Eq 0 (5) for P and F by using the measured polarizabilities. Thus, in units of MHz/(kV/cm) 2 , we obtainF = 711.2(9.1) and P = 101.0(10.6). In the Coulomb calculation, 98% of the 4d level shifts come from interaction with 5p and 4/. Thus the measured P and F are very nearly the radial integrals for the nearest states only. Then the experimentally determined P and F can be used to evaluate the oscillator strengths, giving/ 4d _ >5jf) = 0.274(29) and /*r-4 /= 0.018 85(24). Two-photon spectroscopy has provided a convenient means to study Stark shifts in highly excited states with high resolution. Further im-There has been considerable interest recently in the nature of the nematic-smectic-A liquidcrystal phase transition, since mean-field theo-provements, such as the use of laser frequency locking and heterodyne techniques, should increase this resolution even more c .ries 1 " 3 indicate that this transition is allowed to be second order. Although clearly first order in most compounds, 4 possible second-order behav-We have measured a proton NMR dipolar splitting, AH, proportional to the "nematic order parameter," in the nematic and smectic-^4 phases of ^-cyanobenzylidene-^-nonylaniline, at pressures up to 3.5 kbar. The discontinuity in AH at the nematic-smectic-^l phase transition decreases from a relatively large value at 1 atm to an unobservably small value at higher pressures. We tentatively identify a tricritical point for this transition at*, =140±4°C and P t = 2.89 ±0.23 kbar. 1076

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Efficiency loss due to high electron beam current density in green‐emitting false(normalZn,normalCdfalse)S:normalCu,normalAl phosphors has been measured for Cu and Al concentrations ranging from 27 to 1500 ppm. The severe loss of efficiency exhibited by commercial phosphors with activator concentrations in the 20–75 ppm range is somewhat alleviated at higher concentration. However, the improvement is far less than expected from a simple activator‐depletion model, and saturation effects are nearly constant from 100 to 500 ppm, showing that a second mechanism for saturation is important in this material. It is also found that saturation is independent of repetition rate, but depends rather on the net dose of electron beam charge delivered in a single pulse or scan. Laser photoexcitation experiments in the blue and u.v. show that analogous saturation occurs for all modes of excitation. Thus the second saturation mechanism is neither a thermal effect nor electric field quenching. The second paper in this series will show that the second saturation mechanism in normalZnS‐normaltype phosphors is excited‐state absorption.