The quadratic Stark effect in the hyperfine structure of the ground levels of samarium, europium and aluminium has been investigated by the method of atomic beams. Frequency shifts caused by the application of an electric field have been interpreted, to second order in perturbation theory, in terms of an effective Stark operator with which is associated a parameter called the tensor polarizability, α
ten.
. In the
LS
coupling approximation the results for five lines in samarium give a value α
ten.
(
7
F
;
J
═ 6) ═ – 3·64 ± 0·17
a
0
3
. Measurements on one line in europium give, for the ground level, α
ten.
(
J
═
7
/
2
) ═ 0·0141 ± 0·0007
a
0
3
. In the
IJ
coupling and
LS
coupling approximations the theory of the Stark effect in hyperfine structure has been tested and confirmed, but on the more stringent assumption that the central-field approximation is also valid, an attempt to evaluate a parameter α
ten.
(4
f
) common to both samarium and europium has only limited success. This result leads to the expected conclusion that in the rare earths the central-field model breaks down. Measurements on one line in aluminium give α
ten.
(
J
═ 3/2) ═ α
ten.
(3
p
) ═ – 8·15 ± 0·40
a
0
3
. In the course of this work new values of the hyperfine structure interaction constants and of the
g
-factor have been found for the 3
p
2
P
3/2
level of
27
Al. These results are
A
═ + 94·27767 ± 0·00010 Mc/s,
B
═ + 18·91526 ± 0·00070 Mc/s,
gj
═ 1·33474 ± 0·00005.
Color Hues in Fluorescent Proteins with the Same Chromophore are due to Internal Quadratic Stark Effect