Splitting isotope shift in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mi>s</mml:mi> <mml:mn>2</mml:mn> <mml:mi>p</mml:mi> <mml:mspace width="0.28em"/> <mml:msup> <mml:mrow/> <mml:mn>3</mml:mn> </m

We report measurements and theoretical calculations of the fine-structure splittings in all three <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mn>1</a:mn> <a:mi>s</a:mi> <a:mn>2</a:mn> <a:mi>s</a:mi> <a:mspace width="0.16em"/> <a:mmultiscripts> <a:mi>S</a:mi> <a:mn>1</a:mn> <a:none/> <a:mprescripts/> <a:none/> <a:mn>3</a:mn> </a:mmultiscripts> <a:mo>→</a:mo> <a:mspace width="0.16em"/> <a:mn>1</a:mn> <a:mi>s</a:mi> <a:mn>2</a:mn> <a:mi>p</a:mi> <a:mspace width="0.16em"/> <a:mmultiscripts> <a:mi>P</a:mi> <a:mrow> <a:mn>0</a:mn> <a:mo>,</a:mo> <a:mn>1</a:mn> <a:mo>,</a:mo> <a:mn>2</a:mn> </a:mrow> <a:none/> <a:mprescripts/> <a:none/> <a:mn>3</a:mn> </a:mmultiscripts> </a:mrow> </a:math> transitions in the heliumlike systems of the isotopes <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mmultiscripts> <e:mi mathvariant="normal">C</e:mi> <e:mprescripts/> <e:none/> <e:mrow> <e:mn>12</e:mn> <e:mo>,</e:mo> <e:mn>13</e:mn> <e:mo>,</e:mo> <e:mn>14</e:mn> </e:mrow> </e:mmultiscripts> </e:math> . The metastable triplet state was efficiently populated in an electron beam ion source and the <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:msup> <g:mrow> <g:mi mathvariant="normal">C</g:mi> </g:mrow> <g:mrow> <g:mn>4</g:mn> <g:mo>+</g:mo> </g:mrow> </g:msup> </g:math> ions were electrostatically accelerated to 50 keV to perform collinear laser spectroscopy. From the determined transition frequencies, the splitting isotope shift (SIS), i.e., the difference in fine-structure splittings between different isotopes of the same element, was extracted. In the SIS, theoretical uncertainties due to higher-order quantum electrodynamic corrections are strongly suppressed since they are independent of both nuclear mass and the fine-structure quantum number <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:mi>J</i:mi> </i:math> in lowest order. Comparison with theory provides an important test of experimental accuracy, particularly in the <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"> <j:mmultiscripts> <j:mi mathvariant="normal">C</j:mi> <j:none/> <j:mrow> <j:mn>4</j:mn> <j:mo>+</j:mo> </j:mrow> <j:mprescripts/> <j:none/> <j:mn>13</j:mn> </j:mmultiscripts> </j:math> case, for which the nuclear spin leads to hyperfine-induced fine-structure mixing. At the same time, the even-even isotopes <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"> <l:mmultiscripts> <l:mi mathvariant="normal">C</l:mi> <l:none/> <l:mrow> <l:mn>4</l:mn> <l:mo>+</l:mo> </l:mrow> <l:mprescripts/> <l:none/> <l:mrow> <l:mn>12</l:mn> <l:mo>,</l:mo> <l:mn>14</l:mn> </l:mrow> </l:mmultiscripts> </l:math> without nuclear spin can be used to confirm theory. Theoretical values of the SIS are given for all the heliumlike ions with <n:math xmlns:n="http://www.w3.org/1998/Math/MathML"> <n:mrow> <n:mn>2</n:mn> <n:mo>≤</n:mo> <n:mi>Z</n:mi> <n:mo>≤</n:mo> <n:mn>10</n:mn> </n:mrow> </n:math> .