Optical transitions near the elusive <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>5</mml:mn> <mml:mi>s</mml:mi> <mml:mtext>−</mml:mtext> <mml:mn>4</mml:mn> <mml:mi>f</mml:mi> </mml:mrow> </mml:math> level crossing in h

Optical transitions in highly charged ions can be very sensitive to hypothetical beyond-the-standard-model phenomena. Those near the degeneracy due to the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mn>5</a:mn> <a:mi>s</a:mi> <a:mtext>−</a:mtext> <a:mn>4</a:mn> <a:mi>f</a:mi> </a:mrow> </a:math> level crossing are especially promising. We present measurements of <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:msup> <b:mrow> <b:mi>Os</b:mi> </b:mrow> <b:mrow> <b:mn>15</b:mn> <b:mo>,</b:mo> <b:mn>16</b:mn> <b:mo>,</b:mo> <b:mn>17</b:mn> <b:mo>+</b:mo> </b:mrow> </b:msup> </b:math> ions at an electron beam ion trap and corresponding predictions of several transitions that show a high sensitivity to hypothetical fifth forces and possible violations of local Lorentz invariance. We found electric quadrupole transitions in <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:msup> <c:mrow> <c:mi>Os</c:mi> </c:mrow> <c:mrow> <c:mn>16</c:mn> <c:mo>+</c:mo> </c:mrow> </c:msup> </c:math> , which are suitable for frequency metrology due to linewidths down to <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:mn>44</d:mn> <d:mspace width="0.16em"/> <d:mo>µ</d:mo> <d:mi>Hz</d:mi> </d:mrow> </d:math> . Our calculations show the need for including enough inner-shell excitations to avoid an overestimation of interconfiguration transition rates, which were too faint for detection at the present signal-to-noise ratio.