Laser-fluence-dependent production of molecular thorium ions in different charge states for trapped-ion experiments

Thorium ions and molecules, recognized for their distinctive nuclear and atomic attributes, are central to numerous trapped-ion experiments globally. Our study introduces an effective, compact source of thorium atomic and molecular ions produced via laser ablation of microgram-scale, salt-based samples. We thoroughly analyze the variety of ion species and charge states generated at varying laser fluences. Utilizing 10 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>µ</a:mi><a:mtext>g</a:mtext></a:mrow></a:math> of thorium fluoride crystals and laser fluences between 1.00(5) and 6.75(35) J/<b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msup><b:mrow><b:mi>cm</b:mi></b:mrow><b:mn>2</b:mn></b:msup></b:math>, we produce thorium molecular ions <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:msup><c:mrow><c:mmultiscripts><c:mi>ThF</c:mi><c:mi>x</c:mi><c:none/><c:mprescripts/><c:none/><c:mn>232</c:mn></c:mmultiscripts></c:mrow><c:mrow><c:mi>n</c:mi><c:mo>+</c:mo></c:mrow></c:msup></c:mrow></c:math> (with <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mi>x</d:mi><d:mo>=</d:mo><d:mn>0</d:mn><d:mo>–</d:mo><d:mn>3</d:mn></d:mrow></d:math> and charge states up to <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mi>n</e:mi><e:mo>=</e:mo><e:mn>3</e:mn></e:mrow></e:math>), including <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:msup><f:mrow><f:mi>ThF</f:mi></f:mrow><f:mrow><f:mn>2</f:mn><f:mo>+</f:mo></f:mrow></f:msup></f:math> and <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:msup><g:mrow><g:mi>ThF</g:mi></g:mrow><g:mrow><g:mn>3</g:mn><g:mo>+</g:mo></g:mrow></g:msup></g:math>. These species are particularly relevant for spectroscopy; <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:msup><h:mrow><h:mi>ThF</h:mi></h:mrow><h:mrow><h:mn>3</h:mn><h:mo>+</h:mo></h:mrow></h:msup></h:math> is valuable due to its stable closed-shell configuration, while <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:msup><i:mrow><i:mi>ThF</i:mi></i:mrow><i:mrow><i:mn>2</i:mn><i:mo>+</i:mo></i:mrow></i:msup></i:math>, which is isoelectronic to RaF, offers a unique probe for studying nuclear structure and fundamental symmetries due to its simple electronic structure with a single unpaired electron. Density-functional-theory-based calculations of the electric charge distribution aid in understanding the formation of the observed species. Positive charges are found to be mainly located on the thorium atom, which points to an inhibition of immediate Coulomb explosion, thus supporting the existence of the observed ion species. The simplicity of the method and similarities among the actinide elements suggest that our approach will also be applicable to other actinide species.