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Photoionization of Cl III ions into Cl IV was studied theoretically using the ab initio relativistic Breit–Pauli R-matrix (BPRM) method and experimentally at the Advanced Light Source (ALS) synchrotron at the Lawrence Berkeley National Laboratory. A relative-ion-yield spectrum of Cl IV was measured with a photon energy resolution of 10 meV. The theoretical study was carried out using a large wave-function expansion of 45 levels of configurations <inline-formula><math display="inline"><semantics><mrow><mn>3</mn><msup><mi>s</mi><mn>2</mn></msup><mn>3</mn><msup><mi>p</mi><mn>2</mn></msup></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><mn>3</mn><mi>s</mi><mn>3</mn><msup><mi>p</mi><mn>3</mn></msup></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><mn>3</mn><msup><mi>s</mi><mn>2</mn></msup><mn>3</mn><mi>p</mi><mn>3</mn><mi>d</mi></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><mn>3</mn><msup><mi>s</mi><mn>2</mn></msup><mn>3</mn><mi>p</mi><mn>4</mn><mi>s</mi></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><mn>3</mn><mi>s</mi><mn>3</mn><msup><mi>p</mi><mn>2</mn></msup><mn>3</mn><mi>d</mi></mrow></semantics></math></inline-formula>, and <inline-formula><math display="inline"><semantics><mrow><mn>3</mn><msup><mi>p</mi><mn>4</mn></msup></mrow></semantics></math></inline-formula>. The resulting spectra are complex. We have compared the observed spectrum with photoionization cross sections (<inline-formula><math display="inline"><semantics><msub><mi>σ</mi><mrow><mi>P</mi><mi>I</mi></mrow></msub></semantics></math></inline-formula>) of the ground state <inline-formula><math display="inline"><semantics><mrow><mn>3</mn><msup><mi>s</mi><mn>2</mn></msup><mn>3</mn><msup><mi>p</mi><mn>3</mn></msup><mrow><msup><mo>(</mo><mn>4</mn></msup><msubsup><mi>S</mi><mrow><mn>3</mn><mo>/</mo><mn>2</mn></mrow><mi>o</mi></msubsup><mo>)</mo></mrow></mrow></semantics></math></inline-formula> and the seven lowest excited levels <inline-formula><math display="inline"><semantics><mrow><mn>3</mn><msup><mi>s</mi><mn>2</mn></msup><mn>3</mn><msup><mi>p</mi><mn>3</mn></msup><mrow><msup><mo>(</mo><mn>2</mn></msup><msubsup><mi>D</mi><mrow><mn>5</mn><mo>/</mo><mn>2</mn></mrow><mi>o</mi></msubsup><mo>)</mo></mrow></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><mn>3</mn><msup><mi>s</mi><mn>2</mn></msup><mn>3</mn><msup><mi>p</mi><mn>3</mn></msup><mrow><msup><mo>(</mo><mn>2</mn></msup><msubsup><mi>D</mi><mrow><mn>3</mn><mo>/</mo><mn>2</mn></mrow><mi>o</mi></msubsup><mo>)</mo></mrow></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><mn>3</mn><msup><mi>s</mi><mn>2</mn></msup><mn>3</mn><msup><mi>p</mi><mn>3</mn></msup><mrow><msup><mo>(</mo><mn>2</mn></msup><msubsup><mi>P</mi><mrow><mn>3</mn><mo>/</mo><mn>2</mn></mrow><mi>o</mi></msubsup><mo>)</mo></mrow></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><mn>3</mn><msup><mi>s</mi><mn>2</mn></msup><mn>3</mn><msup><mi>p</mi><mn>3</mn></msup><mrow><msup><mo>(</mo><mn>2</mn></msup><msubsup><mi>P</mi><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow><mi>o</mi></msubsup><mo>)</mo></mrow></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><mn>3</mn><mi>s</mi><mn>3</mn><msup><mi>p</mi><mn>4</mn></msup><mrow><msup><mo>(</mo><mn>4</mn></msup><msub><mi>P</mi><mrow><mn>5</mn><mo>/</mo><mn>2</mn></mrow></msub><mo>)</mo></mrow></mrow></semantics></math></inline-formula>, <inline-formula><math display="inline"><semantics><mrow><mn>3</mn><mi>s</mi><mn>3</mn><msup><mi>p</mi><mn>4</mn></msup><mrow><msup><mo>(</mo><mn>4</mn></msup><msub><mi>P</mi><mrow><mn>3</mn><mo>/</mo><mn>2</mn></mrow></msub><mo>)</mo></mrow></mrow></semantics></math></inline-formula> and <inline-formula><math display="inline"><semantics><mrow><mn>3</mn><mi>s</mi><mn>3</mn><msup><mi>p</mi><mn>4</mn></msup><mrow><msup><mo>(</mo><mn>4</mn></msup><msub><mi>P</mi><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msub><mo>)</mo></mrow></mrow></semantics></math></inline-formula> of Cl III, as these can generate resonances within the energy range of the experiment. We were able to identify most of the resonances as belonging to various specific initial levels within the primary Cl III ion beam. Compared to the first five levels, resonant structures in the <inline-formula><math display="inline"><semantics><msub><mi>σ</mi><mrow><mi>P</mi><mi>I</mi></mrow></msub></semantics></math></inline-formula> of excited levels of <inline-formula><math display="inline"><semantics><mrow><mn>3</mn><mi>s</mi><mn>3</mn><msup><mi>p</mi><mn>4</mn></msup></mrow></semantics></math></inline-formula> appear to have a weaker presence. We have also produced combined theoretical spectra of the levels by convolving the cross sections with a Gaussian profile of experimental width and summing them using statistical weight factors. The theoretical and experimental features show good agreement with the first five levels of Cl III. These features are also expected to elucidate the recent observed spectra of Cl III by Sloan Digital Scan Survey project.