| Summary: | Carbon structures comprising sp<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mn>1</mn> </msup> </semantics> </math> </inline-formula> chains (e.g., polyynes or cumulenes) can be synthesized by exploiting on-surface chemistry and molecular self-assembly of organic precursors, opening to the use of the full experimental and theoretical surface-science toolbox for their characterization. In particular, polarized near-edge X-ray absorption fine structure (NEXAFS) can be used to determine molecular adsorption angles and is here also suggested as a probe to discriminate sp<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mn>1</mn> </msup> </semantics> </math> </inline-formula>/sp<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mn>2</mn> </msup> </semantics> </math> </inline-formula> character in the structures. We present an ab initio study of the polarized NEXAFS spectrum of model and real sp<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mn>1</mn> </msup> </semantics> </math> </inline-formula>/sp<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mn>2</mn> </msup> </semantics> </math> </inline-formula> materials. Calculations are performed within density functional theory with plane waves and pseudopotentials, and spectra are computed by core-excited C potentials. We evaluate the dichroism in the spectrum for ideal carbynes and highlight the main differences relative to typical sp<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mn>2</mn> </msup> </semantics> </math> </inline-formula> systems. We then consider a mixed polymer alternating sp<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mn>1</mn> </msup> </semantics> </math> </inline-formula> C<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>4</mn> </msub> </semantics> </math> </inline-formula> units with sp<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mn>2</mn> </msup> </semantics> </math> </inline-formula> biphenyl groups, recently synthesized on Au(111), as well as other linear structures and two-dimensional networks, pointing out a spectral line shape specifically due to the the presence of linear C chains. Our study suggests that the measurements of polarized NEXAFS spectra could be used to distinctly fingerprint the presence of sp<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mn>1</mn> </msup> </semantics> </math> </inline-formula> hybridization in surface-grown C structures.
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