Polyynes are appealing one-dimensional carbon chains, made by the alternation of triple and single sp-hybridized bonds. Polyynes feature impressive properties predicted by theory, that make these systems suitable in a wide range of technological fields. The simplicity of polyynes allows the verification of one-dimensional models, particularly regarding the analysis of their vibrational modes. Indeed, polyynes possess a characteristic Raman fingerprint that modulates with their chain structure and carries information about their optoelectronic properties. This mode, also called α mode, represents the collective motion of all the CC bonds of the chain. Moreover, polyynes possess an outstanding optical activity with a very peculiar vibronic absorption pattern in the UV.<br/>We employed the fine tunability of the synchrotron radiation (IUVS beamline at ELETTRA, Italy) to exploit UV resonant Raman (UVRR) spectroscopy of size-selected H-capped polyynes (HC<sub>2n</sub>H, n=4-6). We excited each polyyne at the maxima of its vibronic pattern (Fig. 1a) that corresponds to transitions to a specific vibrational level of the excited electronic state. We detect for the first time up to the fifth-order overtone of the α mode, along with combinational modes with another collective vibration (β mode). We applied Albrecht’s theory of resonant Raman to interpret the singular intensity patterns of the overtones of the α mode (Fig. 1b). This theory allowed us to shed light on the physical processes determining the Raman intensities of vibrational transitions with high quantum numbers. Indeed, Albrecht’s theory predicted the main features of the UVRR spectra of polyynes (Fig. 1b). The observable discrepancies may be due to anharmonic effects that gain importance for excited vibrational states.<br/>The theoretical analysis accomplished here could be extended to analyze other carbon-based compounds with π-conjugated electrons, for whom Raman spectroscopy is a fundamental spectroscopic tool.

High order Raman overtones of polyynes by synchrotron based UV resonant Raman spectroscopy

ACS Spring 2022

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