The Raman spectrum up to the fifth-order shows a peculiar behavior of the overtone intensity as a function of the excitation at selected vibronic transitions that can be explained by Albrecht’s theory of resonance Raman scattering in a simple and analytic approach. We evaluated the electron-phonon coupling (Huang–Rhys factor) and the vibrational structure of the excited state through the analysis of the UV-Vis absorption spectra. We exploited synchrotron radiation as a tunable coherent UV light source to precisely excite the size-dependent absorption vibronic maxima resolving by resonance Raman scattering the vibrational fine structure (up to the fifth level) of the ground state. Such systems represent the simplest linear carbon chain comprising sp-hybridized carbon atoms only and prove to be the ideal system to investigate resonance Raman processes in detail. In this work, we provide a resonance Raman investigation of size-selected H-terminated polyynes, as the simplest model of sp-carbon wires in the short size limit. In this framework, the size-dependent resonance Raman behavior and the electron-phonon coupling in the limit of short sp-carbon wires is an open issue. The evaluation of an average Huang–Rhys factor of 1.82 indicates a large electron-phonon coupling and accounts for the extremely high Raman activity of this system 8, 9. Recently resonance Raman in the visible range has been used to infer the excitation profile of confined carbynes 9. Pre-resonance and resonance conditions have been exploited for polyynes absorbing in the UV 31, 35. Resonance Raman spectroscopy has been rarely employed to investigate the vibrational spectra of short sp-carbon systems 31, 32, 33, 34. The resonance Raman cross section exceeding any other material by two orders of magnitude makes confined carbyne a candidate for nanoscale temperature monitoring by exploiting the Stokes/anti-Stokes ratio 30. Sp-carbon features a larger Raman cross section than sp 2 amorphous carbon and confined carbyne has been recently identified as the strongest Raman scatterer ever reported 28, 29. The main vibrational mode, also called α or C-mode, involves a collective motion of all the CC bonds of the chain, as discussed in the framework of the Effective Conjugation Coordinate ECC model, and carries fundamental information about the structure and properties of sp-carbon chains 3, 23, 24, 25, 26, 27. Sp-carbon chains feature a specific Raman fingerprint in a spectral region (i.e., 1800–2300 cm −1) where other carbon systems do not present any signal. While Raman excitation profiles of the fundamental Raman transition have been extensively investigated in the past for many systems including carbon nanotubes and conjugated molecules 18, 19, 20, 21, 22, the effect of the resonance with different vibronic states in the region of the overtones has not been observed so far, to the best of our knowledge. The so-called 2D mode, sensitive to the number of graphene layers, is a second-order Raman mode (i.e., the overtone of the D peak) showing a larger intensity than the first order only in single layer graphene 12, 17. For instance, resonance Raman scattering and electron-phonon coupling are relevant in understanding the spectrum of graphite, single-walled carbon nanotubes, and graphene 11, 12, 13, 14, 15, 16, 17. Resonance Raman scattering can easily detect higher-order scattering processes involving more than one phonon or vibrational quantum. The Raman process, based on the inelastic scattering of photons by phonons or molecular vibrations, can be enhanced in resonance Raman scattering by using photon energies matching the absorption edges of the material and thus involving excited electronic states. Raman spectroscopy has proven to be a fundamental technique for the characterization of materials. However, the question of how the properties change from short carbon wires to carbyne remains largely unanswered. Recently, the optical gap behavior in finite systems (i.e., oligoynes) as a function of chain length has been shown to reach saturation, thus pointing to the possible value of the ideal carbyne 8, 10. Long carbon wires exceeding thousands of atoms encapsulated in carbon nanotubes (i.e., confined carbyne) have provided a way to approach the model carbyne whose properties should no longer depend on the length of the system 1, 8, 9. Finite sp-carbon wires can be synthesized up to a few tens of carbon atoms and show size-dependent optical and electronic properties 6, 7. Besides the most known carbon allotropes, mainly based on sp 2 hybridization, linear sp-carbon chains have recently attracted the interest of the scientific community as carbon atomic wires modeling the elusive carbon allotrope carbyne and showing appealing mechanical, thermal, and optoelectronic properties 1, 2, 3, 4, 5.
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