Simulating vibrationally resolved x-ray photoelectron spectra of flexible molecules: Linear alkanes CnH2n+2 (n=1-8)

We integrate full core-hole density-functional theory with Franck-Condon calculations, considering Duschinsky rotation, to simulate vibrationally resolved C 1s x-ray photoelectron spectra of eight linear alkanes, from methane to octane (CnH2n+2, n = 1-8). Results align excellently with experimental absolute binding energies and profiles. The spectrum of ethane serves as a "spectral seed," with each longer alkane's atom-specific spectrum displaying similar characteristics, albeit with shifts and slight intensity adjustments. Detailed assignments clarify the distinct spectra in short alkanes (n = 1-4) and their stabilization in long alkanes (n = 5-8). Carbons are classified as central or distal (C1 and C2), with central carbons contributing nearly identically to the lowest-energy feature A, while distal carbons contribute to the second-lowest-energy feature B (both are 0-0 transitions). Increasing molecule size adds more central carbons, enhancing feature A and weakening feature B. Our analysis identifies two dominant Franck-Condon active vibrations: a localized C & lowast;-H stretching (approximately 3400-3500 cm-1) and a delocalized C-H bending (approximately 1400-1500 cm-1) modes. Structural analysis shows that core ionization minimally affects alkane geometry, less than in ring compounds from previous studies. This work extends our protocol from rigid ring compounds to flexible molecules, contributing to building a high-resolution theoretical x-ray photoelectron spectroscopy library and enhancing the understanding of vibronic coupling.