Simulating Vibrationally-Resolved X-ray Photoelectron Spectra of Flexible Molecules: Linear Alkanes C$_{n}$H$_{2n+2}$ ($n$=1-8)

TL;DR

This study combines advanced computational methods to accurately simulate vibrationally-resolved X-ray photoelectron spectra of linear alkanes, revealing detailed spectral features and vibrational contributions, thus expanding the theoretical XPS library for flexible molecules.

Contribution

It introduces an integrated computational protocol for simulating vibrationally-resolved XPS of flexible molecules, extending previous methods from rigid rings to alkanes, and clarifies spectral features and vibrational effects.

Findings

Excellent agreement with experimental binding energies and spectra.

Identification of key vibrational modes influencing spectra.

Structural stability upon core ionization in alkanes.

Abstract

We integrated full core-hole density functional theory with Franck-Condon calculations, considering Duschinsky rotation, to simulate vibrationally-resolved C1s X-ray photoelectron spectra (XPS) of eight linear alkanes, from methane to octane (C$_{n}$H$_{2n+2}$, $n$=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 (C$_1$ and C$_2$), 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: C$^*$--H stretching ($\sim$3400-3500 cm$^{-1}$) and bending ($\sim$1400-1500 cm$^{-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 XPS library and enhancing the understanding of vibronic coupling.