Probing the photoabsorption features and electronic excited states of propylene oxide: an experimental and theoretical study

Abstract

Propylene oxide is the first chiral molecule identified in the interstellar medium, which has resulted in growing interest in it as a prototypical molecule to study the origin of life on Earth. Numerous spectroscopic studies have investigated the excitation, ionization and dissociation of propylene oxide by photons, electrons and/or ions. However, for vacuum ultraviolet (VUV) photoabsorption spectroscopy, data are available only for energies between 6 and 9 eV with low energy resolution. Here, we present the high-resolution VUV photoabsorption cross-sections in the 6.0–10.8 eV range through an experimental and theoretical approach. The measurements were carried out using a VUV synchrotron radiation light source and are supported by quantum chemical calculations performed using time-dependent density functional theory. There is good agreement between experiment and theory, allowing us to characterize the main absorption bands assigned to electronic transitions involving mainly oxygen lone pairs and lower-lying Rydberg states. At higher energy, there are several Rydberg states observable, characterized by superimposed features with different vibrational progressions. Some features observed in the spectrum are assigned to vibrational modes involving the methyl group, namely CH₃ bending (υ₂₂ and υ₂₃) and CH₃ torsion (υ₂₄). Additionally, we report a vibrational progression which may be related to the cation ring CC stretching with an average frequency of about 565 cm⁻¹. Calculated potential energy curves for the low-lying excited states along the C–CH₃ stretching coordinate reveal that the initial Rydberg states evolve into dissociative states at larger bond distances, as the σ* valence character increases.