Is there anybody out there? Ultrafast Rydberg–valence interactions in the photodissociation of trimethylamine

Abstract

Trimethylamine (TMA) is a tertiary aliphatic amine that stands as a potential marker for life beyond Earth due to only being naturally produced via biotic means. However, its propensity to undergo photodissociation in the gas phase when excited by a deep ultraviolet photon means that its amine daughter product could serve as an additional biomarker and confirmational spectral signature of TMA in exoplanetary atmospheres. The photochemistry of TMA is dominated by strong Rydberg-valence state interactions. To understand how these interactions lead to its amine photoproduct, we employ time-resolved extreme ultraviolet photoelectron spectroscopy where TMA is pumped by a 200 nm femtosecond laser pulse and analyze the results with the help of electronic structure calculations of the excited state potential energy surface relevant to the process. Our combined experimental and theoretical study indicates that from the decay of the initially prepared 3p _z state (time-constant 400 fs), internal conversion through the remaining 3p manifold (4.4 ps) and the 3s state (67 ps) states competes with ultrafast photodissociation, forming ground state dimethyl amidogen (DMA) and CH ₃ (ν ₂ = 4) radical products. Decay of the 3s state reveals the formation of a second product pair, forming DMA in a low-lying excited state, DMA ( A ²A ₁) , and vibrationally cold CH ₃. We suggest that the rapid dissociation channel arises from a near-planar geometry accessed in the 3p _z state and the longer time channel arises from the excited state population, accessing a pyramidal geometry in the 3s state.

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