Abstract We present a laboratory study of X-ray-induced chemistry of organic molecules (CH 4 , C 2 H 4 , and C 2 H 6 ) irradiated with synchrotron X-rays in the 200–1000 eV range from the Advanced Light Source beamline 9.3.2 at the Lawrence Berkeley National Laboratory. Broadband nonmonochromated X-rays beams used in this study partially simulate X-rays emitted from low-mass young stellar objects (0.2–2 keV). The experiment is conducted by irradiating a vacuum chamber prefilled with the selected hydrocarbon at a pressure of 100 mTorr at room temperature during predefined exposure times of 5, 10, 20, and 30 minutes and 1 hr. After each exposure, the chemical photoproducts deposited on Si substrates are analyzed in situ using X-ray photoelectron spectroscopy (XPS). Secondary-ion mass spectrometry is also performed ex situ to determine the hydrogen content of the deposited samples. Our analysis indicates that the photoproducts formed from X-ray irradiation of C 2 H 4 undergo more extensive photoprocessing than those derived from CH 4 and C 2 H 6 . XPS results show that sp 2 - and sp 3 -fitted XPS bands correspond to amorphous carbon particles with hybridization that depends on the parent precursor molecules. When irradiating pure CH 4 , a fully sp 3 -hybridized molecule, we found that X-ray photons induce photochemical reactions leading to the formation of heavier organics with sp 2 C=C bonds. Possible chemical pathways and astrophysical implications are discussed.
Bejaoui et al. (Fri,) studied this question.
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