We report here on the coupling of a gas reactor with a VUV beamline at the SOLEIL
synchrotron radiation facility. The reactor may be irradiated window-less with gas
pressure up to the atmosphere. The photochemistry is monitored by a mass spectrometer gas
analyzer. This set up, termed APSIS for Atmospheric Photochemistry SImulated by
Synchrotron, has been used to simulate the atmosphere of Titan and to study the formation
of the photochemical smog and the formation of tholins.