Experimental work was carried out in a flow reactor at an undulatorbased vacuum ultraviolet (VUV) beamline (BL03U) of the Hefei Light Source at the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. The apparatus used for this work has been described in previous publications. The samples were vaporized into the 10-mm-diameter flow reactor at 220 -250 ℃ to ensure full vaporization, the pressure difference between the flow reactor (266 Pa) and differential chamber (0.05 Pa) was large enough so that a molecular beam formed at the quartz nozzle.