Fluorescence yields from photodissociative excitation of chloromethanes by vacuum ultraviolet radiation

The photoabsorption and fluorescence cross sections of chloromethanes were measured in the 105-220 nm region using synchrotron radiation as a light source. The fluorescence threshold for CCl4 is at 152 nm with a maximum yield of 3 percent at 113 nm. The fluorescence results from the CCl2(A-X) system. For CHCl3, the fluorescence threshold is at 155 nm with a maximum yield of 0.6 percent at 110 nm. For CH2Cl2, the threshold is at 137 nm with a maximum yield of 0.35 percent at 107 nm. The fluorescence yield of CH3Cl is very small with an upper limit of 0.02 percent. The photodissociation processes are discussed in accord with the fluorescence data observed. Vibrational structures in CHCl3 and CH3Cl2 are observed and classified into progressions.