Optical absorption changes induced by 308-nm laser pulses and by electron pulses at low temperatures were studied for crystalline 5,7-dodecadiyne-1,12-diol bis[phenylcarbamate](TCDU; R-C≡C-C≡C-R, R=(CH 2 ) 4 O(CO)C 6 H 5 ). The dimer diradical which shows an absorption band at 3.05 eV is not formed by the electron-pulse irradiation, but is formed efficiently by irradiation with the 308-nm laser pulses. By exciting crystals with an electron pulse and a laser pulse sequentially, it is shown that the collision of excited species with the lowest triplet state is a crucial step in generating the dimer diradical. The mechanism of the formation of the dimer diradical is discussed.