A key factor in parent and fragment ion formation on irradiation with an intense femtosecond laser pulse

Two pairs of organic molecules (2,3-dimethyl-1,3-butadiene, 2,5-dimethyl-2,4-hexadiene; 1,4-cyclohexadiene, 1,3-cyclohexadiene) were irradiated with a high-intensity 120-fs laser pulse in the intensity region of 10(14) W cm(-2). One molecule in each pair had no allowed electronic transition in the cation at the excitation wavelength of 800 mn, resulting in parent ion dominance. In contrast, the counter molecule, which had a similar structure but with absorption in the cation at the excitation wavelength, showed heavy fragmentation with a negligibly small signal of the parent ion. The previously reported observations are explainable on the basis of the proposed mechanism for fragmentation in femtosecond ionization. (C) 2001 Elsevier Science B.V. All rights reserved.