Anisotropic bulletlike emission of terminal ethynyl fragment ions: ionization of ethynylbenzene-d under intense femtosecond laser fields

The authors investigated Coulomb explosions of ethynylbenzenes under intense femtosecond laser fields. Deuteration on the edge of the triple bond gave information about specific fragment emissions and the contribution of hydrogen migration. Some fragments not resulting from migration were emitted in the direction of laser polarization. These were ethynyl fragment ions (D(+), CD(+), C(2)D(+), and C(3)D(+)). Although two bonds have to be cleaved to produce C(3)D(+), the rigid character of the triple bond was maintained in the Coulomb explosion process. In contrast, fragment ions, which are formed after single or double hydrogen migration, showed isotropic emissions with distinct kinetic energies. The character of the substituents has been found to hold even under strong laser light fields where violent fragmentation took place. The ethynyl parts were emitted like bullets from the molecular frame of ethynylbenzene despite the explosion into pieces of the main body of benzene ring.