Anisotropic Distributions of Ion Fragments Produced by Dissociative Ionization of Halogenated Ethylenes in Intense Laser Fields

The angular distributions of fragments arising from dissociative ionization of chloroethylene (H₂CCHCl), bromoethylene (H₂CCHBr), and 2-chloroethenylsilane (H₃SiHCCHCl) by a linearly polarized pulsed laser field (800 nm, 50 fs, 2 × 10¹⁴ W cm^-2) have been measured using a time-of-flight mass spectrometer. The angular distributions of the multiply charged halogen and silicon ions originating from different molecules are markedly anisotropic, yielding a maximum signal for a laser polarization along the axis of the time-of-flight detector. In contrast, the anisotropy of the distributions of the multiply charged carbon ions depends on the parent molecule:  for chloroethylene and bromoethylene they are peaked preferentially in the direction parallel to the laser polarization, for 2-chloroethenylsilane they are peaked perpendicular to it. The anisotropy of all the above distributions is found to increase with the charge multiplicity of the ion fragment. The fragment anisotropies are discussed in terms of the molecular alignment, due to the nonresonant polarizability interaction, and of the selection of orientation in the ionization and dissociation dynamics in the strong laser field.