Ultrafast laser induced molecular alignment and deformation: Experimental evidence from neutral molecules and from fragment ions

The alignment and molecular structure deformations induced by intense off-resonance excitation with ultrafast laser pulses are examined using femtosecond transient grating spectroscopy and by angle resolved multiphoton ionization in a molecular beam. The goal of this study is to correlate evidence obtained from the angular dependence of multiphoton ionization and from rotational recurrences observed in neutral molecules regarding alignment and molecular structure deformation. Structural parameters are determined by analysis of the field-free rotational recurrences, obtained by transient grating measurements, or by analysis of the anisotropy in the detected fragment ions, in the molecular beam experiments. Experimental data were obtained for CS2, CO2, acetylene, and benzene, for pulse intensities ranging from 10(11) to 10(14) W/cm(2). The experimental results are consistent with molecular alignment resulting from a "kick" induced by the ultrafast off-resonance field. The results also provide evidence of molecular structure deformations. Results from transient grating experiments indicate that the electric field can induce alignment and bending in polyatomic molecules and that these effects can take place in the absence of ionization.