Optimizing Transient Laser Alignment of Diatomic Molecules

Intense, short laser pulses have been used to transiently align diatomic molecules for use as targets in subsequent strong-field experiments. The laser induces a series of Raman transitions within each molecule, forming a rotational wavepacket that exhibits periodic angular localization along the laser polarization axis [1]. We probe this alignment by exploding the molecule using a more intense, time-delayed circularly polarized 30 fsec 780 nm pulse, and measuring the momenta of the multiply charged ion fragments using a mass spectrometer equipped with a helical wire-anode detector. We have observed transient alignment in N$_{2}$, O$_{2}$, and CO and find that the use of multiple laser kicks, with appropriate relative delays, can significantly enhance the degree of alignment while reducing the level of ionization. We are now using transiently aligned targets to compare intense laser ionization rates for atoms with those for molecules aligned parallel or perpendicular to the laser polarization [2]. In addition we are exploring the use of a laser pulse-shaper and genetic feedback algorithm [3] to further enhance alignment efficiency through a closed-loop optimization of the aligning pulse shapes. This work is supported by DOE BES and the UVa FEST. [1] H. Stapelfeldt and T. Seideman, Rev. Mod. Phys. \textbf{75, }543 (2003). [2] I.V. Litvinyuk et al., Phys. Rev. Lett. \textbf{90}, 233003 (2003). [3] B.J. Pearson et al., Phys. Rev. A \textbf{63}, 063412 (2001).