Direct Measurement of Dynamic Alignment in Strong Femtosecond Fields

Coulomb explosion imaging provides a unique window into molecular structure and dynamics on a timescale commensurate with intramolecular motion. This techniques combines the rapid removal of electrons from a molecular target, induced by intense ultrashort laser pulses, with position sensitive detection for coincidence capture of all fragments over $4\pi$ sr. We employed to measure the degree of dynamic alignment (the excess alignment beyond that due to geometric alignment) induced by linearly polarized, 100 fs pulses in the $10^{15}$ W/cm$^2$ intensity range. Exploiting circular polarization to turn off dynamic alignment a quantitative measure of the excess alignment was extracted from the relative atomic ion yields subsequent to Coulomb explosion in linearly and circularly polarized fields for several linear molecules (H$_2$, N$_2$, O$_2$ and CO$_2$). The degree of dynamic alignment was measured to be about 0.90 (H$_2$), 0.16 (N$_2$ and O$_2$) and 0.29 (CO$_2$). The anomalously large value of CO$_2$ implies a torque enhancement that we show is consistent with CO$_2$ interacting with the field longer than N$_2$ and O$_2$ prior to enhanced ionization. These results will be summarized and compared with other approaches and previous measurements.