Hyperfine-Structure-Induced Depolarization of Impulsively Aligned ${\mathrm{I}}_2$ Molecules

A moderately intense 450 fs laser pulse is used to create rotational wave packets in gas phase ${\mathrm{I}}_2$ molecules. The ensuing time-dependent alignment, measured by Coulomb explosion imaging with a delayed probe pulse, exhibits the characteristic revival structures expected for rotational wave packets but also a complex nonperiodic substructure and decreasing mean alignment not observed before. A quantum mechanical model attributes the phenomena to coupling between the rotational angular momenta and the nuclear spins through the electric quadrupole interaction. The calculated alignment trace agrees very well with the experimental results.

Figure 1

Experimental results (black line) and the model calculations (red line) calculated at $T=0.8\mathrm{K}$. An alignment trace calculated without quadrupole coupling is also shown (dotted black line).

Figure 2

In (a), the sum of $J\ne J^{\prime }$ matrix elements comprising the theoretical quadrupole coupled alignment trace $⟨{\cos }^2{\theta }_{2\mathrm{D}}{⟩}_{\mathrm{coh}}$ is shown in blue. The blue trace in (b) shows $⟨{\cos }^2{\theta }_{2\mathrm{D}}{⟩}_{\mathrm{coh}}^{\neg \delta \omega }$, where the frequency and phase shifts caused by the hyperfine energy splitting have been suppressed. Both (a) and (b) are superposed with equivalent traces calculated without quadrupole coupling (dotted black lines).

Figure 3

(a) Occupancy of initial state $|2,-2⟩\otimes |6,0⟩$ and the $|I{M}_I⟩\otimes |J{M}_J⟩$ states it couples to evolving over the timescale of the experiment. (b) Sketch of the state energy splittings and relative coupling strengths ($\delta =57\mathrm{MHz}$). (c) Schematic classical interpretation of the system dynamics.

Figure 4

Quadrupole-coupled alignment traces simulated to 70 ns, with alignment pulse intensities set to (a) the experimental and (b) $2.5\times$ the experimental value.