Ultrafast Angular Momentum Orientation by Linearly Polarized Laser Fields

We theoretically show and experimentally verify that a pair of linearly polarized intense femtosecond pulses can create molecular ensembles with oriented rotational angular momentum on an ultrafast ($\sim \mathrm{ps}$) time scale, when the delay and the mutual polarization between them are appropriately arranged. An asymmetric distribution for $+M$ and $-M$ sublevels relies on quantum interference between rotational wave packets created in stimulated Raman excitation by the first and second pulses. The present approach provides spatiotemporally propagating ensembles, of which the classical perspective is molecules rotating in a clockwise or counterclockwise direction.

Figure 1

Experimental scheme to create and observe the angular-momentum orientation in molecular rotation.

Figure 2

Calculated populations in a rotational wave packet created by double-pulse excitation of the $|0,0,0⟩$ initial state, plotted against the delay, $\tau$, between the two pulses. $\beta$ is set to $-\pi /4$. A Gaussian time profile with duration of 200 fs (FWHM) is used for the calculation. The laser peak intensity for each pulse is (a) $0.1\mathrm{TW}/{\mathrm{cm}}^2$, and (b),(c) $1.2\mathrm{TW}/{\mathrm{cm}}^2$. In panel (a), the vertical scale is expanded so that the population of $|0,0,0⟩$ is out of the range. For $1.2\mathrm{TW}/{\mathrm{cm}}^2$, populations of $J=0$ and 2 are shown in panel (b) while those of $J\ge 4$ in (c) for clarity.

Figure 3

Excitation spectra of the $P$-branch region in the $S_1-S_0$ $6_0^1$ band of benzene after fs double-pulse excitation with $\tau$ set to 7.3 ps, recorded with the right-handed [dark gray (blue) lines] and left-handed [light gray (red) lines] polarized probe pulse. The middle and lower panels show experimentally observed ones for $\beta =-\pi /4$ and $\pi /4$, respectively, while the upper one is simulated for $\beta =-\pi /4$ with a laser peak intensity of $1.2\mathrm{TW}/{\mathrm{cm}}^2$ for each pulse.

Figure 4

Delay-time dependence of the degree of orientation, $⟨{\widehat{J}}_Z⟩$, for wave packets created from the five lowest rotational levels, as well as their nuclear-spin weighted average. $\beta$ is set to $-\pi /4$ and the laser peak intensity is $1.2\mathrm{TW}/{\mathrm{cm}}^2$ for each pulse. Vertical broken lines indicate delay time at zero and $T_{\mathrm{rev}}\approx 87.9\mathrm{ps}$.