Abstract
We directly measure the field-free molecular alignment of various room-temperature molecular gases based on alignment-induced spatial focusing and defocusing effects. By imaging the spatial profile of a time-delayed probe pulse with increased and decreased local intensity at the beam center, the parallel and perpendicular molecular alignments are clearly characterized. Meanwhile, the electronic Kerr effect, weak plasma contribution and field-free molecular alignment impact could be distinguished from the measured signals.
Highlights
Periodic revivals of field-free molecular alignment [1,2,3,4,5] after impulsive rotational Raman excitation by an ultrashort laser pulse have attracted growing interest for its interesting applications in chemistry and ultrafast optics [6,7,8,9,10,11,12]
Throughout the paper, a positive signal signifies the spatial focusing phenomenon, indicating that the molecules were aligned parallel to the polarization of the probe pulse for which a bright spot pattern in the center was observed, while on the other hand a ring probe beam pattern and a negative signal corresponds to spatial defocusing with perpendicular alignment
The parallel molecular alignment revival with increased refractive index introduced a spatial focusing effect which was equivalent to a positive lens and increased the local intensity at the interaction part of the probe pulse
Summary
Periodic revivals of field-free molecular alignment [1,2,3,4,5] after impulsive rotational Raman excitation by an ultrashort laser pulse have attracted growing interest for its interesting applications in chemistry and ultrafast optics [6,7,8,9,10,11,12]. The weak field polarization technique [3,13] and cross-defocusing measurement [14,15] were successfully used to measure the alignment signals, obtaining a signal proportional to (-1/3) rather than , where the parallel and perpendicular alignment revivals were hard to be directly distinguished since both of them showed increased signals from the measurement with respect to the random molecular orientation. Other methods such as supercontinuum spectral interferometry [5,10], Coulomb explosion of pre-aligned molecules [15,16], and a revised weak field polarization technique [17] could be used to get alignment signals proportional to or (-1/3). The electronic Kerr effect, the weak plasma and field-free molecular alignment impact on the modulation of the probe pulse differ in both time and intensity scales, which can be clearly distinguished
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