Laser-induced field-free alignment of OCS molecule

We investigate the dynamical alignment of jet-cooled OCS molecules induced by a short laser pulse. The alignment is measured through the orientational contribution of the optical Kerr effect using a second weak laser pulse as a probe. Maximum alignment is observed at conditions close to saturation of ionization. The results are analysed with a quantum mechanical model solving for the rotational dynamics. Laser-induced field-free alignment of the OCS molecule 2 Molecular field-free alignment has received enhanced attention (for a review see reference [1]) after its first observation in I2 [2]. In principle, any molecule of anisotropic polarizability kicked by an intense and sufficiently short laser pulse can be aligned in field-free conditions, i.e. after extinction of the pulse. Even molecules that are highly symmetrical in their ground vibronic state (like for instance CH4) would be aligned, providing that they are excited in a state of non-spherical symmetry. However, most of the studies have been performed through the non-resonant interaction of the laser field with the ground vibronic state of the molecule [3]. When one uses laser pulses of duration τL much longer than the rotational period defined by Tr = 1/(2Bc), with B one of the rotational constants and c the speed of light, the alignment is adiabatic and occurs therefore only during the interaction [4]. On the other hand, a short pulse τL ≪ Tr produces post-pulse alignment transients of duration much shorter than the rotational period. For linear molecules, the transients are spaced in time by a fraction of the rotational period. If one considers the rigid rotor approximation, the effect lasts as long as the coherence of the medium is maintained. The main advantage of the short pulse is that applications based on aligned molecules can be conducted in field-free conditions. Most of the studies devoted to laser-induced alignment of small molecules have been focused on non-polar molecules [1]. In fact, the permanent dipole plays a negligible role when the alignment occurs through the non-resonant excitation of the polarizability. However, alignment of polar molecules is an interesting issue due to its close relation to orientation. Laser-induced orientation has been achieved by combining an electrostatic and a pulsed laser field in the adiabatic regime [5, 6]. So far, observation of orientation in the short pulse regime [7] remains out of reach. Orientation presents important prospects with respect to control of steric effects in physical chemistry but also to nonlinear optics. In the latter, processes for instance prohibited in centrosymmetric media could be allowed in the gas phase, like the generation of even order harmonics. The scope of this work is to present a detailed study of field-free alignment of a polar molecule using carbonyl sulfide as a target molecule. The OCS molecule is aligned nonresonantly by a short pump pulse of duration∼ 70 fs and intensity 10−20 TW/cm. The detection is based on the optical Kerr effect. The alignment is probed by using a second pulse that interacts with the molecules after they have been exposed to the pump pulse. The anisotropic angular distribution of the molecular axis results in the depolarization of the probe that can be observed in time by changing the delay between the two pulses. Because the second pulse is weak, we can safely assume that the alignment is not affected by the probe and therefore that the signal reflects only the alignment produced by the first pulse. The pump and probe pulses are delivered by a Ti:Sapphire laser chirped pulse amplified system operating at 10 Hz with a central wavelength of 800 nm, delivering pulses with energy up to 150 mJ and duration down to 50 fs. Although we use a vacuum setup designed for the generation of high order harmonics through large interaction volume [8], the scheme of the experiment is similar to the one presented in reference [9]. Laser-induced field-free alignment of the OCS molecule 3 A delay line is used in order to adjust the temporal delay between the pump and probe pulse. Both pulses are linearly polarized at 45 to each other. For improved alignment efficiency, the molecules are cooled in a jet expansion of pure OCS operating at a backing pressure of 2-4 bars. Both beams are focused with a lens of 3 m focal length and cross each other at a small angle (∼ 1) into the jet expansion, close to the nozzle where the number density is large. The jet axis is intersected by the two laser beams at 90. At the exit of the interaction chamber, the pump is blocked by a beam stop and the depolarization of the probe is detected through a polarizer set at 90 with respect to its initial polarization direction. The depolarized probe field is then collected with a photomultiplier, sampled by a boxcar integrator, and sent to a computer that is also used to control the delay line. In the limit of small birefringence, the intensity signal S measured at the pumpprobe delay τ is approximated by