Ionization of oriented carbonyl sulfide molecules by intense circularly polarized laser pulses

We present combined experimental and theoretical results on strong-field ionization of oriented carbonyl sulfide molecules by circularly polarized laser pulses. The obtained molecular frame photoelectron angular distributions show pronounced asymmetries perpendicular to the direction of the molecular electric dipole moment. These findings are explained by a tunneling model invoking the laser-induced Stark shifts associated with the dipoles and polarizabilities of the molecule and its unrelaxed cation. The focus of the present article is to understand the strong-field ionization of one-dimensionally-oriented polar molecules, in particular asymmetries in the emission direction of the photoelectrons. In the following article [Phys. Rev. A 83, 023406 (2011)] the focus is to understand strong-field ionization from three-dimensionally-oriented asymmetric top molecules, in particular the suppression of electron emission in nodal planes of molecular orbitals.

Figure 1

(a) Schematic of the velocity map imaging spectrometer used to detect ions or electrons. The alignment of the molecules, illustrated by the OCS model, is determined by the alignment laser polarization, here shown for $\beta ={45}^{°}$. The static electric field of the spectrometer, pointing from the repeller to the extractor electrode for ion detection, breaks the head-for-tail symmetry by preferentially placing the O end toward the repeller. When detecting electrons the polarity of the electrodes is inverted, forcing the S end of the molecules toward the repeller electrode. [(b) and (c)] Images of ${\mathrm{S}}^{+}$ ions for $\beta ={45}^{°}$ and $-$45${}^{°}$, respectively, and ${\mathbf{F}}_{\text{stat}}=345$ V/cm. In (b) 73% of all ${\mathrm{S}}^{+}$ ions appear in the upper half of the detector. In (c) 28% of all ${\mathrm{S}}^{+}$ ions appear in the upper half of the detector. [(d) and (e)] Images of ${\mathrm{S}}^{+}$ ions for $\beta ={45}^{°}$ and $-$45${}^{°}$, respectively, with the static field increased to ${\mathbf{F}}_{\text{stat}}=594$ V/cm. In (d) and (e), respectively, 80% and 19% of all ${\mathrm{S}}^{+}$ ions appear in the upper part of the detector. The intensity of the probe laser is $~$$5.4\times {10}^{14}$ W/cm${}^2$.

Figure 2

Schematic of the experimental setup showing an OCS molecule oriented with its permanent dipole moment (bold red arrow) pointing in the direction of the static electric field. The left circularly polarized (LCP) probe pulse ionizes the molecule and imparts an upward momentum to the freed electron resulting in recording on the upper part of the detector (see text for details).

Figure 3

Two-dimensional momentum image of electrons produced when a randomly oriented sample of OCS molecules are ionized by (a) the LCP probe pulse. The polarization plane of the probe pulse is in the Y,Z plane, i.e., perpendicular to the image (detector). (b) Same as in (a) but for an RCP probe pulse. [(c) and (d)] As in (a) and (b) but with the OCS molecules aligned along the $Y$ direction by the alignment pulse polarized parallel to the image plane. [(e) and (f)] As in (a) and (b) but with the OCS molecules aligned along the $Z$ direction by the alignment pulse polarized perpendicular to the image plane. [(g) and (h)] As in (e) and (f) but with the intensity of the alignment pulse lowered from $I_{\text{align, YAG}}=8.4\times {10}^{11}$ W/cm${}^2$ to $I_{\text{align, YAG}}=2.8\times {10}^{11}$ W/cm${}^2$. The intensity of the 800-nm, 30-fs probe pulse is kept at $2.44\times {10}^{14}$ W/cm${}^2$ in all pictures.

Figure 4

Two-dimensional momentum image of electrons produced when a randomly oriented sample of OCS molecules is ionized by the LCP for increasing intensity of the ionization pulse, corresponding to $I_{\text{probe}}=1.76\times {10}^{14}$ W/cm${}^2$ in (a), $I_{\text{probe}}=2.44\times {10}^{14}$ W/cm${}^2$ in (d), and $I_{\text{probe}}=2.83\times {10}^{14}$ W/cm${}^2$ in (g). [(b), (e), and (h)] Same as in (a), (d), and (g), respectively, but with the OCS molecules aligned along the $Z$ direction by the alignment pulse polarized perpendicular to the image plane. In (c), (f), and (i) the radial distributions of the upper and lower parts of images (b), (e), and (h) are shown. $I_{\text{align, YAG}}=8.4\times {10}^{11}$ W/cm${}^2$.

Figure 5

Visualization of the degenerate HOMO orbitals of OCS, at an isodensity contour value of 0.1. The S end of the molecule is on the left in these images. The orbital to the left lies in the polarization plane of the laser field and contributes more to the total ionization yield than the orbital to the right, which has a nodal structure in the polarization plane and is obtained by 90${}^{°}$ rotation of the orbital to the left around the molecular axis. The circles with arrows pointing counterclockwise illustrate the LCP field.

Figure 6

The saddle points of the OCS potential at the intensity of $2.44\times {10}^{14}$ W/${\mathrm{cm}}^2$ in the polarization $Y$,$Z$ plane. The dots are the saddle points of the effective potential made up of the molecular single-active electron potential and the external field. The values of the saddle points are taken at 5${}^{°}$ steps with respect to the $z$ axis and are connected with the dotted lines to guide the eyes. The origin is the center of mass of the molecule. The S end of the molecule is to the left of this figure, in accordance with the geometry of Fig. 2. The explicit nuclear positions are as follows: S ($-2$ a.u.), C (0.99 a.u.), and O (3.2 a.u.) (see the Appendix).

Figure 7

Momentum distributions obtained by the present model for peak intensities of (a) $2.44\times {10}^{14}$ and (b) $2.83\times {10}^{14}$ W/${\mathrm{cm}}^2$ of the 800-nm, 30-fs probe laser pulse. Compare with the experiments in Figs. 4e and 4h.

Figure 8

Comparison of experimental (full lines) and theoretical (dashed lines) radial distributions for peak intensities of (a) $2.44\times {10}^{14}$ and (b) $2.83\times {10}^{14}$ W/${\mathrm{cm}}^2$ of the 800-nm, 30-fs probe laser pulse. The upper curves are the radial distributions in the upper ($p_Y>0$) and the lower curves are radial distribution in the $p_Y<0$ half plane; seeEqs. (22) and (23).

Figure 9

Comparison of experimental (full lines) and theoretical (dashed lines) angular distributions [Eq. (21)] for peak intensities of (a) $2.44\times {10}^{14}$ and (b) $2.83\times {10}^{14}$ W/${\mathrm{cm}}^2$ of the 800-nm, 30-fs probe laser pulse.

Figure 10

Comparison of experimental (dots) and theoretical (dashed lines) asymmetry $A^{+}$ [Eq. (24)] as a function of radial momentum for peak intensity of (a) $2.44\times {10}^{14}$ and (b) $2.83\times {10}^{14}$ $\text{W}/{\text{cm}}^2$ of the 800-nm, 30-fs probe laser pulse.