Experimental investigation of left-right asymmetry in photon-atom interaction

Single ionization of noble-gas atoms by linearly polarized synchrotron radiation has been studied by employing angle- and energy-resolved photoelectron spectroscopy. The measurements were carried out in the plane defined by the momentum and polarization vectors of the photon. Parameters describing the left-right asymmetry (LRA; relative to the photon propagation direction) of the photoelectron angular distribution were determined experimentally for the $s$ shells of He, Ne, Ar, Kr, and Xe atoms and $\mathrm{H}{}_2$ molecules and for the $p$ shells of Ne, Ar, Kr, and Xe atoms. The values of the left-right asymmetry differ significantly from zero for both subshells. The photon and photoelectron energy dependence of the LRA parameters is presented also. Possible experimental and instrumental sources that could generate asymmetry are discussed and excluded as well.

Figure 1

(a) Definition of the coordinate system. (b) An example of the double-differential cross section of photoelectrons in the scattering $(\mathbf{P},\mathbf{k})$ plane. The solid and the dashed lines represent the angular distribution of emitted photoelectrons for linearly polarized and unpolarized light calculated from Eqs. (1) and (2), respectively, for the given parameter values.

Figure 2

Schematic cross section of an ESA-22-type electron spectrometer.

Figure 3

The Ar $L_{2,3}-M_{2,3}{M}_{2,3}$ Auger group measured at 461.2-eV photon energy with a 500-$\mu$m monochromator slit size. The colored Auger peaks were used to normalize the photoelectron spectra. The dark red (dark gray) shaded peak ($\approx$203.3 eV) was employed in the DORIS III experiment, while in the MAX II experiment [3] the light orange (light gray) line ($\approx$207.0 eV) was used.

Figure 4

Comparison of the experimental LRA parameters $A_{LR}$ for $\mathrm{H}{}_2$ molecule and outer $s$ shells of noble gases. Three data sets were collected at MAX II [3], and one was measured at DORIS III [11]: solid circles, first observation of the nonzero asymmetry parameters at MAX II; squares, results of the repeated experiment after the careful test of the experimental system at MAX II; inverted triangles, asymmetry parameters after rotation of the spectrometer system by ${180}^{\circ }$ horizontally at MAX II; open circles, data measured at DORIS III. The solid line shows the average LRA parameters obtained from the results of different measurements. The dashed line is the linear fit for the mean values of the noble gases.

Figure 5

The experimental values of the asymmetry parameters $A_{LR}$ for the $p$ shells of Ne, Ar, Kr, and Xe atoms. Circles, present work; squares, previously measured data for Ar $2p$ shells [3]; triangles, Ar $2p$ data measured by Heinasmaki et al. [19].

Figure 6

Dipole anisotropy $\beta$ and LRA parameters $A_{LR}$ for the angular distribution of $3p$ photoelectrons of Ar as a function of the photon energy. Circles show experimental $\beta$ anisotropy parameters from [20]. The line is to guide the eye. Squares show experimental LRA parameters. The dashed red line shows the theoretical calculation for the anisotropy $\beta$ parameter by Derevianko and Johnson [2]. The arrow denotes the photon energy where the asymmetry parameter was presented in Fig. 5.

Figure 7

Dipole anisotropy $\beta$ (top panel for Ar $3p_{1/2}$) [20] and LRA parameters $A_{LR}$ of Ar $3p_{1/2}$ (middle panel) and $3p_{3/2}$ shells (bottom panel) in the photon energy range where the resonant excitations of Ar $2p$ shell exist. The vertical lines denote the positions of the resonances. The dashed line in the top panel is the theoretical calculation of Gorczyca and Robicheaux [21].

Figure 8

The photoelectron energy dependence of the experimental LRA parameter $A_{LR}$ in the cases of $s$ and $p$ shells for Ar and Xe atoms. The solid line has been obtained by smoothing the data.