Photon echo measurements in SF6and SiF4

Abstract

The polarization of photon echoes stimulated by linear-linear, circular-linear, and linear-circular excitation sequences has been studied in S${\mathrm{F}}_6$ with the $P\left(12\right)\mathrm{}$ through $P\left(26\right)\mathrm{}$ laser lines of C${\mathrm{O}}_2$ near 10.6 μm and in Si${\mathrm{F}}_4$ with the $P\left(28\right)\mathrm{}$ through $P\left(34\right)\mathrm{}$ lines near 9.6 μm. A linear-linear pulse sequence with angle $\beta$ between the linear polarizations produces an echo at angle $\varphi$ with respect to the second pulse. $\varphi$ rotates in the same sense as $\beta$ with $P\left(16\right)\mathrm{}$ in S${\mathrm{F}}_6$ and in the opposite sense as $\beta$ with the other lines in S${\mathrm{F}}_6$, and with the lines in Si${\mathrm{F}}_4$. A left-circular-linear sequence produces a right-elliptic echo with $P\left(16\right)\mathrm{}$ in S${\mathrm{F}}_6$, and a left-elliptic echo in the other cases. A linear-left-circular sequence produces a left-circular echo in all cases. These results are in accord with the large-$J$ polarization theory of Heer and Nordstrom. For a linear-linear excitation sequence, this theory predicts an unnormalized echo electric field $\overrightarrow{\mathrm{E}}\cong \mp \hat{X}\left(\frac{1}{2}sin\beta \right)+\hat{Z}cos\beta$, where the negative sign applies to a $P$- or $R$-branch transition, and the plus sign to a $Q$-branch transition. For a left-circular-linear sequence, the theory predicts an unnormalized echo electric field $\overrightarrow{\mathrm{E}}\cong \pm \frac{1}{2}i\hat{X}+\hat{Z}$, where the plus sign refers to a $P$- or $R$-branch transition, and the negative sign to a $Q$-branch transition. Echo decay rates determined from relaxation measurements are used to calculate molecular cross sections, and these cross sections are compared with calculations based on the refractive indices and the Slater-Kirkwood equation. Experimental cross sections are in general smaller than those found from the Slater-Kirkwood equation.