Anomalous cooling of the parallel velocity in seeded beams

We have measured the parallel velocity distribution of a lithium supersonic beam produced by seeding lithium in argon. The parallel temperature for lithium is considerably lower than the calculated parallel temperature of the argon carrier gas. We have extended the theory of supersonic cooling to calculate the parallel temperature of the seeded gas, in the limit of high dilution. The theoretical result thus obtained is in good agreement with our observations.

Figure 1

Laser induced fluorescence signal as a function of the laser frequency: The dots represent the experimental data and the full curves are the Gaussian fits to each line. The Fabry-Pérot used for calibration has a free spectral range equal to $251.4\pm 0.5\mathrm{MHz}$. The fluorescence peaks are labeled by the ground state $F$ value ($F=1$ or $2$) and by a letter corresponding to the laser beam: A for the beam near $45°$ incidence, B for the beam near normal incidence.

Figure 2

The reduced temperatures $\tau ={\Xi }^{-9∕11}T∕T_0$ are plotted as a function of the reduced $z$ coordinate $\zeta =2.48{\Xi }^{12∕11}z∕d$ with $\Xi =0.813n_{0}d{\left(C_6∕k_B{T}_0\right)}^{1∕3}$ (to give an example, for our argon beam seeded with lithium, $\Xi =32.4$). This plot, which is identical to Fig. 5 of Beijerinck and Verster [3] for ${\tau }_{\Vert 1}$ and ${\tau }_{\perp 1}$, shows the anomalous cooling effect for species $2$. We have used ${\rho }_o=1.32$ and ${\rho }_s=2.55$ corresponding to lithium seeded in argon.

Figure 3

The calculated parallel temperature ratio $T_{\Vert 2\infty }∕T_{\Vert 1\infty }$ is plotted as a function of the mass ratio $m_2∕m_1$ (spanning the range from hydrogen to rubidium seeded in argon) for several values of the ratio ${\rho }_s$, with ${\rho }_o$ fixed, ${\rho }_o=1.32$. From bottom to top, ${\rho }_s=\infty$, ${\rho }_s=2.55$ (lithium case), ${\rho }_s=1.67$ (sodium case). The experimental points corresponding to lithium and sodium seeded in argon are represented by stars.