Enhancement of Blackbody Friction due to the Finite Lifetime of Atomic Levels

The thermal friction force acting on an atom moving relative to a thermal photon bath is known to be proportional to an integral over the imaginary part of the frequency-dependent atomic (dipole) polarizability. Using a numerical approach, we find that blackbody friction on atoms either in dilute environments or in hot ovens is larger than previously thought by orders of magnitude. This enhancement is due to far off-resonant driving of transitions by low-frequency thermal radiation. At typical temperatures, the blackbody radiation maximum lies far below the atomic transition wavelengths. Surprisingly, due to the finite lifetime of atomic levels, which gives rise to Lorentzian line profiles, far off-resonant excitation leads to the dominant contribution for blackbody friction.

Figure 1

Plot of the integrand $x^5{e}^{-bx}{L}_s(x)$ defined in Eq. (11b) in the interval characteristic of the thermal peak $x\approx 5/b=0.01583$ [(a), parameter $s=1$] and near the resonant peak $x\approx 0.375$ [(b), indistinguishable curves for $s=1$ and $s=3$]. The thermal peak (a) yields the dominant contribution to the model integral $J$ defined in Eq. (11a).

Figure 2

The characteristic slowdown time due to friction for ground state atomic hydrogen as a function of the blackbody radiation temperature. The solid line shows the results using the dynamic polarizability of Eq. (4) in length-gauge form [Eq. (9b)], the long-dashed line is the velocity-gauge form (8b), the shaded area is in between, and the short-dashed line results from Dirac $\delta$ peaks given in Eq. (3).

Figure 3

Same as Fig. 2 for a ground state helium atom.

Figure 4

Same as Figs. 2, 3 for metastable helium ($2{}^{3}S_1$).