Non-sticking of helium buffer gas to hydrocarbons

Lifetimes of complexes formed during helium-hydrocarbon collisions at low temperature are estimated for symmetric-top hydrocarbons. The lifetimes are obtained using a density-of-states approach. In general the lifetimes are less than 10–100 ns and are found to decrease with increasing hydrocarbon size. This suggests that clustering will not limit precision spectroscopy in helium-buffer-gas experiments. Lifetimes are computed for noble-gas benzene collisions and are found to be in reasonable agreement with lifetimes obtained from classical trajectories as reported by J. Cui et al. [J. Chem. Phys. 141, 164315 (2014)].

Figure 1

Rotational energy levels for (top) hexane and (bottom) benzene. The dashed lines show the collision energy $E_c=10$ K and the highest possible threshold energy that can contribute to the DOS, $|V_{L=0}\left(R_{min}\right)|+E_c$. While hexane has more levels above $E_c$ that can contribute to sticking, it also possesses more levels below $E_c$ that can lead to dissociation of the atom-molecules complex.

Figure 2

Schematic showing contributions to $N_m$ and $N_o$. The dashed horizontal lines represent zero energy and the collision energy. The horizontal solid black lines on the right represent rotational levels of the molecule $E\left(N\right)$. Combinations of $N$ and $L$ consistent with angular momentum conservation lead to potentials $V_{N,L}$, only a few of which are shown for clarity. Potentials $V_{N,L}$ whose minimum lies below the collision energy contribute to $N_m$; these potentials are colored green (light gray). Likewise, potentials $V_{N,L}$ with a threshold and centrifugal barrier below the collision energy contribute to $N_o$ and are colored red (medium gray). Potentials which cannot contribute to either are colored blue (dark gray).

Figure 3

Lifetime of helium symmetric-top molecule clusters as a function of rotational constants $A$ and $B$ at (top) $E_c=1$ K and (bottom) $E_c=10$ K, using ${\rho }_{am}$ and $V_{L,N}$ as for hexane. Labeled are the lifetimes for the prolate-top hexane and oblate-top benzene; the point labeled is at the bottom left of the word.