Infrared light on molecule-molecule and molecule-surface collisions

By analyzing measured infrared absorption of pure $\mathrm{C}{\mathrm{H}}_4$ gas under both “free” (large sample cell) and “confined” (inside the pores of a silica xerogel sample) conditions we give a demonstration that molecule-molecule and molecule-surface collisions lead to very different propensity rules for rotational-state changes. Whereas the efficiency of collisions to change the rotational state (observed through the broadening of the absorption lines) decreases with increasing rotational quantum number $J$ for $\mathrm{C}{\mathrm{H}}_4\text{-C}{\mathrm{H}}_4$ interactions, $\mathrm{C}{\mathrm{H}}_4$-surface collisions lead to $J$-independent linewidths. In the former case, some (weak) collisions are inefficient whereas, in the latter case, a single collision is sufficient to remove the molecule from its initial rotational level. Furthermore, although some gas-phase collisions leave $J$ unchanged and only modify the angular momentum orientation and/or symmetry of the level (as observed through the spectral effects of line mixing), this is not the case for the molecule-surface collisions since they always change $J$ (in the studied $J=0–14$ range).

Figure 1

Top: $R$-branch absorption of $\mathrm{C}{\mathrm{H}}_4$ measured (a) for the free gas and a pressure of 680 hPa and (b) for the confined gas at 9.91 hPa. The lower panels display differences (magnified by a factor of 100) between measured spectra and those obtained from calculations that disregard [(c) and (d)] or take into account [(e) and (f)] the collisional transfers among the sublevels of each manifold.

Figure 2

Absorption spectra of the $R\left(5\right)$ and $R\left(9\right)$ manifolds of $\mathrm{C}{\mathrm{H}}_4$ measured (top) for the confined gas and (bottom) for the free gas. For the free gas, the $R\left(5\right)$ and $R\left(9\right)$ were measured at 631 and 680 hPa, respectively. The black dots (not all plotted) are measured values whereas the simulations with and without the inclusion of collisional transfers among sublevels are in blue (dahsed line) and red (solid line), respectively.

Figure 3

(a) Measured $R$-branch transmission spectrum of $\mathrm{C}{\mathrm{H}}_4$ confined in the silica xerogel pores at 9.91 hPa and measure-to-fit residuals shifted by $+0.55$. (b) Half-widths in the $R\left(J\right)$ manifolds: The full red circles (with $5\%$ error bars) have been retrieved from the fit of the spectrum in (a); the thick horizontal line is deduced from the half-widths of CO, $\mathrm{C}{\mathrm{O}}_2$, and ${\mathrm{N}}_2\mathrm{O}$ gases confined in the same xerogel [17]; the dashed lines defining the confidence interval; the open blue circles are the averaged half-widths [3] of the lines of the $R$($J$) manifolds for the free gas at 600 hPa.