The angular distributions at two surface temperatures, Ts = 1023 K and 1623 K are shown in Figure (Supplementary Material). The IS angular distributions are fairly broad, as seen for atoms scattering from a liquid interface, which is rough on an atomic scale . The TD-prompt angular distribution is fit well with a cos n θf distribution, with n = 0.67 at Ts = 1023 K and n = 0.69 at Ts = 1623 K. For thermal desorption of Ar atoms, which should have a relatively weak interaction with a carbon surface, n would be expected to be equal to 1.