Magnetic relaxation rates are then determined using our Tau code (commit e058b24959), considering Orbach and Raman rates, given by Equations 40, 41 and 46 -49 in reference 58. There are two forms of the Raman mechanism, which arise from their derivation using different orders of perturbation theory: the Raman-I mechanism (first-order in spinphonon coupling, second-order in time) does not depend on the magnitude of an external magnetic field, while the Raman-II mechanism (second-order in spin-phonon coupling, firstorder in time) has a quadratic dependence on the field and vanishes in zero field. Since our experiments are performed in zero field, we do not consider the Raman-II mechanism, and we therefore refer to the Raman-I mechanism simply as "the Raman mechanism" throughout. In the context of magnetic relaxation in SMMs, the two-phonon Raman mechanism concerns coupling between the two states of the ground Kramers doublet, and our calculations are thus restricted to this pair of states. Derivation of the Raman rate expressions adopt the secular approximation which assumes no degeneracies exist in the electronic eigenstates, and thus we must introduce an energy gap between the two states of the ground Kramers doublet.