When the coherent quantum tunneling arises solely from the electronic coupling between the electrode and the redox mediator, then β exclusively describes the tunneling decay, is independent of potential and temperature, and its inverse has the physical meaning of a reaction zone thickness. Eq. ( ) provides the means to estimate ν el 0 from the intercepts in Fig. . Using an arbitrary value of λ = 1.0 eV for both mediators 50 yields ν el 0 of 6.8 × 10 7 s -1 and 2.3 × 10 11 s -1 for [IrCl 6 ] 3- and [Ru(NH 3 ) 6 ] 3+ , respectively. We then use these ν el 0 values and Eq. ( ) to calculate ρ E 0 at x = 0, i.e. the DOS of the bare graphite (assuming H ER 0 ≈ 0.1 eV). The ρ E 0 of 1.6 × 10 -5 atom -1 eV -1 for [IrCl 6 ] 3-is in agreement with recently determined values for basal plane graphite, while the ρ E 0 of 5.4 × 10 -4 atom -1 eV -1 for [Ru(NH 3 ) 6 ] 3+ is closer to the edge plane of graphite, consistent with the above discussion. 36