Fig. 1.

Single oscillator. a The χ ⁽³⁾ phase angle φ computed for ω = 3000 cm⁻¹ (solid line) and 2100 cm⁻¹ (dashed line) from equation (4) as described in the main text for varying ionic strength. SFG intensity spectra of a single oscillator on a surface simulated for ω ₁ = 2100 cm⁻¹, A _v M _v = 10, Γ _v = 15 cm⁻¹, γ ₁ = 0°, χ ⁽³⁾ = 1.0, and Φ(0) = −240 mV (purple), −170 mV (blue), < 1 mV (green), + 170 mV (orange), and + 240 mV (red), with an ionic strength of 1 mM, calculated without b and with c absorptive-dispersive interactions. d Peak frequency computed from equation (1) as a function of interfacial potential, Φ(0), and linear least squares fit (red line)