Fig. 4. Energy transfer processes at the water-INP interface.

(A) Time-resolved difference sum frequency spectrum for the water–P. syringae interface after excitation with a 2470-cm⁻¹ pump pulse near the weakly H-bonded water resonances (dashed line). The signal bleach is very intense in the low-frequency water peak related to strongly H-bonded water. This shows that energy transfer is very rapid and efficient. For clarity, any spectral changes due to thermal effects have been removed. (B) Time-dependent bleach integrated over two spectral regions, 2330 to 2430 cm⁻¹ (strongly H-bonded) and 2480 to 2580 cm⁻¹ (weakly H-bonded). Fits of the data using a coupled differential equation model reveal extremely efficient (80 ± 50 fs) energy transfer between more weakly and more strongly H-bonded water molecules. (C) Time-resolved populations of the more weakly and more strongly H-bonded water molecules extracted from the coupled differential equations. The states become populated from the excitation pulse, energy transfer, and decay to the ground states (not plotted). For the water–P. syringae interface, the more strongly H-bonded state’s population is higher than that of the initially excited peak, which proves extremely efficient energy transfer.