Static and dynamic solvation at the air/water interface

The electronic excited-state solvation dynamics of coumarin 314 (C314) adso rbed at the air/water interface was investigated by femtosecond time-resolv ed second-harmonic generation (TRSHG). The second-harmonic spectrum of C314 at the air/water interface was determined using a tunable optical parametr ic generator, giving a spectral peak at 419 +/- 3 nm. The solvatochromic sh ift is in good agreement with the recently proposed interfacial polarity sc ale, which predicts that the empirical polarity parameter of the interface is the arithmetic mean of the two bulk phases. The time-resolved second-har monic response to the excited-state solvation was measured at two fundament al wavelengths (840 and 900 nm) and with two pump polarizations (S and P). The results indicate that the static spectral peaks of the S- and P-pumped distributions are offset similar to5 nm, while the dynamic interfacial solv ation proceeds with two time constants, tau (1) = similar to 200 fs and tau (2) = 1.2 ps, which are independent of C314 interfacial orientation. The s light blue shift of the C314 molecules closer to the normal incidence indic ates that the electronic spectra of species at the air/water interface is i nhomogeneously broadened by orientation, an effect which had not been obser ved previously. This can be described by a static solvation that depends on the orientation of the molecule's dipole moment at the interface. The dyna mic solvation times are consistent with bulk experiment, as well as theory, which indicates that the air/water relaxation times, which are controlled by water/water interactions, should not be different from the bulk values.