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.