文摘
We elucidated the phase-sensitive sum frequency generation (SFG) spectrum of NaOH aqueous solution by molecular dynamics simulation. The MD analysis reproduced the effects of NaOH on the imaginary 蠂(2) spectrum, characterized with a positive (upward) shift in the amplitude in midfrequency region (3300鈥?600 cm鈥?) and a negative (downward) shift in low frequency (3000鈥?200 cm鈥?). We found that the positive shift is a consequence of electric double layers, while the negative shift is attributed to the first solvation shell of OH鈥?/sup>. The latter mechanism generally holds for electrolyte solutions, though it becomes conspicuous for the NaOH solution in the low-frequency tail region. These perturbation mechanisms evidence that OH鈥?/sup> at the surface is fully hydrated and not preferentially exposed to the surface.