若手のショートレビュー

中赤外高速分光法で観る水の分子振動緩和現象

Vibrational relaxations in liquid water observed by mid-infrared ultrafast spectroscopy

芦原　聡
Satoshi ASHIHARA

中赤外波長域のフェムト秒パルスを利用した非線形分光法により，分子の振動緩和や構造変化といったダイナミクスを，高い時間分解能で観測できるようになった．本稿では，液相における水の分子振動緩和現象について，最近得られた実験結果を紹介する．水の中では，あらゆる振動モードのエネルギーが100 fsオーダーという短い時間スケールで緩和し，そのエネルギーが瞬くうちに水素結合ネットワークへと放出されることが明らかになった．

Nonlinear vibrational spectroscopy by use of mid-infrared femtosecond pulses enables the observation of ultrafast vibrational/structural dynamics of molecular systems. This article presents the recent experimental studies on vibrational energy relaxations and energy transfer in liquid water. In pure liquid water, vibrational energy relaxation of OH-stretching and OH-bending excitation occurs at about 200 fs and 170 fs, respectively. When vibrational energy from individually excited water molecules is transferred to intermolecular modes, a sub-100 fs nuclear rearrangement occurs, leaving the local hydrogen bonds weakened but unbroken. Subsequent energy delocalization over many molecules proceeds with the breaking of hydrogen bonds on an 1-ps time scales. The observed vibrational/structral response should be relevant for chemical reactions and structural changes in aqueous environments.

Keywords: Water, Vibrational spectroscopy, Molecular dynamics, Ultrashort laser, Mid-infrared light