Ultrafast Vibrational Dephasing of Liquid Water

We present the first infrared photon echo study of liquid water. For the O-H stretch vibration of HDO in ${\mathrm{D}}_2\mathrm{O}$, femtosecond four-wave mixing experiments reveal an extraordinary short pure dephasing time $T_2^{*}$ on the order of 90 fs. This unusually fast time scale of dephasing is rationalized by taking into account the large anharmonicity of the hydrogen bonded O-H stretching mode.