Anisotropic energy absorption from mid-infrared laser pulses in constrained water systems

Abstract

The energy acquisition based on resonant excitations is of great importance in chemical and biological systems. Here, the intramolecular resonant absorption of polarized mid-infrared pulses by bulk water and surface water is investigated using molecular dynamics simulation. The consequent heating based on the OH stretching vibrations is found to be very prompt, achieving more than 100 K temperature jump under irradiation of a pulse with 1 ps width and maximum intensity of 0.5 V/nm. A general anisotropic phenomenon is manifested as a result of preferential resonant excitation of symmetric or asymmetric OH stretching vibration, depending on the relationships between the orientations of water molecules and the polarization direction of the pulse. In the case of water molecules with the preferred dipole orientation, constrained by applied static electric field or spacial confinement, parallel to the polarization direction of the pulses, the energy absorption is dominated by the symmetric OH stretching mode near 3300 cm-1, while in the perpendicular case, the asymmetric stretching mode around 3370 cm-1 is more efficient. Since oriented water molecules are prevalent in chemical and biological systems, these findings concerning direction-dependent excitation of intramolecular vibrations are of special significance to understand the energy absorption and transition in relevant biochemical processes.