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ND-stretching vibrational energy relaxation of NH(2)D in liquid-to-supercritical ammonia studied by femtosecond midinfrared spectroscopy.

Femtosecond midinfrared pump-probe spectroscopy was carried out to explore the dynamics of vibrational energy relaxation of NH(2)D in fluid ammonia NH(3). The ND-stretching fundamental of the partially deuterated solute NH(2)D was excited by femtosecond pulses centered at 2450 cm(-1), and both the ground-state bleach and the anharmonically shifted transient absorption of the same vibration was probed. The temperature of the sample was varied between 230 and 450 K, while the pressure was tuned from 10 to 1500 bar, thereby entering both the liquid and the supercritical phase of the fluid solution. The density and temperature dependence of the ND-stretching lifetime suggests that hydrogen bonding is of negligible importance for vibrational energy relaxation. Rather, the energy transfer dynamics can be understood qualitatively in terms of a simple Landau-Teller description for vibrational energy relaxation using molecular dynamics simulations to estimate the spectral density of the fluctuating forces exerted by a weakly interacting Lennard-Jones solvent (NH(3)) onto the vibrationally excited solute (NH(2)D).