The vibrational relaxation time of the cyanide ion in H2O and in D2O w
as measured by IR-pump-IR-probe experiments. The isotopic composition
of the ion was varied in order vary the oscillation frequency of the C
N-vibrational mode. In D2O, the vibrational relaxation rate is acceler
ated from 120 to 71 ps when increasing the vibrational frequency from
2004 cm(-1) ((CN)-C-13-N-15) to 2079 cm(-1) ((CN-)-C-12-N-14). In H2O,
time constants between 31 and 28 ps were observed. The systematic dep
endence of the relaxation rates on the vibrational frequency provides
a small portion of the friction spectrum. A significant correlation be
tween vibrational relaxation time of the solute and the IR absorption
cross section of the solvent was found, providing experimental evidenc
es for a dominating contribution to vibrational relaxation of Coulomb
interactions and the importance of coupling to internal solvent modes.
In addition, the infrared bandwidths and the orientational diffusion
times are reported. All experimental observables T-1, T-2, and tau(2R)
related to the vibrational transitions of CN- are now experimentally
available and can be compared with model theoretical calculations. (C)
1997 American Institute of Physics.