Simulation of the time dependent infrared ν2 mode absorptions of (oH2)n:H2O clusters in O2 doped solid hydrogen at 4.2 K

Using Fourier transform infrared spectroscopy, we have analyzed the time evolution of the nu(2) mode of (oH(2))(n):H2O clusters (n=11 to 1) embedded in solid normal hydrogen at 4.2 K over a period of 150 h using paramagnetic O-2 to speed up the ortho to para nuclear spin conversion process. For concentrations H2O/O-2/H-2=1/20/4000, at time t=0 right after the solid is prepared, all the H2O molecules are preferentially clustered by large numbers of oH(2). With time the cluster distribution irreversibly shifts toward smaller cluster sizes and also generates freely rotating H2O (n=0) which is solvated completely by pH(2) molecules. From a spectral decomposition of the nu(2) (oH(2))(n):H2O cluster spectra, a phenomenological simulation of the time behavior of the clusters has been developed. The time evolution is modeled using coupled rate equations in a step by step n to n-1 cluster cascade fashion and analyzed over nine successive time periods. It shows that rotating H2O grows only at the expense of cluster n=1 and that the process dramatically slows down as the conversion of orthohydrogen proceeds. At the end of the conversion process, it was found that cluster n=1 remained with a very slow decrease.