Knowledge Bank

A pure parahydrogen crystal was irradiated with $\gamma$-rays from a $\mathrm{<mrow\; data-mjx-texclass="ORD">60</mrow>}Co$ source at Argonne National Laboratory, and the high resolution infrared absorption of the irradiated crystal around the $Q1(0)$ region was studied by using a difference frequency laser system. After the irradiation, a new sharp transition was observed around $4149.7cm-1$. The same transition was also detected by Chan and others using electron bombardment ionnization of solid $hydrogen.(1)$ Under high resolution, the transition consisted of many sharp spectral lines but the frequency of the strongest feature was exactly the same as that of the $Q1(0)$ Raman transition. This was confirmed by observing the infrared absorption spectrum and the stimulated Raman spectrum simultaneously. The exact coincidence of the IR and Raman frequencies shows that this new transition is a $Q1(0)$ field induced $transition(2)$ with the $\Delta k=0$ selection rule. The electric field of ions produced by $\gamma$-ray irradiation induces an infrared transition which has the same selection rule as the Raman transition. There was additional structure on the low frequency side of the strongest transition which may be due to a breakdown of the $\Delta k=0$ selection rule caused by the presence of ion impurities. Many other sharp weak features were also observed and are most likely due to the simultaneous transitions with $J=1$ hydrogen pair. The ions appear to be extrmely stable in solid parahydrogen; the sample was maintained at 4K for several days and no decay of the field induced transition was observed. The spectra of the ion species, however, have not yet been assigned.