FT-IR investigation of proton transfer in irradiated ice at 90 K in the absence of mobile bjerrum defects

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Abstract
Samples of cubic and amorphous ice containing isolated D2O molecules have been prepared using established methods. Radiolysis with 1.7 MeV electrons at 90 K has been observed to convert the D2O in H2O cubic ice to primarily coupled HOD pairs (HOD)2 rather than isolated HOD. Based on the assumption that radiolysis produces an abundance of mobile protons but relatively few mobile orientational defects this result was predictable, i.e., the motion of protons through D2O sites converts the D2O to coupled (HOD)2, while extensive conversion of (HOD)2 units to isolated HOD requires the passage of orientational defects. Thus, the vanishingly small amount of isolated HOD formed during irradiation has been interpreted as evidence that no significant number of mobile orientational defects are produced by radiolysis. The ability to observe (HOD)2 as a dominant spectroscopic species has added to the credibility of an earlier thermal study of the kinetics of proton motion in ice that was dependent on the quantitative determination of the (HOD)2 concentration as a function of reaction time. Furthermore, the observation that protons produced radiolytically fall rapidly into shallow traps, from which they are released by modest warming (to ∼130 K), supports a suggestion by Warman that, for a brief period following a radiolysis pulse, mobile protons are in pseudoequilibrium with protons immobilized by association with L defects. The D2O isolated in amorphous ice was similarly converted to HOD by mobile protons produced during radiolysis. However, the broader infrared bands of the amorphous samples have prevented assigning the HOD to a particular form [(HOD)2 or isolated HOD]. The interesting result was the indication that the proton mobility in amorphous ice is comparable to that for cubic ice. By contrast, radiolysis of ND3 isolated in crystalline cubic NH3 caused only very limited formation of mixed isotopically substituted ammonia molecules, a clear indication that either few protons (or NH+4) are formed or the protons produced lack mobility. The latter situation appears to be most likely.