Temperature dependence of the IR driven cis-trans isomerization of nitrous acid (HONO).

With the help of ultrafast time-resolved infrared spectroscopy, we investigate the temperature dependence of the IR driven cis-->trans isomerization of nitrous acid (HONO) in solid Kr. We find that the lifetime of the OH-stretch vibration, as well as the final cooling into the matrix, is affected only minimally (if at all) by temperature. Nevertheless, the quantum yield of the cis-->trans isomerization reaction increases by approximately 30% to a total of 50%-70% when lowering the temperature from 30 to 15 K, whereas the trans-->cis back yield is reduced by approximately 40%. The results are discussed in analogy to Marcus theory of nonadiabatic electron transfer for the essentially barrierless case. We present a unified view of this important prototype proton transfer reaction that can explain the high cis-->trans quantum yield of close to 1.