Stepwise internal energy control for protonated methanol clusters by using the inert gas tagging

Preferred isomer structures of hydrogen-bonded clusters should depend on their temperature because of the entropy term in the free energy. To observe such temperature dependence, we propose a new approach to control the internal energy (vibrational temperature) of protonated clusters in the gas phase. We performed IR spectroscopy of protonated methanol clusters, H$^{+}$ (CH${_3}$OH) ${_n}$, n= 5 and 7, with the tagging by various inert gas species (Ar, CO${_2}$, CO, CS${_2}$, C${_2}$H${_2}$, and C${_6}$H${_6}$). We found that vibrational temperature of the tagged clusters raises with increase of the interaction energy with the tag species, and the observed cluster structures follow the theoretical prediction of the temperature dependence of the isomer population