Knowledge Bank

Photodissociation spectra of the protonated methanol cluster ions, H$+$(CH$3$OH) ($n=4-8$), were measured in the range of $900-2300$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ by using a infrared free electron laser. A broad spectral feature was observed at about 1600 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ in each spectrum. We assign this feature to the OH$+$ stretching vibration, which is coupled with the OH bending motion in the HOH$+$ group. The small vibrational wavenumber suggests that the O-H$+$ bond is very weak. Structure optimizations by the density functional theory show that H$+$ is equally shared by two oxygen atoms at $n=4,5$. This result also demonstrates the weak bonding between H$+$ and oxygen. The binding features of H$+$ in the cluster ions will be discussed.