Knowledge Bank

The hydroxyl radical and its complex with acetylene have been characterized in superfluid helium nanodroplets by infrared depletion spectroscopy. For OH, two sharp Q(3/2) lines were observed, with a separation that is consistent with a fivefold increase in the parity (lambda type) doubling of the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 3/2$ state in helium droplets relative to the gas phase. This increase is rationalized in terms of the differences in the potential energy surfaces for He-OH(A) and He-OH(A), which are most pronounced at around 90. Switching to the T-shaped OH-C$2$H$2$ complex, depletion signals corresponding to OH and CH stretching transitions were observed. The spectra reveal that the electronic angular momentum of OH in the complex is only partially quenched, and to a similar degree as observed in the gas phase, \textbf{121}, 3009 (2004).}$\mathrm{<mrow\; data-mjx-texclass="ORD">,</mrow>}$, \textbf{121}, 5845 (2004).}. This indicates that the helium droplet environment does not significantly affect the electronic intermolecular interactions in OH-C$2$H$2$.