Anomalous Λ-doubling in the infrared spectrum of the hydroxyl radical in helium nanodroplets.

The X(2)Π3/2 hydroxyl (OH) radical has been isolated in superfluid (4)He nanodroplets and probed with infrared laser depletion spectroscopy. From an analysis of the Stark spectrum of the Q(3/2) transition, the Λ-doublet splittings are determined to be 0.198(3) and 0.369(2) cm(-1) in the ground and first excited vibrational states, respectively. These splittings are 3.6 and 7.2 times larger than their respective gas phase values. A factor of 1.6 increase in the Q(1/2) Λ-doublet splitting was previously reported for the He solvated X(2)Π1/2 NO radical [von Haeften, K.; Metzelthin, A.; Rudolph, S.; Staemmler, V.; Havenith, M. Phys. Rev. Lett. 2005, 95, 215301]. A simple model is presented that reproduces the observed Λ-doublet splittings in He-solvated OH and NO. The model assumes a realistic parity dependence of the rotor's effective moment of inertia and predicts a factor of 3.6 increase in the OH ground state (J = 3/2) Λ-doubling when the B0(e) and B0(f) rotational constants differ by less than one percent.