Rotovibrational spectroscopy of hydrogen peroxide embedded in superfluid helium nanodroplets

We report the infrared depletion spectrum of para-and ortho-hydrogen peroxide embedded in superfluid helium nanodroplets in the OH stretching region. Six transitions were observed in the antisymmetric stretching band (v(5)) of H2O2, and three in the weaker symmetric stretching band (v(1)). While rotations about the b- and c-axes are slowed by a factor of similar to 0.4 relative to the gas phase, rotations about the a-axis are not significantly affected; this relates to the rotational speed about the a-axis being too fast for helium density to adiabatically follow. The trans tunneling splitting does not appear to be considerably affected by the helium droplet environment, and is reduced by only 6% relative to the gas phase, under the assumption that the vibrational shifts of the v(5) and v(1) torsional subbands are the same. The linewidths increase with increasing rotorsional energies, and are significantly narrower for energies which fall within the "phonon gap" of superfluid helium. These narrower lines are asymmetrically broadened, indicative of a dynamical coupling between the H2O2 rotor and surrounding helium density.