Infrared Spectroscopy of Small Molecules and Hydrogen-Bonded Dimers in Helium Nanodroplets

  • Author / Creator
    Knapp, Christy J.
  • As a spectroscopic matrix, helium nanodroplets exhibit many advantages over frozen rare gas matrices or other seeded clusters. Helium nanodroplets are superfluid, and interact only very weakly with embedded dopants, meaning high-resolution spectra are possible. Further, dopants are rapidly cooled to the droplet temperature of ~0.4 K, such that only the ground vibrational state is populated. Finally, helium nanodroplets are transparent to all wavelengths above ~60 nm. In this thesis, the infrared laser spectra of hydrogen peroxide, singly deuterated hydrogen peroxide, and hydrogen bonded dimers embedded in superfluid helium nanodroplets are presented. The infrared spectrum of hydrogen peroxide reveals much about the interaction between the helium bath and the dopant. First, the B rotational constant is reduced by ~42% of the gas phase value, while the A constant is largely unaffected due to fast rotation about the a-inertial axis. The extent of renormalization was found to be the same for singly deuterated hydrogen peroxide. The trans-tunneling splitting is reduced by only 6% of the gas phase value. Rovibrational linewidths vary based on the density of phonon states available in the droplet at various rotational energies. Linewidths are narrower for lines with rotational energies falling within the “phonon gap.” Finally, dynamic coupling between the H2O2 rotor and the surrounding helium density resulted in asymmetrically skewed lines. Infrared spectra of two hydrogen bonded dimers in helium droplets are reported. The hydrogen peroxide dimer exhibits two OH vibrational modes corresponding to the bound and free OH asymmetric stretch. The bound OH stretch is significantly redshifted compared to the free OH stretch, in good agreement with previously reported spectra in a solid argon matrix. Both lines were broadened to an extent that rotationally resolved lines were not observed, and the precise configuration of the dimer could not be determined beyond having a cyclic shape. For the formic acid dimer, rotational analysis was possible, and the structure was found to be consistent with a non-polar, cyclic structure.

  • Subjects / Keywords
  • Graduation date
    Fall 2018
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
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