ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
A tunable microwave-sideband CO2 laser has been used with a molecular-beam electric-resonance optothermal spectrometer to observe the infrared spectrum of the NH3 umbrella fundamental vibration (ν5 in Cs ) of HOH––NH3 at a resolution of ∼3 MHz. Ground- and excited-state assignments were verified and extended using microwave–infrared double-resonance spectroscopy, with microwave transitions observed in both the ground and the excited states. The spectrum exhibits numerous perturbations, as evidenced by the observation of a minimum of 13 subbands originating from the (K,m)=(0,0) ground NH3 internal-rotor state and the (K,m)=(±1,±1) first excited NH3 internal-rotor state. For an unperturbed spectrum, only four such subbands are expected, two for the symmetric H2O tunneling state and two for the antisymmetric H2O tunneling state. The rotational progressions within the excited states are poorly fit to polynomial series in J(J+1), in contrast to the ground-state progressions which are well characterized by such series. The B rotational constants in the excited states are smaller than in the ground state, indicating an extension of the hydrogen-bonding interaction distance upon vibrational excitation. This is consistent with the observed infrared band origin for the (K,m)=(0,0) state of ∼1021 cm−1, which is blue shifted by 71 cm−1 from the hypothetical inversion-free 950 cm−1 ν2 band origin of uncomplexed NH3. The observed ν5 band origin is also in good agreement with matrix-isolation results scaled to correct for the matrix shift of the NH3 umbrella frequency found in the recently studied NH3–HCN complex. The complex does not dissociate upon vibrational excitation, implying that the binding energy is greater than the laser frequency of ∼1021 cm−1.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.462433
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