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OBSERVATION OF THE LOWEST BENDING STATE OF HEHCN BY MOLECULAR BEAM ELECTRIC RESONANCE

Please use this identifier to cite or link to this item: http://hdl.handle.net/1811/13157

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Title: OBSERVATION OF THE LOWEST BENDING STATE OF HEHCN BY MOLECULAR BEAM ELECTRIC RESONANCE
Creators: Drucker, Stephen; Tao, Fu-Ming; Klemperer, William
Issue Date: 1994
Abstract: The lowest bending state of HeHCN has been observed by mm-wave electric resonance optothermal $spectroscopy.^{1}$ Transitions originating in the ground vibrational state were recorded at 101.4 GHz and 105.8 GHz. Pure rotational transitions were observed at 15.9 GHz and 31.1 GHz in double resonance with the 101.4 GHz line. The bending state correlates to $j_{HCN}$ = 1 in the limit of free internal rotation. Fine structure in the spectrum arises from the rotation of the pseudodiatomic complex comprised of He and HCN subunits. Coupling to the HCN rotation splits these levels and prevents a spectral pattern with simple dependence on the total angular momentum {J}. To aid the J assignment, we have computed rovibrational energies and wave-functions arising from an {ab initio} intermolecular potential, calculated at the MP4 level using a very large basis set containing bond $functions.^{2}$ Calculated and observed transition frequencies, including hyperfine structure, agree to within 10%. This comparison, used in conjuction with calculated transition intensities, establishes rotational assignments for the observed lines. The 15.9 GHz line is assigned as the ground state $J=1 \leftarrow 0$ transition. Its observation requires double resonance with the 101.4 GHz line, suggesting that only the vibrationally excited molecules strike the detector. This implies a nearly free internal rotor description of the ground state, in which the vibrationally averaged dipole moment is too small to produce the required deflection in the electrostatic hexapole focuser. Resolved hyperfine structure in the 101.4 GHz transition also indicates large angular excursion of the HCN in the ground state of this complex. The ground state value of $eq_{ua}Q$ is -0.567(60) MHz, giving $(P_{2}(\cos \theta)) = 0.120.$ This value is very close to $(P_{2}(\cos \theta)) = 0$, characteristic of a $j = 0$ free internal rotor.
URI: http://hdl.handle.net/1811/13157
Other Identifiers: 1994-RE-05
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