Knowledge Bank

Using our slit-jet, infrared diode laser spectrometer, we have observed a rotationally resolved parallel band centered at $714cm-1$ in a pulsed molecular beam formed by expanding a mixture of 4% HF in helium through a $200\mu m\times 12$ cm slit with a backing pressure of ca. 1000 Torr. A set of excited and ground state spectroscopic constants were fit simultaneously to a data set of 204 transitions using a symmetric top, semi-rigid rotor Hamiltonian. The fit constants are $\nu 0714.7849(1)cm-1,B\prime =0.129614(5)cm-1,(C\prime -B\prime )-(C\prime \prime -B\prime \prime )=0.001339(6)cm-1,Dj\prime =4.9(2)\times 10-7cm-1,DJK\prime =-1.1(2)\times 10-6cm-1,DK\prime -DK\prime \prime =-2.81(8)\times 10-6cm-1,HJKK\prime =-2.0(2)\times 10-8cm-1,HKKK\prime -HKKK\prime \prime -4.68(6)\times 10-8cm-1,B\prime \prime =0.132059(5)cm-1,DJ\prime \prime =5.1(2)\times 10-7cm-1,DJK\prime \prime =-1.3(2)\times 10-6cm-1,HJKK\prime \prime =-6(2)\times 10-9cm-1$, and the RMS deviation of the obs - cales is $0.00045cm-1$. We have also observed two other bands in the 700 to $775cm-1$ region: one band is a strongly perturbed perpendicular band centered at $752cm-1$ with $B\prime \prime =0.133cm-1$ and the other band is a perturbed parallel band centered at $741cm-1$ with $B\prime \prime =0.0755cm-1$. For hydrogen fluoride tetramer and pentamer, theory predicts (Maerker, $etal.)a$ planar, oblate symmetric top structures at their global minima with $Bc$ values of 0.136 and $0.0767cm-1$ for $(HF)4(C4h$ symmetry) and for $(HF)5(C5h$ symmetry), respectively. We have tentatively assigned the $714cm-1$ band as the $\nu 4(Au)$ HF out-of-plane torsion of the tetramer, the $752cm-1$ band as the $\nu 13(Eu)$ HF in-plane torsion of the tetramer, and the $741cm-1$ band as the $\nu 4(A\prime \prime )$ HF out-of-plane torsion of the pentamer.