Kelly J. Higgins

THE INTERMOLECULAR POTENTIAL OF HE-OCS

The dynamics of molecules trapped inside or bound to the surface of liquid helium droplets is a most exciting new development. As an aid toward the understanding of the spectrum of OCS in liquid helium, we report the intermolecular potential between He and OCS studied by ab initio calculations and high-resolution microwave spectroscopy. The potential is found to have three minima, corresponding to a T-shaped configuration for the global minimum and secondary minima at either end of the OCS molecule. The three lowest calculated bound states are loosely localized in each of the minima, with the ground state being T-shaped. Ten rotational transitions of the ground state are observed in the frequency region 1.5–45.0 GHz. Comparison of theory with experiment shows good agreement. The agreement improves substantially if the calculated He–OCS intermolecular potential is made uniformly 10% deeper and the He–OCS separation is reduced by 0.05 A.

Dynamics of linear and T-shaped Ar–I2 dissociation upon B←X optical excitation: A dispersed fluorescence study of the linear isomer

We report the dispersed fluorescence spectra of the linear and the previously well-studied T-shaped isomers of Ar–I2 following B←X optical excitation for vpump=16–26, below the I2 dissociation limit. The linear isomer has a continuum excitation spectrum. For excitation at the highest pumping energy (vpump=26), the product vibrational state distribution is nearly identical to that observed for excitation above the I2(B) dissociation limit; it shows a broad, nearly Gaussian distribution of I2(B) vibrational states, with about 22% of the available excess energy deposited in translation of the Ar+I2. This gives direct evidence that the “one-atom cage” effect seen above the I2(B) dissociation limit is attributable to the linear Ar–I2 isomer. The product vibrational state distribution becomes increasingly Poisson for decreasing excitation energies, and only about 7% of the excess energy is deposited in translation for vpump=16. The bond energy in the linear isomer is determined from the spectra, 170(±1.5)⩽D0″(l...

The intermolecular potential between an inert gas and a halogen: Prediction and observation of transitions between the linear and T-shaped isomers of HeClF

The intermolecular potential surface of He and ClF is calculated with a large basis at the fourth-order Mo/ller–Plesset level. The rotation–vibration levels calculated from the intermolecular potential surface serve as an excellent guide for finding the experimental spectra. Pure rotational transitions are observed for the lowest linear Σ0 state and for an excited T-shaped K=0 Σ1 state of He35ClF and He37ClF. Direct transitions between the linear ground state and the T-shaped state are observed for He35ClF. The observed energy difference between the J=0 level of the linear state and the J=0 level of the T-shaped state is 2.320 cm−1. In addition, transitions into the two J=1 levels and one J=2 level of the K=1 T-shaped state, Π1, are observed for He35ClF. The He–ClF complex is highly nonrigid, undergoing large amplitude oscillation in both angular and radial coordinates. The effect of zero-point oscillation is seen in the large difference, 22.9 cm−1, between the calculated potential energy minima of −58.1 ...

Structure, binding energy, and equilibrium constant of the nitric acid‐Water complex

Author Institution: Harvard University, Cambridge, MA 02138.; Aerodyne Research Inc., 45 Manning Rd., Billerica, MA 01821.

Nuclear hyperfine interaction of rotating hydrogen: A spectroscopic investigation of hydrogen-OCS van der Waals complexes

The rotational spectra of five weakly bonded hydrogen-OCS complexes (paraH(2), orthoH(2), HD, orthoD(2), and paraD(2)) are measured. Hyperfine structure is resolved and analyzed in all except the complex with paraH(2), where I=0. For the two j=1 species, orthoH(2)-OCS and paraD(2)-OCS, nuclear hyperfine coupling constants are found to be d(a)=21.2(2) and 8.4(2) kHz, respectively, indicative of nearly free uniaxial rotation of the hydrogen around the b-inertial axis. Similar analyses for HD-OCS and orthoD(2)-OCS yield the quadrupole coupling constants eqQ(a)=16(2) and 30(2) kHz, respectively, showing that the internal rotational motions of HD and orthoD(2) in the complex are slightly hindered producing a small nonspherical distribution. For orthoD(2)-OCS, the observed hyperfine structure indicates that the nuclear spin states I=0 and 2 are strongly coupled in the rotation of the complex.

Rotational spectra of the van der Waals complexes of molecular hydrogen and OCS

The a- and b-type rotational transitions of the weakly bound complexes formed by molecular hydrogen and OCS, para-H2-OCS, ortho-H2-OCS, HD-OCS, para-D2-OCS, and ortho-D2-OCS, have been measured by Fourier transform microwave spectroscopy. All five species have ground rotational states with total rotational angular momentum J=0, regardless of whether the hydrogen rotational angular momentum is j=0 as in para-H2, ortho-D2, and HD or j=1 as in ortho-H2 and para-D2. This indicates quenching of the hydrogen angular momentum for the ortho-H2 and para-D2 species by the anisotropy of the intermolecular potential. The ground states of these complexes are slightly asymmetric prolate tops, with the hydrogen center of mass located on the side of the OCS, giving a planar T-shaped molecular geometry. The hydrogen spatial distribution is spherical in the three j=0 species, while it is bilobal and oriented nearly parallel to the OCS in the ground state of the two j=1 species. The j=1 species show strong Coriolis coupling with unobserved low-lying excited states. The abundance of para-H2-OCS relative to ortho-H2-OCS increases exponentially with decreasing normal H2 component in H2He gas mixtures, making the observation of para-H2-OCS in the presence of the more strongly bound ortho-H2-OCS dependent on using lower concentrations of H2. The determined rotational constants are A=22 401.889(4) MHz, B=5993.774(2) MHz, and C=4602.038(2) MHz for para-H2-OCS; A=22 942.218(6) MHz, B=5675.156(7) MHz, and C=4542.960(7) MHz for ortho-H2-OCS; A=15 970.010(3) MHz, B=5847.595(1) MHz, and C=4177.699(1) MHz for HD-OCS; A=12 829.2875(9) MHz, B=5671.3573(7) MHz, and C=3846.7041(6) MHz for ortho-D2-OCS; and A=13 046.800(3) MHz, B=5454.612(2) MHz, and C=3834.590(2) MHz for para-D2-OCS.

Weak bond stretching for three orientations of Ar–HF at vHF=3

Three new ArHF (vHF=3) states, (3001), (3101), and (3111), have been observed between 11 350 and 11 420 cm−1 by the hot band transitions from (0001) using intracavity laser induced fluorescence. The term values and rotational constants of these levels are: (3001) ν0=11 385.928 98(28) cm−1, B=0.095 546(32) cm−1; (3101) ν0=11 444.258 12(68) cm−1, B=0.090 617(37) cm−1; and (3111) ν0=11 456.076 51(36) cm−1, B=0.091 863(14) cm−1. Observation of the ArHF (3001) state provides the van der Waals stretching frequency for ArHF at v=3, namely 46.8945(4) cm−1=(3001)–(3000). This value shows an increase of 8.208 cm−1 (21%) upon HF v=3←0 valence excitation. The stretching frequency for the T shaped ArHF is (3111)–(3110)=33.7055(5) cm−1. This value is only 7% greater than that observed at v=1. The (vHF101) Σ bend-stretch combination state, corresponding to (νs=1) of the Ar–FH configuration, has not been observed at vHF=0–2. The stretching frequency here is (3101)–(3100)=31.8178(8) cm−1. The soft-mode frequencies reveal ...