The structure of the C4 cluster radical

The first infrared spectrum of gas phase, jet-cooled C4 has been measured by high resolution diode laser absorption spectroscopy. Twelve rovibrational transitions are assigned to the nu3(sigmau) antisymmetric stretch of linear 3Sigma - g C4. No evidence is observed for the bent structure of triplet C4 recently observed in a matrix study by Cheung and Graham [J. Chem. Phys. 91, 6664 (1989)]. Indeed, the measured band origin (1548.9368(21) cm^–1) and effective ground state C–C bond length [1.304 31(21)A] are consistent with several ab initio predictions of a rigid, linear, cumulenic structure for this cluster radical

The C9 cluster: Structure and infrared frequencies

The high resolution infrared spectrum of the C9 cluster has been measured in direct absorption by infrared diode laser spectroscopy of a pulsed supersonic carbon cluster jet. Fifty-one rovibrational transitions have been assigned to the nu6 (sigmau ) antisymmetric stretch fundamental of the 1Sigma + 9 linear ground state of C9. The measured rotational constant [429.30(50) MHz] is in good agreement with ab initio calculations and indicates an effective bond length of 1.278 68(75) Å, consistent with cumulenic bonding in this cluster. Several perturbations are observed in the upper state, and the upper- and lower-state centrifugal distortion constants are observed to be anomolously large, evidencing a high degree of Coriolis mixing of the normal modes

Measurement of the spectral signature of small carbon clusters at near and far infrared wavelengths

A significant percentage of the carbon inventory of the circumstellar and interstellar media may be in the form of large refractory molecules (or small grains) referred to as carbon clusters. At the small end, uneven numbers of carbon atoms seem to be preferred, whereas above 12 atoms, clusters containing an even number of carbon atoms appear to be preferred in laboratory chemistry. In the lab, the cluster C-60 appears to be a particularly stable form and has been nicknamed Bucky Balls because of its resemblance to a soccer ball and to geodesic domes designed by Buckminster Fuller. In order to investigate the prevalence of these clusters, and their relationship to the polycyclic aromatic hydrocarbons (PAHs) that have become the newest focus of IR astronomy, it is necessary to determine the spectroscopic characteristics of these clusters at near and far infrared wavelengths. Described here is the construction of a near to far IR laser magnetic resonance spectrometer that has been built at the University of California Berkeley in order to detect and characterize these spectra. The equipment produces carbon clusters by laser evaporation of a graphitic target. The clusters are then cooled in a supersonic expansion beam in order to simulate conditions in the interstellar medium (ISM). The expansion beam feeds into the spectrometer chamber and permits concentrations of clusters sufficiently high as to permit ultra-high resolution spectroscopy at near and far IR wavelengths. The first successful demonstration of this apparatus occurred last year when the laboratory studies permitted the observational detection of C-5 in the stellar outflow surrounding IRC+10216 in the near-IR. Current efforts focus on reducing the temperature of the supersonic expansion beam that transport the C clusters evaporated from a graphite target into the spectrometer down to temperatures as low as 1 K

Cavity ringdown laser absorption spectroscopy and time-of-flight mass spectroscopy of jet-cooled silver silicides

The cavity ringdown technique has been employed for the first spectroscopic characterization of the AgSi molecule, which is generated in a pulsed laser vaporization plasma reactor. A total of 20 rovibronic bands between 365 and 385 nm have been measured and analyzed to yield molecular properties for the X, B, and C 2Sigma states of AgSi. A time-of-flight mass spectrometer simultaneously monitors species produced in the molecular beam and has provided the first direct evidence for the existence of polyatomic silver silicides. Comparison of the AgSi data to our recent results for the CuSi diatom reveals very similar chemical bonding in the two coinage metal silicides, apparently dominated by covalent interactions

Experimental determination of dipole moments for molecular ions: Improved measurements for ArH^+

An improved value for the dipole moment of ArH^+ has been obtained from new measurements of the rotational g factors of ArH^+ and ArD^+ made with tunable far‐IR laser spectroscopy. Systematic errors present in earlier measurements have been eliminated. The new result (μ=3.0±0.6 D) is slightly higher than the ab initio value of Rosmus (2.2 D) at the 2σ limits of precision

Laboratory measurement of the pure rotational spectrum of vibrationally excited HCO^+ (v_2 = 1) by far-infrared laser sideband spectroscopy

Laboratory observations of the pure rotational spectrum of HCO^+ in its lowest excited bending state (v_1, v^l_2 v_3)_= (0,1^1,0) are reported. Because of their severe excitation requirements, such vibrational satellites and the high-J ground-state lines also measured here sample only hot, dense regions of matter in active molecular cloud cores and circumstellar envelopes. As the HCO^+ abundance is tied directly to the gas fractional ionization, it is probable that the vibrationally excited formyl ion transitions will provide high-contrast observations of shocked molecular material, rather than the more quiescent, radiatively heated gas surrounding stellar sources detected with the few vibrationally excited neutral species observed to date

On the ground electronic states of copper silicide and its ions

The low-lying electronic states of SiCu, SiCu^+, and SiCu^− have been studied using a variety of high-level ab initio techniques. As expected on the basis of simple orbital occupancy and bond forming for Si(s^2p^2)+Cu(s^1) species, ^2Π_r, ^1Σ^+, and ^3Σ^− states were found to be the ground electronic states for SiCu, SiCu^+, and SiCu^−, respectively; the ^2Π_r state is not that suggested in most recent experimental studies. All of these molecules were found to be quite strongly bound although the bond lengths, bond energies, and harmonic frequencies vary slightly among them, as a result of the nonbonding character of the 2π-MO (molecular orbital) [composed almost entirely of the Si 3p-AO (atomic orbital)], the occupation of which varies from 0 to 2 within the ^1Σ^+, ^2Π_r, and ^3Σ^− series. The neutral SiCu is found to have bound excited electronic states of ^4Σ^−, ^2Δ, ^2Σ^+, and ^2Π_i symmetry lying 0.5, 1.2, 1.8, and 3.2 eV above the ^2Π_r ground state. It is possible but not yet certain that the ^2Π_i state is, in fact, the “B state” observed in the recent experimental studies by Scherer, Paul, Collier, and Saykally

Tunable far-infrared laser spectroscopy of hydrogen bonds: The K_a = O(u)→1(g) rotation-tunneling spectrum of the HCI dimer

The ground state K_a =0(u)→1(g) b‐type subband of the rotation–tunneling spectrum of the symmetric ^(35)Cl–^(35)Cl,^(37)Cl–^(37)Cl, and the mixed ^(35)Cl–^(37)Cl hydrogen chloride dimers have been recorded near 26.3 cm^(−1) with sub‐Doppler resolution in a continuous two‐dimensional supersonic jet with a tunable far‐infrared laser spectrometer. Quadrupole hyperfine structure from the chlorine nuclei has been resolved. From the fitted rotational constants a (H^(35)Cl)_2 center‐of‐mass separation of 3.81 Å is derived for the K_a =1(g) levels, while the nuclear quadrupole coupling constants yield a vibrationally averaged angular structure for both tunneling states of approximately 20–25 deg for the hydrogen bonded proton and at least 70–75 deg for the external proton. This nearly orthogonal structure agrees well with that predicted by ab initio theoretical calculations, but the observed splittings and intensity alterations of the lines indicate that the chlorine nuclei are made equivalent by a large amplitude tunneling motion of the HCl monomers. A similar geared internal rotation tunneling motion has been found for the HF dimer, but here the effect is much greater. The ground state tunneling splittings are estimated to lie between 15–18 cm^(−1), and the selection rules observed indicate that the trans tunneling path dominates the large amplitude motion, as expected, provided the dimer remains planar. From the observed hyperfine constants, we judge the dimer and its associated tunneling motion to be planar to within 10°