Biomedical subjects

A R W McKellar

Publications and source records attributed to A R W McKellar.

18 recordsLinked to original sources

PubMed2006

Spectroscopy and potential energy surface of the H2-CO2 van der Waals complex: experimental and theoretical studies.

A 4-D ab initio potential energy surface is calculated for the intermolecular interaction of hydrogen and carbon dioxide, using the CCSD(T) method with a large basis set. The surface has a global minimum with a well depth of 212 cm(-1) and an intermolecular distance of 2.98 A for a planar configuration with both the O-C-O and H-H axes perpendicular to the intermolecular axis. Bound state calculations are performed for the H(2)-CO(2) van der Waals complex with H(2) in both the para and ortho spin states, and the binding energy of paraH(2)-CO(2)(50.4 cm(-1)) is found to be significantly less than that of orthoH(2)-CO(2)(71.7 cm(-1)). The surface supports 7 bound intermolecular vibrational states for paraH(2)-CO(2) and 19 for orthoH(2)-CO(2), and the lower rotational levels with J< or = 4 follow an asymmetric rotor pattern. The calculated infrared spectrum of paraH(2)-CO(2) agrees well with experiment. For orthoH(2)-CO(2), the ground state rotational levels allowed by symmetry are found to have (K(a), K(c))=(even, odd) or (odd, even). This somewhat unexpected fact enables the previously observed experimental spectrum to be assigned for the first time, in good agreement with theory, and indicates that the orientation of hydrogen is perpendicular to the intermolecular axis in the ground state of the orthoH(2)-CO(2) complex.

Journal Article↗

PubMed2006

Spectroscopic exploration of atomic scale superfluidity in doped helium nanoclusters.

We present high resolution spectra of He(N)-OCS clusters with N up to 39 in the microwave and 72 in the infrared regions, observed with apparatus-limited line widths of about 15 kHz and 0.001 cm(-1), respectively. The derived rotational constant, B (proportional to the inverse moment of inertia), passes through a minimum at N=9, then rises due to onset of superfluid effects, and exhibits broad oscillations with maxima at N=24, 47 and minima at 36, 62. We interpret these unexpected oscillations as a manifestation of the aufbau of a nonclassical helium solvation shell structure. These results bridge an important part of the gap between individual molecules and bulk matter with atom by atom resolution, providing new insight into microscopic superfluidity and a critical challenge for theory.

Journal Article↗

PubMed2006

Helium clusters seeded with CO molecules: new results for HeN-13C18O and the approach to the nanodroplet limit.

Infrared spectra of helium clusters seeded with doubly substituted carbon monoxide molecules, 13C18O, have been studied in order to complement recent helium nanocluster results and to determine whether additional isotopic data would help to separate vibrational and rotational contributions to the observed transitions. The experiments were made by direct infrared absorption in pulsed supersonic jet expansions using a tunable diode laser probe in the region of the fundamental band (approximately 2045 cm-1 for 13C18O). Even with data on the R0 transitions from four CO isotopomers, it was found that a clear and consistent separation of vibration and rotation could not be achieved for HeN-CO clusters in the size range N approximately 10-20. Isotope shifts observed for clusters with 13C18O (relative to 12C16O) were found to be close to the sums of the shifts previously determined for 13C16O and 12C18O. The new measurements generally supported previous assignments of cluster size, but some modifications for the range N=14-16 are suggested here. New measurements for HeN-12C16O under conditions favoring larger clusters (high backing pressure and low jet temperature) showed that individual transitions could be resolved even at N approximately 50. For larger clusters, a partly resolved "lump" of transitions was observed to approach the nanodroplet limit.

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PubMed2006

Isotope effects in the infrared spectra of OCS-He complexes and clusters.

Infrared spectra of the OCS-He van der Waals complex and of OCS-He(N) clusters have been studied in the region of the OCS nu1 fundamental band using a tunable diode laser to probe a pulsed supersonic slit jet. For the complex, the spectrum of the normal isotope, 16O12C32S-4He, has been considerably extended and the 34S- and 13C-substituted forms have been recorded for the first time. The data could be analyzed satisfactorily using a conventional asymmetric rotor Hamiltonian with sextic centrifugal distortion terms. For the clusters, the 34S- and 13C-substituted forms have been observed and assigned for N = 2-7, including some transitions with higher J values than previously reported for the normal isotope, e.g., R5. The observed vibrational shifts, relative to the free OCS molecule, were very similar to those of the normal isotope, and most of the difference could be explained by simple scaling. These results constitute a subtle and precise probe of intermolecular forces and dynamical effects in a system which is of current interest for cluster studies.

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PubMed2006

Infrared spectra of isotopic CO2-He complexes.

Infrared spectra of three isotopic forms of the weakly bound CO(2)-He van der Waals complex have been studied in the region of the CO(2) nu(3) fundamental band around 2300 cm(-1), using a tunable diode laser to probe a pulsed supersonic expansion. The complex is a T-shaped near-oblate asymmetric rotor, and it is found that (18)O isotopic substitution is sufficient to interchange the a and b inertial axes. For the symmetric isotopes, such as the normal species and the (16)O(13)C(16)O and (18)O(13)C(18)O forms studied here, half of the normal rotational levels of the complex are missing due to the effects of (16)O (or (18)O) interchange symmetry. However, for asymmetrically substituted ones, such as (16)O(13)C(18)O, all rotational levels are present. Moreover, for the asymmetric isotope, both a- and b-type transitions were observed, so that the spectrum was much richer. The CO(2)-He system is of interest both as a benchmark for intermolecular potential energy surface calculations, and because CO(2) is a valuable probe molecule for helium cluster spectroscopy.

Journal Article↗

PubMed2006

Spectroscopic tests of a 2.3 microm tunable diode laser.

A 2.3 microm near-room temperature tunable diode laser was tested for applications in high-resolution laboratory spectroscopy. It was mounted using a simple adaptor in a spectrometer usually used with lead-salt diode lasers, and was found to be electrically and optically compatible with the system. Good output power (several milliwatts) was observed, and a tuning range of 4460-4150 cm(-1) was achieved for laser temperatures of 210-310 K. Some spectra of N(2)O and NH(3) were recorded in the 4300-4430 cm(-1) region. However, the laser was not generally useful due to noisy and unstable output and high sensitivity to optical feedback.

Ammonia↗

PubMed2005

Infrared spectra of seeded hydrogen clusters: (para-H2)N-N2O and (ortho-H2)N-N2O, N = 2-13.

High-resolution infrared spectra of clusters containing para-H2 and/or ortho-H2 and a single nitrous oxide molecule are studied in the 2225-cm(-1) region of the upsilon1 fundamental band of N2O. The clusters are formed in pulsed supersonic jet expansions from a cooled nozzle and probed using a tunable infrared diode laser spectrometer. The simple symmetric rotor-type spectra generally show no resolved K structure, with prominent Q-branch features for ortho-H2 but not para-H2 clusters. The observed vibrational shifts and rotational constants are reported. There is no obvious indication of superfluid effects for para-H2 clusters up to N=13. Sharp transitions due to even larger clusters are observed, but no definite assignments are possible. Mixed (para-H2)N-(ortho-H2)M-N2O cluster line positions can be well predicted by linear interpolation between the corresponding transitions of the pure clusters.

Journal Article↗

PubMed2005

Theoretical and experimental studies of the infrared rovibrational spectrum of He2-N2O.

Rovibrational spectra of the He(2)-N(2)O complex in the nu(1) fundamental band of N(2)O (2224 cm(-1)) have been observed using a tunable infrared laser to probe a pulsed supersonic jet expansion, and calculated using five coordinates that specify the positions of the He atoms with respect to the NNO molecule, a product basis, and a Lanczos eigensolver. Vibrational dynamics of the complex are dominated by the torsional motion of the two He atoms on a ring encircling the N(2)O molecule. The resulting torsional states could be readily identified, and they are relatively uncoupled to other He motions up to at least upsilon(t) = 7. Good agreement between experiment and theory was obtained with only one adjustable parameter, the band origin. The calculated results were crucial in assigning many weaker observed transitions because the effective rotational constants depend strongly on the torsional state. The observed spectra had effective temperatures around 0.7 K and involved transitions with J < or =3, with upsilon(t) = 0 and 1, and (with one possible exception) with Deltaupsilon(t)=0. Mixing of the torsion-rotation states is small but significant: some transitions with Deltaupsilon(t) not equal 0 were predicted to have appreciable intensity even assuming that the dipole transition moment coincides perfectly with the NNO axis. One such transition was tentatively assigned in the observed spectra, but confirmation will require further work.

Journal Article↗

PubMed2005

Infrared spectra of CO2-H2 complexes.

Infrared spectra of weakly bound CO(2)-H(2) complexes have been studied in the region of the CO(2) v(3) asymmetric stretch, using a tunable diode laser probe and a pulsed supersonic jet expansion. For CO(2)-paraH(2), results were obtained for three isotopic species, (12)C(16)O(2), (13)C(16)O(2), and (12)C(18)O(2). These spectra were analyzed using an asymmetric rotor Hamiltonian, with results that resembled those obtained previously for OCS- and N(2)O-paraH(2), except that half the rotational levels were missing due to the symmetry of CO(2) and the spin statistics of the (16)O or (18)O nuclei. However, for CO(2)-orthoH(2), more complicated spectra were observed which could not be assigned, in contrast with OCS- and N(2)O-H(2) where the paraH(2) and orthoH(2) spectra were similar, though distinct. The CO(2)-paraH(2) complex has a T-shaped structure with and intermolecular distance of about 3.5 Angstroms, and the CO(2) v(3) vibration exhibits a small redshift (-0.20 cm(-1)) in the complex.

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PubMed2005

Small para-hydrogen clusters doped with carbon monoxide: quantum Monte Carlo simulations and observed infrared spectra.

The structures and rotational dynamics of clusters of a single carbon monoxide molecule solvated in para-hydrogen, (paraH(2))(N)-CO, have been simulated for sizes up to N=17 using the reptation Monte Carlo technique. The calculations indicate the presence of two series of R(0) rotational transitions with J=1<--0 for cold clusters, similar to those predicted and observed in the case of He(N)-CO. Infrared spectra of these clusters have been observed in the region of the C-O stretch ( approximately 2143 cm(-1)) in a pulsed supersonic jet expansion using a tunable diode laser probe. With the help of the calculations, the observed R(0) rotational transitions have been assigned up to N=9 for the b-type series and N=14 for the a-type series. Theory and experiment agree rather well, except that theory tends to overestimate the b-type energies. The (paraH(2))(12)-CO cluster is calculated to be particularly stable and (relatively) rigid, corresponding to completion of the first solvation shell, and it is observed to have the strongest a-type transition.

Journal Article↗

PubMed2005

High resolution infrared spectra of H2-Kr and D2-Kr van der Waals complexes.

Infrared spectra of weakly bound hydrogen-krypton complexes have been studied at high spectral resolution (0.04 cm(-1)) using a long-path (154 m) low temperature (100 K) absorption cell and a Fourier transform spectrometer. In addition to spectra from the regions of the H(2) and D(2) fundamental vibrational bands in the midinfrared, the results also include the region of the pure rotational S(0)(0) transition of H(2) in the far infrared. A total of 219 measured line positions from these spectra have been fully assigned to specific quantum transitions and form the basis for determining a greatly improved semiempirical three-dimensional intermolecular potential energy surface for hydrogen-krypton in an accompanying paper.

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PubMed2004

High resolution infrared spectra of helium clusters seeded with isotopic carbon monoxide, HeN-13C 16O and HeN-12C 18O.

Infrared spectra of isotopically substituted HeN-CO clusters (1 < N < 19) have been studied in order to extend the original results on the normal isotope. The same two series of R(0) transitions were observed, correlating with the a- and b-type transitions of He1-CO, with only small shifts in relative position. The previously obscured a-type line for He6-CO was detected. Examination of the small shifts among isotopomers showed remarkably smooth behavior, except in the "unstable" regions around N=7 (b-type series) and 15 (a-type series). The overall results firmly support the assignments and analysis given for the normal isotope.

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PubMed2004

Infrared spectra of seeded hydrogen clusters: (paraH2)N - OCS, (orthoH2)N - OCS, and (HD)N - OCS, N = 2 - 7.

Infrared spectra of hydrogen-carbonyl sulfide clusters containing paraH2, orthoH2, or HD have been studied in the 2060 cm(-1) region of the C-O stretching vibration. The clusters were formed in pulsed supersonic jet expansions and probed using a tunable infrared diode laser spectrometer. Simple symmetric rotor type spectra were observed and assigned for clusters containing up to N = 7 hydrogen molecules. There was no resolved K structure, and Q-branch features were present for orthoH2 and HD but absent for paraH2. These characteristics can be rationalized in terms of near symmetric rotor structures, very low effective rotational temperatures (0.15 to 0.6 K), and nuclear spin statistics. The observed vibrational shifts were compared with those from recent observations on the same clusters embedded in helium nanodroplets. The observed rotational constants for the paraH2 clusters are in good agreement with a recent quantum Monte Carlo simulation. Some mixed clusters were also observed, such as HD-HD-He-OCS and paraH2 - orthoH2 - OCS.

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PubMed2004

High resolution infrared spectra of a carbon dioxide molecule solvated with helium atoms.

Infrared spectra of He(N)-CO(2) clusters with N up to about 20 have been studied in the region of the CO(2) nu(3) fundamental band ( approximately 2350 cm(-1)) using a tunable diode laser spectrometer and pulsed supersonic jet source with cooled (>-150 degrees C) pinhole or slit nozzles and high backing pressures (<40 atm). Compared to previous studies of He(N)-OCS and -N(2)O clusters, the higher symmetry of CO(2) results in simpler spectra but less information content. Discrete rotation-vibration transitions have been assigned for N=3-17, and their analysis yields the variation of the vibrational band origin and B rotational constant over this size range. The band origin variation is similar to He(N)-OCS, with an initial blueshift up to N=5, followed by a monotonic redshift, consistent with a model where the first five He atoms fill a ring around the equator of the molecule, forcing subsequent He atom density to locate closer to the ends. The B value initially drops as expected for a normal molecule, reaching a minimum for N=5. Its subsequent rise for N=6 to 11 can be interpreted as the transition from a normal (though floppy) molecule to a quantum solvation regime, where the CO(2) molecule starts to rotate separately from the He atoms. For N>13, the B value becomes approximately constant with a value about 17% larger than that measured in much larger helium nanodroplets.

Journal Article↗

PubMed2004

Bridging the gap between small clusters and nanodroplets: spectroscopic study and computer simulation of carbon dioxide solvated with helium atoms.

High resolution infrared spectra of He(N)-CO2 clusters with N up to 17 have been studied in the region of the CO2 nu(3) fundamental band. The B rotational constant initially drops as expected for a normal molecule, reaching a minimum for N=5. Its subsequent rise for N=6 to 11 can be interpreted as the transition from a normal (though floppy) molecule to a quantum solvation regime. For N>13, the B value becomes approximately constant with a value about 17% larger than that measured in much larger helium nanodroplets. Quantum Monte Carlo calculations of pure rotational spectra are in excellent agreement with the measured B in this size range and complement the experimental study with detailed structural information. For a larger cluster size (N=30-50) the simulations show a clear sign of convergence towards the nanodroplet B value.

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PubMed2004

Tunable diode laser spectrometer for pulsed supersonic jets: application to weakly-bound complexes and clusters.

The design and operation of an apparatus for studying infrared spectra of weakly-bound complexes is described in detail. A pulsed supersonic jet expansion is probed using a tunable Pb-salt diode laser spectrometer operated in a rapid-scan mode. The jet may be fitted with either pinhole or slit shaped nozzles, the former giving lower effective rotational temperatures, and the latter giving sharper spectral lines. Notable features of the apparatus include use of a toroidal multi-pass mirror system to give over 100 passes of the laser through the supersonic jet, use of the normal laser controller for laser sweeping during both setup and data acquisition, and use of a simple semi-automated wavenumber calibration procedure. Performance of the apparatus is illustrated with observed spectra of the van der Waals complex He-OCS, and the seeded helium clusters He(N)-OCS and He(N)-CO.

Chemistry Techniques, Analytical↗

PubMed2003

Spectroscopic studies of quantum solvation in 4HeN-N2O clusters.

High resolution microwave and infrared spectra of He(N)-N2O clusters were studied in the range N=3 to 12. The apparent cluster moments of inertia increase from N=3 to 6, but then decrease, showing oscillatory behavior for N=7 to 12. This provides direct experimental evidence for the decoupling of helium atoms from the rotation of the dopant molecule in this size regime, signaling the transition from a molecular complex to a quantum solvated system and directly exploring the microscopic evolution of "molecular superfluidity."

Journal Article↗

PubMed2002

Quantum solvation of carbonyl sulfide with helium atoms.

High-resolution infrared and microwave spectra of He(N)-carbonyl sulfide (He(N)-OCS) clusters with N ranging from 2 to 8 have been detected and unambiguously assigned. The spectra show the formation of a solvation layer beginning with an equatorial "donut" of five helium atoms around the OCS molecule. The cluster moment of inertia increases as a function of N and overshoots the liquid droplet limit for N > 5, implying that even atoms in the first solvation shell are decoupled from the OCS rotation in helium nanodroplets. To the extent that this is due to superfluidity, the results directly explore the microscopic evolution of a phenomenon that is formally macroscopic in nature.

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