Biomedical subjects

Michael C Heaven

Publications and source records attributed to Michael C Heaven.

17 recordsLinked to original sources

PubMed2006

The permanent electric dipole moments and magnetic g factors of uranium monoxide.

Permanent electric dipole moments and magnetic g factors for uranium monoxide (UO) have been determined from analyses of optical Stark and Zeeman spectra recorded at a spectral resolution that approaches the natural linewidth limit. Numerous branch features in the previously characterized [L. A. Kaledin et al., J. Mol. Spectrosc. 164, 27 (1994)] (0,0) [18403]5-X(1)4 and (0,0) [18404]5-X(1)4 electronic transitions were recorded in the presence of tunable static electric (Stark effect) or magnetic (Zeeman effect) fields. The lines exhibited unusually large Zeeman tuning effects. A ligand field model and an ab initio electronic structure calculation [R. Tyagi, Ph.D. thesis, The Ohio State University (2005)] were used to interpret the ground state properties. The results indicate that the low energy electronic states of UO are sufficiently ionic for the meaningful application of ligand field theory models. The dipole moments and g factors were distinctly different for the three electronic states examined, which implies that these properties may be used to deduce the underlying electronic state configurations.

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PubMed2006

Probing the electronic structure of UO+ with high-resolution photoelectron spectroscopy.

The pulsed field ionization-zero kinetic energy photoelectron technique has been used to observe the low-lying energy levels of UO+. Rotationally resolved spectra were recorded for the ground state and the first nine electronically excited states. Extensive vibrational progressions were characterized. Omega+ assignments were unambiguously determined from the first rotational lines identified in each vibronic band. Term energies, vibrational frequencies, and anharmonicity constants for low-lying energy levels of UO+ are reported. In addition, accurate values for the ionization energies for UO [48,643.8(2) cm(-1)] and U [49,957.6(2) cm(-1)] were determined. The pattern of low-lying electronic states for UO+ indicates that they originate from the U3+(5f3)O2- configuration, where the uranium ion-centered interactions between the 5f electrons are significantly stronger than interactions with the intramolecular electric field. The latter lifts the degeneracy of U3+ ion-core states, but the atomic angular momentum quantum numbers remain reasonably well defined.

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PubMed2006

Universal scaling features of spectroscopic constants for diatomic systems.

Based on a new criterion that was proposed to search for the universality of spectroscopic constants for bound ground-state diatomics [R. H. Xie and P. S. Hsu, Phys. Rev. Lett. 96, 243201 (2006)], we have found universal scaling relations between spectroscopic constants of diatomic systems with s-, p-, and d-type valence-shell constituents. Our study suggests a useful empirical approach for the prediction of molecular spectroscopic constants.

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PubMed2006

Probing actinide electronic structure using fluorescence and multi-photon ionization spectroscopy.

The role of the 5f orbitals in actinide bond formation is one of the central issues of actinide chemistry. This question can be explored using relativistic quantum chemistry calculations, but the theoretical methods must be validated using definitive experimental data. For simple ionic compounds such as actinide oxides and halides, the electronic structure can be examined using electronic and photoelectron spectroscopy of gas phase molecules. The present article surveys recent spectroscopic and theoretical studies of the oxides of thorium and uranium. The results show that the 5f-like states are spectators and that the complex patterns of low-lying electronic states for these molecules can be understood in terms of a semi-empirical ligand field model. Comparisons with recent high-level theoretical calculations are presented and discussed.

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PubMed2006

Spectroscopy of the ground and low-lying excited states of ThO+.

The ThO(+) cation is of interest as it is a useful prototype for experimental and theoretical studies of bonding in a simple actinide compound. Formally the ground state of ThO(+) has the configuration Th(3+)(7s)O(2-), where there is a single unpaired electron associated with a closed-shell Th(4+)-ion core. The first tier of excited states above the X (2)Sigma(+) ground state is expected to be 1 (2)Delta, 1 (2)Pi, and 2 (2)Sigma(+) derived from the Th(3+)(6d)O(2-) configuration. Spectroscopic observations of ThO(+) using the pulsed field ionization-zero kinetic-energy photoelectron technique are reported here. Rotationally resolved spectra were recorded for the X (2)Sigma(+), 1 (2)Delta, and 1 (2)Pi states. Extensive vibrational progressions were observed. Surprisingly, it was found that ionization of ThO decreases the dissociation energy, while increasing the vibrational frequency and decreasing the bond length. Accurate values for the ionization energies of ThO [53 253.8(2) cm(-1)] and Th [50 868.71(8) cm(-1)] were determined as part of this investigation.

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PubMed2006

Spectroscopic characterization of the C2-Ne van der Waals complex.

Binary complexes of C2 with rare-gas atoms (C2-Rg) have attracted theoretical interest as their potential-energy surfaces are predicted to support linear equilibrium geometries, without the local minimum for the T-shaped geometry that would be expected using a standard pair-potential model. In the present work we have explored the properties of C2-Ne using laser-induced fluorescence detection of the D 1Sigmau +-X 1Sigmag + transition. Bands of the complex were observed in association with the monomer 0-0 and 1-1 transitions. Rotationally resolved data yielded rotational constants of B'=0.099(3) cm(-1) and B"=0.100(3) cm(-1) for the excited and ground states, respectively. Analysis of the rovibrational energy-level structure for C2(D)-Ne indicates that the complex has a linear equilibrium structure with a barrier to internal rotation of approximately 15 cm(-1). Data for the ground state validate a recent high-level ab initio calculation of the potential-energy surface for C2(X)-Ne.

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PubMed2005

Bound states and scattering resonances of OH(A)-He.

The OH-He complex has been observed using laser excitation of the A 2sigma+-X 2pi transition. The bands of the complex were close to the monomer rotational lines that terminate on the n = 0, 1, and 2 levels of OH(A). The unresolved band associated with He.OH (A,n=0) was redshifted from the OH parent line by 1.6 cm(-1), providing a direct measurement of D0'-D0". The complex features associated with n = 1 and 2 were identified as scattering resonances. They have been assigned by comparison with resonance structures derived from close-coupling calculations. The ab initio potential energy surface of H.-S. Lee, A.B. McCoy, R.R. Toczylowski, and S.M. Cybulski, [J. Chem. Phys. 113, 5736 (2000)] was used in these calculations. The level of agreement between the observed and predicted resonances indicated that the ab initio surface is reasonably accurate.

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PubMed2005

Spectroscopic and theoretical characterization of the A2delta-X2pi transition of CH-Ne.

The A2delta-X2pi transition of CH-Ne was examined using laser-induced fluorescence and fluorescence depletion techniques. The spectrum was found to be particularly congested due to the large number of bound states derived from the CH(A,n=2)+Ne interaction, and the small energy spacings between these states resulting from the relatively weak anisotropy of the van der Waals bond. High-level ab initio calculations were used to generate two-dimensional potential energy surfaces for CH(X)-Ne and CH(A)-Ne. The equilibrium structures from these surfaces were bent and linear for the X and A states, respectively. Variational calculations were used to predict the bound states supported by the ab initio surfaces. Empirical modification of the potential energy surfaces for the A state was used to obtain energy-level predictions that were in good agreement with the experimental results. Transitions to all of the optically accessible internal rotor states of CH(A,n=2)-Ne were identified, indicating that CH performs hindered internal rotations in the lowest-energy levels of the A and X states. The characteristics of the potential energy surfaces for CH-Ne in the X,A,B, and C states suggest that dispersion and exchange repulsion forces dominate the van der Waals interaction.

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PubMed2005

Ionization energy measurements and electronic spectra for ThO.

The ionization energy (IE) for ThO has been determined using photoionization efficiency and mass-analyzed threshold ionization measurements. An IE of 6.6038(12) eV was obtained, which was appreciably higher than the result from previous appearance potential measurements [6.1(1) eV]. The revised IE is 0.3 eV greater than that of atomic Th, indicating that neutral ThO is more tightly bound than ThO(+). The one-color two-photon resonant ionization spectrum of ThO was examined in the range of 315-370 nm. Rotationally resolved bands were recorded for three new electronic states (designated as E('),F('), and G(')). In addition, transitions to the A(')(v=1,2,3) levels and the N(v=2) level were observed for the first time. Ligand field theory predictions [L. A. Kaledin, J. E. McCord, and M. C. Heaven, J. Mol. Spectrosc. 164, 27 (1994)] were used to propose configurational assignments for 20 electronically excited states.

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PubMed2005

Experimental detection and theoretical characterization of the H2-NHX van der Waals complex.

The H2-NH(X) van der Waals complex has been examined using ab initio theory and detected via fluorescence excitation spectroscopy of the A(3)Pi-X(3)Sigma(-) transition. Electronic structure calculations show that the minimum energy geometry corresponds to collinear H2-NH(X), with a well depth of D(e)=116 cm(-1). The potential-energy surface supports a secondary minimum for a T-shaped geometry, where the H atom of NH points towards the middle of the H2 bond (C(2v) point group). For this geometry the well depth is 73 cm(-1). The laser excitation spectra for the complex show transitions to the H2+NH(A) dissociative continuum. The onset of the continuum establishes a binding energy of D(0)=32+/-2 cm(-1) for H2-NH(X). The fluorescence from bound levels of H2-NH(A) was not detected, most probably due to the rapid reactive decay [H2-NH(A)-->H+NH2]. The complex appears to be a promising candidate for studies of the photoinitiated H2+NH abstraction reaction under conditions were the reactants are prealigned by the van der Waals forces.

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PubMed2005

The electronic spectrum of the UO2 molecule.

The electronic spectrum of the UO(2) molecule has been determined using multiconfigurational wave functions together with the inclusion spin-orbit coupling. The molecule has been found to have a (5fphi)(7s), (3)Phi(2u), ground state. The lowest state of gerade symmetry,( 3)H(4g), corresponding to the electronic configuration (5f)(2) was found 3330 cm(-1) above the ground state. The computed energy levels and oscillator strengths were used for the assignment of the experimental spectrum in the energy range 17,000-19,000 and 27,000-32,000 cm(-1).

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PubMed2004

Bound state spectroscopy of NH-He.

The NH-He van der Waals complex was characterized via laser excitation of bands associated with the NH A (3)Pi-X (3)Sigma(-) transition. It was demonstrated that the ground state supports a bound level with a rotational constant of B"=0.334(2) cm(-1). These results are in agreement with the predictions of recent high-level theoretical calculations. Spin-orbit predissociation of the excited complex was observed, and the spectra yield insights regarding the NH(A)+He potential energy surfaces.

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PubMed2004

State-to-state rotational relaxation rate constants for CO+Ne from IR-IR double-resonance experiments: comparing theory to experiment.

IR-IR double-resonance experiments were used to study the state-to-state rotational relaxation of CO with Ne as a collision partner. Rotational levels in the range Ji=2-9 were excited and collisional energy transfer of population to the levels Jf=2-8 was monitored. The resulting data set was analyzed by fitting to numerical solutions of the master equation. State-to-state rate constant matrices were generated using fitting law functions. Fitting laws based on the modified exponential gap (MEG) and statistical power exponential gap (SPEG) models were used; the MEG model performed better than the SPEG model. A rate constant matrix was also generated from scattering calculations that employed the ab initio potential energy surface of McBane and Cybulski [J. Chem. Phys. 110, 11 734 (1999)]. This theoretical rate constant matrix yielded kinetic simulations that agreed with the data nearly as well as the fitted MEG model and was unique in its ability to reproduce both the rotational energy transfer and pressure broadening data for Ne-CO. The theoretical rate coefficients varied more slowly with the energy gap than coefficients from either of the fitting laws.

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PubMed2004

Electronic spectroscopy and ionization potential of UO2 in the gas phase.

The electronic spectroscopy of UO(2) has been examined using multiphoton ionization with mass-selected detection of the UO(2) (+) ions. Supersonic jet cooling was used to reduce the spectral congestion. Twenty-two vibronic bands of neutral UO(2) were observed in the range from 17,400 to 32,000 cm(-1). These bands originated from the U(5fphi(u)7ssigma(g))O(2) X (3)Phi(2u) and (3)Phi(3u) states. The stronger band systems are attributed to metal-centered 7p<--7s transitions. Threshold ionization measurements were used to determine the ionization potentials of UO and UO(2). These were found to be higher than the values obtained previously from electron impact measurements but in agreement with the results of recent theoretical calculations.

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PubMed2004

Electronic spectroscopy of UO2 isolated in a solid Ar matrix.

Dispersed fluorescence spectra have been recorded for UO(2) isolated in a solid Ar matrix. Near UV excitation produced groups of emission bands in the 370-420 and 465-645 nm spectral regions. These bands originated from two energetically close upper levels and terminated on a range of low-lying electronic states. Comparisons with electronic structure calculations indicate that the ground and low-lying electronic states of UO(2) are derived from the U(5f7s) configuration. The level of agreement between the observed and predicted electronic energies provides an impressive validation of the calculations. In previous studies of matrix isolated UO(2), it had been suggested that the ground state in solid Ar is U(5f(2)) (3)H. The present results do not appear to be consistent with this hypothesis.

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PubMed2004

State-to-state rotational rate constants for CO + He: infrared double resonance measurements and simulation of the data using the SAPT theoretical potential energy surface.

An extensive data set of 54 time-resolved pump-probe measurements was used to examine CO + He rotational energy transfer within the CO v = 2 rotational manifold. Rotational levels in the range Ji = 2-9 were excited and collisional energy transfer of population to the levels Jf = 1-10 was monitored. The resulting data set was analyzed by fitting to numerical solutions of the master equation. State-to-state rate constant matrices were generated using fitting law functions and ab initio theoretical calculations that employed the SAPT potential energy surface of Heijmen et al. [J. Chem. Phys. 107, 9921 (1997)]. Fitting laws based on the modified exponential gap (MEG), statistical power exponential gap (SPEG), and energy corrected sudden with exponential power (ECS-EP) models all yielded acceptable simulations of the kinetic data, as did the theoretical rate constants. However, the latter were unique in their ability to reproduce both our kinetic data and the pressure broadening coefficients for CO + He. These results provide an impressive demonstration of the quality of the symmetry adapted perturbation theory (SAPT) potential energy surface.

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PubMed2003

Accurate ionization potentials for UO and UO2: a rigorous test of relativistic quantum chemistry calculations.

Accurate ionization potential (IP) measurements provide essential thermodynamic information and benchmark data that can be used to evaluate the validity of electronic structure models. Calculations of the first IP of UO2 using relativistic methods consistently predict values that are approximately 0.7 eV higher than the accepted experimental value. The present measurements validate the theoretical calculations and show that the previous determinations corresponded to the ionization of thermally excited molecules. Similarly, new measurements of the IP for UO show that the currently accepted value is too low by 0.4 eV.

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