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The effects of permanent dipoles, and the relative effects of the direct permanent dipole and the virtual state excitation mechanisms, are discussed for excitations involving the simultaneous absorption of two identical photons. Two molecular models for twophoton excitation, one dominated by the direct permanent dipole mechanism and the other having significant contributions from both excitation mechanisms, are used for this purpose. Resonance profiles, as a function of laser intensity, are evaluated for both models by employing the full Generalized Rotating Wave Approximation method and the recently developed Analytic Generalized Rotating Wave Approximation (AGRWA). The profiles are used to assess (1) the nature of the effects of permanent molecular dipoles, (2) the relative contributions of the two excitation mechanisms, and (3) the validity of the AGRWA for twophoton excitations. The AGRWA is a very useful interpretive∕predictive tool even for higher laser intensities where its validity becomes questionable. It can be used to suggest how to exploit the effects of molecular permanent dipoles to enhance two photon excitations using both excitation mechanisms.
The Journal of Chemical Physics 10/2013; 139(14):144104. DOI:10.1063/1.4824382 · 3.12 Impact Factor

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One of the purposes of this paper is to develop an analytical manylevel generalized rotating wave approximation (GRWA), including the effects of permanent dipoles, for the excitation of manylevel molecules through the simultaneous absorption of two photons. Included are expressions for the twophoton laser–molecule coupling C, and its two components Cd and Cv corresponding to the direct permanent dipole and the virtual state excitation mechanisms, respectively, and related observables such as the timedependent populations of the initial and final states of the excitation process and resonance profiles. This GRWA treatment also includes an energy shift parameter ε, which causes shifts in the position of the resonance energy as the laser intensity increases. The effects of permanent dipoles are very different in Cv and ε versus Cd. These effects have been discussed previously for Cd using analytic twolevel RWA approaches. The analytical results for Cv and ε obtained here are new as is their use in discussing the influence of permanent dipoles in the parts of the twophoton excitation process involving virtual states. In the absence of permanent dipoles Cd is zero whereas Cv and ε are not; they equal the corresponding perturbation theory results. The GRWA and perturbative results are related by Bessel function damping functions which, for dipolar molecules, damp out the divergence of the perturbative results as the laser intensity increases. Illustrative examples are given for a twophoton excitation involving a model chromophore which has a significant virtual state contribution through both the laser–molecule coupling and the energy shift parameter.
Journal of Physics B Atomic Molecular and Optical Physics 09/2011; 44(20):205401. DOI:10.1088/09534075/44/20/205401 · 1.92 Impact Factor

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Dipole oscillator strength distributions (DOSDs) have been constructed for ground state ethane, propane, nbutane, npentane, nhexane, nheptane, and noctane. Each DOSD is constructed by using available experimental and theoretical photoabsorption cross sections and by constraining the resulting dipole oscillator strength data to satisfy the Thomas–Reiche–Kuhn sum rule and molar refractivity constraints. The latter were obtained using available experimental refractive index measurements of relevant dilute gases. The recommended DOSDs, and the values of integrated "band" oscillator strengths and the dipole oscillator strength sums Sk and Lk (for a variety of k values) obtained from them, are reported. The discussion includes an analysis of the accuracy of the results and a comparison with available literature values, which are scarce, for the dipole properties of the alkanes.
Canadian Journal of Physics 02/2011; 59(2):185197. DOI:10.1139/p81023 · 0.93 Impact Factor

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Dipole oscillator strength distributions (DOSDs) have been constructed for ground state ethylene, propene, and 1butene. Each DOSD is constructed by using available experimental and theoretical photoabsorption cross sections and by constraining the resulting dipole oscillator strength data to satisfy the Thomas – Reiche–Kuhn sum rule and molar refractivity constraints. The latter were obtained from experimental refractive index measurements of relevant dilute gases. The recommended DOSDs, and the values of integrated "band" oscillator strengths, and the dipole oscillator strength sums Sk and Lk (for a variety of k values) obtained from them, are reported. The discussion includes an analysis of the reliability of the results using 1butene as a detailed model.
Canadian Journal of Physics 02/2011; 61(7):10271034. DOI:10.1139/p83129 · 0.93 Impact Factor

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Dipole oscillator strength distributions have been constructed and used to evaluate integrated oscillator strengths, and a variety of dipole oscillator strength properties, for ground state SO2, CS2, and OCS. Each distribution has been constructed by using experimental and theoretical photoabsorption cross sections and by subjecting the resulting dipole oscillator strength data to constraints provided by the Thomas–Reiche–Kuhn sum rule and molar refractivity data for the relevant dilute gases. The discussion includes graphical presentations of how various spectral regions of the dipole oscillator strength distributions contribute to the more important dipole properties.
Canadian Journal of Physics 02/2011; 63(3):417427. DOI:10.1139/p85065 · 0.93 Impact Factor

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Dipole oscillator strength distributions (DOSDs) have been constructed for ground state Li, N, O, H2, N2, O2, NH3, H2O, NO, and N2O by using experimental and theoretical photoabsoiption and high energy electron inelastic scattering cross sections. Each DOSD is required to satisfy the Thomas–Reiche–Kuhn sum rule and additional constraints derived from available accurate experimental refractivity and dispersion measurements. The DOSDs, the data and procedure used to construct the DOSDs, and the values of the dipole oscillator strength sums Sk and Lk (for a variety of k values) and related atomic and molecular properties obtained from the DOSDs are reported. The discussion includes comments regarding the importance of the constraints imposed on the DOSD with respect to the evaluation of various dipole sums and properties and the accuracy of the results.
Canadian Journal of Physics 02/2011; 55(23):20802100. DOI:10.1139/p77252 · 0.93 Impact Factor

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Dipole oscillator strength distributions (DOSDs) have been constructed for ground state methanol, ethanol, and npropanol and used to evaluate integrated ("band") oscillator strengths and the dipole oscillator strength sums Sk and Lk (for a variety of k values) for these molecules. Each DOSD is constructed by using available experimental and theoretical photoabsorption cross sections and by constraining the resulting dipole oscillator strength data to satisfy the Thomas–Reiche–Kuhn sum rule and molar refractivity constraints. The discussion includes an analysis of the reliability of the results.
Canadian Journal of Chemistry 02/2011; 62(2):373381. DOI:10.1139/v84066 · 1.01 Impact Factor

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Globally reliable dipole oscillator strength distributions (DOSDs) have been constructed for the H2S molecule in its ground state. An adopted DOSD is used to evaluate a variety of dipole oscillator strength sums Sk, logarithmic dipole sums Lk, and mean excitation energies Ik, for H2S; these dipole properties are important in various physical processes. It is also used to obtain reliable results for the dipole–dipole dispersion energy coefficients C6 for the interaction of H2S with itself, and with thirtynine other atoms and (mostly) molecules, and the tripledipole dispersion energy coefficient C9 for (H2S)3. A pseudoDOSD for H2S is presented which facilitates the evaluation of C6's, and in particular C9's.
Canadian Journal of Chemistry 02/2011; 66(4):615619. DOI:10.1139/v88105 · 1.01 Impact Factor

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Newly available and highly accurate oscillator strength data, extending over the continuous energy range from the first excitation threshold to 200 eV, are used together with mixture rule estimates and other photoabsorption data to construct a refined dipole oscillator strength distribution to infinite photon energy using Thomas–Reiche–Kuhn sum rule and molar refractivity constraints. This constrained dipole oscillator strength distribution has been used to calculate a wide range of related dipole properties (Sk, k = 2, 1, 0, −1/2, −1, −3/2, −2, −5/2, −3, −4, −5, −6, −8, −10, −12; Lk and Ik, k = 2, 1, 0, −1, −2). The theoretical analysis and associated consistency checks support the high accuracy of the newly available absolute oscillator strengths for the photoabsorption of NH3.
Canadian Journal of Chemistry 02/2011; 71(3):341351. DOI:10.1139/v93051 · 1.01 Impact Factor

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Dipole oscillator strength distributions have been constructed, and used to evaluate integrated dipole oscillator strengths and a variety of dipole oscillator strength properties, for ground state Ne, Ar, Kr, Xe, HF, HCl, and HBr. Each distribution has been constructed by using experimental and theoretical photoabsorption cross sections and by subjecting the resulting dipole oscillator strength data to constraints provided by the Thomas–Reiche–Kuhn sum rule and molar refractivity or related data for the relevant dilute gases. The resulting dipole properties are generally the most or only reliable values available for the hydrogen halides, for the rare gases they largely augment reliable results for most of the properties already available in the literature. The results for HCl are used to discuss an interesting molecular effect in the photoabsorption spectrum of this molecule betwen ~200 eV and ~205 eV.
Canadian Journal of Chemistry 02/2011; 63(7):16161630. DOI:10.1139/v85272 · 1.01 Impact Factor

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Formulae for the computation of isotropic and anisotropic dipolar dispersion energy coefficients, for twobody and threebody interactions involving H2, N2, CO, and the rare gases, are presented in an average energy approximation. These coefficients are computed to within 1% of the reliable values for these coefficients, which are obtained by using the relevant dipole oscillator strength distributions, with the exception of a few that are recorded in tabular form. The input data required for these formulae are the isotropic and anisotropic polarizabilities and average energies for the interacting species. The results provide the first reliable anisotropic tripledipole dispersion energy coefficients for interactions involving molecules. Key words: nonadditive, anisotropic, interaction energies, tripledipole dispersion energies.
Canadian Journal of Chemistry 02/2011; 74(6):11801186. DOI:10.1139/v96132 · 1.01 Impact Factor

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Dipole (e,e) spectroscopy has been used to measure the absolute photoabsorption oscillator strengths (cross sections) for the valence shells of CH3NH2, (CH3)2NH, and (CH3)3N from the photoabsorption threshold to 250 eV at low resolution (1 eV fwhm) and to 31 eV at high resolution (0.048 eV fwhm). The observed peaks in the photoabsorption spectra of the methylamines have been assigned to transitions to Rydberg orbital upper states. Our measured photoabsorption data, augmented by mixture rule estimates for high photon energies, have been used in conjunction with Thomas–Reiche–Kuhn sum rule and molar refractivity constraints, to construct constrained dipole oscillator strength distributions for each of the methylamines. From these constrained dipole oscillator strength distributions a wide range of related dipole properties have been calculated for each of the methylamines, and in most cases the results so obtained represent the first (reliable) determination of these properties.
Canadian Journal of Chemistry 02/2011; 72(3):529546. DOI:10.1139/v94077 · 1.01 Impact Factor

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Dipole oscillator strength distributions have been constructed for acetone, acetaldehyde and formaldehyde, by using constrained leastsquares methods, experimental and mixture rule dipole oscillator strength data, and molar refractivity data. The distributions have been used to evaluate recommended values for a wide variety of isotropic dipole oscillator strength sums, logarithmic dipole sums, and mean excitation energies for each molecule. Often these results are the only (reliable) values available for many of these properties which are of importance in a variety of research areas.
Molecular Physics 12/2010; February 20(31997):389398. DOI:10.1080/002689797172507 · 1.64 Impact Factor

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Four potential energy surfaces are of current interest for the NeCO interaction. Two are highlevel fully ab initio surfaces obtained a decade ago using symmetryadapted perturbation theory and supermolecule coupledcluster methods. The other two are very recent exchangeCoulomb (XC) model potential energy surfaces constructed by using ab initio HeitlerLondon interaction energies and literature long range dispersion and induction energies, followed by the determination of a small number of adjustable parameters to reproduce a selected subset of pure rotational transition frequencies for the (20)Ne(12)C(16)O van der Waals cluster. Testing of the four potential energy surfaces against a wide range of available experimental microwave, millimeterwave, and midinfrared NeCO transition frequencies indicated that the XC potential energy surfaces gave results that were generally far superior to the earlier fully ab initio surfaces. In this paper, two XC model surfaces and the two fully ab initio surfaces are tested for their abilities to reproduce experiment for a wide range of nonspectroscopic NeCO gas mixture properties. The properties considered here are relative integral cross sections and the angle dependence of rotational statetostate differential cross sections, rotational relaxation rate constants for CO(v=2) in NeCO mixtures at T=296 K, pressure broadening of two pure rotational lines and of the rovibrational lines in the CO fundamental and first overtone transitions at 300 K, and the temperature and, where appropriate, mole fraction dependencies of the interaction second virial coefficient, the binary diffusion coefficient, the interaction viscosity, the mixture shear viscosity and thermal conductivity coefficients, and the thermal diffusion factor. The XC model potential energy surfaces give results that lie within or very nearly within the experimental uncertainties for all properties considered, while the coupledcluster ab initio surface gives results that agree similarly well for all but one of the properties considered. When the present comparisons are combined with the ability to give accurate spectroscopic transition frequencies for the NeCO van der Waals complex, only the XC potential energy surfaces give results that agree well with all extant experimental data for the NeCO interaction.
The Journal of Chemical Physics 01/2010; 132(2):024308. DOI:10.1063/1.3285721 · 3.12 Impact Factor

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ExchangeCoulomb model potential energy surfaces have been developed for the NeCO interaction. The initial model is a threedimensional potential energy surface based upon computed HeitlerLondon interaction energies and literature results for the longrange induction and dispersion energies, all as functions of interspecies distance, the orientation of CO relative to the interspecies axis, and the bond length of the CO molecule. Both a rigidrotor model potential energy surface, obtained by setting the CO bond length equal to its experimental spectroscopic equilibrium value, and a vibrationally averaged model potential energy surface, obtained by averaging the stretching dependence over the ground vibrational motion of the CO molecule, have been constructed from the full data set. Adjustable parameters in each model potential energy surface have been determined through fitting a selected subset of pure rotational transition frequencies calculated for the (20)Ne(12)C(12)O isotopolog to precisely known experimental values. Both potential energy surfaces provide calculated results for a wide range of available experimental microwave, millimeterwave, and midinfrared NeCO transition frequencies that are generally far superior to those obtained using the best current literature potential energy surfaces. The vibrationally averaged CO ground state potential energy surface, employed together with a potential energy surface obtained from it by replacing the ground vibrational state average of the CO stretching dependence of the potential energy surface by an average over the first excited CO vibrational state, has been found to be particularly useful for computing and/or interpreting midIR transition frequencies in the NeCO dimer.
The Journal of Chemical Physics 07/2009; 130(24):244310. DOI:10.1063/1.3157169 · 3.12 Impact Factor

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Isotropic dipole oscillator strength distributions (DOSDs) have been constructed for the dimethyl, diethyl and methyl–propyl ether molecules through the use of quantum mechanical constraint techniques and experimental dipole oscillator strength data. The constraints are furnished by molar refractivity data and the Thomas–Reiche–Kuhn sum rule. The DOSDs are used to obtain recommended values for a variety of isotropic dipole oscillator strength sums, logarithmic dipole oscillator strength sums, and mean excitation energies for the molecules. PseudoDOSDs for the ethers are also constructed and used to obtain reliable results for the isotropic dipole–dipole dispersion energy coefficients for all twobody interactions of the ethers with each other and with fifty other species. In addition reliable results are also obtained for the triple–dipole dispersion energy coefficients for all threebody interactions involving the ethers. Dedicated to Anthony Stone, an excellent scientist and friend, on the occasion of his 70th birthday.
Molecular Physics 06/2008; 106(12):15311544. DOI:10.1080/00268970802139916 · 1.64 Impact Factor

William J. Meath
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The dependence of the population of the target state, and the excitation cross section and rate, on pulse duration Q, laser intensity I, and a molecular matrix element A is discussed for twophoton molecular excitation. Perturbative and rotating wave approximation (RWA) expressions for the observables are obtained; the latter are used to discuss the validity of the former. For example, the perturbative cross section and rate increasingly underestimate the RWA results as I increases for given A and Q. Two and tenlevel model dipolar molecules are employed for illustrative purposes. The results are relevant for understanding twophoton excitation processes and their enhancement and include discussions of the roles the permanent dipole and virtualstate mechanisms in such processes and of the validity of using intensityindependent cross sections to gauge the strength of such excitations.
Journal of the Optical Society of America B 04/2008; 25(5):865876. DOI:10.1364/JOSAB.25.000865 · 1.81 Impact Factor

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A recommended isotropic dipole oscillator strength distribution (DOSD) has been constructed for the ethylene molecule through the use of quantum mechanical constraint techniques and experimental dipole oscillator strength (DOS) data; the DOS data employed are recent experimental results not available at the time of the original constrained DOSD analysis of this molecule. The constraints are furnished by molar refractivity data and the ThomasReicheKuhn sum rule. The DOSD is used to evaluate a variety of isotropic dipole oscillator strength sums, logarithmic dipole oscillator strength sums, and mean excitation energies for ethylene. PseudoDOSDs for this molecule, and for propene and 1butene, which are based on an earlier constrained DOSD analysis for these molecules, are developed. They are used to obtain reliable results for the isotropic dipoledipole dispersionenergy coefficients C6, for the interactions of the alkenes with each other and with 47 other species, and the tripledipole dispersionenergy coefficients C9 for interactions involving any triple of molecules taken from ethylene, propene, and 1butene.
Canadian Journal of Chemistry 09/2007; 85(10):724737. DOI:10.1139/V07057 · 1.01 Impact Factor

Advances in Chemical Physics: Lasers, Molecules, and Methods, Volume 73, 03/2007: pages 307  349; , ISBN: 9780470141229

Advances in Chemical Physics: Intermolecular Forces, Volume 12, 03/2007: pages 3  106; , ISBN: 9780470143582