Polar Diatomic Molecules Research Articles

Electric-field-induced Q branch of the vibrational fundamental of CO

The electric-field-induced Q branch of the vibrational fundamental has been observed for the first time in a polar diatomic molecule in a study of 12C16O near 2143.−1 Intensity measurements of the Q(0), Q(1), and Q(2) transitions with a diode laser spectrometer have shown that these normally forbidden lines become allowed through the mixing of rotational levels generated by the interaction of the dipole moment with the externally applied electric field. Measurements have been made both with a static field and with a square-wave-modulated field. The electric dipole moment of 12C16O in the first-excited vibrational state, not previously measured experimentally, has been found to equal −0.0830 (23) D. The transition moment for the vibrational fundamental has been determined to be 0.1077 (38) D, which is in agreement with previous, more precise determinations based on the intensity of the allowed P and R branches. The results obtained here are in excellent agreement with earlier calculations of the dipole moment function.

Far-infrared spectra of diatomic polar molecules in simple liquids: Accounting for the anisotropic interaction

A theoretical expression giving the far-infrared spectrum of a dilute solution of diatomic polar molecules in a nonpolar liquid solvent is derived in order to analyze the influence on this spectrum from the J=1 and J=2 components of the Legendre polynomial expansion of the anisotropic solute–solvent potential. A non-Markovian spectral theory incorporating finite correlation of the interaction, mixing effects between rotational lines and quantum intermolecular potential correlation functions is used. This theory allows one to analyze the influence of each anisotropic part of the interaction in terms of a very reduced set of parameters: the strength and the width of its time correlation function. The zero and finite frequency components of the J=2 contribution are also studied. In particular, we show that the use of quantum potential correlation functions gives a zero frequency component not only in the width, but also in the shift of the different rotational lines. Numerical results for DCl, HCl, and HF in SF6 liquid at 273 K are obtained.

Moleculelike metastable states of antiprotonic and mesic helium.

The exotic helium atom, consisting of a helium nucleus He{sup 2+}, an electron {ital e}{sup {minus}}, and a massive, negatively charged particle {ital X}{sup {minus}}, such as an antiproton ({ital {bar p}}) or a kaon ({ital K}{sup {minus}}), is studied theoretically. This exotic atom is more like a polar molecule with two nuclei He{sup 2+} and {ital X}{sup {minus}}, if {ital e}{sup {minus}} is in a low-lying orbital and {ital X}{sup {minus}} is in a highly excited orbital. Such states are generally formed in the capture of {ital X}{sup {minus}} by helium. The Born-Oppenheimer (BO) separation of the {ital e}{sup {minus}} motion and the He{sup 2+}-{ital X}{sup {minus}} motion is a good approximation for these states, and affords a transparent, unified perspective of exotic helium with different {ital X}{sup {minus}} and different isotopes of He{sup 2+}. A propensity rule that favors small transition energies (except for the smallest one) is found for radiative emission by low vibrational and high rotational levels. This rule resembles the selection rule for infrared emission by the usual diatomic polar molecules, and contradicts the conventional expectation that radiative transitions in exotic atoms favor larger transition energies. The radiative lifetimes {tau} of highly excited states formedmore » immediately after the capture of {ital X}{sup {minus}} are of the order of {mu}sec for {sup 4}He{sup 2+}-{ital {bar p}}-{ital e}{sup {minus}} and {sup 3}He{sup 2+}-{ital {bar p}}-{ital e}{sup {minus}}, and those for {sup 4}He{sup 2+}-{ital K}{sup {minus}}-{ital e}{sup {minus}} are about half the values for the antiprotonic helium.« less

The physical bases of spectroscopic measurements of electrical fields in a plasma

Consideration is given to the chief spectroscopic methods of measuring electric fields in plasma media. These methods are based on the effects of Stark splitting of spectral lines of hydrogen atoms, and the appearance of forbidden (by parity) components in the emission spectra of helium atoms. An analysis is made of methods of measuring weak electric fields (with intensities from 10 to 100 V/cm) in plasma. The methods are based on the appearance of forbidden components in the spectra of laser-induced fluorescence of diatomic polar molecules and the Stark effect of Rydberg atoms.

Rovibrational line width of polar diatomic molecules diluted in rare gases. Application to HCl in dense Ar.

The rovibrational line widhts of HCl in Ar have been calculated using an analytical expression deduced from a spectral theory previously developed (J. Pérez J. Chem. Phys. 91 (1989) 3435). Quantitative values for two statistical parameters appearing in the theory regarding the diatomic-solvent interaction, are obtained by comparison between calculated and experimental half widths, and contrasted with values obtained by comparison of calculated and experimental rotational line shapes.

Memory effects on the rotational relaxation of diatomic polar molecules in non-polar liquids

A quantum study of the rotational energy relaxation ( T 1) and dephasing ( T 2) processes for diatomic polar molecules in a non-polar liquid host is given without invoking the Markovian hypothesis. The study is based on a model consisting of a rigid rotor stochastically coupled to a thermal bath. The time evolution of the reduced density operator of the rotor is described by a master equation with time-dependent matrix coefficients, which contains all information about the time dependence of the T 1 and T 2 rotational relaxation processes. Explicit expressions for the times characterizing these processes are derived on the basis of a quantum treatment for the time autocorrelation functions (TAF) of the rotor-bath interaction Hamiltonian. A numerical application for HCl in liquid Ar is also reported.

Time dependent rotational relaxation transition rates (T1 processses) for diatomic polar molecules in rare-gas liquids

The time-dependent population decay rates (T 1 processes) corresponding to the rotational relaxation of a diatomic molecule immersed in a liquid medium are studied. The analysis is based on two approaches (quantum-mechanical and semiclassical) recently reported by the authors to calculate time-dependent transition rates for a multilevel quantum system stochastically coupled to a thermal bath. The free rotation of the diatomic molecule is described by means of a quantum rigid rotor whereas the rotor-liquid coupling is taken to be a stochastic Ornstein-Uhlenbeck process. Numerical application for the HCl-Kr system is presented and comparison between both approaches for the same system is also discussed.

Infrared spectra of diatomic polar molecules in rare-gas liquids. I. Spectral theory

A theoretical model for the infrared (0–1) band of dilute solutions of diatomic polar molecules in nonpolar rare-gas liquids is presented. The model is based on decomposition of the rotational motion of the diatomic molecule into two limiting cases, according to the Bratos model: quasifree rotation and rotational diffusion. Contribution to the infrared absorption coefficient due to quasifree rotation is analyzed within a non-Markovian formalism using a stochastic directing intermolecular field (DIF) model to describe the diatomic molecule–solvent interaction. The P and R branches appear as a consequence of the quasifree contribution, which is also important in the Q-branch region. The contribution due to rotational diffusion is calculated making use of the Debye model and is mainly significant in the Q-branch region.

Infrared spectra of diatomic polar molecules in rare-gas liquids. II. Application to the HCl in Ar, Kr, and Xe solutions

In paper I (the preceding paper), a spectral theory describing the near infrared fundamental band spectra of diatomic polar molecules impurities in nonpolar solvents has been developed in terms of a reduced set of parameters. In this paper, this spectral theory is applied to HCl in Ar, Kr, and Xe liquid solutions. The parameters obtained by fitting the experimental and calculated profiles are in good agreement with those calculated using several microscopic theories.

Theoretical far-infrared spectrum of HCl in liquid Ar: A density dependence study

A non-Markovian theory for the dipolar absorption coefficient of polar diatomic molecules diluted in a non-polar liquid medium is applied to obtain the far-infrared spectra of HCl in liquid Ar at densities between 480 and 100 amagat and T=162.5 K. The time correlation functions involved in the theory are calculated by describing the time evolution of the anisotropic diatomicliquid medium interaction by a stochastic Ornstein-Uhlenbeck process involving only two statistical parameters: the mean-square intensity of the diatomic-medium interaction, λ 2, and its correlation time, t c. It is shown that (i) both λ 2 and t c depend linearly on the Ar density; (ii) an overlapping effect among the basic rotational transitions is noticeable even for the lowest Ar densities; and (iii) the influence of non-Markovian effects on the spectra is negligible over the entire density range.

On new possibilities of measuring electric fields in plasmas using molecule emission spectra

A theoretical consideration is given of the spontaneous emission spectrum at electric-dipole transition in a diatomic polar molecule between two electronic states in a static or an oscillating electric field. Intensities of forbidden spectral components appearing under the influence of an external electric field have been calculated. The results obtained allow, by using the experimental ratio of intensities of the forbidden and the allowed spectral components registered at different positions of the polarizer axis, to determine the strength and the direction of the electric fields in low temperature plasmas.

The density integration approach to populations. A critical comparison of projection populations to populations defined by the theory of atoms in molecules

AbstractThe theory of atoms in molecules defines an unambiguous partitioning of the three‐dimensional electron density into atomic basins based on the zero‐flux surfaces of the gradient of the electron density, ∇(r). Integrations of the electron density within such basins yield integrated Bader populations (IBP) that have a rigorous foundation in quantum mechanics. In the density integration technique based on the two‐dimensional electron density projection function, P(x,z), integrated projection populations (IPP) are obtained by integration within regions demarked by steepest descent lines Dp of P(x,z). These density integration techniques are compared by an analysis of the electron density of diatomic molecules that is based on the properties of the zero‐flux surface that partitions the electron density between the atoms. The conventional method for the partitioning of regions of P(x,z) approximates the virial partitioning. Differences between IPP and IBP can be quantitatively described by two terms. One term reflects the error intrinsic to projection populations as a result of the loss of all information about the electron distribution in the third dimension in the calculation of P(x,z). The second term accounts for the effects of the displacement of the demarcation lines Dp toward the less polarizable atom compared with the cross‐section of the density with the plane of projection, Dd. The analysis suggests the definition of a projection population IPP2 that is based on the cross‐section Dd instead of the demarcation lines Dp. Relations between the populations IPP, IPP2, and IBP are derived for diatomic molecules and numerical results are presented for a series of diatomic molecules. Several polyatomic anions are also discussed. The values of IPP are found to be good approximations of IBP in highly polar diatomic molecules. In cases where the bonding involves comparatively little intramolecular charge transfer IPP2 is the better and equally satisfactory projection population. In the intermediate semipolar bonding situations projection populations provide qualitatively correct descriptions of the charge distributions but the numerical agreement with the IBP values is less satisfactory.

Non-markovian far-infrared spectra of HF in liquid SF6

Non-markovian far-infrared spectra calculations for dilute solutions of HF in liquid SF6 at two different temperatures are presented in the range 0 to 250 cm-1. The fine rotational structure appearing in these spectra is discussed as function of the parameters involved in the theoretical absorption coefficient. A comparison with the spectra of other diatomic polar molecules (HCl, DCl) is also reported.

Rotational relaxation and dephasing of diatomic polar molecules in rare-gas liquids

Abstract The rotational relaxation of a diatomic polar molecule in a rare-gas liquid host is studied by a consideration of a quantum rigid rotor weakly interacting with a thermal bath. Dephasing (T 2 ) and population decay (T 1 ) are evaluated through second order in the Markovian limit. The rates are expressed in terms of temperature and frequency dependent bath correlation functions of the interaction. Explicit expressions are presented using a stochastic Hamiltonian to describe diatomic-solvent molecular interactions. Numerical applications for HCl in liquid Ar, Kr and Xe are reported.

Comparison between two models (stochastic and dynamics) calculations for the far-infrared spectra of a HClAr solution

Abstract A comparative study between the theoretical far-infrared spectra of diatomic polar molecules dissolved in rare-gas liquids obtained from two different models (one stochastic and other dynamics in nature) is presented. In both models the theoretical absorption coefficient is calculated within a non-Markovian formalism called PTOC (Partial Time Ordering Cumulants). A numerical application to a HClAr solution shows that a slightly better agreement between calculated and observed spectra is obtained using the stochastic model

Numerical study ofT1andT2rotational relaxation times of HCl in liquid Ar

Energy relaxation (T 1) and dephasing (T 2) processes are analysed for the rotational relaxation of diatomic polar molecules in rare-gas liquids under markovian assumption. Bath autocorrelation functions defining the markovian relaxation superoperator, which contains all information about T 1 and T 2 processes, are derived for an intermolecular potential approximated by a truncated series of Legendre polynomials P J . Hence, analytical expressions for T 1 and T 2 are obtained in terms of a reduced set of parameters regarding both the diatomic and the liquid as their mutual interaction. Numerical contribution to T 1 and T 2 processes, from P 1 and P 2 terms, is given for a HCl-Ar solution by using a dynamical quasiharmonic model to describe the solvent liquid structure.

Hexapole focusing and orientation of HCl and HBr via their hydrogen-bonded complexes with benzene

Polar diatomic molecules such as the hydrogen halides (HX) cannot be readily oriented by conventional electrostatic field techniques. However, by forming the 1:1 hydrogen-bonded complex with benzene (a floppy C 6v symmetric top) the HX becomes orientable via the electric hexapole focusing method. Oriented-molecule beams of C 6H 6-HCl and C 6H 6-HBr have been prepared and studied. From the threshold focusing voltage their electric dipole moments have been estimated: 1.6 ± 0.1 and 1.2 ± 0.1 D for C 6H 6-HCl and -HBr, respectively. These results confirm the substantial dipole moment enhancement of HX upon hydrogen bonding, measured accurately for the C 6H 6-HF complex by Klemperer and co-workers (1983).

The electronic structure of LaF: A multiconfiguration ligand field calculation

A zero-free-parameter ligand field model is used to account for the observed low-lying (E<10 000 cm−1) electronic states of LaF. The electronic structure of LaF is represented as the effect of a nonpolarizable point F− ligand on the two valence electrons of a free La+ atomic ion. Free-ion configuration interaction (CI) effects (represented by a parametric fit of the free La+ energy levels to Fk,Gk Slater–Condon electrostatic and ζ spin–orbit parameters) and ligand-driven CI (treated by Bk0 ligand field radial integrals evaluated using Hartree–Fock La+ orbitals) are included in the model. A series of calculations is described, from the simplest ‘‘primitive LFT model’’ which includes the three lowest-lying and most important configurations (6s2, 5d6s, 5d2) to the most elaborate ‘‘balanced s-polarized LFT model’’ which includes six configurations (6s2, 5d6s, 5d2, 6s6p, 5d6p, and 6p2) in order to account properly for s–p polarization effects. The inclusion of ligand-driven CI (metal-centered orbital polarization) effects shows that simple electrostatic atomic-ion-in-molecule models are capable of accounting for the electronic structure of many highly polar diatomic molecules having more than a single metal-centered valence electron.

A quasiharmonic model calculation for two non-Markovian far-infrared spectra of diatomic polar molecules in a rare-gas liquid. II. Correlation functions of the interaction and spectral application to a HCl–Ar solution

In paper I, the theoretical TTOC and PTOC far-infrared absorption coefficients of diatomic polar molecules in rare-gas liquid have been expressed in terms of correlation functions of the Hamiltonian which describes the diatomic–medium interaction. In this paper these correlation functions are derived using a harmonic model for the liquid structure which takes into account some peculiarities such as dynamical behavior and geometrical arrangement of atoms in the liquid. The correlation functions and the different spectral contributions which appear in the spectral theory of paper I are numerically analyzed for a HCl–Ar solution. Comparison between theoretical and experimental spectra shows that the best agreement is obtained with the PTOC approach.

A quasiharmonic model calculation for two non-Markovian far-infrared spectra of diatomic polar molecules in a rare-gas liquid. I. Spectral theory

A model consisting in a quantum rigid rotor interacting with a quasiharmonic medium is presented to obtain the far-infrared spectra of diatomic polar molecules dissolved in rare-gas liquids. The absorption coefficient is calculated within the framework of two non-Markovian approaches called TTOC (total time ordered cumulants) and PTOC (partial time ordered cumulants). Both non-Markovian spectra contain the same correlation functions of the diatomic–medium interaction, which is expressed in terms of a Legendre polynomial series truncated up to second order. These correlation functions are calculated in paper II. The absorption cofficients appear as a sum of two spectral contributions. The first one, called secular contribution, is given as a juxtaposition of basic resonances, different in each of two non-Markovian approaches, associated to the absorption lines j→j+1. The second one, called interference contribution, takes into account the existence of a overlapping line effect. The contributions to the absorption coefficients from the P1 and P2 Legendre polynomials of the interaction are explicitly reported.