Ground State Dissociation Energy Research Articles

On the ground and electronically excited states of Na3O: Theory and experiment

Na 3 O has been generated by reacting preformed sodium clusters with O2 and N2O in a crossed beam pickup arrangement. This “superalkali” species was probed by photodepletion spectroscopy coupled with one-photon ionization mass spectroscopy to yield: (i) a first measure of visible/NIR region photodissociation cross sections showing several broad absorption features, (ii) a rough determination of the ground state dissociation energy (1.48±0.04 eV), as well as (iii) a remeasurement of the ionization potential (3.69±0.15 eV). The experimental investigations were supplemented by quantum chemical ab initio calculations employing coupled-cluster methods for ground and excited states of Na3O. Experiment and theory are in good agreement, allowing a tentative assignment of the Na3O depletion spectrum while providing further evidence for the computed D3h ground state. Observed and calculated dipole-allowed electronic transitions are discussed in terms of the unusual electronic structure of this nominally one excess-electron species.

Ab initio study of the O2(X 3Σg−)+Ar(1S) van der Waals interaction

A potential energy surface for the Ar(1S)+O2(X 3Σg−) interaction is calculated using the supermolecular unrestricted Mo/ller–Plesset (UMP) perturbation theory and analyzed via the perturbation theory of intermolecular forces. The global minimum occurs for the T-shaped geometry, around 6.7 a0. Our UMP4 estimate of the well depth of the global minimum is De=117 cm−1 and the related ground state dissociation energy obtained by diffusion Monte Carlo calculations is 88 cm−1. These values are expected to be accurate to within a few percent. The potential energy surface also reveals a local minimum for the collinear geometry at ca∼7.6 a0. The well depth for the secondary minimum at the UMP4 level is estimated at De=104 cm−1. The minima are separated by a barrier of 23 cm−1. The global minimum is determined by the minimum in the exchange repulsion in the direction perpendicular to the O–O bond. The secondary, linear minimum is enhanced by a slight flattening of the electron density near the ends of the interoxygen axis.

Fluorescence excitation and depletion spectroscopy of the BAr complex: Electronic states correlating with the excited valence B(2s2p2 2D) asymptote

Laser fluorescence excitation and depletion spectra of the BAr van der Waals complex, in the spectral region around the B atomic 2s2p2 2D←2s22p 2P transition at 208.9 nm, are reported. The fluorescence excitation spectrum displays a series of structured bands, whose excited levels are weakly bound (⩽60 cm−1), and an unstructured continuum to the blue. The onset of the latter allows determination of the ground state dissociation energy D0″=102.4±0.3 cm−1. The fluorescence depletion spectrum reveals, in addition, bands at lower wave numbers to a strongly bound, predissociating state, assigned as C2Δ. The bands observed by fluorescence excitation are assigned to the D2Π, E2Σ+–X2Π transitions. The derived spectroscopic data are used to construct potential energy curves for the C2Δ and D2Π states. The predissociation of the C2Δ state is due to spin–orbit coupling to a repulsive 4Π state correlating with the B(2s2p2 4P)+Ar asymptote, as shown in the accompanying paper [K. Sohlberg and D. R. Yarkony, J. Chem. Phys. 106, 6607 (1997)]. A weak nonradiative decay process is also observed for the D2Π state.

Photoionization spectroscopy of ionic metal dimers: LiCu and LiAg

Electronic spectra are reported for the heteronuclear metal dimers LiCu and LiAg, with resonant one-color two-photon ionization (R2PI). The dimers are produced in a pulsed supersonic molecular beam by laser vaporization of either a copper or silver rod coated with a thin film of vacuum deposited lithium metal. A total of twelve excited electronic states for LiCu and seven for LiAg are observed. Analysis of the vibrational progressions yields ground and excited state vibrational frequencies and dissociation energies for both LiCu and LiAg. In addition, selected vibronic bands are rotationally resolved. This data, together with that obtained by Morse and co-workers for LiCu [J. Chem. Phys. (to be published)], gives bond lengths for LiCu and LiAg (r0″=2.26 and 2.41 Å, respectively). The bond lengths for LiCu and LiAg are significantly shorter than expected by comparison to the homonuclear diatomics Li2 and Cu2 or Ag2. Dissociation energies in the heteronuclear dimers are also much greater than the mean of the corresponding homonuclear dimer values. These trends indicate that ionic character plays a leading role in the ground-state bonding.

Ab initio calculations of the ground and excited states of I 2− and ICl −

We performed all-electron ab initio calculations of the first six states of I 2 − and ICl − using a multi-reference configuration interaction method. Spin-orbit coupling is included via an empirical one-electron operator and has a large effect on the dissociation energy. The ground state dissociation energies were in error by 20–30%, probably due to deficiencies in the one electron basis sets. The electronic wavefunctions at the equilibrium geometry were used to calculate the electronic absorption spectrum from the ground state, and good agreement was found with the experimental data.

Direct measurement of the ground-state dissociation energy of Na2.

We report a direct measurement of the Na{sub 2} {ital X}{sup 1}{Sigma}{sub {ital g}}{sup +} ground-state dissociation energy. Three spectroscopic measurements are combined to directly yield the energy of the lowest ({ital v}=0) vibrational level relative to the free-atom limit. The {ital X}{sup 1}{Sigma}{sub {ital g}}{sup +}({ital v}=0{r_arrow}31) splitting is measured by double-resonance spectroscopy; {ital X}{sup 1}{Sigma}{sup +}{sub {ital g}}({ital v}=31){r_arrow}{ital A}{sup 1}{Sigma}{sup +}{sub {ital u}}({ital v}{prime}=165) by laser-induced fluorescence; and free atoms{r_arrow}{ital A}{sup 1}{Sigma}{sub {ital u}}{sup +}({ital v}{sup {prime}}=165) by photoassociation of ultracold atoms. The result is {ital D}{sub 0}=5942.6880(49) cm{sup {minus}1}. The uncertainty is a factor of 6 improvement over a previous (indirect) determination. {copyright} {ital 1996 The American Physical Society.}

Photodissociation spectroscopy of Ca+–rare gas complexes

Weakly bound complexes of the form Ca+–RG (RG=Ar, Kr, Xe) are prepared in a pulsed nozzle/laser vaporization cluster source and studied with mass-selected resonance enhanced photodissociation spectroscopy. The Ca+ (2P←2S) atomic resonance line is the chromophore giving rise to the molecular spectra in these complexes. Vibrationally resolved spectra are measured for these complexes in the corresponding 2Π←X 2Σ+ molecular electronic transition. These spectra are red shifted from the atomic resonance line, indicating that each complex is more strongly bound in its excited 2Π state than it is in the ground state. Vibronic progressions allow determination of the excited state vibrational constants: Ca+–Ar, ωe′=165 cm−1; Ca+–Kr, ωe′=149 cm−1; Ca+–Xe, ωe′=142 cm−1. Extrapolation of the excited state vibrational progressions, and combination with the known atomic asymptotes and spectral shifts, leads to determination of the ground state dissociation energies Ca+–Ar, D0″=700±100 cm−1 (0.09 eV); Ca+–Kr, D0″=1400±150 cm−1 (0.17 eV); Ca+–Xe, D0″=2300±150 cm−1 (0.29 eV). The spin–orbit splitting in the 2Π1/2,3/2 state for these complexes is larger than expected by comparison to the Ca+ atomic value.

Accessing a low-lying bound electronic state of the alkali oxides, LiO and NaO, using laser induced fluorescence

Abstract The first laser based probe for the sodium and lithium monoxides is established. The Li(Na)+N2O reactions studied in a multiple collision entrainment mode produce the LiO and NaO ground X2Π and low-lying monoxide excited states. In contrast to the alkali halides, laser induced excitation spectroscopy confirms that the LiO and NaO B 2Π states, counter to recent predictions, are located at energies well below the ground state dissociation asymptote and, as predicted, possess significant binding energies. An assignment of the laser induced excitation spectra (LIF) for the B 2Π-X 2Π transitions of LiO in the region 3940–4300 A is based on a direct correlation with the observed chemiluminescence (CL) from the lowest level of the LiO B 2Π state ( ∼4000–7000 A ) and high quality ab initio calculations for the ground state. The self-consistent assignment of the observed LIF and CL spectra makes use of the complimentary extended progressions in the X 2Π (CL) and B 2Π (LIF) vibrational level structure which results from the significant shift of the B 2Π excited state potential relative to that of the ground state. The experimental data are consistent with an excited state vibrational frequency separation of order 130 cm−1, and T e ( B 2 Π) ≈ 26078 ± 800 cm −1 . The latter value, in correlation with the ground state dissociation energy of LiO, suggests a B 2Π excited state dissociation energy well in excess of 2000 cm−1. The radiative lifetimes of the lowest levels of the LiO B 2Π state, isoergic with the highest levels of the LiO ground state, are determined to be in excess of 600 ns. The corresponding NaO excitation spectra in the range 6680–7250 A also correlate well with ab initio calculations for the ground electronic state of NaO. Within this study, we provide optical signatures which one might consider to monitor LiO or NaO in process streams. In correlation with the observed chemiluminescence from B 2Π states of the higher alkali oxides KO, RbO, and CsO, this study suggests regions where one might probe their excitation spectra.

An improved potential energy surface for the degenerate rearrangement of (HF) 2

We report a new potential energy surface for the (HF) 2 van der Waals molecule and quantum mechanical nine-dimensional eigenvalue calculations of vibrational energy levels on this surface. The new surface has been adjusted to give an accurate tunneling splitting but is otherwise based on the SQSBDE potential energy surface of Quack and Suhm. We obtain a ground-state dissociation energy of 1076.1 cm −1 and a ground-state tunneling splitting of 0.65 cm −1, in good agreement with the experimental values of 1062 cm −1 and 0.659 cm −1, respectively.

Dissociation energies of PH and PH +

Dissociation energies for the ground electronic states of diatomic PH and PH + are determined by fitting empirical potential functions to the respective RKRV curves using correlation coefficients. The estimated ground state dissociation energies of PH and PH + are 3.10 and 3.20 eV respectively by the curve fitting procedure using Lippincott potential function. The computed values are in good agreement with experimental values.

Photoionization spectroscopy of the In–N2 van der Waals complex

A vibrationally resolved electronic spectrum is observed for the metal atom van der Waals complex In–N2. Two electronic band systems are detected with mass resolved two-color photoionization spectroscopy. A lower energy system is observed slightly to the blue of the In ( 2D←P) atomic asymptote. It is characterized by a progression in the In–N2 stretching mode with a frequency of ω′e=76.7 cm−1. The higher energy system is slightly to the blue of the In (4P←2P) asymptote. It also exhibits a progression in the In–N2 stretch with a frequency of ω′e=87.7 cm−1. Extrapolation of the vibrational progressions leads to determination of the excited state dissociation energies. Energetic cycles based on the electronic transition energies, excited state dissociation energies, and atomic asymptotes lead to a determination of the ground state dissociation energy of D″0=1519 cm−1 (0.188 eV). A single-photon photoionization experiment determines the ionization potential to be 43 372 cm−1 (5.377 eV). This IP value, together with the atomic IP and the ground state neutral dissociation energy, yields a dissociation energy of D″0=4817 cm−1 (0.597 eV) for the In+–N2 ion–molecule complex.

Multiphoton studies of jet-cooled Xe2 near the Xe*5d[5/2]3 state: Characterization of the ground and excited state potential curves

The two-photon resonant, three-photon (2+1) ionization spectra of jet-cooled mXenXe, at energies near the Xe* 5d[5/2]03 state, are reported. A new progression has been observed and is attributed to transitions from the van der Waals ground state, X 1Σ+g(0+g), through bound vibrational levels of an excited state of gerade symmetry. The analysis of some 26 closely spaced vibronic bands and isotope effects provides information on the excited and ground state potential energy curves. The vibrational quantum number of the lowest frequency band near 82 539.1 cm−1 is assigned to v′=6±1. For v′=6 this leads to molecular constants Te′ ≂ 82 514.9 cm−1, ωe′ ≂ 5.7955 cm−1, and ωexe′ ≂ 0.07491 cm−1. The upper state can be described by a Morse potential with dissociation energy De′ ≂ 112.10 ± 0.05 cm−1 and internuclear separation Re′ ≂ 5.51 ± 0.03 Å. This is consistent with assignment to a Rydberg molecular state of either the B 2Π1/2g or D 2Σ+1/2g ion core. At the Xe 1S0+Xe* 5d[5/2]03 threshold the molecular spectrum terminates and continuum absorption is evidenced by a rise and fall in the fragment ion yield. The direct determination of the dissociation limit for the excited state is used to derive the ground state dissociation energy De″≂ 196.32 ± 0.05 cm−1.

A laser-induced fluorescence study of the BiF A O+→X1 O+ system in the region 6550 to 7400 Å: Rotational analysis and evidence for an A O+ state potential maximum

Eighteen previously unobserved bands belonging to the BiF A O+→X1 O+ system have been identified in the region between 6550 and 7400 Å. Nine bands representing v′=27–29 and v″=40–45 have been rotationally analyzed, and the v′=27 and v′=29 states exhibit strong homogeneous perturbations. These upper state levels are also observed to predissociate, providing evidence for an A O+ state potential maximum which has been theoretically predicted. A limiting curve of dissociation suggests a rotationless potential maximum at 32 674±822 cm−1. This energy must correspond to a potential maximum if it is to be reconciled with recent thermochemical measurements of the ground state dissociation energy, D00=3.76±0.13 eV. The new spectroscopic constants have been included in a calculation of Rydberg–Klein–Rees potential curves for both states. Franck–Condon factors calculated from these potentials are qualitatively consistent with the observed intensity distribution. Measured collision free radiative lifetimes are consistent with known A O+ state lifetimes for nonpredissociated states and are found to be shorter for predissociated states.

Hydrogen bonding between the water molecule and the hydroxyl radical (H2O⋅HO): The global minimum

Ab initio quantum mechanical self-consistent-field (SCF) and single and double excitation configuration interaction (CISD) methods have been used in a new study of hydrogen bonding between the water molecule and the hydroxyl radical. The basis sets used are double-zeta plus polarization (DZP) and triple-zeta plus double polarization (TZ2P). The two energetically low-lying minima are 2A′ and 2A″ states, with hydrogen bonding occurring between the oxygen atom in the water molecule and the hydrogen atom in the hydroxyl radical; the 2A′ state has a slightly (∼0.3 kcal/mol) lower energy. The comparable planar C2v symmetry structure lies <0.1 kcal/mol higher in energy. The hydrogen bond distance H...O of the 2A′ is ∼1.94 Å, which is close to that of the water dimer. The ground state dissociation energy is 5.6 kcal/mol, which is larger than that of the water dimer. The predicted infrared spectra for these structures are also reported.

Ab initio theoretical study of the electronic structure, stability and bonding of dialkali halide cations

The ground state electronic structures and dissociation energies of the dialkali halide cations M1M2M+ (M1, M2 = Li, Na; X = F, Cl) have been calculated at the HF and post-HF (MPn) levels using a variety of basis sets. The calculated values are in good agreement with the corresponding experimental data. All the species are predicted to be linear with only one minimum in the corresponding potential energy surface. In the formation of mixed halides, the reaction between M+1 and M2X, where M1 is lighter than M2, is more exothermic than the alternative reaction. A bonding analysis has been made on the basis of Mulliken and natural atomic charges and bond orders.

Valence bond model analysis of alkali and halogen diatomics on the basis of exchange perturbation theory. Part 2. Heteronuclear compounds

Exchange perturbation theory is applied, in the framework of the valence bond method, to study chemical bonding in heteronuclear alkali and halogen diatomics. It is shown that, using a basis set of atomic functions limited to a very few spherical or polarized Gaussian-type orbitals, whose characteristic parameters are determined a priori from those of the corresponding homonuclear diatomics given in a previous publication [1], it is possible to reproduce experimental data on the equilibrium distances, ground state dissociation energies, and vibrational constants with an accuracy comparable to that obtained on the basis of extensive ab initio calculations.

Valence bond model analysis of alkali metal hydrides and hydrogen halides on the basis of exchange perturbation theory

Exchange perturbation theory is applied, in the framework of the valence bond method, to study chemical bonding in the series of alkali metal hydrides and hydrogen halides. It is shown that, using a basis set of atomic functions limited to a very few spherical or polarized Gaussian-type orbitals, with optimal parameters determined a priori under the constancy of first-order “β R” products given in a previous paper (G. Dotelli, E. Lombardi and L. Jansen, J. Mol. Struct. (Theochem), 276 (1992) 159, it is possible to reproduce experimental data on the equilibrium distances, ground state dissociation energies and vibrational constants, with an accuracy comparable to that obtained by extensive ab initio calculations.

Valence bond model analysis of homonuclear alkali and halogen diatomics on the basis of exchange perturbation theory

First- and second-order exchange perturbation theory was applied, within the framework of the valence-bond method, to the study of chemical bonding in homonuclear alkali and halogen diatomics. The ultimate aim of the analysis was to derive accurate, yet simple, methods applicable to conformational and reactivity problems in biomolecules. Using a basis set of atomic functions limited to a very few, spherical or polarized, gaussian-type orbitals and two variable parameters with a clear physical interpretation, it was shown that it is possible to reproduce, through a scanning procedure, experimental data on the equilibrium distances, ground-state dissociation energies, and vibrational constants with an accuracy comparable to that obtained with extensive ab initio calculations.

Electronic spectroscopy of silver dimer rare gas complexes

Vibrationally resolved electronic spectra are reported for the metal dimer-rare gas complexes Ag2–Ar and Ag2–Kr. These spectra are obtained using resonant two-photon photoionization in the energy region near the Ag2 B←X electronic transition (280–285 nm). Both complexes exhibit extensive activity in three vibrational modes, making it possible to determine vibrational constants, anharmonicities, and cross-mode couplings. An unusual cancellation of factors results in the Kr complex (ω′e =72.6 cm−1) having nearly the same metal-rare gas stretching frequency as the Ar complex (ωe=73.9 cm−1). Progressions extending over a significant range of the excited state potential surfaces make it possible to derive the excited state dissociation energies (D′0=755 and 1205 cm−1 for Ar and Kr, respectively). Combination with the red-shifted electronic state origins yields the corresponding ground state dissociation energies (D■0=275 and 394 cm−1 for Ar and Kr, respectively). Potential energy surfaces are investigated for excited and ground states of both complexes.

Spectroscopy and Structure of the Alkali Hydride Diatomic Molecules and their Ions

All significant experimental measurements and theoretical calculations of the spectroscopy and structure of the alkali hydrides NaH, KH, RbH, and CsH, and the corresponding alkali deuterides, are identified and reviewed. Published molecular constant determinations from conventional and laser spectroscopy are evaluated; recommended spectroscopic constants for X 1Σ+ and A 1Σ+ states are tabulated. RKR and hybrid potential energy curves are evaluated; recommended RKR curves for X 1Σ+ and A 1Σ+ states are tabulated. Ground state dissociation energy (De) estimates are evaluated; recommended X 1Σ+ and A 1Σ+ state De and D0 values are tabulated. Accurate electronic structure calculations (Hartree–Fock or better) are listed and described briefly; all excited electron states considered are included. Experimental and theoretical radiative and dipole properties are noted and discussed. Calculations on the positive and negative ions of the four diatomic alkali hydrides are also listed and described briefly.