Atomic Asymptote Research Articles

Zinc oxide and its anion: A negative ion photoelectron spectroscopic study

We have recorded, assigned, and analyzed the photoelectron spectrum of ZnO−. The adiabatic electron affinity (E.A.a) of ZnO and the vibrational frequencies of both ZnO and ZnO− were determined directly from the spectrum, with a Franck–Condon analysis of its vibrational profile providing additional refinements to these parameters along with structural information. As a result, we found that E.A.a(ZnO)=2.088±0.010 eV, ωe(ZnO)=805±40 cm−1, ωe(ZnO−)=625±40 cm−1, and that re(ZnO−)>re(ZnO) by 0.07 Å. Since our measured value of E.A.a(ZnO) is 0.63 eV larger than the literature value of E.A.(O), it was also evident, through a thermochemical cycle, that D0(ZnO−)>D0(ZnO) by 0.63 eV. This, together with the literature value of D0(ZnO), gives a value for D0(ZnO−) of 2.24 eV. Since the extra electron in ZnO− is expected to occupy an antibonding orbital, the combination of D0(ZnO−)>D0(ZnO), ωe(ZnO−)<ωe(ZnO), and re(ZnO−)>re(ZnO) was initially puzzling. An explanation was provided by the calculations of Bauschlicher and Partridge, which are presented in the accompanying paper. Their work showed that our experimental findings can be understood in terms of the a 3Π state of ZnO dissociating to its ground-state atoms, while the X 1Σ+ state of ZnO formally dissociates to a higher energy atomic asymptote.

A pair potentials study of matrix-isolated atomic zinc. II. Intersystem crossing in rare-gas clusters and matrices

The mechanism of 4p 1P1→4p 3PJ intersystem crossing (ISC) following excitation of the 4p 1P1 level of matrix-isolated atomic zinc is investigated using a pair potentials approach. This is achieved by extending earlier ISC calculations on the Zn⋅RG2 and Zn⋅RG3 complexes to the square planar Zn⋅RG4 and square pyramidal Zn⋅RG5 species which are the building blocks of the Zn⋅RG18 cluster used to represent the isolation of atomic zinc in the substitutional site of a solid rare-gas host. ISC predictions in these clusters are based on whether crossing of the strongly bound 1A1 states, having a 4p 1P1 atomic asymptote, occurs with the repulsive 3E states correlating with the 4p 3PJ atomic level of atomic zinc. Predictions based on 1A1/3E curve crossings for 3E states generated with the calculated ab initio points for the Zn⋅RG 3Σ(pz) states do not agree with matrix observations. Based on similar overestimation of ISC in the Zn⋅RG diatomics, less repulsive Zn⋅RG 3Σ(pz) potential curves are used resulting in excellent agreement between theory and observations in the Zn–RG matrix systems. 1A1/3E curve crossings do not occur in the Zn–Ar system which shows only singlet emission. Curve crossings are found for the Zn–Xe system which exhibits only triplet emission. The Zn–Kr system does not show a crossing of the body mode Q2, which exhibits a strong singlet emission at 258 nm while the waist mode Q3, does have a crossing, resulting in a weak singlet emission at 239 nm and a stronger triplet emission at 312 nm. The efficiency of ISC is determined from Landau–Zener estimates of the surface hopping probabilities between the 1A1 and the 3E states. Differences in the application of this theory in the gas and solid phase are highlighted, indicating that the rapid dissipation of the excited-state energy which occurs in the solid must be included to obtain agreement with observations.

Experimental and theoretical study of the AlNe complex

The laser fluorescence excitation spectrum of the AlNe complex, in the vicinity of the Al atomic 3d←3p and 5s←3p atomic transitions, is reported. Transitions out of the v=0 vibrational levels of both lower-state spin-orbit levels, X1 2Π1/2 and X2 2Π3/2, to vibrational levels of the C 2Δ, D 2Π, and H 2Σ+ AlNe electronic states were observed. From observations of the onset of excitation to the Al(3d)+Ne dissociation continuum, dissociation energies for the various AlNe electronic states were determined. Ab initio calculations of AlNe electronic states correlating with the ground Al(3p)+Ne atomic asymptote were also carried out. The X1 2Π1/2 and X2 2Π3/2 binding energies computed using the calculated AlNe(X 2Π, A 2Σ+) potential energy curves were in reasonable agreement with the experimental determinations. The experimentally determined dissociation energy for the X2 2Π3/2 level is significantly larger than that of the ground X1 2Π1/2 level (D0=32.3±0.3 and 14.1±0.3 cm−1, respectively).

Photoionization spectroscopy of AuAr

Resonant photoionization spectroscopy probes the new metal van der Waals complex AuAr. Band systems observed near 270 nm and 245 nm correlate to the spin-orbit components ( 2 P 1 2 → 2 S and 2 P 3 2 → 2 S ) of the Au (6p ← 6s) atomic transition. Extrapolated progressions yield the excited state convergence energy, which is combined with the atomic asymptote to yield the ground state bond energy. D O ″ = 130 ± 15 cm −1. Lower limits for the excited state binding energies are D O ′( 2 II 1 2 )⩾338 cm −1 and D O ′( 2, II 3 2 )⩾654 cm −1 . The ionization potential is ⩽9.208 eV. AuAr is compared to the CuRG and AgRG complexes studied previously.

Laser fluorescence excitation spectroscopy of BNe electronic states correlating with the excited valence B(2s2p2 2D) atomic asymptote

The laser fluorescence excitation spectrum of the BNe van der Waals complex, in the vicinity of the B atom 2s2p2 2D←2s22p 2P transition at 208.9 nm, is reported. A total of six partially resolved molecular bands, as well as a broad, unstructured feature to the blue of these bands, have been observed. Three BNe electronic states, denoted as C 2Δ, D 2Π, and E 2Σ+, correlate with the B(2s2p2 2D)+Ne atomic asymptote, and the observed bands are assigned as (v′,0) progressions of the C 2Δ−X 2Π1/2 and D 2Π–X 2Π1/2 band systems. Rotational analysis of the C–X bands has been carried out, and spectroscopic constants characterizing the upper and lower states determined. The onset of the continuous excitation is assigned as the energy to reach the B(2s2p2 2D)+Ne atomic asymptote. Identification of this threshold has allowed the determination of dissociation energies of the X, C, and D states. The observation of banded features in this wavelength range contrasts sharply with the continuous free←bound excitation in the B 2Σ+–X 2Π1/2 transition, because of the purely repulsive B(2s23s 2S)–Ne interaction [X. Yang, E. Hwang, P. J. Dagdigian, M. Yang, and M. H. Alexander, J. Chem. Phys. 103, 2779 (1995)]. The differences in the binding energies of the BNe electronic states are discussed in terms of their expected electronic structures.

A comparison of density functional theory withab initio approaches for systems involving first transition row metals

Density functional theory (DFT), using the B3LYP hybrid functional, is found to give a better description of the geometries and vibrational frequencies of FeL and FeL+ systems that second-order Moller Plesset perturbation theory (MP2). Namely, DFT correctly predicts the shift in the CO vibrational frequency between free CO and the5Σ− state of FeCO and yields a good result for the Fe-C distance in the quartet states of FeCH 4 + . These are properties where the MP2 results are unsatisfactory. Thus DFT appears to be an excellent approach for optimizing the geometries and computing the zero-point energies of systems containing first transition row atoms. Because the DFT approach is biased in favor of the 3d 7 occupation, whereas the more traditional approaches are biased in favor of the 3d 6 occupation, differences are found in the relative ordering of states. It is shown that if the dissociation energy is computed relative to the most appropriate atomic asymptote and corrected to the ground state asymptote using the experimental separations, the DFT results are in good agreement with high levels of theory. The energetics at the DFT level are much superior to MP2 and in most cases they are in good agreement with high levels of theory.

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.

Laser induced fluorescence spectroscopy of the B′ 1u–X 0+g transition of Xe2: Determination of the B′ state potential and evidence for a barrier to dissociation

We report the observation of discrete and continuous laser induced fluorescence (LIF) spectra of the B′ 1u–X 0+g transition of Xe2, near 68 000 cm−1. The discrete features continue 5 cm−1 above the predicted atomic asymptote, which indicates the presence of a barrier to dissociation in the excited state. The dissociation energy (De′=48±12 cm−1), and excited state constants (re′=5.46±0.05 Å, ωe′=5.9±0.7 cm−1, and ωexe′=0.17±0.02 cm−1) for the B′ state were obtained from a Franck–Condon fit to the spectrum. The resulting potential is more shallow and has a longer equilibrium bond length compared with a previous experimentally derived potential. The barrier to dissociation (2 cm−1≤h≤10 cm−1, r≊10 Å) is attributed to the presence of a long-range (∝1/r3) repulsion, arising from a dipole–dipole resonant interaction.

Double resonance spectroscopy of NaI and KI: the van der Waals states Ω=1 and Ω=2 of the first atomic asymptote

The energy region of the lowest atomic asymptote of NaI and KI was investigated in a high-resolution Λ-type optical double resonance experiment with the highly excited covalent C0 + state as resonant intermediate. For the first time two van der Waals states with electronic angular momenta Ω=1 and Ω=2 were observed correlating to the ground states of sodium and potassium with iodine. The electronic system C0 +−Ω=2 shows a significant transition moment because of the heterogeneous coupling with Ω=1 at large rotational quantum numbers. For the new states the Dunham parameters and the RKR potential curves were determined. In addition, for the Ω=1 states modified Hartree—Fock dispersion potentials, were calculated by a least-squares fit of the observed energy levels and the dissociation energy of both molecules could be derived with an accuracy of a few wavenumbers.

Bound–free B2Σ+ –X2Π, A2Σ+ emission in the BAr van der Waals complex

Spectrally resolved bound–free fluorescence emission spectra for the excitation of several vibrational levels in the excited B2Σ+ electronic state of the van der Waals molecule 11BAr are presented. This excited state emits to the ground X2Π and low-lying A2Σ+ states, both of which correlate with the ground state atomic asymptote B(2p2P) + Ar. Because of the large differences in equilibrium internuclear separations, the emission occurs mainly to the repulsive walls. In order to gain more information on this portion of the potential energy curves, the experimental emission spectra were compared with simulated spectra derived from ab initio calculated B–Ar interaction potentials. The simulated spectra reproduce the experimental spectra well if the lower-state potential energy curves are shifted slightly inward. This discrepancy is consistent with our previous observation that the ab initio calculations slightly overestimate the vibrationally averaged internuclear separation, which we determined experimentally. This reflects the difficulty of accurately calculating weak van der Waals interaction energies.

High-Resolution Spectroscopy of Kl Around the Atomic Asymptote K(4 p2P3/2) + I(5 p5 2P3/2)

A bound slate CO + of KI is found, which correlates to the atomic asymptote K(4 p 2 P 3/2) + I(5 p 5 2 P 3/2) and has an internuclear distance of 3.7356( 88) Å and a dissociation energy of 893.1(50) cm −1.This is the first clearly characterized excited bound state in KI and will serve as an intermediate state for multiphoton experiments.

Two-photon spectroscopy and predissociation of TlCl to the asymptote Tl 7 s

TlCl was investigated by two-photon spectroscopy with two tunable pulsed dye lasers between 55 000 and 62 000 cm −1. Potential curves of three electronic states, labeled with E 1, E 2 with Ω = 0 + , and F 1 with Ω = 1, could be determined. Predissociation of the E 1 state by the E 2 state to the atomic asymptote Tl 7s 2S 1 2 with linewidths up to 55 cm −1 was observed. Agreement between observations and theory is good for vibrational quantum numbers v ≤ 9. From the fluorescence of the perturbed E 2 state, two repulsive potential curves in the energy region of the A0 + state were determined.

A theoretical study of the low-lying states of Ti2 and Zr2

The 1Σ+g, 3Δg, 3Σ+u, and 7Σ+u states of Ti2 and Zr2 have been studied using a multireference configuration-interaction (MRCI) approach. Although our best calculation produces a 1Σ+g ground state for Zr2, the 3Σ+u and 3Δg states are found to be very low lying. Additional support for a 1Σ+g ground state assignment comes from the fact that the resulting vertical excitation spectrum is consistent with the optical spectrum of Zr2 observed in noble gas matrices. For Ti2, it proved more difficult to make a definitive assignment of the ground state, because of the many low-lying states and the large effect of inner-shell (3s and 3p) correlation. With only valence correlation included, the ground state is predicted to be 7Σ+u at both the MRCI and averaged coupled-pair functional (ACPF) levels of correlation treatment. However, inner-shell correlation effects, estimated based on modified coupled-pair functional (MCPF) and contracted configuration-interaction (CCI) calculations, preferentially lower the 1Σ+g and 3Δg states, resulting in a 3Δg ground state and a very low-lying 1Σ+g state. Since the 3Δg state lies below the 1Σ+g state at all levels of correlation treatment and both states are derived from the same atomic asymptote, we prefer a 3Δg assignment for the ground state of Ti2. This is consistent with the failure to observe an electron-spin resonance (ESR) signal, but such an assignment requires an explanation for the absence of subcomponents on the lines observed in the resonance Raman spectrum of Ti2.

Laser spectroscopy of the 3s 2 Sigma +p 2 Pi transition in LiNe.

Measurements of the rotationally resolved absorption spectrum of the $^{6}\mathrm{Li}^{20}$Ne and $^{7}\mathrm{Li}^{20}$Ne 3${\mathit{s}}^{2}$${\mathrm{\ensuremath{\Sigma}}}^{+}$\ensuremath{\leftarrow}2p $^{2}\mathrm{\ensuremath{\Pi}}$ transition have been made. For each isotope, 13 vibrational transitions from the lower 2 $^{2}\mathrm{\ensuremath{\Pi}}$ (v''=0,1,2,3) to the upper 3${\mathit{s}}^{2}$${\mathrm{\ensuremath{\Sigma}}}^{+}$ (v'=0,1,2,3,4,5) electronic state have been observed and rotationally analyzed. For the 2 $^{2}\mathrm{\ensuremath{\Pi}}$ state a well depth of 212(5) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and an equilibrium separation of 4.36(2)${\mathit{a}}_{0}$ have been obtained. Analysis of the 3${\mathit{s}}^{2}$${\mathrm{\ensuremath{\Sigma}}}^{+}$ electronic state using an inverted perturbation analysis has revealed that the potential for this state displays, at large internuclear separations, a barrier greater than 60.6 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ above the atomic Li 3s asymptote. The well region has a minimum of 570(7) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ located at 4.11(1)${\mathit{a}}_{0}$. An anomalously large fine-structure constant of A=2.77(3) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ was determined for the 2${\mathit{p}}^{2}$\ensuremath{\Pi} (v''=0) level.

Heteronuclear transition metal diatomics: The bonding and electronic structure of ScNi, YNi, ScPd, and YPd

High quality ab initio calculations show that ScNi, YNi, ScPd, and YPd all have 2Σ + ground states in agreement with electron spin resonance experiments. For ScNi and YNi, this is expected based on the lowest atomic asymptote. For ScPd and YPd, the lowest atomic asymptote would give the order of stability 2Δ > 2Π≈ 2Σ +, but the calculations show that mixing in of the excited asymptotes preferentially lowers the 2Σ + state. The calculations show that the quartet states are about 20–30 kcal/mol above the ground state, and therefore probably do not contribute significantly to the unexpected g ⊥ values found in experiment. Calculations of excited states for YPd reveal some strong transitions that should be amenable to spectroscopic studies.

The influence of the spin-orbit and the hyperfine interaction on the asymptotic behaviour of the A 1Σ u+ state of Na 2

The vibrational structure of the A 1Σ u + excited state of Na 2 has been investigated using a molecular beam experiment. Vibrational levels with quantum numbers up to 184 have been accessed. The highest is about 0.025 cm −1 below the dissociation limit and its outer turning point corresponds to a separation of the two atoms of about 200 Å. A more precise dissociation energy of 22978.21(10) cm −1 with respect to the minimum of the ground state is derived. The spin-orbit and the hyperfine interaction split the manifold of the atomic asymptote Na( 2S)+Na( 2P). Their influence on the asymptotic behaviour of the A 1Σ u + state of Na 2 is qualitatively discussed.

Laser spectroscopy on the A0 +-X1Σ + transition of T1I

The first excited electronic state of TII was investigated by laser excitation and laser fluorescence spectroscopy. Almost completely resolved rotational fine structure was observed with the help of a collimated molecular beam and a single-mode cw dye laser. From the derived Dunham parameters Y lk a RKR potential for the excited state is calculated. The unusual shape of the potential energy curve can be understood as originating from an avoided crossing of the ionic ground-state potential and a non-bonding covalent state of the atomic asymptote (6p) 2P 1 2 (TI)+(5p) 5 2P 3 2 (I).

Theoretical studies of the transition metal–carbonyl systems MCO and M(CO)2, M=Ti, Sc, and V

A b initio calculations on the transition metal–carbonyl systems MCO and M(CO)2, M=Ti, Sc, and V, have been carried out using large Gaussian basis sets and an extensive treatment of electron correlation. The dissociation energies (De) and geometries of these molecules are given, and the bonding mechanisms are discussed. High-spin ground states are favored for the monocarbonyl molecules, whereas for the dicarbonyl molecules there is a competition between high-, intermediate-, and low-spin states, which are found to be very close in energy. The computed De(Ti–CO) is 0.62 eV whereas for Ti(CO)2 it is 1.02 eV, relative to the ground state Ti atomic asymptote and CO(1Σ+). This suggests that the recent experiment giving a value of ≊1.75 eV for De[Ti–(CO)x] should be interpreted as giving the De for Ti(CO)x, x≥2. For the three metal atoms the binding energy per carbonyl is found to be significantly lower for the dicarbonyl than the monocarbonyl molecules. This is in contrast to the Ni(CO)x molecules, where each CO is bound with approximately the same energy.