2Δ State Research Articles

Influence of the electronic asymmetry in NH (1Δ) state Λ doublets on the photodissociation dynamics of HN3 and DN3

The influence of the electronic asymmetry in the 1Δ(A′) and 1Δ(A″) Λ doublets of NR (R=H,D) on the photodissociation dynamics of hydrazoic acid (RN3) has been investigated. Hydrazoic acid was prepared in its first excited electronic state, Ã1A″. A variety of scalar (internal state and translational energy distribution) and vectorial (angular distribution, rotational alignment, correlation between translational and rotational motion) properties of the ejected NH or ND fragment were analyzed by Λ-doublet-specific Doppler profile measurements. While the population of the 1Δ(A′) and 1Δ(A″) states are equal, the vector correlations for both Λ sublevels are different. NR(A″) products are preferentially ejected in the original plane formed by the parent, and the recoil of NR fragments in the symmetric Δ(A′) state is preferentially perpendicular to that plane. The vector correlation between the translational and the rotational motion of the fragment also indicates a strong nonplanar dissociation geometry for NR products in the Δ(A′) state. About 50% of the ND(A′) product rotation is generated by a torsional motion, while 80% of the ND(A″) fragments are formed with J being aligned perpendicular to the recoil direction (MJ=0).

Resonance-enhanced multiphoton ionisation spectroscopy of the NH (ND) radical. Part 2.—Singlet members of the 3p Rydberg complex

We report new, or improved, spectroscopic data for three singlet excited electronic states, the f1Π, g1Δ and h1Σ states of the imidogen radical (NH and ND). Each of these states is observed via the two-photon resonance enhancements they provide in the multiphoton ionisation spectrum of a 1Δ state NH (ND) radicals. All three states are assigned as members of the Rydberg 3p complex, with leading configuration … 3σ21π13pλ1. The available evidence points to the h 1Σ state having + parity, but this cannot be confirmed in the present study. The separation of the f 1Π and g 1Δ origins is comparable to the energy of one quantum of N—D stretching vibration; as a result, transitions to vibrationally excited (v′>0) levels of the f 1Π state of ND are seen to gain intensity from, and to be perturbed by, the near-resonant g 1Δ levels with quantum number v′–1.

A rotationally resolved REMPI-PES study of the NH radical

The NH radical produced selectively in its a 1Δ excited state by photodissociation of HN 3 has been investigated by (2+1) REMPI-PES in the range 246–286 nm. Vibrational and rotational structure is evident in the photoelectron spectra of all the 3p Rydberg states previously assigned, thereby permitting the determination of accurate values for the first ionization energy. New states further to the blue permit determination of the ionization energy to the next ionic state. A 2Σ −. Wavelength scans for the new g 1Δ(ν′=1)←a 1Δ(ν″=0) transition provided a more precise determination of the spectroscopic constants for the g 1Δ(ν′=1) state. The vibrational structure in the kinetic energy scans for this intermediate 3pπ Rydberg state shows a Δν=0 propensity, with the resolved rotational structure exhibiting both a Δ N=even and odd character. The f 1Π 3pσ Rydberg state also shows the expected Franck-Condon behaviour, but with a predominant Δ N=even propensity.

Laser-induced fluorescence of BaCl: the A′ 2Δ state

The 514.5 nm Ar + laser line and the 530.8 and 520.8 nm Kr + laser lines have been used to excite infrared fluorescence of the BaCl molecule. Spectra of transitions from the C 2Π excited state to both components Ω=3/2, 5/2 of the metastable A′ 2Δ electronic state were recorded by Fourier transform spectrometry. Molecular constants for this state have been determined and those of the C 2Π state are improved.

A study of the F 2Δ Rydberg state of NS by resonance-enhanced multiphoton-ionization spectroscopy

The F 2Δ state of NS has been observed as a two-photon resonance in the overall three-photon MPI spectrum recorded for this molecule in the single-photon wavelength range 337–365 nm. The state is Rydberg in character and arises from the electronic excitation·7σ 22π 43dδ 1 ←…7σ 22π 43λ 1. Analysis of the spectra has yielded vibrational and rotational constants for the F 2Δ state which are consistent with its Rydberg character.

A determination of the molecular parameters for NiH in its 2Δ ground state by laser magnetic resonance

The rotational spectrum of the NiH radical in the v = 0 level of its X 2Δ state has been studied by laser magnetic resonance (LMR) at far-infrared wavelengths. Transitions have been detected for all five isotopes of nickel and within both the lower ( 2 Δ 5 2 ) and upper ( 2 Δ 3 2 ) spin components. Nuclear hyperfine splittings for both the proton and 61Ni ( I = 3 2 ) are observed. The effective Zeeman Hamiltonian for a molecule in a 2 S+1 Δ state is developed and used to fit all the available data for NiH in the v = 0 level. This includes the magnetic resonance observations of the (1,0) vibration-rotation band and the fine structure transitions in the v = 0 level (also by LMR) and the lambda-doubling intervals for the 2 Δ 3 2 component. There are several indications that this exercise is only just possible in the case of NiH in its X 2Δ state. The results are used to determine the equilibrium bond length of NiH, r e = 0.14694493(63) nm.

The spectrum of NiF: Identification of the three lower 2Δ, 2Σ, and 2Π states

Revisiting the experimental spectra of three transitions of NiF already analyzed ( J. Mol. Spectrosc. 77, 29 (1979); J. Phys. B 14 2569 (1981)) we show that two transition 2Π 3 2 - 2Δ 5 2 (22 125 cm −1) and 2Π 3 2 - 2Σ (21 381 cm −1) share a common upper electronic level as observed by Bai and Hilborn ( Chem. Phys. Lett. 128, 133 (1986)). The third transition lying at 21 275 cm −1 is a 2Δ 3 2 - 2Π 1 2 or a 2Π 1 2 - 2Δ 3 2 transition. Our analysis shows that the Λ-doubling responsible for the fine structure arises from the lower state. A fourth transition (21 931 cm −1) is partially analyzed as a 2Π 1 2 - 2Σ transition. Three different lower electronic states ( 2Δ, 2Σ, 2Π) can be supposed to arise from the 3 d 9 lower orbital of Ni + as expected in a strongly ionic molecule. Two of them are identified ( 2Δ and 2Σ). The 2Π state can be the lower state of the 21 275 cm −1 transition or possibly an unobserved state strongly mixed with the 2 Δ 3 2 lower level in which it induces a large fine structure under the hypothesis that the 21 275 cm −1 band is that of a 2Π 1 2 - 2Δ 3 2 transition.

Semiempirical electrostatic polarization model of the ionic bonding in alkali and alkaline earth hydroxides and halides

An electrostatic polarization model is proposed that allows calculations of (i) the binding energy and dipole moment of ground state alkali (Li to Cs) and alkaline earth (Mg to Ba) hydroxides (M-OH) and halides (M-X), as well as (ii) the energy of the electronic terms A 2Π, A′ 2Δ and B 2Σ of hydroxides and halides of Ca, Sr and Ba. The model is an extension of the one proposed by Törring and co-workers for alkaline earth halides. Its predictions compare well to experimental or ab initio data when available. Notably, several quantities that are not currently available either from experiments or other calculations have been evaluated by the present model: dipole moments of ground electronic state RbOH (7.5 D), MgOH (1.2 D), SiOH (1.8 D), and BaOH (1.7 D), electronic energies of A′ 2Δ states of CaOH (18362 cm −), and SrOH (20221 cm −1), and dipole moments of electronically excited SrOH and BaOH.

Electronic energy transfer from B ′ 2Δ to B 2Σ+ in SiCl

Efficient electronic energy transfer to the B 2Σ+ state is observed by collision induced B–X emission when SiCl molecules in the B′ 2Δ state collide with rare gas atoms. The rate constants for the collisional removal from B′ 2Δ v′=0 and 1 are measured by time resolved laser-induced fluorescence and found to be large for He, Ne, and Ar ranging from 1–8×10−11 cm3/s. The fraction of the total quenching if the B′ 2Δ which results in electronic-to-electronic energy transfer from B′ 2Δ to B 2Σ+ is measured to be between 0.3 and 0.6. The nascent vibrational distribution of the product B 2Σ+ shows a surprising variation with collider.

Perturbations in the multiphoton ionization spectrum of the F 1Δ state of HCl

The 2+1 REMPI spectra of the F(0–0), F(1–0), E(0–0), and V(11–0) transitions of HCl were measured in a time-of-flight molecular beam machine. Both room temperature HCl, and rotationally and vibrationally excited HCl obtained by photodissociating vinyl chloride at 193 nm, were used. Several anomalies in the F state were observed. These include a decrease in intensity of the P, Q, and R branches between J′=3 and J′=10 for the 0–0 transition, a falloff in intensity for J′>5 for the P, Q, and R branches of the 1–0 transition, and an enhanced intensity loss of the Q(9) line of the 0–0 transition. Accurate values of the lambda-doublet splitting constants for these transitions were also obtained. The intensity loss in the 0–0 transition is due to an interaction with some unobserved bound state, which could also explain the 1–0 perturbation. A candidate for the perturbing state is the e 3Σ+(1). The isolated perturbation of the Q(9), 0–0 line could be caused by an interaction with the g(0+) state.

Electronic states and potential energy curves of ZrH + and NbH +

State-averaged complete active space MCSCF (CASSCF) followed by second-order configuration interaction (SOCI) calculations are carried out on several low-lying electronic states of ZrH + and NbH +. The ground state of ZrH + is found to be a 3Δ state ( r e = 1.825 A ̊ , ω e = 1752 cm −1) with a very low-lying excited state of 3Φ symmetry ( r e = 1.841 A ̊ , ω e = 1714 cm −1). The NbH + ion has 4Δ state as the ground state ( r e = 1.746 A ̊ , ω e = 1841 cm −1) with a nearly degenerate state of the 4Π symmetry ( r e = 1.768 A ̊ , ω e = 1826 cm −1). The dissociation energies D 0 (ZrH +) D 0 (NbH +) are calculated to be 56.8 and 54 kcal/mol, compared to experimental values of 54 ± 3 and 53 ± 3 for ZrH + and NbH +, respectively. The nature of low-lying states is analyzed.

Spectroscopic constants and potential energy curves of RhH +

Spectroscopic constants and potential energy curves of 14 electronic states of RhH + are computed. Several spectroscopic transitions are predicted in the IR-UV region none of which is observed. Our computed ground state D e of the X 2Δ state of RhH + [2.07 eV] is in good agreement with the experimental value deduced by Elkind and Armentrout [1.91 ± 0.1 eV].

The A′2Δ-X2Σ+ transition in BaH

Of the twelve possible branches in the 0-0 bands of the forbidden systems A′2Δ-X2Σ+ of BaH and BaD, only three are found to be strong [1], namely R12, Q12 and Q21. These systems become allowed by mixing of the 2Δ state with both the nearby states A2Π and B2Σ+, and interference between transition moments leads to loss of intensity in many branches and to reinforcement in the branches observed to be strong. Semiquantitative agreement with the observed intensity distribution in BaH is obtained if μ‖ is taken to be between -μ⊥ and -2μ⊥.

The X 21 → X 10 + electronic band systems of bismuth monohalides in the near infrared

Emission spectra of the transitions X 21 → X 10 + between the fine-structure components of the X 3Σ − ground states of BiF, BiCl, BiBr and BiI have been observed in the near-infrared spectral region between 1.3 and 1.7 μm. The X 21 states of the radicals were excited by near-resonant electronic-to-electronic ( E— E) energy exchange from metastable O 2 (a 1Δ g) molecules in a fast- flow system. For each molecule, a number of blue-degraded bands with sharp heads in the P branches were measured in the Δ v = 0, ± 1 and −2 sequences at spectral resolutions between 0.1 and 1 cm −1 using a Bomem Fourier-transform spectrometer. The following electronic term values and vibrational constants of the X 21 states were obtained (in cm −1): T e = 6752.7944(12), 6670.83(15), 6526.44(24), 6181.64(6), ω e = 543.0558(24), 327.44(15), 220.38(21), 168.68(6), ω eχ e = 2.3996(15), 1.004(27), 0.546(36), 0.386(6), for BiF, Bi 35Cl, Bi 79Br and BiI, respectively, where the error limits are 3σ. From a re-interpretation of near-UV spectra of BiF, the a2(a 1Δ) state of BiF is calculated to lie at 11555 cm −1.

Resonances and bound rovibrational levels in the interacting X, A, C, and D states of HeH, HeD, 3HeH, and 3HeD

Potential energy curves are calculated for the ten lowest states in HeH which correlate with the hydrogen asymptote in the n=1, 2, 3 occupation; these are X, A, C, D, 5 2Σ+, 6 2Σ+, and B, E, 3 2Π as well as the 1 2Δ states. Multireference configuration interaction calculations are employed thereby in an atomic orbital (AO) basis of contracted Gaussians. Extensive calculations of the ∂/∂R, ∂2/∂R2, Lx, and L2 matrix elements are carried out to account explicitly for the effects beyond the Born–Oppenheimer approximation. The positions of rovibrational levels are thereby determined in pairwise close-coupling calculations for the X/A and C/D states of 2Σ+ symmetry for the four isotopomers 4HeH, 3HeH, 4HeD, and 3HeD. Radial, angular, and mass polarization corrections affect the A and C states differently, so that the A–C energy gap increases by 39 cm−1 in 3HeD and by 53 cm−1 in HeH upon introduction of these terms, e.g., whereby the contribution of the mass polarization is by far the smallest. By employing a two-parameter correction function to the calculated electronic potential energy and making use of the calculated non-Born–Oppenheimer terms, a large number of levels for the A, C, and D states as a function of (v,J) quantum numbers are computed which agree with those, which are experimentally available for the C–A and D–A transitions within wave number accuracy.

Measurements of the radiative lifetimes of MgO(B 1Σ +, d 3Δ, D 1Δ) states

MgO molecules have been produced in X 1Σ +, a 3Π, and a 1Π dark states by the Mg ( 1S 0) + N 2O (X 1Σ +) reaction in an experiment performed with pulsed, crossed, supersonic molecular beams. Analysis of fluorescence decays following pulsed dye-laser excitation of the MgO reaction product has resulted in radiative lifetime determinations of the B 1Σ + v=0–3, d 3Δ and D 1Δ states. The following values have been found at 95% confidence level: B 1Σ + state, τ υ=0=21.5±1.8 ns, τ υ=1=21.9±2.1 ns, τ υ=2=21.7±2.0 ns, τ υ=3=21.5±3.2 ns; d 3Δ state, τ υ=0=7.8±1.8 ns; D 1Δ state, τ υ=0=4.3±1.0 ns.

Potential energy surfaces of LaH+ and LaH+2

Using the complete active space multiconfiguration self-consistent field (CAS-MCSCF) followed by full second-order configuration interaction (SOCI) calculations, 16 electronic states of LaH+ and 8 electronic states of LaH+2 are investigated. The potential energy surface of these electronic states of LaH+2 and LaH+ are computed. These calculations show that the 3F(5d2) ground state of La+ ion forms a weak complex with H2. The La+(1D) excited state inserts into H2 with a small barrier (<8 kcal/mol) to form the 1A1 ground state of LaH+2 (re=2.057 Å, θe=106°). At the SOCI level of theory LaH+2 is found to be 11 kcal/mol more stable than La+(3F)+H2. Our calculations explain the experimental observations on La++H2→LaH++H reaction. The adiabatic ionization potential (IP) of LaH2 and LaH are calculated as 5.23 and 5.33 eV, respectively. The ground state of LaH+ was found to be a 2Δ state. We compute De(LaH+) and De(HLa–H+) as 2.54 eV in excellent agreement with the experimental De(LaH+)=2.57 eV measured by Armentrout and co-workers. The spin–orbit effects of LaH+ were also studied using the relativistic configuration interaction (RCI) method.

Quenching of angular momentum in the ground states of VC, NbC, VSi, and NbSi molecules

The electron spin resonance spectra of VC, VSi, NbC, and NbSi trapped in the solid rare gases (Ne, Ar, Kr) show, principally through the large negative values of Δg∥=g∥−ge, that the orbital angular moments of these molecules in their (proposed) ground 2Δ states are partially quenched. Observed metal hyperfine splittings also vary with the matrix gas. This unprecedented effect in rare-gas matrices is attributed to strong unsymmetrical interaction of these ionic molecules with the matrix such that each is situated in an orthorhombic crystal field. The crystal-field-splitting parameter δ is both molecule and matrix dependent, varying from approximately 1400 to 7800 cm−1.

Rotational structure and splitting phenomena in electronic spectra of matrix-isolated NH

The forbidden transition a 1Δ ⇌ X 3Σ − of matrix-isolated NH has been analysed in Ne, Ar, Kr, and Xe. The rotational structure including levels up to N″ = 4 strongly supports the rotation-translation-coupling (RTC) model of Friedmann and Kimel. Translational mode frequencies ν t of NH in the electronic ground state are 68.5 cm −1 in Ar, 54 cm −1 in Kr, and 39 cm −1 in Xe. Similar frequencies ν t in the a 1Δ state, which show up as sharp features in the phonon sideband of the RTC-induced b 1Σ + →a 1Δ transition, were identified in Ar and Kr. A narrow matrix-induced quartet/quintet splitting of the a 1Δ state has been utilized to identify two trapping sites. NH radicals in these sites are nearly degenerate in the a 1Δ and b 1Σ + states (Δ E in the b state is only 0.05 cm −1 in argon), but can be individually excited and detected at sufficient spectral resolution due to the splitting. The width of the splitting decreases by less than 30% in the order Ar>Kr>Ne>Xe, and is totally unaffected by deuteration. It is proposed that the NH radical is trapped with about equal probability in substitutional sites of O h and D 3h symmetry. However, trapping in two interstitial O h, sites, which differ in the second coordination sphere in fcc and local hcp lattices, cannot be excluded in Ar, Kr and Xe.

An MRD-CI study of low-lying electronic states in CaF

Dipole moments and various spectroscopic constants of some low-lying electronic states of the CaF molecule have been calculated using the multireference single- and double-excitation configuration-interaction (MRD-CI) method. The electronic structure of the highly ionic molecule in various excited states can be explained in terms of different polarisations of the mainly Ca-centered valence electron in the field of the F − anion. Plots of natural orbitals occupied by the valence electron in the different states give a qualitative picture of the charge distribution and provide a visualisation of the different polarisations of the valence electron in the various states. Comparisons with the electrostatic polarisation model of Törring, Ernst and Kändler (TEK model) are made. The unknown A′ 2Δ state is predicted to lie about 21200 cm −1 above the ground state.