2Δ State Research Articles

Multireference configuration interaction studies on metastable states of the dication BN2+

Multireference CI calculations predict fourteen states of BN2+ to be quasibound, an uncommonly large number for a first-row diatomic, confirming the important role played by the electropositive B atom in stabilizing molecular dications. About two-thirds of the metastable potentials have dissociation barriers (Deff) between 0.59 and 2.25 eV, accommodating several vibrational levels and therefore being kinetically stable. The ground state X 3Σ−, however, might be difficult to study experimentally because it is only weakly bound (Deff=0.23 eV), and its formation via ionization of BN or BN+ has unfavorable Franck–Condon factors (FCF). The 1 5Σ− state is the best candidate for detecting BN2+: Its barrier is the highest (Deff=2.25 eV), sustaining about 30 vibrational levels (with tunneling lifetimes τTυ≊∞ for υ<20), and the ionization FCFs are favorable. The 1 1Δ state might also be detectable since its properties are similar to those of 1 5Σ−. Near equilibrium, most metastable states are described by the configurations 3σ24σ5σ1π2 and 3σ24σ1π3, both having charge distributions B2+N. The adiabatic ionization potential into BN2+ (X 3Σ−) is 21.4 eV for ionization from BN+ (X 4Σ−) and 32.9 eV from BN(X 3Π). The corresponding IPs into 1 5Σ− lie 1.12 eV higher.

Laser Spectroscopy of TiO: Accurate Term Energies for the Singlet States and Ligand Field Assignment of States in the Range 0 to 4 eV

The wavelength-resolved fluorescence excitation technique has been used to record the (2,0) band of the C 3Δ 3- a 1Δ transition, and the (2,3) and (2,4) bands of the C 3Δ 3- X 3Δ 3 electronic transition of TiO at a resolution of 0.03 cm −1. These data were combined with a previous high-resolution analysis of the C 3Δ- X 3Δ system (T. Gustavsson, C. Amiot, and J. Verges, J. Mol. Spectrosc. 145, 56-65, 1991) to obtain accurate term energies for the singlet states of TiO. The term energy for the first excited a 1Δ state is T 0( a 1Δ) = 3444.367 ± 0.020 cm −1. This value differs significantly from the previously derived value of T 0( a 1Δ) = 3448.33 ± 0.04 cm −1 (A. I. Kobylyanskii, A. N. Kulikov, and L. V. Gurvich, Opt. Spectrosc. 54, 254-255, 1983). Ligand field theory models were used to analyze the electronic structures of TiO, TiS, and TiCl +.

Electronic spectroscopy of jet-cooled iron monocarbide. The 3Δi←3Δi transition near 493 nm

We report the first gas phase spectroscopic study of iron monocarbide. FeC molecules were generated in a laser vaporization molecular beam source and detected by laser induced fluorescence. Twenty-six vibronic bands have been recorded in the 430–500 nm region. Rotational analyses have been carried out for 22 of the bands. There are three lower states involved, two with Ω″=2 and one with Ω″=3. Based on our experimental observations and the ab initio calculations on RuC [Shim et al. J. Phys. Chem. 91, 3171 (1987)] the observed bands are interpreted as 3Δi←3Δi and 1Δ←1Δ transitions. The ground state electronic configuration is assigned to 1δ39σ1. Evidence for three excited electronic states (one 3Δi state, one 1Δ state and another state with Ω′=3) and for perturbations in several of their vibrational levels is presented.

Photoelectron spectroscopy of FeO− and FeO−2: Observation of low-spin excited states of FeO and determination of the electron affinity of FeO2

The photoelectron spectra of FeO− and FeO−2 are obtained at 3.49 eV photon energy. Transitions to the ground state (5Δ) and three low-lying excited states (5Σ+, 3Σ+, and 3Δ) of FeO are observed. The two low-spin excited states found at 6770 and 8310 cm−1 above the ground state, respectively, have not been observed before. The two Σ states, characteristic of detachment of a nonbonding electron from the FeO− anion, exhibit no vibrational progressions while a vibrational progression is observed for each of the two Δ states. The two high-spin states 5Δ and 5Σ+ are in agreement with a previous photoelectron study [P.C. Engelking and W. C. Lineberger, J. Chem. Phys. 66, 5054 (1977)]. The 3Δ state has a vibrational frequency of 800 (50) cm−1. The spectrum of FeO−2 only shows one major feature with little vibrational structure at this photon energy. The electron affinity of FeO2 is determined to be 2.358 (0.030) eV.

Photodissociation of ketene with a narrow-band tunable laser around 212.5 nm

The photodissociation of ketene with a narrow-band tunable laser has been studied at 218.0, 212.5 and 207.0 nm. A CH photofragment is formed in the C 2Σ + (ν′ = 0), B 2Σ − (ν′ = 0, 1) and A 2Δ (ν′ = 0, 1) states through a process involving the absorption of two laser photons. The visible emission observed above 580 nm is attributed to the CH 2(b 1B 1) photofragment formed in a single-photon process. The experimental results strongly suggest that electronically excited CH could be formed in a sequential mechanism involving the absorption of a second photon by the CH 2(b 1B 1) intermediate. Nevertheless, the simultaneous existence of several mechanisms must be considered to explain the formation of the CH fragment in the various observed electronic states.

Adiabatic Potentials of the Lowest in 1Π g and 1,3Δ g States of the Hydrogen Molecule

The Born-Oppenheimer energies and adiabatic corrections have been computed for the I and R 1Π g , J and S 1 Δ g and j and s 3Δ g electronic states. For the R state these are the first accurate ab initio results. For the remaining states the energies are the lowest ever obtained, except for the J 1Δ state, where our energies are higher. It is argued that the present results are the correct ones. Theoretical term values of the lowest singlet states are compared with experiment, and the remaining convergence errors in the Born-Oppenheimer potentials near equilibrium are estimated to be below 0.5 cm −1 for the I and R states and about 2-4 cm −1 for the J and S states.

Resonance enhanced multiphoton ionization spectroscopy of the NF molecule: 1,3Φ 3d and 4d Rydberg states

The (3dδ)1,3Φ and (4dδ)1,3Φ Rydberg states of NF have been investigated by multiphoton ionization (MPI) spectroscopy. These states were observed as two-photon resonances in an overall (2+1) MPI process from NF a1Δ produced from the F+N3 reaction. Ab initio calculations performed at the multireference double excitation configuration interaction level showed that the excited Φ states were of Rydberg character with configurations of ...1π45σ22π1(3dδ)1 and ...1π45σ22π1(4dδ)1. The 3Φ←a1Δ two-photon transitions were found to derive their intensities from spin–orbit and spin-uncoupling interactions in the 1Φ3, 3Φ4, 3Φ3, and 3Φ2 upper states. Analysis of the rotationally resolved bands, using a model which includes these factors, allowed the spin–orbit constant, aπ, for the 2π valence orbital to be derived as (159.0±1.0) cm−1. Rotationally resolved envelopes recorded for the (3dδ)1Φ, v′=0, 1←a1Δ, v″=0 and (3pπ)1Σ+, v′=0←a1Δ, v″=0 (2+1) NF MPI bands, could be simulated reasonably well assuming a Boltzmann rotational distribution for the a 1Δ state at ≊180 K. Experiments showed, however, that this was not a true measure of the initial state distribution because of predissociation or perturbation effects in the resonant intermediate state. Assignments for other two-photon resonant NF MPI bands observed in this work are also suggested.

Quenching rate constants for NCl(b1Σ+) and PCl(b1Σ+) and radiative lifetimes of NCl(b1Σ+), PCl(b1Σ+) and PBr(b1Σ+)

NCl(b 1Σ+), PCl(b 1Σ+) and PBr(b 1Σ+) molecules have been generated in a flow reactor by passing dilute flows of NFCl2, PCl3 and PBr3 in He through a dc discharge, and the total rate constants for quenching of NCl(b 1Σ+) and PCl(b 1Σ+) by several diatomic and small polyatomic molecules have been measured at 300 K. Except for O2, NO2 and molecular halogens, the rate constants are in the 10–13– 10–14 cm3 molecule–1 s–1 range. The dependence of the rate constants on the properties of the reagent suggests that the dominant quenching mechanism for PCl(b) and NCl(b) is electronic-to-vibrational (E–V) energy transfer with the a 1Δ state as the product, just as for the reactions of NF(b 1Σ+) and PF(b 1Σ+). The rate constants for quenching of NCl(b 1Σ+) and PCl(b 1Σ+) by O2 are 1.0 × 10–12 and ⩽ 10–15 cm3 molecule–1 s–1, respectively, and are in accord with an electronic-to-electronic (E–E) energy transfer with the final states being O2(b 1Σ+)+ NCl(X 3Σ–) or PCl(X 3Σ–). Formation of PBr(b 1Σ+) from the PCl(b 1Σ+)+ Br2 reaction was observed, and quenching of PCl(b 1Σ+) by F2, Cl2 and Br2 proceeds by a chemical mechanism with rate constants of 2.3 × 10–12, 0.79 × 10–12 and 16 × 10–12 cm3 molecule–1 s–1, respectively. The radiative lifetimes for NCl(b 1Σ+), PCl(b 1Σ+) and PBr(b 1Σ+) are 2.0 ± 0.4, 4.9 ± 0.8 and 0.8 ± 0.2 ms, respectively, as determined from the decay of the respective b 1Σ+→ X 1Σ– emission intensities along the flow reactor.

Vibronic spectra of the matrix-isolated van der Waals complexes NH·N2 and NH·CO

Abstract NH · N2 and NH · CO van der Waals complexes have been prepared in rare gas matrices by in situ photolysis of the precursor molecules HN3 and HNCO, and by in situ photolysis of NH3 in rare gas matrices doped with N2 and CO, respectively. NH · CO has been detected in absorption on the A 3Π ← X 3Σ− transition. Its a → X fluorescence yield is very low, but not zero. The perturbing N2 neighbour red shifts the NH b 1Σ ↔ +X3Σ− transition by 0.07%, and the NH a 1Δ ↔ X 3Σ− transition by 0.25%, from their origins in pure argon matrices. Rotation of the NH moiety in argon is suppressed by the N2 neighbour. High resolution spectra of the a 1Δ ← X 3Σ− transition consist of 10 closely spaced lines in argon ( widths between 0.18 and 0.35 cm−1), and of 6 lines in krypton. The lines are assigned to five double substitutional sites which host the NH · N2 van der Waals molecule in constant proportions of 3.6:3.2:1.2:1:1 in argon, and to three sites in krypton, where the proportions are 4.9:3.0:2.1. Each site contributes a doublet, which is due to electronic degeneracy lifting in the a 1Δ state. The relative abundances of the sites and the splittings (ranging from 2 to 7.6 cm−1) are practically unaffected by deuteration. The number of sites can be related with our previous conclusion that NH is trapped in substitutional sites of (local) Oh and D3h, symmetry. Annealing experiments reveal that NH is perturbed by N2 only in its first coordination shell. Local modes, including site-specific modes of very low frequencies, are tentatively interpreted by means of model calculations.

The a3Σ+ (a11)→X1Σ+ (X0+) Transitions of BiP, BiAs, and BiSb

Chemiluminescence of the a3Σ+ (a11)→X1Σ+ (X0+) transitions of BiP, BiAs, and BiSb has been observed in the near-infrared spectral region between 6000 and 10 000 cm−1 from the reactions of microwave-discharged phosphorus, arsenic, and antimony vapor with bismuth vapor in a fast-flow system. The spectra consisting of short sequences of red-degraded bands were measured at a resolution of 0.5 cm−1 with a Fourier-transform spectrometer. Vibrational analyses of the systems have yielded the following molecular constants of the X0+ and a11 states of BiP, BiAs, and Bi121Sb (in cm−1): [formula] Here the numbers in parentheses give the standard deviations of the parameters in units of the last digit. Weak bands of a second transition to the ground state of BiP were observed in the region 11 000-15 000 cm−1. The upper state with constants Te = 15093.1 (2) cm−1, ωe = 334.8 (2) cm−1, and ωexe = 2.79(5) cm−1 is tentatively assigned to the Ω = 1 component of the lowest 3Δ state of BiP.

State-specific collisional energy transfer in electronically excited SiF radicals: dramatic contrasts with SiCl

The collisional removal of state-selected SiF (C 2Δ, υ′ = 0 or 1) has been studied by time- and wavelength-resolved laser-induced fluorescence spectroscopy. A series of non-polar quenchers (Ar, He, H 2, N 2, CO 2 and CH 4) was examined. Rate constants are generally large for the molecular quenchers, but are negligible for the noble gases. A collisionless radiative lifetime of 94±2 ns (2σ statistical uncertainty) is also reported for the SiF C 2Δ state, independent of vibrational level. A fraction of the removed C 2Δ excited valence state population, varying from 0 to 20% with quencher, is deposited in the nearby B 2Σ + Rydberg state. The resultant B 2Σ + state vibrational populations are largely quencher independent and correlate well with the Franck-Condon overlap between the initial and product vibronic states, despite the substantial energy gaps involved. These results contrast dramatically with those for the nominally isoelectronic states in SiCl: possible explanations for the distinctions are discussed.

The smithsonian astrophysical observatory database SAO92

The Smithsonian Astrophysical Observatory (SAO) maintains a molecular line database (the SAO line database) for analysis of atmospheric spectra in the far i.r. and longer-wave mid i.r., from 10-800 cm -1. This database combines the best currently available line parameters, including the HITRAN 1.2 molecular database, the JPL submillimeter, millimeter, and microwave spectral line catalog 3 (JPLSMM), and other available measurements and calculations. The current version of the database contains 154.895 lines from H 2O, CO 2, O 3, N 2O, CO, CH 4, O 2 (including the 1Δ state), NO, SO 2, NO 2, NH 3, HNO 3, OH, HF, HCl, HBr, HI, ClO, OCS, H 2CO, HOCl, HCN, H 2O 2, O( 3 P), and HO 2. The database is available in both HITRAN-type 80 character (SAO92) and 100 character (SAO92A) formats. 1. L.S. Rothman, R.R. Gamache, A. Goldman, L.R. Brown, R.A. Toth, H.M. Pickett, R.L. Poynter, J.-M. Flaud, C. Camy-Peyret, A. Barbe, N. Husson, C.P. Rinsland, and M.A.H. Smith, Appl. Opt. 26, 4058 (1987). 2. L.S. Rothman, R.R. Gamache, R.H. Tipping, C.P. Rinsland, M.A.H. Smith, D. Chris Benner, V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S.T. Massie, L.R. Brown, and R.A. Toth, JQSRT. 48, 469 (1992). 3. R.L. Poynter and H.M. Pickett, Submillimeter millimeter and microwave spectral line catalog, Publ. 80-23, revision 2, Jet Propulsion Laboratory, Pasadena, CA (1984).

Vibrational transition probabilities in the B-X and C-X systems of the SiF radical

Vibrational transition probabilities have been measured for the B 2Σ + (υ′ = 0, 1, 2, 4, 5)-X 2Π(υ″=0–10) and C 2Δ(υ′ = 0, 1)-X 2Π(υ″ = 0–5) band systems of the SiF radical. Individual B 2Σ + and C 2Δ state vibrational levels were selectively populated by laser excitation of ground state SiF X 2Π prepared in a discharge-flow system. The resultant fluorescence was then dispersed and recorded. Comparison of the observed band strengths with Franck-Condon factors calculated from Rydberg-Klein-Rees potentials allowed the form of the respective electronic transition dipole moment functions to be estimated. The B-X system was found to have an electronic transition probability which did not vary significantly with internuclear separation. In contrast, the observations for the C-X system could only be reproduced by assuming a rapidly decreasing function. These conclusions are consistent with the accepted dominant electronic configurations of the states involved.

Observation and identification of the g 6Φ–a 6Δ transition in the blue system of FeH at 493 nm

The blue system of FeH, at 493 nm, has been recorded with a sample at near-ambient temperatures by laser excitation spectroscopy. With the help of dispersed fluorescence studies, all the strong lines in this region of the spectrum have been assigned to the (0,0) band of the g 6Φ–a 6Δ transition, where the lower state lies about 1890 cm−1 above the ground 4Δ state. Assignments have been made to ΔΩ=+1 and ΔΩ=0 subbands involving the lowest three spin components in each state. Term values have been determined for the levels of the three lowest spin components of the a 6Δ state and thereby for the g 6Φ11/2 spin component, which were hitherto unknown. It is established that the a 6Δ state of FeH is inverted and that the vibrational interval ΔG1/2 varies markedly between the spin components of this state: 6ΔΩ 9/2 7/2 5/2 ΔG1/2/cm−1 1622 1569 1582. There is evidence that the a 6Δ state is perturbed by levels of other, nearby electronic states and it is not possible to model its levels with a simple effective Hamiltonian. The radiative lifetime of FeH in the two lowest spin components of the g 6Φ state has been measured as 39±2 ns.

Potential energy surfaces for Rh–CO, Rh–OC, Ir–CO, and Ir–OC interactions

The potential energy surfaces for eight low-lying electronic states of the Rh–CO and Rh–OC complexes and of the Ir–CO and Ir–OC complexes have been studied using the complete active space multiconfiguration self-consistent field method followed by the multireference singles+doubles configuration interaction calculations which included up to 1.6 million configurations. In addition, spin–orbit effects are included through the relativistic configuration interaction method for the Ir–CO complex. It is found that the Rh(2F) and Ir(2F) states react spontaneously with CO to form stable RhCO and IrCO molecules in which the 2Δ ground states are 42.4 and 75.1 kcal/mol more stable than Rh(2F)+CO(1Σ+) and Ir(2F)+CO(1Σ+) states, respectively, in the absence of spin–orbit effects. The RhOC and IrOC complexes in the 2Δ states were found to be less stable than Rh(4F)+CO(1Σ+) and Ir(4F)+CO(1Σ+), respectively. The nature of the chemical bond is discussed in terms of the wave function composition, and the electronic population analysis.

Explicit construction of a convergent MBPT series for the 1Δ state of C 2 and the H 2 ground state at large bond distance

We explicitly construct a convergent perturbation series for intrinsically divergent Møller—Plesset cases. The means to accomplish this goal is an explicit reduction of the size of the perturbation. This is achieved by means of a Λ transformation and an unlinked-graph shift. Generic multi-reference cases are treated by single-reference methods after a simple and feasible transformation of the Hamiltonian matrix to a basis comprising a suitable reference zero-order vector. This effectively solves the intruder-state problem. We present the 1Δ state of C 2 and the H 2 ground state at large bond distance as typical examples.

Deexcitation electron spectroscopy of core-excited NO as a function of excitation energy

Deexcitation electron spectra of core-excited NO have been measured at several excitation energies in the N 1s→2π and O 1s→2π resonances. The nitrogen spectra exhibit significant variation with excitation energy; the oxygen spectra vary only slightly. Sensitivity to excitation energy occurs because each resonance represents the overlap of three transitions to 2Σ+, 2Δ, and 2Σ− states, and each of these excited states decays to a unique set of levels in the final-state ion. We have analyzed all spectra by taking into account excitation energy, lifetime-vibrational interference, and the ordering and splitting of the core-excited levels. Good agreement between calculated line shapes and experiment occurs if it assumed that the level ordering is 2Δ, 2Σ−, 2Σ+ for core-excited nitrogen and 2Σ−, 2Δ, 2Σ+ for core-excited oxygen. Photoexcitation data for oxygen have been analyzed to determine the energies of these states 531.7, 532.7, and 533.7 eV. The deexcitation spectrum from the 2Δ state of nitrogen core-excited NO to the ground state of NO+ has been analyzed using the theory of lifetime-vibrational interference to give a lifetime width for the core-excited state of 146 meV. A similar analysis for the deexcitation of the oxygen core-excited state is less conclusive, but is consistent with a lifetime width of 180 meV.

An Energy Level Diagram of the Known Electronic States of NiF Determined by Radiative and Collisional Transfers

A study Of collisional and radiative energy transfers between the electronic states of NiF has made possible the determination of the relative positions of most of the electronic states involved in the already rotationally analyzed bands. The presence of three lower 2Π, 2Δ, and 2Σ states is now precisely determined. This experimental work is in good agreement with a recent theoretical study (P. Carette, C. Dufour, and B. Pinchemel, J. Mol. Spectrosc. 161, 323, 1993).

One- and two-photon dissociation of acetyl chloride at 193 nm

The photolysis of CH 3COCl by 193 nm radiation yields Cl, CH 3, CO, and CH amongst its photodissociation products. The Cl and CH 3 fragments are formed in equal amounts with a quantum yield of 0.90±0.07 relative to the photodissociation of methyl chloride at 193 nm. The CO is born with a vibrational temperature of 2600 +400 −300 K. The concentrations of Cl, CH 3, and CO are first order in laser power indicating that they arise from a one-photon process. The CH fragment is formed in the B 2Σ − and A 2Δ states. Its formation varies quadratically with laser power and is attributed to resonance-enhanced two-photon dissociation of acetyl chloride.

Detection of OH+ in its a 1Δ state by far infrared laser magnetic resonance

The spectrum associated with the J=3←2 transition of OH+ in the a 1Δ(v=0) state has been observed by far infrared laser magnetic resonance spectroscopy. A new microwave discharge source enabled the detection of this spectrum, which is the first observation of the rotational spectrum of an ion in a metastable state. Assignment and least-squares fitting of the observed transitions have determined the following molecular constants: B0=494.420 388 (22) GHz, the proton hyperfine parameter a=74.84 (32) MHz, gL′ = 1.000 915 (15), and gr=−0.001 815 (18), with the 1σ uncertainties of the last digits in parentheses. The relationship of these parameters to the geometric and electronic structure of OH+ is discussed.