2Πr State Research Articles

On the ground electronic states of copper silicide and its ions

The low-lying electronic states of SiCu, SiCu+, and SiCu− have been studied using a variety of high-level ab initio techniques. As expected on the basis of simple orbital occupancy and bond forming for Si(s2p2)+Cu(s1) species, 2Πr, 1Σ+, and 3Σ− states were found to be the ground electronic states for SiCu, SiCu+, and SiCu−, respectively; the 2Πr state is not that suggested in most recent experimental studies. All of these molecules were found to be quite strongly bound although the bond lengths, bond energies, and harmonic frequencies vary slightly among them, as a result of the nonbonding character of the 2π-MO (molecular orbital) [composed almost entirely of the Si 3p-AO (atomic orbital)], the occupation of which varies from 0 to 2 within the 1Σ+, 2Πr, and 3Σ− series. The neutral SiCu is found to have bound excited electronic states of 4Σ−, 2Δ, 2Σ+, and 2Πi symmetry lying 0.5, 1.2, 1.8, and 3.2 eV above the 2Πr ground state. It is possible but not yet certain that the 2Πi state is, in fact, the “B state” observed in the recent experimental studies by Scherer, Paul, Collier, and Saykally.

Interaction of an aluminum atom with a closed subshell metal atom: Spectroscopic analysis of AlZn

Resonant two-photon ionization spectroscopy has been employed to investigate diatomic AlZn produced by laser vaporization of a 1:2 Al:Zn alloy target disk in a supersonic expansion of helium. Several discrete transitions are reported in the energy range from 18 400 to 19 100 cm−1. Most of these are assigned as members of the B 2Π←X 2Π system, although an isolated band has been observed and assigned as the 2-0 band of the A Ω′=0.5←X 2Π1/2 system. A pair of strongly mixed levels are identified as resulting from a homogeneous spin–orbit perturbation between the A Ω=0.5, v′=3 and the B 2Π1/2, v′=1 levels, and the perturbation matrix element has been deduced to be 8.11 cm−1 for 27Al64Zn, 8.23 cm−1 for 27Al66Zn. The ground state has been unambiguously identified as a 2Πr state with a bond length of 2.6957±0.0004 Å. Comparisons to the results of the preceding article on the spectroscopy of AlCa are also provided, along with a discussion of the chemical bonding in AlZn in relation to AlCa, AlAr, and AlKr.

Interaction of an aluminum atom with an alkaline earth atom: Spectroscopic and ab initio investigations of AlCa

A spectroscopic analysis of diatomic AlCa generated by laser vaporization of a 2:1 Al:Ca metal alloy followed by supersonic expansion has been completed using resonant two-photon ionization spectroscopy. Four excited electronic states have been identified and investigated in the energy region from 13 500 to 17 900 cm−1. These are the [13.5] 2Πr, the [15.8] 2Σ, the [17.0] 2Δ3/2(?), and the [17.6] 2Δ3/2 states. From rotational analysis excited state bond lengths have been measured for three of the four excited states, and the ground state has been unambiguously determined as a 2Πr state with a weighted least squares value of the ground state bond length of r0″ = 3.1479± 0.0010 Å. The ionization energy of the molecule has also been directly determined as 5.072±0.028 eV. Ab initio calculations for the potential energy curves of seven low-lying states of AlCa [X 2Πr, 2Σ+, 4Σ−, 4Πr, 2Πr(2), 2Δ, and 2Σ−] and for the X 1Σ+ ground electronic state of AlCa+ have been carried out. In agreement with experiment, 2Πr is calculated to be the ground electronic state of the neutral molecule. The dissociation energies of AlCa (X 2Πr) into Al(3s23p1,2P0)+Ca(4s2,1S) and for AlCa+ (X 1Σ+) into Al+(3s2,1S)+Ca(4s2,1S) are calculated to be 0.47 and 1.50 eV, respectively. The excited 2Σ+, 4Σ−, 4Πr, 2Πr(2), 2Δ, and 2Σ− states are calculated to lie 0.2, 0.7, 0.7, 1.1, 1.1, and 1.1 eV above X 2Πr, respectively, and the vertical and adiabatic ionization energies of AlCa have been calculated to be 5.03 and 4.97 eV, respectively.

A b i n i t i o calculations on the four lowest electronic states of AlF+ and AlCl+

The potential energy curves of the two lowest 2Σ+ (X,B) and the two lowest 2Π (A,C) electronic states of the AlF+ and AlCl+ molecular ions have been calculated using highly correlated multireference configuration interaction (MR-CI) wave functions. It is found that both 2Σ+ states and the C 2Πr state are bound, whereas the A 2Πi state is repulsive. Electronic transition moment functions for all six pairs of states of both ions have also been calculated and used for evaluation of the radiative transition probabilities between bound vibrational states. The calculated charge distributions show that the X and C states are dominated by Coulombic attraction between a doubly charged positive aluminum and the singly charged negative halogen ion. The results provide a new assignment of the photoelectron spectra of the neutral AlF. They are also in good agreement with the recently observed optical B–X emission spectra of both molecular ions and C–X bands of AlCl+. The absence of C–X emission of AlF+ is most likely due to predissociation of the C 2Πr state by the repulsive A 2Πi state.

Multiphoton ionization of vibrationally hot nitric oxide produced in a pulsed supersonic glow discharge

Mass-resolved, resonant multiphoton ionization (MPI) spectroscopy has been used to identify and characterize transient species produced in a pulsed, supersonic glow discharge source. Vibrationally hot (up to v″=9), but rotationally cold nitric oxide is characterized by (1+1) MPI via the A 2Σ+ state and (2+1) MPI through the C 2Πr state. Nine A←X and six C←X hot bands are observed; only four of these have been previously characterized. Accidentally overlapping C and A state hot bands can be separately studied by using different order MPI schemes. Implications for several previous studies are discussed. Additionally, the 3P0,2 metastable rare gas atoms are readily formed and detected by MPI as are metal atoms sputtered from the electrodes.

Resonance enhanced multiphoton ionization spectra of the GeF and GeCl radicals from 400 to 500 nm

The resonance enhanced multiphoton ionization spectra from 400 to 500 nm of the GeF and GeCl radicals are reported. In GeF bands involving transitions to the A 2Σ+, C′ 2Π, D′ 2Π, E 2Σ+, and G 2Δ states were observed. The bands involving transitions to the valence A 2Σ+ state were accounted for by 1+2 and 1+1+1 resonance enhanced multiphoton ionization (REMPI) mechanisms. Bands associated with the other Rydberg states were generated by a 2+1 REMPI mechanism. The spectrum of the GeCl radical arose from 2+1 REMPI excitation through the C′ 2Πr (5pπ) and C 2Πr (4pπ) Rydberg states. The C′ 2Πr state spin–orbit coupling constant was determined by direct measurement of C′ 2Π1/2←←X 2Π1/2 transitions and ‘‘one photon forbidden’’ C′ 2Π3/2←←X 2Π1/2 transitions. A least-squares fit determined the spectroscopic constants of the C′ 2Πr 5pπ Rydberg state of GeCl: Te=42 190±5 cm−1, Ae=81.0±3.5 cm−1, ωe=509.2±5.3 cm−1, and ωexe=3.3±1.1 cm−1.

The generation of 12C31P and 13C31P by reactive laser vaporization for rare gas matrix electron spin resonance studies: Comparison with a b i n i t i o theoretical calculations

The 12C31P and 13C31P diatomic radicals have been generated by the laser vaporization reaction between carbon and phosphorus which were pressed into a pellet to form the laser target. This method is applicable to a wide range of nonmetallic systems for generating new reactive intermediates which cannot be done with more conventional approaches. The radicals were isolated in neon and argon matrices at 4 K for detailed ESR investigations. The magnetic parameters (MHz) for neon were: g∥=2.0009(3); g⊥=1.9902(3); A∥(31P)=145.0(3); A⊥(31P)=−269.0(2); A∥(13C)=580.0(3); and A⊥(13C)=422.0(3). The argon A and g tensors were virtually indistinguishable from these neon results. Extensive ab initio theoretical calculations were conducted for CP which yielded nuclear hyperfine A values in close agreement with the experimental results. Valence orbital spin populations extracted from the calculated CI wave functions are compared with those determined directly from the ESR hyperfine parameters for 13C and 31P. The observed g shift for CP agreed with the gas phase spin–rotation constant and indicates the presence of a low lying 2πr state that has not been previously observed.

Theoretical study of AlH+: Spin splitting, core polarization, and interstellar chemistry

The spin splitting constant for the X 2Σ+ state of AlH+ is calculated to be γ0=0.058 cm−1. The favorable comparison of this result with experiment indicates that the uncertainty in the previously calculated spin splitting in MgH is likely to be of the order of a few percent. Calculations are presented of the so-called core polarization contribution to the spin-orbit coupling constant in the A 2Πr state of AlH+. Results are also given for MgH and SiH. Astronomical applications of such calculations are discussed and the abundances of aluminum-bearing molecules in interstellar clouds are estimated.

Spin–orbit coupling and Λ doubling in NaAr

A b initio values of spin-orbit coupling constants and Λ-doubling parameters have been obtained for the A 2Πr state of NaAr. Values are in reasonable agreement with experiment and, in particular, suggest that calculated potentials may be used to account for the Λ-doubling, provided that a reasonable model is used. The interest in NaAr arises from the fact that the Λ-doubling parameters in the A 2Π state give direct information about the Na+Ar interaction in the B 2Σ+ state. The success of the fine structure calculations presented here encourages application of similar methods to other van der Waals species.

Recovery of the long range potential in BeAr+ by potential inversion methods

The results of the potential inversion methods of Dunham, Simons, Parr, and Finlan, Thakkar, Huffaker, and the Padé approximate technique are presented for the X 2Σ+ state of BeAr+. The method of Thakker is shown to yield reasonable behavior at long range, leading to an ion-induced dipole interaction. A simple model potential is shown to provide a good description of the bonding in both the ground state and the A 2Πr state. The effective nuclear charge of Be+ is found to be 1.29 in the X state and 1.88 in the A state. Integration of the various potentials to determine the vibrational eigenvalues shows the superiority of the Thakkar and Huffaker methods and shows the three parameter model potential to be a very good representation of the true potential.

A Note on the D′2Πr State of the PO Molecule

Observed vibrational levels of the D′2Πr state of the PO molecule at 48 835.4 and 48 854.4 cm−1 corresponding to P16O and P18O are shown to have vibrational numbering ν and ν + 1 respectively, where ν is shown to be large (≈21). From this, T0 of D′2Πr is predicted to be about 38 000 cm−1

Spectral study of the phosphorus glow

A bright flame produced by the reaction of atomic oxygen with white phosphorus has been developed. In addition to a visible continuum, the emission consists of several transitions of PO and the A1Πg–X1Σg+ transition of P2. Atomic oxygen and a third body such as H2O or H2 is found essential for the production of discrete bands. Pressure effects on the intensity of the β bands of PO indicate the presence of a heterogeneous perturbation in the B2Σ (ν = 7) level by a 2Πr state.

Rotational analysis of the red electronic emission system of titanium nitride

An electronic emission spectrum has been obtained from a carrier identified as the diatomic molecule TiN. The system, which occurs in the red, has been analyzed as 2Πr → (X)2Σ, and it appears to be the analogue of the orange–red systems of the isoelectronic molecules CaF and ScO.The lower (ground?) state has B0″ = 0.6211 cm−1 corresponding to r0″ = 1.583 Å, whereas the excited 2Πr state has B0′ = 0.6103 cm−1. The system has a well-defined homogeneous perturbation in the excited state, showing the presence of a second, unobserved, 2Π term.