A2Πu State Research Articles

Transition probabilities and Franck‐Condon factors for the second negative band system of O2+

Transition probabilities for the second negative band system of O2+ are computed using the dipole transition moment presented by Wetmore et al. [1984]. Vibrational levels υ″ = 0 − 54 of the X²Πg ground state and υ′ = 0 − 33 of the excited A²Πu state are included. Franck‐Condon factors for ionization‐excitation of O2 (X³Σg−; υ = 0 − 25) to O2+(A²Πu;υ′ = 0 − 33) are also presented.

High resolution spectrum of the N_2^+ Meinel system to 11 250 Å

This spectrometric investigation of the emission of the A2Πui→X2Σg+ band system of the nitrogen molecular ion has yielded the wave numbers of 1267 individual lines in eight bands. Cooled photo-multipliers were used up to the 1-0 band at 9200 Å and a cooled intrinsic germanium photodiode was used for the 0-0 band. The spectra were obtained using a specialized hollow cathode source and a 5-m Jarrell-Ash spectrograph adapted to spectrometric operation through the installation of an exit slit translated with a lead screw assembly driven by a stepping motor. Improved molecular constants and vibrational term values have been determined as well as the origins of the 0-0 and 1-0 bands which have not previously been subject to direct measurement and which specify the bottom of the A2Πu potential curve. Observation of a very intense 0-0 band indicates a large population for the υ = 0 level of the A2Πu state under the conditions of the experiment. It is concluded that collisional transfer of excitation between electronic states is responsible for the accumulation of population in this lowest excited level of N2+.

Angular distributions, energy disposal, and branching studied by photoelectron–photoion coincidence spectroscopy: O2+, NO+, ICl+, IBr+, and I2+ fragmentation

It is shown that some levels of the A2Πu states of I2+, IBr+, and ICl+ are stable toward dissociation, and ought to decay by emission. The B2Σg+ state of I2+ is fully dissociated to ground state I++I, while the B2Σg+ states of IBr+ and ICl (probably) yield excited decay products. The c3Π and B1Π states of NO+ dissociate to ground state N++O fragments, while the c4Σg− and 2Πu states of O2+ near 24 eV dissociate to excited levels of O++O. The origin of anomalous time-of-flight peak shapes is discussed, and several examples are attributed to anisotropic fragment angular distributions.

CS2+(B2Σu+, A2Πu → X2Πg) Fluorescence from Photoionization Excitation of CS2 Vapor

CS2+(B2Σu+, A2Πu → X2Πg) fluorescence bands produced by irradiation of CS2 vapor with emission lines from λλ462–977 Å were analyzed and their production cross sections measured. The assignments for the emission bands of the bending transitions, B2Σu+(0ν0) → X2Πg(0ν0), have been suggested. Using the tentatively assigned positions of the band heads for the A2Πu(ν00) → X2Πg(000) transitions the ionization potentials for the A2Πu(ν00) vibrational levels were obtained and found to be in good agreement with the photoelectron data. The splitting constant of the spin–orbit interaction for the A2Πu state was thereby determined to be 169 ± 20 cm−1. The CS2+(A2Πu, B2Σu+ → X2Πg) fluorescence has exceptionally high production cross sections at the absorption bands associated with the n = 3 members of the Rydberg series III and V.

Formation of electronically excited CO2+ in collisions of H+, H, He+, and Ne+ with CO2

Electronic excitation in collisions of 200 eV–25 keV H+, H, He+ and Ne+ with CO2 has been investigated under single-collision conditions in an atomic beam experiment. For each of these projectiles, the B 2Σu+ and A2πu states of CO2+ are the dominant electronically excited species emitting in the 200–500 nm wavelength range owing to target excitation. Although dissociative electron capture by He+ and Ne+ to form electronically excited CO+ minimizes the energy defect, no emission is observed from CO+ A2π, and formation of CO+B2Σ+ is of only minor importance. The latter observations are in direct contradiction to predictions derived from the adiabatic criterion and demonstrate that, in the competition among allowed excitation processes, details of the electronic structure of the collision partners override energy considerations. The qualitative features of the emission spectrum excited by H+, H, and He+ are insensitive to relative velocity over the energy range covered. The ratio of the cross sections for excitation of CO2+ B−X and A−X emissions is about 0.20 for H+ and 0.40 for He+. In Ne+ impact, the CO2+ A state vibrational distribution changes significantly, the (B−X)/A−X) cross section ratio decreases from 0.46 to 0.07, and the relative importance of Ne and Ne+ emissions diminishes markedly as the energy is reduced from 4 keV to 200 eV. Our results are compared with those for electronic excitation produced by photoionization, electron impact, and Penning ionization of CO2.

On the Photoelectron Spectrum of O2

The photoelectron spectrum of molecular oxygen has been studied in the range 12-28 eV with the 584 Å and 304 Å He lines, and the O2+ states have been measured accurately. Due to the high resolution of our apparatus we have been able to observe the A2 Πu state which has been considered as absent in photoelectron spectroscopy work. Its ionization energy is 17.045 eV. Our result is compared with spectroscopic observations of the second negative band system A2Πu → X2Πg.The relative intensity of the A2Πu state is in good agreement with a recent calculation by Dixon and Hull.

Excitation of the A2πu state of N 2+ by electrons

Absolute values of the emission cross sections for five vibrational bands in the Meinel system of N 2 +, A 2 π u to X 2 Σ g +, excited by electron impact are presented. From these, a value was obtained for the total excitation cross section of the A 2 π u state at 100 eV of 26·5 × 10 −18 cm 2. The results are compared with those of other workers and with theory. Collisional transfer of the excitation energy from the levels of the A 2 π u state was also observed with a transfer cross section of approximately 10 −14 cm 2.

Electronic Structure of Diatomic Molecules. III. A. Hartree—Fock Wavefunctions and Energy Quantities for N2(X1Σg+) and N2+(X2Σg+, A2Πu, B2Σu+) Molecular Ions

The problem of the convergence of a sequence of Hartree—Fock—Roothaan wavefunctions and energy values to the true Hartree—Fock results is examined for N2(X1Σg+). This critical study is based on a hierarchy of Hartree—Fock—Roothaan wavefunctions which differ in the size and composition of the expansion basis set in terms of STO symmetry orbitals. The concluding basis set gives a total Hartree—Fock energy of −108.9956 hartree and Re(HF)=2.0132 bohr for N2(X1Σg+). Results are also presented from direct calculations for three states of the N2+ molecular ion (X2Σg+, A2Πu, B2Σu+) which are also thought to be very close approximations to the true Hartree—Fock values. The results give EHF=−108.4079, −108.4320, and −108.2702 hartree and Re(HF)=2.0385, 2.134, and 1.934 bohr for the X2Σg+, A2Πu, and B2Σu+ states of N2+, respectively. Extensive calculations for various R values establish that the X2Σg+ and A2Πu states are reversed in order relative to experiment, a short-coming ascribed to the Hartree—Fock approximation.

VIBRATIONAL ISOTOPE SHIFTS OF ABSORPTION BANDS OF N2 IN THE SPECTRAL REGION 720–830 Å

Using a normal-incidence vacuum spectrograph with a 3-meter concave grating, the absorption spectra of 14N2 and 15N2 in the 720–830 Å region have been investigated. From the vibrational isotope shifts, it has been confirmed that the previous assignment given by Ogawa and Tanaka for the Rydberg series converging to the A2Πu state of N2+ was correct. The observed series limits converging to the [Formula: see text] state of 15N2+ are 134735(0,0), 136540(1,0), 138320(2,0), and 140070(3,0) cm−1. The observed vibrational isotope shifts at the series limits in both Rydberg series converging to the A2Πu and the X2Σg+ states of N2+ agree with those values calculated from the known vibrational constants of the A2Πu, and the X2Σg+ states of 14N2+ and the X1Σg+ state of 14N2. In addition to the Rydberg series, four new progressions have been identified tentatively. The observed vibrational quanta of the upper states of these progressions are almost the same, and are, moreover, almost the same as those of the A2Πu state of N2+.