Ch. Jungen

Rovibronic interactions in the photoabsorption spectrum of molecular hydrogen and deuterium: An application of multichannel quantum defect methods

Multichannel quantum defect theory (MQDT) is applied to the treatment of electron motion in molecular Rydberg states and its coupling to the vibrational and rotational motions of the nuclei. The full rovibronic Hamiltonian is taken into account and the theory formulated in terms of parameters which relate to the standard Born–Oppenheimer (BO) theory. The development is an extension of Seaton’s quantum defect theory and relies on Fano’s frame transformation method. MQDT is not restricted to the BO approximation, but includes the so‐called adiabatic corrections and nonadiabatic effects (i.e., vibronic coupling and rotational l uncoupling), however strong, without requiring the explicit evaluation of the interactions between individual levels. The general formalism can be simplified for the H2 and D2 molecules in the range below or near the ionization threshold and for vibrational excitation well below the dissociation energy. The only input data required are the potential energy curve of the ion core in its...

Orbital angular momentum in triatomic molecules

The vibrational and K-type rotational levels of the [Xtilde] 2 B 1 and A 2 A 1 states of NH2 and H2O+ have been fitted by least squares to give sets of Born-Oppenheimer potential curves for the combining electronic states. The model used allows a simultaneous fit to all the observed vibronic levels for each of the two molecules, using a large amplitude formalism and making no approximations other than to neglect Fermi (anharmonic) resonance effects. An accurate description of the effects of orbital angular momentum on the level positions in the two molecules has been obtained, and relative transition moments have been calculated for the vibronic bands of the A 2 A 1-[Xtilde] 2 B 1 transitions. For NH2 the relative transition moments permit the calculation of the emission lifetimes (with the absolute value of the electronic transition moment taken from the ab initio calculations of Peyerimhoff and Buenker); the calculated lifetimes are found to agree very well with the experimental values of Halpern et al....

Theoretical study of competing photoionization and photodissociation processes in the NO molecule

The multichannel quantum defect theory of dissociative recombination is reviewed and adapted to treat simultaneous vibrational preionization and electronic predissociation in NO. The photoionization, photodissociation, and photoabsorption cross sections of NO corresponding to 2Π excited states are computed in the range from the N(2D)+O(3P) dissociation limit up beyond the second vibrational level of NO+(X 1Σ+). The calculations are based on the 2Π Rydberg and valence state potential curves and Rydberg‐valence state interaction energies extracted by Gallusser and Dressler from the high‐resolution spectroscopic data of Miescher and co‐workers. These data pertain to the discrete range and furnish precise information on numerous spectral perturbations involving levels of the npπ Rydberg series and the B and L valence states. The calculated cross section profiles reproduce with reasonable accuracy the observed absorption and photoionization curves, and in particular the widths of the Rydberg resonances. The co...

Calculation of rotational–vibrational preionization in H2 by multichannel quantum defect theory

Multichannel quantum defect theory is adapted to treat simultaneous rotational and vibrational preionization in H2. The strongly preionized spectrum between the N+=0 and N+=2 rotational thresholds of photoionization of H2X1Σg+(J″=0, v″=0) to produce H2+X2Σg+(N+, v+=0) is computed as example and good agreement is obtained with the photoionization data of Dehmer and Chupka.

Calculation of vibrational preionization in H2 by multichannel quantum defect theory: Total and partial cross sections and photoelectron angular distributions

Multichannel quantum defect theory has been used to calculate the effect of vibrational preionization on the total and partial oscillator strength distributions and photoelectron angular distribution in H2 for excitation between 790 and 760 A. The total oscillator‐strength distribution obtained agrees well with the high‐resolution photoionization data of Dehmer and Chupka. The partial oscillator strength resonance profiles are predicted to have different shapes in different vibrational ionization channels, while their widths change little with channel. The preionization resonances are also predicted to affect the angular distribution asymmetry parameters b over a broader range than they affect the oscillator strength distribution. The gross features of the preionization resonances are discussed in terms of approximate solutions of the MQD equations.

Orbital angular momentum in triatomic molecules: III. Spin and rotational fine structure effects in the à 2 A 1-[Xtilde] 2 B 1 transitions of NH2 and H2O+

The effects of orbital angular momentum on the details of the spin and rotational fine structure of the [Xtilde] 2 B 1 and A 2 A 1 states of NH2 and H2O+ are considered. It is found that the erratic spin-orbit splittings and asymmetry parameters of the A 2 A 1 states can be reproduced with good accuracy by calculations that also account for the regular spin and rotational structures of the ground states; the only input parameters are the shapes of the Born-Oppenheimer potential curves, the bond lengths and the spin-orbit coupling constants. The same calculations give an almost quantitative explanation of various perturbations in the A 2 A 1 state of NH2. These perturbations are caused by the ground state and result from the presence of orbital angular momentum. Three types have been documented; one is direct spin-orbit interaction between the A and [Xtilde] states, and the others involve rotational asymmetry, which is found to be an important mechanism for causing perturbations between electronic states.

High‐resolution jet absorption study of nitrogen near 800 Å

The absorption spectrum of a supersonic free jet expansion of 14N2 in He has been photographically recorded in the region of the He continuum from ∼900 to 650 A at reciprocal dispersions of 0.26 and 0.19 A/mm. The low rotational temperature of close to 20 K results in a drastic simplification of the band structures compared to room or even liquid nitrogen temperature spectra. This report concentrates on the region from 835 to 790 A and presents a survey of vibronic structures up to the first ionization potential. The Rydberg series of np complexes converging to v=0 in the ground state X 2Σ+g of the ion can be followed to n=39, the low‐J rotational structure being clearly discernible in complexes as high as n=19. The application of multichannel quantum defect theory to a least‐squares analysis of four series of rovibronic levels associated with the lowest rotational levels N+=0, 1, and 2 of the ion leads to very substantially reduced error limits for the ionization potential of 14N2 and reveals a number of...

Orbital angular momentum in triatomic molecules: IV. The Ã1II u state of C3

The electronic orbital angular momentum effects in the A 1II u state of C3 have been considered. A very good fit to the observed vibronic energy levels and rotational constants has been obtained using a full matrix treatment of the orbital angular momentum in conjunction with a large amplitude formalism for the bending motion. The importance of the variation of the expectation value with bond angle is emphasized; the quality of the final fit is very much improved when this is included. Franck-Condon overlap integrals have been calculated for bands of the A 1II u -[Xtilde] 1Σ g + transition, which is the carrier of the 4050 A group of bands prominent in cometary spectra. A comparison has been made between the vibronic energy levels of linear and quasi-linear molecules in orbitally degenerate electronic states.

Quantum defect theory for asymmetric tops: Application to the Rydberg spectrum of H2O

The multichannel quantum defect theory of rotational–electronic channel interactions and spectroscopic absorption intensities for rigid asymmetric tops is described. The symmetry selection rules are given in terms of the overall rotational–electronic parity and the even or odd character of K+a=Ka−λ. The angular momentum recoupling from Hund’s case (b) to (d) is shown to be accompanied by increasingly strong propensity rules K+a=‖K‘a−λ‘‖ and ‖N+−J‘‖≤l‘, which explain the simplicity of the rotationally resolved photoionization spectra of H2O and D2O without recourse to any predissociation mechanism. The main features of this spectrum are well reproduced, and ionization limits of 101 772(2) and 101 920(5) cm−1 are derived for H2O and D2O.

High orbital angular momentum states in H2 and D2

A characteristic group of lines observed in the infrared emission spectrum of H2 has been identified as representing the transitions between the 5g and the 4f group of levels. Because of nearly complete uncoupling of l from the molecular axis the appearance of this group of lines is very different from a normal band spectrum. An analysis was possible on the basis of ab initio computations of the spectrum using the knowledge of the vibrational levels, quadrupole moment, and the polarizability of the H+2 core. The validity of the assignments was further corroborated by agreement of the predicted spectra of D2 with the observed spectra. In addition, similar transitions corresponding to 4f–3d have been observed.