Rydberg states of calcium fluoride

Analysis of vibrational autoionization of CaF Rydberg states

We report calculations of vibrational autoionization rates of CaF Rydberg states, based on the results of a global multi-channel quantum defect theory (MQDT) fit. Our goal is to use intuitive physical models to interpret and extend the results from the MQDT calculations and, in particular, to characterize the physical mechanisms for the interaction between the Rydberg electron and the ion-core. The calculations indicate that, among the six strongly l-mixed core-penetrating (CP) Rydberg series of CaF, the n.36 p^Π Rydberg series has the fastest Δv = 1 vibrational autoionization rate, which is at least four times larger than that for the other CP Rydberg series, in agreement with experimental results. We first demonstrate that the rotational level dependence of the vibrational autoionization rate of the n.36 p^Π series is satisfactorily explained by l-uncoupling interactions, which differ for the positive and negative Kronig symmetry levels. Next, we interpret the relative vibrational autoionization rates of all six CP Rydberg series in the context of a valence-precursor (VP) model. The VP model is a consequence of Mulliken's rule, which states that the innermost lobe of the Rydberg wavefunction remains invariant in both the nodal position and shape for members of the same Rydberg series. The electronic properties of the six VP states, which are the terminus states (lowest-n) of each of the six CP Rydberg series, are further characterized in terms of a ligand-field model, providing insight into the intimate relationship between the Rydberg electron density in the ion-core region and the vibrational autoionization rate.

Towards a rational design of laser-coolable molecules: insights from equation-of-motion coupled-cluster calculations

Access to cold molecules is critical for quantum information science, design of new sensors, ultracold chemistry, and search of new phenomena.

Electronic structure of the polar molecules XF (X: Be, Mg, Ca) with rovibrational and dipole moment calculations

A theoretical investigation for the feasibility of laser-cooling is performed through the calculation of accurate potential energy curves, static dipole moments, spectroscopic constants and rovibrational calculations for 24, 26 and 27 highly excited electronic states for BeF, CaF and MgF molecules respectively. In order to understand the electronic structure of their lowest lying electronic states and to learn the characteristic behavior of their chemical bonding, a high level of calculation is realized by using the complete active space self-consistent field (CASSCF) with multi-reference configuration interaction MRCI method including single and double excitations with Davidson correction (+Q) for the three considered molecules. The comparison between the values of the present work and those available in the literature for several electronic states shows a good agreement. Fifty new excited electronic states have been investigated, in the present work, for the first time for the three studied molecules.

Broad shape resonance effects in CaF Rydberg states

Results of ab initio R-matrix calculations [S. N. Altunata et al., J. Chem. Phys. 123, 084319 (2005)] indicate the presence of a broad shape resonance in electron-CaF(+) scattering for the (2)Sigma(+) electronic symmetry near the ionization threshold. The properties of this shape resonance are analyzed using the adiabatic partial-wave expansion of the scattered electron wave function introduced by Le Dourneuf et al. [J. Phys. B 15, L685 (1982)]. The qualitative aspects of the shape resonance are explained by an adiabatic approximation on the electronic motion. Mulliken's rule for the structure of the Rydberg state wave functions [R. S. Mulliken, J. Am. Chem. Soc. 86, 3183 (1964)] specifies that, except for an (n*)(-32) amplitude scale factor, every excited state wave function within one Rydberg series is built on an innermost lobe that remains invariant in shape and nodal position as a function of the excitation energy. Mulliken's rule implies a weak energy dependence of the quantum defects for an unperturbed molecular Rydberg series, which is given by the Rydberg-Ritz formula. This zero-order picture is violated by a single (2)Sigma(+) CaF Rydberg series at all Rydberg state energies (n*=5-->infinity, more so with increasing n*) below the ionization threshold, under the broad width of the shape resonance. Such a violation is diagnostic of a global "scarring" of the Rydberg spectrum, which is distinct from the more familiar local level perturbations.

Properties of nearly one-electron molecules. II. Application to the Rydberg spectrum of CaF

The ab initio K matrix method described in the preceding paper (Part I) is applied to the Rydberg electronic structure of calcium monofluoride. The spectroscopic quantum defects for the 2Sigma+, 2Pi, 2Delta, and 2Phi states of CaF are computed using the effective potential of Arif et al. [M. Arif, Ch. Jungen, and A. L. Roche, J. Chem. Phys. 106, 4102 (1997)]. Satisfactory agreement with the experimental values is obtained. The eigenquantum defects obtained from the reaction matrix for the CaF++e- system are found to be strongly energy dependent. The analysis shows that the main features of the energy-dependent structure in the eigenphases are a consequence of a broad molecular shape resonance. Partial-l (orbital angular momentum) characters of two interacting collision eigenchannels vary rapidly as a function of increasing collision energy. This prominent variation leads to interference structure in the intensities for transitions into the ionization continuum, manifesting nodal points in the total ionization cross section in the continuum above the shape resonance. The usefulness of this structure in the ionization cross section as a direct probe of the l-character of the bound state is discussed. In addition, ab initio results for the photoelectron angular distribution and the anisotropy parameter are presented. These computed results are susceptible to direct experimental verification.

Electron–molecule scattering calculations in a 3D finite element R-matrix approach

We have implemented a three-dimensional finite element approach, based on tricubic polynomials in spherical coordinates, which solves the Schrodinger equation for scattering of a low energy electron from a molecule, approximating the electron exchange as a local potential. The potential is treated as a sum of three terms: electrostatic, exchange, and polarization. The electrostatic term can be extracted directly from ab initio codes (GAUSSIAN 98 in the work described here), while the exchange term is approximated using different local density functionals. A local polarization potential approximately describes the long range attraction to the molecular target induced by the scattering electron.

"Spectrum-only" assignment of core-penetrating and core-nonpenetrating Rydberg states of calcium monofluoride

Rydberg states of calcium monofluoride in the n* = 17–20 region have been observed by ionization-detected optical–optical double-resonance spectroscopy via the D2Σ+ v = 1 intermediate state. All members of the six core-penetrating Rydberg series in the n* = 17–20 region and several components of the 17f and 17g core-nonpenetrating Rydberg states have been assigned. While the assignment of core-penetrating Rydberg states is straightforward without use of an effective Hamiltonian model, "spectrum-only" assignment of core-nonpenetrating states is complicated because strong l-uncoupling causes the core-nonpenetrating states to evolve rapidly from Hund's case (b) to Hund's case (d) coupling. We describe "spectrum-only" assignment procedures, developed in the spirit of Gerhard Herzberg, that can be used to assign optical–optical double-resonance spectra of core-penetrating and core-nonpenetrating Rydberg states using only information contained in the spectrum rather than predictions derived from an effective Hamiltonian model. The ambiguities that arise in the assignment of each class of states are discussed in detail.Key words: CaF, electric quadrupole moment, Rydberg states, laser spectroscopy.

Core-Penetrating Rydberg Series of BaF: Single-State and Two-State Fits of New Electronic States in the 4.4 </= n* </= 14.3 Region

We report results of optical-optical double-resonance studies of the Rydberg states of the BaF molecule in the energy region of E = 33 100-38 200 cm(-1) (4.4 </= n* </= 14.3). Barium monofluoride molecules have been produced in a resistively heated high-temperature oven from BaF(2) powder mixed with a small amount of boron powder. We have observed more than 50 new electronic states. Forty-nine of them have been rotationally analyzed and assigned to eight Rydberg series (four (2)Sigma(+), one (2)Pi, two (2)Delta, and one (2)Phi). Single-state and two-state fits have been carried out. Multiple local perturbations have been analyzed. Formation of supercomplexes in the Rydberg spectrum of BaF has been discussed. Copyright 2001 Academic Press.

Analysis and Deperturbation of the A(2)Pi and B(2)Sigma(+) States of CaF

A laser excitation spectrum of the (0, 0) and (1, 1) bands of the CaF A(2)Pi-X(2)Sigma(+) system was recorded. The present set of molecular constants for both A(2)Pi and B(2)Sigma(&plus:) states represents a vast improvement in precision over previously reported values because of the larger number of line position measurements and the use of a more reasonable effective Hamiltonian model which accounts explicitly for the A approximately B interaction via off-diagonal matrix elements. The principal constants (in cm(-1)) are where uncertainties of 1varsigma are given in parentheses. Copyright 1999 Academic Press.

The Predissociation Mechanism for 2Sigma+ Rydberg States of CaCl

This work summarizes experimental results from recent ion-dip spectroscopy studies of CaCl and from previously unpublished optical-optical double-resonance work with specific regard to predissociation processes of 2Sigma+ Rydberg states in the low n* (n* = 7, IP - En* = 2240 cm-1) region. A single repulsive state (assigned as 2Sigma+) was found to be responsible for all observed predissociations of 2Sigma+ Rydberg states. The n*-dependent internuclear distances of the intersections between Rydberg states and the repulsive 2Sigma+ state were determined through the use of trial-and-error Franck-Condon calculations. Values of energy-descaled electronic matrix elements governing the Rydberg left and right arrow repulsive state interaction were obtained from the measured linewidths (0.6 < Gamma < 1.2 cm-1) and computed Franck-Condon densities. Copyright 1999 Academic Press.