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Rydberg states of calcium monofluoride in the n* = 17-20 region have been observed by ionization-detected optical--optical double-resonance spectroscopy via the [D.sup.2][[SIGMA].sup.+] 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 stores 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, etectric quadrupole moment, Rydberg states, laser spectroscopy.

We present an improved quantum defect theory model for the 's,' 'p,' 'd,' and 'f' Rydberg series of CaF. The model, which is the result of an exhaustive fit of high-resolution spectroscopic data, parameterizes the electronic structure of the ten ('s'Σ, 'p'Σ, 'p'Π, 'd'Σ, 'd'Π, 'd'Δ, 'f'Σ, 'f'Π, 'f'Δ, and 'f'[uppercase_phi_synonym]) Rydberg series of CaF in terms of a set of twenty μℓℓ′(Λ) quantum defect matrix elements and their dependence on both internuclear separation and on the binding energy of the outer electron. Over 1000 rovibronic Rydberg levels belonging to 131 observed electronic states of CaF with n* ≥ 5 are included in the fit. The correctness and physical validity of the fit model are assured both by our intuition-guided combinatorial fit strategy and by comparison with R-matrix calculations based on a one-electron effective potential. The power of this quantum defect model lies in its ability to account for the rovibronic energy level structure and nearly all dynamical processes, including structure and dynamics outside of the range of the current observations. Its completeness places CaF at a level of spectroscopic characterization similar to NO and H2. [ABSTRACT FROM AUTHOR]

The Stark effect in molecular Rydberg states is qualitatively different from the Stark effect in atomic Rydberg states because of the anisotropy of the ion core and the existence of rotational and vibrational degrees of freedom. These uniquely molecular features cause the electric-field-induced decoupling of the Rydberg electron from the body frame to proceed in several stages in a molecule. Because the transition dipole moment among the same-n* Rydberg states is much larger than the permanent dipole moment of the ion core, the decoupling of the Rydberg electron from the ion core proceeds gradually. In the first stage, analyzed in detail in this paper, l and N are mixed by the external electric field, while N+ is conserved. In the further stages, as the external electric field increases, N+, n*, and v+ are expected to undergo mixing. We have characterized these stages in n*=13, v+=1 states of CaF. The large permanent dipole moment of CaF+ makes CaF qualitatively different from the other molecules in which the Stark effect in Rydberg states has been described (H2, Na2, Li2, NO, and H3) and makes it an ideal testbed for documenting the competition between the external and CaF+ dipole electric fields. We use the weak-field Stark effect to gain access to the lowest-N rotational levels of f, g, and h states and to assign their actual or nominal N+ quantum number. Lowest-N rotational levels provide information needed to disentangle the short-range and long-range interactions between the Rydberg electron and the ion core. We diagonalize an effective Hamiltonian matrix to determine the l-characters of the 3≤=l≤=5 core-nonpenetrating 2Σ+ states and to characterize their mixing with the core-penetrating states. We conclude that the mixing of the l=4, N-N+=-4(g(-4)) state with... [ABSTRACT FROM AUTHOR]

Observation and analysis of the f(ℓ=3), g(ℓ=4), and h(ℓ=5) Rydberg series of CaF in the range 13≤=n≤=17 is presented. Simultaneous analysis of the f, g, and h Rydberg series of CaF, combined with significant improvements to the long-range model for nonpenetrating Rydberg states, provides a generally applicable scheme for separating the effects of the long-range (electric multipole and polarization) and short-range (core penetration) interactions between the Rydberg electron and ion core in diatomic molecules. Techniques for rigorous assignment of nonpenetrating states, as well as extensions of the long-range model, are discussed. Explicit formulas for the first- and second-order matrix elements of the first four anisotropic electric multipole interactions (electric dipole, quadrupole, octupole, and hexadecapole) are given. The discrepancies between the observed behavior and that predicted by the long-range model, which are particularly significant for the f series, are shown to be due to penetration of the Rydberg electron wavefunction within the ion core. We show that these penetration effects can be understood within the framework of ligand field theory and conclude with a discussion of the relative contributions of the long- and short-range interactions to the quantum defects of the core-penetrating “s,” “p,” and “d” series of CaF. [ABSTRACT FROM AUTHOR]

We report direct doubly differential (quantum state and angle-resolved) scattering measurements involving short-lived electronically excited molecules using crossed molecular beams. In our experiment, supersonic beams of nitric oxide and argon atoms collide at 90°. In the crossing region, NO molecules are excited to the A2Σ+state by a pulsed nanosecond laser, undergo rotationally inelastic collisions with Ar atoms, and are then detected 400 ns later (approximately twice the radiative lifetime of the A2Σ+state) by 1 + 1′ multiphoton ionization via the E2Σ+ state. The velocity distributions of the scattered molecules are recorded using velocity-mapped ion imaging. The resulting images provide a direct measurement of the state-to-state differential scattering cross sections. These results demonstrate that sufficient scattering events occur during the short lifetimes typical of molecular excited states (∼200 ns, in this case) to allow spectroscopically detected quantum-state-resolved measurements of products of excited-state collisions. [ABSTRACT FROM AUTHOR]

The paper reports experimental measurements and theoretical calculations of rotational-state-resolved differential scattering cross-sections (DCS) for collisions between electronically excited NO(A2Σ+) molecules and rare gas atoms. The experimental NO(A2Σ+) + Ar and NO(A2Σ+) + He state-resolved product scattering distributions are determined using velocity-mapped ion imaging. The ion images are analysed to determine the state-resolved DCS, which are compared with new theoretical DCS calculated using quantum scattering methods on ab initio electronic potential energy surfaces. Both collision systems are imaged simultaneously; this constraint on the collision energies of the two systems aids the comparison to theory. The experimental and calculated DCS agree well for the NO(A2Σ+)/He system. For the NO(A2Σ+)/Ar scattering system, the experiments do not recover the degree of forward-scattering theoretically predicted and significant differences in the positions of the observed and predicted rotational rainbow features are apparent at large scattering angles, particularly for the most rotationally inelastic collision channels investigated: ΔN = 12,14. [ABSTRACT FROM AUTHOR]

We report the first measured differential cross sections for rotationally inelastic collisions between ND3and Ne, obtained using velocity-mapped ion imaging. In these experiments, ND3molecules initially in the J= 0, K= 0 and J= 1, K= 1 quantum states collide with Ne atoms at a center-of-mass collision energy of 65 meV, leading to rotational excitation of ND3. Differential cross sections are then determined from images of the rotationally excited scattered molecules using an iterative extraction method. These measurements complement and compare well with previous measurements of differential cross sections for the ammonia−rare gas system (Meyer, H.J. Chem. Phys.1994, 101, 6697.; Meyer, H.J. Phys. Chem.1995, 99, 1101.) and are also relevant to the production of cold ND3molecules by crossed-beam scattering (Kay, J. J.; van de Meerakker, S. Y. T.; Strecker, K. E.; Chandler, D. W.Faraday Discuss.2009, DOI: 10.1039/B819256C). [ABSTRACT FROM AUTHOR]

A high resolution cavity ringdown spectrometer (CRDS) has been constructed using a 1.5 μm continuous-wave external-cavity tunable diode laser, a mode-matched near-confocal ringdown cavity, and 2 cm pulsed slit jet. Without signal averaging, the RMS noise in the absorption signal is 1.7 × 10-9 cm-1. The rotationally resolved overtone spectrum of the OH(ν1) + CH(ν3) stretch combination band of methanol between 6510 and 6550 cm-1 has been observed for J′=0–8 and K′=0–3 at sub-Doppler resolution. In total, 418 lines are assigned and global fits yield molecular torsion–rotation parameters for the upper state. Four K′-localized perturbations are analyzed and the pattern of residuals is discussed. [Copyright &y& Elsevier]