Rydberg states (n = 4–29) of azabicyclo [2.2.2] octane as studied by two-color fluorescence DIP and multiphoton ionization spectroscopies

Two-color fluorescence dip and multiphoton ionization (MPI) spectra have been observed for azabicyclo [2.2.2] octane in a supersonic jet. The spectra showed well-resolved structures consisting of five Rydberg series of n = 4–29. The five Rydberg series were assigned to s, p_z, p_xy, and two d orbitals. From the spectra obtained after exciting the molecule to various vibronic levels in the S₁ (3s) state, the Δν = 0 selection rule was obtained for the Rydberg-Rydberg transition. The same selection rule was found to be preserved also for the transition from the S₁ state to the ion. It was shown that the autoionization of the high Rydberg states to the ion is governed by Δν=?1. The existence of a very fast non-radiative channel was found for the Rydberg state from the ω₂ power dependence on the two-color MPI spectra.     

A precise determination of the first ionization potential of benzene

The first ionization potentials of benzene and benzene-d₆ have been precisely determined by the extrapolation of three-photon resonant Rydberg states in the four-photon ionization spectrum of the jet-cooled molecule. The convergence of resolved transitions in two Rydberg series for principal quantum numbers as high as 14 (-h₆) and 15 (-d₆) establish adiabatic thtesholds of 74573.0 ± 2.0 cm^?1, and 74592.5 = ± 1.2 cm^?1, respectively. These results are crucial for the understanding of the many excited states of benzene in terms of quantum defect theory. Precise quantum defects have been obtained for several Rydberg series and their variation with principal quantum number is reported. The results strongly suggest that the R? series of Wilkinson is derived from aπ(e_1g)→ nf±1 Rydberg excitation.     

Two-color excitation of NO in a supersonic free jet. Autoionization of high rydberg states

High Rydberg states of NO above the ionization limit have been measured for the isolated molecule in a supersonic free jet by two-color multiphoton ionization. Three Rydberg series (ns, np and nf) were identified, which appeared by rotational and the vibrational autoionization. The rotational structures of the 13s(υ = 1), 13p(υ = 1) and 12f(υ = 1) states have been analyzed in detail. The fluorescence dip spectra for the intermediate A²Σ⁺(3sσ) state have been measured simultaneously and the cross sections of the one-photon absorption to the high Rydberg states from the A²Σ⁺(υ = 1) state have been determined.     

The Rydberg Nature And Assignments Of Excited States Of The Water Molecule

We report ab initio theoretical calculation on 32 excited states of H₂ O found to lie below 11.7 eV. Of the eight states observed experimentally, the average discrepancy between theoretical and experimental excitation energies is 0.1 eV. We find that the excited states can each be characterized as arising from an excitation to a Rydberg orbital. Our results indicate that the ? and F? states are both 3d-like excited states rather than one 3d state and one 4s state as previously assumed and similarly for the two Rydberg series joining onto ? and F?. The nsa₁ Rydberg series is found to have a quantum defect of 1.38. joining onto the Ã(¹B₁ state. We have assigned the 9.81 eV transition observed by electron impact as the 1b₁ – 3pb₁ excitation to a ³A₁ state.     

Molecular rydberg transitions in carbon monoxide: term value/ionization energy correlation of BF,CO and N2

The linear correlation between the term value and ionization energy for molecular Rydberg transitions is tested for the sequence of isoelectronic molecules BF, CO and N₂ based on a new measurement of the absorption spectrum of CO and data for BF and N₂. For the npσ series and npπ series converging on the first ionization potential, we find an excellent linear behavior (within 10 meV) corroborating (i) the correlation and (ii) the individual assignments. For Rydberg series leading to the A ²Π and B ²Σ⁺ states, where no data for BF are available, a comparison of term values for CO and N₂ is presented.     

Ab initio MRD-CI study of ethane: The 14–25 eV PES region and Rydberg states of positive ions

Ab initio calculations of the MRD-CI type are reported for various states of the C₂H₆⁺ ion in two different nuclear geometries and the results are compared with the experimentally observed ethane PES in the 14–25 eV region. The calculated vertical IP values for ionization out of the 1e_u, 2a_2u and 2a_1g MO's respectively agree well with the locations of the three ionization maxima in this spectral range. The analogous findings for excitation out of the relaxed ionic ground state find several relatively low-lying species which occupy a 2pσ* MO in addition to states resulting from simple ionization of the neutral molecule. A number of Rydberg states are also calculated at the relaxed-ion geometry, from which results it is determined that the quantum defects for such species are from 0.40–0.45 units smaller than for their counterparts in neutral systems; these findings are clearly consistent with a decrease in the core penetrability of the Rydberg electron as the effective charge is increased to Z = 2.     

Even-parity auto-ionizing states of iridium I observed by resonance laser ionization spectroscopy

Auto-ionizing states of neutral iridium were observed in the continuum structure near the first ionization limit using one-color and two-color two-step resonance laser ionization spectroscopy. The total angular momentum of 20 even-parity auto-ionizing states could be determined from a combined analysis of the two-color spectra obtained with ionization schemes using intermediate states with different total angular momentum. Double-resonant ionization schemes were evaluated by fluence-dependence measurements, and photo-ionization cross-sections for resonant ionization transitions were determined. We could also identify several high-lying members of ns, np and nd Rydberg series converging to the first ionization limit of the atom.     

Multiconfigurational perturbation theory (CASPT2) applied to the study of the low-lying singlet and triplet excited states of cyclopropene

The electronic spectrum of cyclopropene has been studied using multiconfigurational second-order perturbation theory (CASPT2) with extended ANO-type basis sets. The calculation comprises two valence states and the 3s, 3p, 3d members of the Rydberg series converging to the π and σ ionization limits. A total of twenty singlet and twenty triplet excited states have been analyzed. The results confirm the valence nature of the lowest energy singlet-singlet band and yield a conclusive assignment: the first dipole-allowed transition in cyclcopropene is due to absorption to a (σ → π*) state. The (π → π*) (V) state is interleaved among a number of Rydberg states in the most intense band of the system. The remaining spectral bands are due to Rydberg transitions of higher energy. The two lowest singlet-triplet transitions involve the same valence states. The results are in agreement with available experimental data and provide a number of new assignments of the experimental spectra.     

A new detection scheme for synchronous,high resolution ZEKE and MATI spectroscopy demonstrated on the Phenol·Ar complex

We present a new detection scheme for high resolution mass analyzed threshold ionization (MATI) spectroscopy which utilizes fractional Stark state selective electric field ionization of high-n Rydberg states. The method represents a significant advance over previous approaches to MATI spectroscopy, which have been characterized by their low resolution compared to zero kinetic energy (ZEKE) photoelectron spectroscopy. In addition, the scheme can be used to synchronously obtain electrons and ions originating from the same Rydberg states, facilitating the acquisition of synchronous, high resolution ZEKE and MATI spectra. We demonstrate the technique by obtaining MATI dissociation spectra of the phenol·Ar complex, which provide dissociation energies of 364±13, 397±13, and 535±3 cm⁻¹ for the S₀, S₁ and D₀ states, respectively.     

Rydberg states of the K2 molecule studied by laser spectroscopy in a supersonic beam

Rydberg states of potassium dimer have been studied in a crossed laser-molecular beam experiment. The K₂ molecules were formed in a supersonic expansion and excited by low-power cw dye laser. Two different excitation schemes have been used: The first scheme uses a single mode ring dye laser to induce near resonant two-photon transitions while in the second scheme stepwise excitation with two dye lasers is used. In each case excitation of Rydberg levels was detected by monitoring the ionization signal resulting from three-photon absorption. We report a detailed study of 700 two-photon resonances between 625 nm and 650 nm. Most of these signals can be assigned to transitions from the X¹σ _g ⁺ to¹σ _g ⁺ ,¹Π _g , and^1Δ _g states, which are all enhanced by the B¹Π _u intermediate state. Accurate rotational constants are given for the populated vibrational levels of these states. By stepwise excitation of Rydberg levels via theB ¹Π _u state we identify 3 series of Rydberg states as¹Δ _g (4S+nD),¹Σ _g ⁺ (4S+nD), and¹Σ _g ⁺ (4S+nS) with principal quantum numbers 7≦n≦20. Molecular constants of these and other observed but as yet unidentified states are given; quantum defects and dissociation energies are discussed.     

Two-color multiphoton ionization of diazabicyclooctane in a supersonic free jet

Two-color multiphoton ionization (MPI) spectroscopy has been applied for diazabicyclooctane (DABCO) in a supersonic free jet. The MPI spectra due to transitions from the various vibronic levels of the S₁ (3s Rydberg) state which were excited by the first laser revealed the high Rydberg states above the adiabatic ionization potential. The ionization process and the vibrational potential of the ion are discussed.     

Jahn-Teller effect in Rydberg series: a multi-state vibronic coupling problem

Two simple limiting cases of Jahn-Teller (JT) coupling in Rydberg states of polyatomic molecules are considered, namely(i) JT coupling in Rydberg orbitals as well as in the ionization continuum (nondegenerate ion core, degenerate Rydberg series) and(ii) JT coupling in the ion core (degenerate ion core, nondegenerate Rydberg series). For both models simple and efficient algorithms for the computation of spectra (dynamical JT effect) are developed. The orbital JT effect is shown to represent a novel type of multi-state vibronic coupling, giving rise to interesting spectroscopic phenomena, among them resonant inter-Rydberg perturbations and JT induced autoionization. Particular attention is paid to the demonstration of the characteristic spectroscopic signatures of the two types of JT coupling in Rydberg states.     

Analysis of the Jahn-Teller effect in then p 2 E′ Rydberg series of H3 and D3

Spectroscopic signatures of Jahn-Teller (JT) coupling in then p _x,y ² E′ Rydberg series of H₃ and D₃ are analyzed within a simple multi-channel-quantum-defect-type model. The JT coupling constant has been inferred from existing ab initio calculations of the potential-energy surfaces of low Rydberg states of H₃. The model predicts pronounced resonant perturbations in Rydberg members with intermediaten (n=5–7) as well as strong JT-induced autoionization in absorption and ion-yield spectra from vibrationally excited initial levels. The JT-induced effects are shown to exhibit a characteristic dependence on the vibrational angular momentum of the bending mode in the initial state.     

Polarization labelling spectroscopy of molecular Li2 rydberg states

A series ofnd δ ¹ Δ _g Rydberg states of the Li₂ molecule has been measured using the polarization labelling technique. The experimental term values and the quantum defects derived from them, are compared with theoretical values obtained from ab initio calculations. The different physical effects contributing to the quantum defects are discussed. It turns out that perturbations of the Rydberg states by the doubly excited configurations with the atomic asymptotes 2p+2p have a significant influence on the quantum defects.     

Vertical Electronic Spectra of the Isovalent Molecules H2CNH,H2SiNH,H2CPH and H2SiPH on the Basis of MRD-CI Calculations

Abstract

Large-scale multi-reference single and double-excitation configuration interaction (MRD-CI) calculations are employed for the study of the isovalent compounds H₂CNH, HLSiNH, hLCPH and H₂SiPH in their ground state equilibrium geometry. The dipole moments and charge distributions are given. The vertical excitation energies to the intravalence states ^3,1 (n, π*) and ^3,1(π,π*) and to the first members of the Rydberg series originating from n and - MO's respectively are predicted; the first two ionization potentials and the Rydberg term values are also calculated. In H₂CNH, mixing of Rydberg and valence-shell states with CN stretching is analyzed. The trends in relative stability of electronic and ionized states can be directly related to increased orbital stability of n relative to π as soon as a first-row constituent is replaced by a second-row atom. The calculations explain the diffuse character of the uv spectrum of imines; they treat the molecules H₂SiNH and H₂SiPH for the first time and present a large number of data for all four molecules which can serve as a basis for future experimental investigations on these and related compounds.     

Isotope selective two-step photoionization study of K2 in a supersonic molecular beam

The photoionization efficiency curves for ^39,39K₂ and ^39,41K₂ dimers excited to the B ¹Π_u electronic state have been measured using sequential two-photon-ionization techniques. Accurate determinations of nine photoionization thresholds yield an adiabatic ionization energy of 4.06073 ± 0.00016 eV. Autoionizing Rydberg states are assigned and analyzed. Autoionization rates are measured for several Rydberg vibronic states. The isotopic dependence of the autoionization structure is partially analyzed and molecular constants for the K⁺₂ (²Σ⁺_g) state are derived.     

Similarities in the Intensities of Analogous Rydberg–Rydberg Transitions in the Molecular Series CF x Cl y (x=3, 2, 1; y=1, 2, 3)

In this work, the photoabsorption behaviour of the molecular series CF₃Cl, CF₂Cl₂ and CFCl₃, involving their ground state and two different Rydberg series, has been studied. The discrepancies or similarities in the intensities of homologous transitions in the three CF _x Cl _y molecules have been analysed on account of their electronic structure. Absorption oscillator strengths have been calculated with the Molecular quantum defect orbital (MQDO) approach. Electronic transitions between states belonging to two different unperturbed Rydberg series of the same molecule have been calculated by us for the first time. The quality of the achieved oscillator strengths has been assessed by comparison with, to our knowledge, scarce experimental data available in the literature, through analysis of the discrepancies or similarities in the intensities of homologous transitions in the molecular series CF _x Cl _y when states of different type are involved, and by testing the compliance of regularities by the Rydberg series object of our study.Article for the special issue dedicated to J.-P. Malrieu     

Vapor-phase electronic absorption spectrum of (η5-cyclopentadienyl)(η5-cycloheptatrienyl)chromium

The UV and visible absorption spectra of (Cp)(Ch)Cr (Cp = η⁵-C₅H₅, Ch = η⁷-C₇H₇) vapor show surprisingly well-resolved Rydberg structure. The first ionization potential (5.603 ± 0.007 eV) has been determined as a convergence limit of three Rydberg series. (Cp)(Ch)Cr is the first sandwich complex in which the lowest Rydberg transitions terminating at ns and np(σ) orbitals have vibrational structure.     

The Rydberg states of trans-butadiene from generalized valence bond and configuration interaction calculations

Self-consistent ab initio generalized valence bond (GVB) and configuration interaction (Cl) calculations are presented for the Rydberg states of trans-1,3-butadiene. Five Rydberg series were identified, three optically allowed (np, np, and nf_{z ³}) and two optically forbidden (ns and nd_{z ²}). It is shown that, except for the Λ～ and F? bands (which correspond to the non-valence 1 ¹B _u and the valence 2 ¹A _g states, respectively) all the transitions observed in the ultra-violet (UV) and electron-impact (EI) spectra, and in the two-photon spectra, can be assigned to members of these Rydberg series.