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The 1+1 REMPI spectrum of SiO in the 210–220 nm range was recorded. Observed bands were mostly assigned to the vibrational bands v ″ = 0 - 3 , v ′ = 5 - 10 of the A - X electronic transition. Additionally, a band near 216–217 nm was tentatively assigned as a 2-photon transition from X to the n = 12 , 13 [ X 2 Σ + , v + = 1 ] Rydberg states. Based on observed lines we estimated the IP of SiO to be 11.594(5) eV. The SiO+ cation has previously been identified as having a cycling transition useful for state control by optical pumping. Our work allowed us to identify an efficient method for loading an ion trap with rotationally and vibrationally cold SiO+ from an ablated sample of SiO by photoionizing through the (5,0) A - X band at 213.977 nm.

Mass resolved resonance enhanced multiphoton ionization (REMPI) spectra of a mixture of DCl and HCl were recorded for two-photon resonance excitation in the region of 80 500–89 500 cm−1. Spectra due to resonance transitions to rovibrational states of number of Rydberg and ion-pair states of DCl, were identified and assigned. Five new spectral bands due to transitions to Rydberg states and eight new bands due to transitions to vibrational states of the ion-pair state were identified and analysed to derive energetic parameters. Irregularities observed in vibrational energy level spacing, rotational constants and isotope shifts of chlorine isotopologues from what to expect for unperturbed states, as well as relative signal intensities, are indicative of strong homogeneous interaction between the E1Σ+ Rydberg state and the V1Σ+ ion-pair state. These observations are found to be due to level-to-level interaction between E and V rovibrational states of same J′ quantum numbers. Comparison of the data for DCl and HCl is performed.

International audience; VUV photoionization dynamics of the S2 molecule were re-investigated from threshold up to 15.0 eV, using synchrotron radiation coupled with double imaging photoelectron/photoion coincidence featuring high resolution capabilities. We measured the first threshold photoelectron spectrum of S2 achieving higher resolution than previous literature to derive accurate spectroscopic constants for a few electronic states of the cation including the X 2 PW, g ground state and the a 4 Pu, b 4 Sg-, and B 2 Sgstates. We also recorded the total ion yield for S2 up to a photon energy of 15.0 eV which, combined with the threshold photoelectron spectrum, led to the assignment of various autoionizing Rydberg series.

Over a thousand spectral lines in the photoexcitation spectrum of molecular deuterium (D2) to np 1 Σ u + and 1 Π u + Rydberg levels ( n ⩾ 4 ) were measured for rotational levels N ′ = 1 –6 in the 117 000–137 000 cm−1 spectral range by two different types of experiments at two synchrotron radiation sources: a vacuum ultraviolet (VUV) Fourier-transform (FT) spectrometer at SOLEIL, Paris and a 10 m-normal-incidence monochromator (NIM) at BESSY II, Berlin. The experimental energies, the absorption cross sections, Einstein A-coefficients, and line widths are compared with ab initio multi-channel quantum defect (MQDT) calculations for these levels. More than 350 R(0) or P(2) lines were assigned, some 280 R(1) or P(3) lines, some 270 R(2) or P(4) lines, over 100 R(3) or P(5) lines, over 90 R(4) lines, and 24 R(5) lines to extract information on the N ′ = 1 –6 excited levels. Transition energies were determined up to excitation energies of 137 000 cm−1 above the ground state, thereby extending earlier work by various authors and considerably improving the spectral accuracy ( 0.1 cm−1), leading to several reassignments. The absorption and the dissociation, ionization and fluorescence excitation cross sections from the NIM experiment are measured on absolute scale and are used to calibrate intensities in the VUV-FT spectra. The overall agreement between experiment and first principles calculations, without adjustable parameters, is excellent in view of the multi-state interferences treated within the MQDT-framework: For the low N ′ values the averaged deviations between those observed in the FT-SOLEIL spectra and those calculated with MQDT are ∼ 0.1 cm−1 with a spread of ∼ 0.5 cm−1. The line intensities in terms of Einstein coefficients are well represented in the MQDT-framework, as are the level widths representing the lifetimes associated with the sum of the three decay channels. These line intensities follow, in general, the 1 / n 3 scaling behavior as characteristic in Rydberg series, but deviations occur and those are explained by MQDT. The decay dynamics of the excited N Rydberg levels is analyzed on the basis of the measured quantum yields for ionization, dissociation and fluorescence observed in the NIM experiment in terms of absolute cross sections for the distinctive channels. In particular in the n = 4 manifolds dissociation is found to play a major role, where in the n = 5 manifolds the behavior is most erratic due to strong competition between decay channels. At n = 6 , ionization takes over as the dominant channel. Despite the excellent agreement between observations and the outcome of the MQDT calculations for both level energies and dynamics, some pronounced deviations are found as in the splitting of the 5 p π , v = 4 –6, N ′ = 1 levels. The shortcomings of the MQDT calculations are ascribed to the treatment of the excited states in terms of a 1 snp single electron configuration, therewith neglecting possible interferences with 1 snf or 2 s core excited states. Some 27 lines remained unassigned; in view of their observation in fluorescence it is stipulated that these lines probe levels in the nf manifold.

The understanding of the potential curves of a given molecule has expanded in many ways during my last 52 years as an experimental and theoretical molecular chemist/physicist in graduate school in Chemistry at Harvard and in both the Chemistry and Physics Departments at University of Iowa and University of Connecticut. This expansion has been up in energy and vibrational and rotational quantum numbers and away from Re to long range as well. It is clear that Prof. Robert Le Roy, who I have known since the late 1960s, has made many important and greatly appreciated contributions to the landscape I describe below from my personal perspective, especially with regard to long range molecules and solution of the radial Schrodinger equations and related calculations.

Recently, high-resolution spectroscopy of slow beams of metastable helium molecules He 2 ∗ generated by multistage Zeeman deceleration was used in combination with Rydberg-series extrapolation techniques to obtain the lowest rotational interval in the molecular helium ion at a precision of 18 MHz (Jansen et al., 2015), limited by the temporal width of the Fourier-transform-limited laser pulses used to record the spectra. We present here an extension of these measurements in which we have (1) measured higher rotational intervals of He 2 + , (2) replaced the pulsed UV laser by a cw UV laser and improved the resolution of the spectra by a factor of more than five, and (3) studied M J redistribution processes in regions of low magnetic fields of the Zeeman decelerator and shown how these processes can be exploited to assign transitions originating from specific spin–rotational levels ( N ″ , J ″ ) of He 2 ∗ .

With the help of a simple semi-analytical procedure the dipole matrix elements for Rydberg electron-rotational transitions in the SO molecule are calculated with account for dipole moment of rotating core. It results in non-zero matrix elements even for some transitions which are forbidden in conventional models of molecular Rydberg states.

Mass resolved REMPI data was recorded for jet cooled HI(g) for two-photon excitation in the region of 74 200 – 80 300 cm−1. REMPI spectra of the ions (HI+, I+ and H+) were derived from the data. The spectra revealed dense and overlapping rotational peak structure due to two-photon resonant transitions to number of Rydberg and ion-pair molecular vibrational states as well as iodine atom (2 + 1) REMPI peaks. A total of 16 Rydberg state spectra and 9 ion-pair state spectra were identified and simulated to derive spectroscopic constants (band origin (ν0) and 1st and 2nd order rotational constants (B′, D′)) for the excited states. Further analysis of the results for the Rydberg states and inspection of the spectral structure allowed more thorough characterization of the states as well as reassignment of some previously reported spectra. Detailed quantum defect analyses were particularly useful in that respect. The states could be specified by term symbols, ion core symmetries, Rydberg electron principal quantum numbers and configurations as well as vibrational quantum numbers. An attempt was made to assign the ion-pair spectra to vibrational quantum numbers. Relevant results on Rydberg and ion-pair states of HI, from the literature are summarized in the paper.

The photoabsorption spectra of C 4 H 4 O and C 4 D 4 O have been measured between ∼5.5 and 17.7 eV using a synchrotron radiation-based Fourier transform spectrometer. In addition to several broad bands due to transitions into valence states, the spectra exhibit numerous sharp bands associated with Rydberg states belonging to series converging onto the X 2 A 2 or the A 2 B 1 state limits. Vertical excitation energies and oscillator strengths have been computed using the second- and third-order algebraic-diagrammic construction polarisation propagator methods (ADC(2) and ADC(3)), and the equation-of-motion coupled-cluster method at the level of singles and doubles model (EOM-CCSD). Adiabatic excitation energies have been estimated using previously computed corrections. The theoretical predictions have allowed assignments to be proposed for the Rydberg series observed in the present single-photon absorption spectra and for some additional series, mainly of A 2 symmetry, reported in previous multiphoton excitation studies. The assignments of some of the Rydberg series converging onto the A 2 B 1 state limit have been revised and, guided by our calculations, the principal series is ascribed to the 2b 1 → n da 2 1 B 2 and 2b 1 → n db 1 1 A 1 transitions. f-type Rydberg series, previously observed only in the multiphoton absorption spectrum of furan, have been observed and assigned. Such f-type series, converging onto either the X 2 A 2 or the A 2 B 1 state thresholds, contribute significantly to the single-photon absorption spectrum. Many of the absorption bands associated with Rydberg states display vibrational progressions which resemble those in the corresponding photoelectron band. It appears that some of the structure associated with the 1a 2 → 3pb 2 1 B 1 and 1a 2 → 3pb 1 1 B 2 transitions involves excitation of non-totally symmetric vibrational modes.

Effects of electronic orbital angular momentum in triatomic molecules are reviewed, known collectively under the name Renner-Teller effect. The focus is on the years between 1960 and 1980, when numerous examples of the Renner-Teller effect became known experimentally. In the same years, Renner’s original theory was refined and extended in various groups in order to account for the new observations. The method developed in A. J. Merer’s research group in Vancouver is discussed in some detail. This effort, based on the theory of Hougen, Bunker, and Johns for large amplitude nuclear bending motion (1970), led to the first quantitative interpretation of the quasilinear bending level structure of the NH 2 radical - itself the first known example of the Renner-Teller effect observed by Dressler and Ramsay in 1958. Alternative approaches, refinements, and extensions of the theory implemented after 1980 are also described, the most important extension being the inclusion of the stretching vibrational degrees of freedom, which allows the description of anharmonic Fermi interactions. Finally, the manifestations of the Renner-Teller effect expected in Rydberg states are discussed, and a sketch of the corresponding theory is presented. It is shown that the Renner-Teller effect can also play an important role in continuum processes, such as the recombination of electrons with linear ions.

We observed that the two-photon excitation of a v3 hot band (6s Rydberg ) in methyl iodide is promising for isotopic laser separation, because the isotope shift of the multiphoton ionization (MPI) resonance is resolvable. To estimate the excitation cross section, which gives the enrichment factor (final isotope ratio per initial isotope ratio) of the separation method, we devised a method based on a pump-probe MPI procedure. By probing the material remaining after irradiation with the pump pulse, we estimated the cross section of the ground-state transition (6s Rydberg 3;0-0) to be 4.1+/-1.4x10-48 (cm4 s). The enrichment factor predicted from this cross section indicates that a high-performance laser system (20 mJ/pulse, 200 Hz, 10 ns duration) was capable of achieving an enrichment of over 1000-fold in view of the dissociation efficiency under low-pressure gas conditions. Thus, laser separation (elimination) appears to be a promising tool to create positron-emission tomography molecular probes. \n

The electronic energies of several molecular ions in different states have been recently determined by applying Hylleraas’ method along with Wind-Jaffe’s series to the Hamilton–Jacobi equation. In this paper, a continuation of three previous studies is presented: we have fitted to fourth-degree Rydberg generalized functions the potential energy curves for the 8 kπ , 9 kσ , 9 lπ , 9 lσ and 10 oσ electronic states of the H 2 + molecular ion. The correspondent rovibrational energies and spectroscopic constants were then derived by both the Discrete Variable Representation and Dunham’s method. The theoretical results – the first reported in the literature – are expected to provide a reliable comparison source to future works.

Resonance-enhanced multiphoton ionization combined with electronic ground state depletion spectroscopy of jet-cooled 2-methylallyl (C 4 H 7 ) radicals provides vibronic spectra of the 3 s and 3 p Rydberg states. Analysis of the vibronic structure following one-photon and two-photon excitation of rovibronically cold 2-methylallyl radicals and its isotopologues C 4 H 4 D 3 and C 4 D 7 reveals transitions to more than 30 vibrational levels in the 3 s Rydberg state that are identified and reassigned on the basis of predictions from ab initio calculations and results from pulsed-field-ionization zero-kinetic-energy photoelectron spectra obtained with resonant multiphoton excitation via selected intermediate states. Depletion spectroscopy reveals transitions to short-lived 3 p Rydberg states that have a large oscillator strength.

Photoelectron imaging technique has been applied to study (2 + 1) REMPI of atomic iodine through 8p Rydberg states around 253 nm. Full three-dimensional state-specific speed and angular distributions of the photoelectrons were recorded. The branching ratios among the different I(+) levels revealed that the perturbation on ((3)P(2))8p series is particularly large among the ((3)P(2))np series. The violation of core-conserving ionization is attributed to the interactions between the ((3)P(2))8p and ((1)D(2))6p series. The photoelectron angular distributions were found to be well characterized by P(2)(cos theta) and P(4)(cos theta). A relatively high positive beta(2) and a relatively low beta(4) observed in (2 + 1) REMPI process indicated that the ionization process can be approximately considered as single-photon ionization via the weakly aligned ((3)P(2))8P intermediate states. (C) 2009 Elsevier Inc. All rights reserved.

The possibility of simultaneous characterization of N2O and N2O van der Waals complexes in supersonic jet expansions is demonstrated for neat N2O samples. Room temperature and jet-cooled spectra of the Rydberg transitions 3pp 1 D X 1

The gerade autoionizing Rydberg states of Ne2 have been studied in the range 162 000–172 000 cm−1 by 1 + 1′ resonant two-photon excitation from the Ne2 X 0 g + ground state via different vibrational levels of the Ne2 C 0 u + state. A rotationally resolved part of the spectrum allowed the determination of the potential energy functions of two states of 1g and 0 g + characters in the vicinity of the Ne(2p6 1S0) + Ne (2p54p′) dissociation limit. The presence of maxima in these potential energy functions is interpreted as originating from a repulsive interaction between the Rydberg electron and the neutral atom.

In this work, the geometrical structures, the formation energies, and electronic states of the Na( m s)Ar n polyatomic exciplexes with m = 3–6 and n = 2–5 are studied by using a quantum-classical method. The interaction potential between an electronically excited sodium atom and argon atoms are calculated by using a one-electron model involving electron–Ar and electron–Na + pseudopotentials, in which the Hamiltonian is diagonalized at every optimization step in the Basin Hopping algorithm. The relationship between the position of the electronically excited levels and the cluster geometry is investigated as a function of the excitation level and of the spatial extension of the excited electron orbital. We show that the equilibrium structures of the ground state Na(3s)Ar n and those of the electronically excited states Na(4s)Ar n , Na(5s)Ar n , and Na(6s)Ar n are significantly different. As a result of the detailed examination of the relationships between the geometrical structure and density distribution of the Na valence electron of the Na ∗ Ar n with n = 2–5 polyatomic exciplexes, we can see that for the Na(4s)Ar n polyatomic exciplexes, the two extreme geometries, neutral Na(3s)Ar n and ionic Na + Ar n compete. It appears that none of them is the actual one. For Na(5s)Ar n and Na(6s)Ar n the valence electron is very weakly bound to the ionic core and described by a more diffused orbital so that the geometry and the formation energies of this excited state called Rydberg states converge towards those of the ionic cores.

The rovibronic structures of gerade excited states of molecular hydrogen have been investigated between 117 000 cm−1 and the second dissociation limit with extreme ultraviolet visible double resonant excitation technique. Several levels have been unambiguously assigned to the v = 0 state of the 4s and 4d Rydberg members 4 s σ O 1 Σ g + , 4 d σ P 1 Σ g + , 4dπ R1Πg, and 4dδ S1Δg. The fluorescence lifetimes have been measured under collision free conditions for the first time, revealing that the lifetimes for the O 1 Σ g + ( v = 0 ) state, 172.8 and 193.7 ns for J = 0 and 1, respectively, are much longer than those for the 4d Rydberg states. The rotational dependence of the lifetimes for the EF + GK + H 1 Σ g + , I1Πg, and J1Δg states is discussed in terms of the nonadiabatic coupling among the upper gerade manifolds.

The 3 1 Π g state of Na 2 is experimentally investigated by using high resolution cw optical–optical double resonance spectroscopy. A single line Ar + laser (total of 9 lines) is used to pump the sodium dimers from thermally populated ground state X 1 Σ g + to the intermediate B 1 Π u state. Then a single mode Ti:sapphire laser is used to probe the 3 1 Π g state. Violet fluorescence from highly Rydberg excited states (mainly 2 3 Π g or 3 3 Π g states which are transferred from 3 1 Π g state via collisions) to the a 3 Σ u + state is monitored by a filtered photomultiplier tube and a lock-in amplifier. Compared with previous studies [C.C. Tsai, J.T. Bahns, W.C. Stwalley, J. Chem. Phys. 99 (1993) 7417], a wider range of rotational quantum numbers of data field are observed. A set of Dunham coefficients and the Rydberg–Klein–Rees potential energy curve of the 3 1 Π g state are deduced from all the observed rovibrational levels.

Resonance-enhanced multiphoton ionization (REMPI) has been applied to study the n → 3 p Rydberg transition of pyrimidine (jet-cooled sample and mass resolved spectrum). Only the one component, the 3 p z ( B 2 ), appears in the (2 + 1) REMPI and the active vibrations are ν 6 a = 622, ν 1 = 946, and ν 9 a = 1116 cm −1 . The symmetry of the state was determined by polarization measurements (linear, circular polarization). The first ( π ∗ , n ) 3 B 1 triplet state appears as a one-photon resonance in the three-photon ionization process.

Some weak, collisionally induced transitions in 7 Li 2 have been recorded by Fourier transform spectrometry in the near infrared, following excitation of the 5 d 1 Π g state by optical–optical double resonance. They have been assigned as transitions to the 1 1 Δ g state from levels v =0 and 1 of a new ungerade Rydberg state, 5 p 1 Π u . Quantum defect considerations indicate that the principal quantum number for this new state is 5, and that the assignment to 5 p is compatible with a Rydberg series of which the lowest members would be the B 1 Π u and C 1 Π u states.

This paper reports for the first time both, an experimental observation and theoretical calculations of the K 2 4 3 Δ g state. For the experiment we used cw perturbation-facilitated optical–optical double resonance (PFOODR) spectroscopy. A single mode Ti-sapphire laser and a dye laser served as the pump and probe lasers, respectively. A total of 55 PFOODR signals have been assigned to the 4 3 Δ g ← b 3 Π u transitions. Absolute vibrational numbering was determined by using quantum defect analysis combined with comparing observed intensities with calculated Franck–Condon factors (FCF). For the former we used known parameters from the 2 3 Δ g state since the 2 3 Δ g and the 4 3 Δ g states belong to the same Rydberg series. We report here our experimental and calculated spectroscopic constants, the corresponding RKR potential energy curve, the Franck–Condon table for the 4 3 Δ g ↔ b 3 Π u system, as well as a comparison with the theoretical potential energy curve. The T e value is found to be 28408.938(52) cm −1 .

Experimental observations of the gerade (g) states of O2 that exist between the dissociation limit of the B 3 Σ u − state and the X 2 Π g ground state of O2+ are reexamined. Emphasis is placed on the assignment of electronic structures of the [X 2 Π g ]ndλ 1,3 Λ g Rydberg states for which experimental data can be obtained from the literature: mainly, ndπg- and ndδg-based 1,3 Λ g states with n=3–5 and the ndπ g 1 Σ g + and 3 Σ g − series with n=6–11. In many cases, characterizations of these electronic structures are limited by the quality of existing experimental data and, in cases where rotational band structures were analyzed, by the unavailability of most of the rotational energy level data upon which reported parameterizations were based. Thus, while a general picture of some segments of the ndπg- and ndδg-based structures can be presented, quantitative descriptions of these structures and unique identification of the causes of many apparent structural irregularities will not be possible until a new well-calibrated, rotationally resolved database is developed.

Well-calibrated, resonance-enhanced multiphoton-ionization (REMPI) spectra of eight bands of d 1 Π g ←a 1 Δ g and four bands of d 1 Π g ←b 1 Σ g + in O2 have been recorded and analyzed. These data provide detailed characterizations of the complex rotational structures in the five lowest vibrational levels (v=0–4) of the d 1 Π g state. The observed complexities in the rovibronic structure of d 1 Π g are the result of strong perturbations between the d 1 Π g Rydberg and the II 1 Π g valence states. Weaker perturbations resulting from interactions between d and the II 1 Δ g valence state also are observed.

With the help of a simple semi-analytical procedure the oscillator strengths for Rydberg electronic transitions in the ArH molecule are calculated by accounting for the effects of non-spherical symmetry of the molecular core’s field due to its dipole moment. Such effects result in non-zero oscillator strength values for some transitions which are forbidden in the widely used atom-like model of molecular Rydberg states. For the allowed transitions we also report the difference between the atom-like calculations and the calculations which take into account the dipole moment of the molecular core.

The 7 Li 2 5 1 Σ g + and 6 1 Σ g + states have been studied both experimentally and theoretically. Vibrational levels v =1–26 of the 5 1 Σ g + state and v =2–14 of the 6 1 Σ g + state were observed using pulsed optical–optical double resonance technique. The 5 1 Σ g + state has an unusual potential energy curve with a shelf near v =11. Dunham coefficients for the v =0–9 levels of the 5 1 Σ g + state have been obtained. RKR potential energy curves of these two were generated. Ab initio potentials are in good agreement with the RKR potentials.

We calculate oscillator strengths for Rydberg electron transitions in polar molecules with a simple analytical procedure of taking into account the effectsof nonspherical symmetry of molecular core field due to its dipole moment. Sucheffects result in nonzero oscillator strengths values for some transitions which are forbidden in widely used atom-like models of molecular Rydberg states. For the allowed transitions we also report the difference between the atom-like calculations and the calculations that account for the dipole moment of the molecular core.

We report the results of a study on the vibrational structure in the (n, 3s) Rydberg state of iso-butanal, carried out via 2+1 multiphoton ionization spectroscopy in a supersonic expansion. Spectra were recorded for both the normal species, (CH(3))(2)CHCHO, and the deuterated isotopomer, (CH(3))(2)CDCHO. The spectra show a complicated vibrational pattern attributable to the gauche (C(1)) conformer which is known to be 250 cm(-1) lower in energy than the more symmetric trans (C(s)) conformer. The spectra are dominated by the low-frequency formyl torsion which appears in combination with the two methyl torsions (in-phase and out-of-phase) and with several skeletal modes. Vibrational assignments have been aided by ab initio calculations of the (n, 3s) Rydberg state. Copyright 2001 Academic Press.

Analyses of high Rydberg series of HCO converging to the (030) vibrational state of the cation establish rovibrational state-detailed thresholds for HCO(+). UV-visible laser double resonance isolates series for assignment. Strongly vertical Rydberg-Rydberg transitions from photoselected N' = 0 and N' = 2 rotational levels of the Sigma(-) Renner-Teller vibronic component of the 3ppi (2)Pi (030) complex define individual series converging to rotational levels, N(+) = 1 through 5 and 3 through 5 of the HCO(+) vibrational states (03(1)0) and (03(3)0), respectively. Extrapolation of autoionizing series locates the positions of these rovibrational states to within +/-0.01 cm(-1). The use of this information combined with precise ionization limits for lower vibrational states determined from earlier Rydberg extrapolations and spectroscopic information available from infrared absorption measurements enables an estimate of the force-field parameters for HCO(+) bending. These parameters include the harmonic bending frequency, omega(2) = 842.57 cm(-1), the vibrational angular momentum splitting constant, g(22) = 3.26 cm(-1), and the diagonal bending anharmonicity, x(22) = -2.53 cm(-1), separated from the off-diagonal contribution, x(12), by reference to ab initio calculations. Results of experiment on the higher vibrationally excited states of HCO(+) are compared with recent theoretical predictions. Copyright 2000 Academic Press.

The VUV absorption spectrum of PbO vapor down to 300 Å has been observed for the first time. Four ionization energies have been determined from Rydberg series at 8.73, 8.83, 12.81, and 26.28 eV corresponding to the X(2)Pi, A(2)Sigma(+), B(2)X(+), and C(2)Sigma(+) electronic states of the PbO(+) ion. Copyright 2000 Academic Press.

With a sensitive space-charge-limited diode ionization detector, we studied 8–10 s ς, 8–10 d ς 1 Σ + g , and 10–15 d π 1 Π g Rydberg states of Na 2 and obtained their molecular constants for the first time. It is the first successful example of a diode ionization detector that is used in a pulsed OODR molecular spectroscopy experiment.

Five highly excited 1 Σ + g states of Na 2 were observed for the first time by pulsed optical–optical double resonance (OODR) fluorescence excitation spectroscopy. Three of the five states are assigned to the 6 s ς 1 Σ + g , 6 d ς 1 Σ + g , and 7 d ς 1 Σ + g Rydberg states. The other two states are assigned to doubly excited 1 Σ + g states.

Six core-penetrating Rydberg series have been assigned in the CaCl molecule by a combination of double-resonance spectroscopic techniques. Two 2Sigma+ series, with approximate quantum defects (delta) of 0.51 and 0.25, have been observed with effective principal quantum numbers (n*) in the range of 5-8 using the D2Sigma+ state as the resonant intermediate state for REMPI and/or ion-dip detection. A third 2Sigma+ series with delta approximately 0.84 and a 2Delta series with delta approximately 0.95 have been observed with n* = 16-18 using the A2Pi3/2 state as the resonant intermediate state for preparation of v+ = 1 vibrationally autoionizing states. Two additional series in the same region with delta approximately 0.90 and 0.07 are tentatively identified as the expected core-penetrating 2Pi series. Vibrational assignments have been confirmed in many cases on the basis of isotope shifts between the 35Cl and 37Cl isotopomers. The ion-dip and REMPI spectra display linewidths systematically broadened by predissociation. In addition to the assigned core-penetrating series, both the REMPI and the ion-dip spectra display some sharper features that have not yet been assigned. These sharper features most likely arise from perturbations of optically "bright" core-penetrating states by nominally "dark" higher-l core-nonpenetrating states. Copyright 1999 Academic Press.

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 (0.6Gamma1.2 cm-1) and computed Franck-Condon densities. Copyright 1999 Academic Press.

The extreme ultraviolet-visible double resonant excitation method was applied to the gerade Rydberg states of D 2 molecules. Tunable coherent extreme ultraviolet radiation near 103 nm prepared D 2 in the B 1 Σ + u ( v = 7, J ) state. Visible laser light subsequently brought them to the H 1 Σ + g ( v = 0 and 1), EF 1 Σ + g ( v = 28 and 32), and GK 1 Σ + g ( v = 2 and 5) states. Our term values for the GK 1 Σ + g state were in complete agreement with previous values, while deviations beyond the experimental error were found for the H 1 Σ + g ( v = 1) and EF 1 Σ + g ( v = 28 and 32) states. The fluorescence lifetimes, measured under a collision-free environment for the first time, were in reasonable agreement with ab initio calculations.

Rotationally and isotopically resolved single-photon excitation spectra of jet-cooled Cl2 in the wavelength region between 133 and 138 nm were recorded using a tunable vacuum ultraviolet "laser" generated by two-photon resonantly enhanced four-wave difference mixing in Kr gas. The dominant transition (1(1)Sigma+u

We report observations and analyses of bands in the spectra of the ArH and ArD molecules due to emission from the 4f Rydberg complexes. For ArD, the observed bands are 4f --5s and 4f --3dlambda for all three components lambda = 0, 1, and 2 of the 3d state. These give direct information on all 14 spin-orbit components of the 4f complex. For ArH, the data is more limited because the levels of the 5s state are broadened by predissociation, and only the lambda = 2 component of the 4f --3dlambda transitions is seen clearly, although a number of lines could be assigned to the 4f --3dpi band, which is strongly overlapped by the 6s --4p band. The 4f complexes of both isotopes are good example of Hund's case (d) coupling. The 4f orbitals are essentially non-core-penetrating, so that spin-orbit coupling is negligible for them, and the zero-rotation splitting agrees well with calculations from the ab initio quadrupole moment and anisotropic polarizability of the core. The orbit-rotation coupling parameter xi4f gives a value of the core electric dipole moment that is in reasonable agreement with ab initio and previous experimental values. A summary of the presently known states of ArH and ArD is presented. Copyright 1998 Academic Press.

Early assignments of the vacuum UV photoabsorption spectrum of allene (1951–1972) gave a number of Rydberg series, but these series had limiting values for the ionization potential, which are incompatible (by up to 0.4 eV) with more recent adiabatic IPs from molecular beam and high resolution photoelectron spectra (UV-PES). We have performed a re-analysis of the VUV spectrum of allene by a combination of overlay of the VUV and UV-PES spectra which leads to new experimental assignments. Major differences in assignment to the early studies are reported, and these include the lowlying Rydberg states, which are now assigned as follows: 6.55 (3s), 7.2 (3p), 8.15 (3d), and 8.25 eV (4s), respectively, with further assignments up to 6s, 6p, and 6d.

A new singlet-singlet absorption band between Rydberg states of the nitrogen molecule was studied by near-infrared diode laser spectroscopy in the 1.3mgr;m region. An analysis was made for the band to establish line assignments and determine molecular parameters for both the lower and the upper vibronic states. As a result of the analysis, this band was assigned to the c1Piu-a"1Sigmag+, v = 1-0 band. The anomaly of the Lambda-type doubling structure in the c1Piu (v = 1) state was discussed in connection with the interaction with the b'1Sigmau+ (v = 4) state. The predissociation in the c1Piu (v = 1) state was also discussed relating with the line broadening observed. Copyright 1997 Academic Press. Copyright 1997Academic Press

Single-photon transitions of jet-cooled Xe 2 have been recorded using a vacuum ultraviolet laser operating between 82 386–82 587 cm −1 and 83 706–84 006 cm −1 . Molecular ions produced by (1 + 1′) resonance enhanced multiphoton ionization and atomic fragment ions resulting from excited state predissociation were mass selected in a linear time-of-flight mass spectrometer. Subsequent vibrational analyses have led to molecular constants for two Rydberg states which dissociate to Xe( 1 S 0 ) + Xe*5 d [5/2] 0 3 and Xe( 1 S 0 ) +Xe*5 d [3/2] 0 1 , respectively. Excited state equilibrium bond length estimates from Franck–Condon factor calculations are also provided. Possible electronic symmetries for the excited states are proposed using angular momentum and long-range interaction theory arguments.

We report results of our studies of the electronic spectrum of the BaF molecule in the energy region of T 0 = 31 460–32 400 cm −1 (3.88 n * −1 ): K 2 Π 1/2 ( T 0 = 31 996), 4.08 2 Σ + ( T 0 = 32 166.293(2), B 0 = 0.2333892(17)), 3.94 2 Δ ( T 1 = 32 252.426(2), B 1 = 0.2294684(16), A 1 = 0.879(13)), and J 2 Π ( T 0 = 32 297.918(2), B 0 = 0.22952165(51), A 0 = 57.196(1)). In addition, the previously observed G 2 Σ + ( T 0 = 30 969.715(2), B 0 = 0.2297411(13)) and H 2 Σ + ( T 0 = 31 606.238(2), B 0 = 0.23031573(85)) states were rotationally analyzed for the first time. We verified by isotope analysis that the G 2 Σ + state, previously reported by J. Singh and H. Mohan [ Indian J. Pure Appl. Phys. 11, 918–922 (1973)], had been incorrectly assigned vibrationally. The I 2 Σ + state, which Singh and Mohan observed at low resolution, was absent from our spectra and we suspect that this state belongs to a species other than BaF. Electronic term values for the remaining seven not yet observed BaF core-penetrating Rydberg states below n * = 6 were predicted by interpolation of well-characterized Rydberg series. We suggest that the E ″ state, assigned by Effantin et al. as a 2 Π 1/2 state [C. Effantin, A. Bernard, J. D'Incan, E. Andrianavalona, and R. F. Barrow, J. Mol. Spectrosc. 145, 456–457 (1991)] must be reassigned as a 2 Δ state.

Out-of-plane potentials and optimized geometries for excited singlet states of formaldehyde, including n ,π*, σ,π*, π,π*, n ,3 s , and n ,3 p , were obtained by multireference CI methods. The 1 ( n ,π*) and 1 (σ,π*) states were found to be nonplanar, with out-of-plane angles of 34.5° and 42.1°, respectively. Agreement with the experimental geometry and energy of 1 ( n ,π*) is good, and even the inversion barrier is well reproduced. The 1 ( n ,3 s ), 1 (σ,π*), and 1 (π,π*) (mixed with n 2 ) states were found to have minima on different sections of the S 2 surface, separated from one another by low barriers of 0.2 to 0.7 eV. This causes the n → 3 s Rydberg transition to be perturbed by 1 (σ,π*) and 1 (π,π*). At nonplanar geometries, 1 (σ,π*) mixes heavily with 1 (π,π*); therefore, the π → π* band system, which may be observable from the higher vibrational levels of the ground state, is expected to be significantly perturbed. The C ← X system, recently reassigned to n → 3 p z , should be perturbed not only by 1 ( n ,3 p y ), as previously suggested, but also by 1 (π,π*) through the ν 6 (CH 2 rocking) mode. Furthermore, the Q 6 potential of 1 ( n ,3 p z ) is expected to interact with 1 ( n ,σ * CH ). The D ← X system, assigned to n → 3 p y , is predicted to be perturbed by 1 (π,π*), as well as by 1 (σ,π*) through the out-of-plane bending mode.

The vacuum ultraviolet absorption spectrum of methanol and two of its isotopes was recorded in the region 60 000–70 000 cm −1 . The molecules were supersonically expanded and they absorbed the impinging photons 1–2 cm downstream from the pulsed nozzle. The sharp peaks indicate an efficient cooling process. However, we could not detect any new peaks associated with dimers or higher oligomers. We assign the observed peaks as transition to the 3 p Rydberg states.

The VUV absorption spectrum of gas phase styrene has been remeasured and interpreted, taking into consideration the argon matrix-isolated styrene's spectrum and resonance-enhanced multiphoton ionization spectra (REMPI). Of particular interest was the detection of Rydberg states. Three Rydberg states with n = 3 were detected at 5.799 eV (delta = 0.74), 6.456 eV (delta = 0.40), and 6.5281 eV (delta = 0.35). Only the 6.5281-eV state has a strong REMPI signal and a well-defined vibrational structure. This Rydberg state has the same geometry as the ground state (planar styrene). The two origins of the ''benzenoid E(1u)'' valence state were detected at 5.986 and 6.161 eV and therefore the B-a-B-b splitting is 0.175 eV. (C) 1995 Academic Press Inc. Journal of Molecular Spectroscopy

The "sizes" of low-lying molecular Rydberg states, estimated experimentally from the results of pressure perturbation spectroscopy, are compared with the results of ab initio molecular orbital calculations. Lennard-Jones 6-12 parameters associated with absorber-perturber pairs were assigned to the ground and excited states on the basis of pressure perturbation spectroscopy. These data are reported for the lowest Rydberg transitions of NH 3 , acetone, and CH 3 I, as well as for the first two transitions of 1-azabicyclo[2.2.2]octane (ABCO). Increases in the sizes of these excited states relative to the respective ground states were compared with the results of ab initio calculations of the root-mean-square values of the electronic displacement, RMS R . Excited state calculations using configuration interaction (singles) with the 6-31+ G * basis set (augmented in the case of CH 3 I) were performed. Calculations are also reported for CH 2 O, SO 2 , CS 2 , and 1,4-diazabicyclo[2.2.2]octane. Results show that increases in molecular size for intravalence transitions are nearly zero, while for the lower Rydberg transitions, Δ(RMS R ) values range between 0.23 A (for ABCO) and 0.95 A (for NH 3 ). The calculations on ABCO and acetone indicate the involvement of C atom 3 s orbitals in the lowest excited (Rydberg) states. These findings are consistent with the experimental results obtained from pressure perturbation spectroscopy.

The absorption spectrum of S 2 has been observed down to 400 A on the SA63 line of SUPERACO (LURE) in Orsay, France. Two already reported Rydberg Series converging to the X 2 Π g ground state of S + 2 , π 4 u π 2 g X 3 Σ − g → π 4 u π g (π u np ) 3 Σ − u and π 4 u π 2 g X 3 Σ − g → π 4 u π g (σ u np ) 3 Π u , have been extended up to n = 10. Eight new series have been observed, giving ionization energies of 12.1247, 14.5562, 15.5737, 18.1067, 18.660, and 26.5838 eV, in good agreement with the values reported by Dyke et al. ( J. Chem. Soc. Faraday Trans. 7, 1026-1037, 1975) for the 2 Π u , 2 Σ − g , 2 Π u , 3 Σ − u , 2 Π u , and 2 Σ − g , ionic states, respectively.

Out-of-plane potentials for 1 A ′ and 1 A ″ states of thioformaldehyde, H 2 CS, at several CS distances, as well as equilibrium geometries of the singlet ( n ,π*), (π,π*), (σ,π*), ( n 0 ,π* 2 ), ( n , 4 s ), and ( n ,4 p x,y,z ) states were determined by multireference Cl methods. Particular emphasis was given to the S 2 surface, which is related to the unusual photoreactivity of thiones. S 2 is composed of (π,π*), (σ,π*), ( n ,4 s ), and ( n 0 ,π* 2 ). The former three states have very similar equilibrium energies and are separated by low barriers; therefore, internal conversions and vibronic couplings should give rise to complicated spectra. The (diabatic) 1 (π,π*) state is shown to be planar, whereas 1 (σ,π*) and 1 ( n 0 ,π* 2 ) are nonplanar. The latter state has a small HCH and large out-of-plane angle. The 1 (σ,π*) state is considered to be the photoactive state in S 2 since (σ,π*) lies slightly lower than (π,π*), can be easily populated by internal conversion from (π,π*), and has a low oscillator strength with the ground state. Evidence suggesting that ( n 0 ,π* 2 ) is the "dark state," a nonemissive state capable of being populated by internal conversion from (π,π*), is not conclusive. Vibrational frequencies were calculated by the HF/6-311++G** method for the 1 3 B 2 state of H 2 CS and D 2 CS, and the X 2 B 2 state of H 2 CS + and D 2 CS + .

Five bound nd 1 Δ g ( n = 11-15) electronic states have been investigated using optical-optical double resonance excitation followed by ionization detection using an ultrasensitive space charge limited diode, which detects ion signals (presumably K + 3 produced by associative ionization). The molecular constants and the Rydberg-Klein-Rees potential curves for these states have been generated. The dissociation energies, additional binding energies, and molecular constants due to the weakly bound Rydberg bonding δ g orbitals are discussed.

Twenty red-degraded bands of S 2 lying in the region 153-140 nm have been identified for the first time. The rotational analyses of these bands are carried out to obtain the various rotational constants. On the basis of their rotational analyses and intensity distribution, the bands are classified as belonging to eight progressions; five of the progressions are attributed to the electronic transitions [ 4 Π ui (S + 2 ) + σ g 4 s ] ← X O + g and the other three to [ 2 Π u (S + 2 ) + σ g 4 s ] ← X O + g . The upper electronic states are found to approximate (Ω c , ω) coupling. Using the Rydberg formula and the respective quantum defects for the corresponding atomic series 3 p 3 4 s , 5 S and 3 S, viz., 2.12 and 2.028, higher ionization potentials of S 2 have been computed to be 11.86 eV (4Π ui of S + 2 ) and 12.47 eV ( 2 Π u of S + 2 ), which are in close agreement with the respective values of 11.82 and 12.33 eV obtained from the VUV photoelectron spectrum of S 2 .