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We report on the measurement and analysis of photoionisation, mass-analysed-threshold-ionisation and pulsed-field-ionisation zero-kinetic-energy photoelectron spectra of HD in the vicinity of the dissociative-ionisation thresholds H + + D(1s) + e − and H(1s) + D + + e − . The spectra were recorded via the v = 11 − 13 vibrational levels of the H ¯ and B ¯ outer wells of the H H ¯ 1 Σ g + and B ′ ′ B ¯ 1 Σ u + states and provide information on the weakly bound levels of the X + 2 Σ g + ground state of HD + with vibrational quantum numbers v + = 16 − 21 and rotational angular-momentum quantum numbers N + = 0 − 10. The spectra also reveal for the first time shape and Feshbach resonances of HD + located energetically above the H + + D(1s) dissociation limit. The structure and dissociation dynamics of the H ¯ and B ¯ states of HD and of the X + 2 Σ g + and A + 2 Σ u + states of HD + near the dissociation thresholds are strongly influenced by the g/u-symmetry breaking caused by the nuclear-mass asymmetry. [ABSTRACT FROM AUTHOR]

Ab initio calculations of the energy level structure of H 2 + that include relativistic and radiative corrections to nonrelativistic energies and the diagonal part of the hyperfine interaction have predicted the existence of four bound rovibrational levels [(v = 0, N = 0 − 2) and (v = 1, N = 0)] of the first electronically excited ( A + Σ u + 2 ) state of H 2 + , the (v = 1, N = 0) level having a calculated binding energy of only Eb = 1.082 219 8(4)·10−9 Eh and leading to an extremely large scattering length of 750(5) a0 for the H+ + H collision [J. Carbonell et al., J. Phys. B: At., Mol. Opt. Phys. 37, 2997 (2004)]. We present an investigation of the nonadiabatic coupling between the first two electronic states ( X + Σ g + 2 and A + Σ u + 2 ) of H 2 + induced by the Fermi-contact term of the hyperfine-coupling Hamiltonian. This interaction term, which mixes states of total spin quantum number G = 1/2, is rigorously implemented in a close-coupling approach to solve the spin-rovibronic Schrödinger equation. We show that it mixes states of gerade and ungerade electronic symmetry, that it shifts the positions of all weakly bound rovibrational states of H 2 + , and that it affects both the positions and widths of its shape resonances. The calculations demonstrate that the G = 1/2 hyperfine component of the A+ (v = 1, N = 0) state does not exist and that, for G = 1/2, the s-wave scattering lengths of the H+ + H(1s) collision are −578(6) a0 and −43(4) a0 for the F = 0 and F = 1 hyperfine components of the H(1s) atom, respectively. The binding energy of the G = 3/2 hyperfine component of the A+ (v = 1, N = 0) state is not significantly affected by the hyperfine interaction and the corresponding scattering length for the H+ + H(1s, F = 1) collision is 757(7) a0. [ABSTRACT FROM AUTHOR]

The electron affinity of the deuterium atom has been determined to be 6086.81(27) cm−1 from a measurement of the difference between the D+ + H− and H+ + D− ion-pair dissociation energies and a thermochemical cycle involving the electron affinity of H and the ionization energies of H and D. Heavy-Rydberg states and the ion-pair dissociation thresholds of HD were accessed with good efficiency using a three-photon excitation sequence through the B Σ u + 1 ( v = 22, N = 1) and H ¯ Σ g + 1 ( v = 9, N = 0) intermediate levels and the threshold positions were determined using the method of threshold-ion-pair-production spectroscopy. [ABSTRACT FROM AUTHOR]

The structure and dynamics of high-n Rydberg states belonging to series converging to the (v+ = 0,N+ = 0–2) levels of the X + 2Σg+ electronic ground state of HD+ were studied by highresolution spectroscopy from the GK 1Σg+ (v = 1,N = 1) state under field-free conditions. Three effects of g/u-symmetry breaking were detected: (i) Single-photon transitions from the GK (v = 1, N = 1) state of gerade symmetry to the 30d21 and 31g22 Rydberg states of gerade symmetry were observed after careful compensation of the stray electric fields. (ii) The singlet 61p12 Rydberg state of ungerade symmetry was found to autoionize to the N+ = 0,ℓ = 2 ionization continuum of gerade symmetry with a lifetime of 77(10) ns. (iii) Shifts of up to 20MHz induced by g/u-symmetry mixing were measured for members of the np11 Rydberg series which lie close to nd21 Rydberg states. These observations were analyzed in the framework of multichannel quantum-defect theory. From the observed level shifts, the off-diagonal eigenquantum-defect element μpd of singlet-π symmetry was determined to be 0.0023(3) and the corresponding autoionization dynamics could be characterized. The ionization energy of the GK (v = 1,N = 1) state of HD was determined to be 12 710.544 23(10) cm−1. [ABSTRACT FROM AUTHOR]

[366] P.A.B. Haase, D.J. Doeglas, A. Boeschoten, E. Eliav, M. Ilias, P. Aggarwal, H.L. Bethlem, A. Borschevsky, K. Esajas, Y. Hao, S. Hoekstra, V.R. Marshall, Th. Meijknecht, M. Mooij, K. Steinebach, R.G.E. Timmermans, A. Touwen, W. Ubachs, L. Willmann, Y. Yin B Systematic study and uncertainty evaluation of P,T-odd relativistic enhancement factors in BaF b J. Chem. Phys. [25] W. Ubachs, K.S.E. Eikema, W. Hogervorst, B Narrowband extreme ultraviolet radiation tunable in the range 90.5-95 nm: application to spectroscopy of N SB 2 sb b Appl. Phys. [346] O. Versolato, A. Bayerle, M. Bayraktar, L. Behnke, H. Bekker, Z. Bouza, J. Colgan, J.R. Crespo Lopez-Urrutia, M.J. Deuzeman, R. Hoekstra, D. Kurilovich, B. Liu, R. Meijer, A. Neukirch, L. Poirier, S. Rai, A. Ryabtsev, J. Scheers, R. Schupp, J. Sheil, F. Torretti, W. Ubachs, S. Witte, B Spectroscopic investigations of YAG-laser-driven microdroplet-tin plasma sources of extreme ultraviolet radiation for nanolithography b J. Phys. [302] M.J. Deuzeman, A.S. Stodolna, E.E.B. Leerssen, A. Antoncecchi, N. Spook, T. Kleijntjens, J. Versluis, S. Witte, K.S.E. Eikema, W. Ubachs, R. Hoekstra, O.O. Versolato, B Ion distribution and ablation depth measurements of a fs-ps laser-irradiated solid tin target b J. Appl. Phys. [281] M.L. Niu, E.J. Salumbides, W. Ubachs, B Test of quantum chemistry in vibrationally-hot hydrogen molecules b J. Chem. Phys. [Extracted from the article]

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of H 2 has been recorded in the vicinity of the dissociative-ionization threshold following three-photon excitation via selected rotational levels of the B 1 Σ u + ( v = 19) and H ‾ 1 Σ g + ( v = 11) intermediate states. The spectra consist of transitions to bound levels of the X + 2 Σ g + state of H 2 + with v + in the range 14–19 and N + in the range 0–9, of the A + 2 Σ u + state with v + = 0 and N + = 0–2, and of shape resonances corresponding to the X + ( v + = 17, N + = 7) and X + ( v + = 18, N + = 4) quasibound levels. Calculations of the level structure of H 2 + have been carried out and the influence of adiabatic, nonadiabatic, relativistic and radiative corrections on the positions of these levels, and in the case of the shape resonances also on their widths, has been investigated. Different methods of calculating the widths and profiles of the shape resonances have been tested for comparison with the experimental observations. Slow oscillations of the dissociative-ionization yield have been observed and reflect, in first approximation, the Franck-Condon factors of the X + , A + ← H ‾ bound - free transitions. [ABSTRACT FROM AUTHOR]

We report a global study of the 3p Rydberg complex of the MgAr+ molecular ion. High-resolution spectroscopic data on the two spin–orbit components of the A+ electronic state were obtained by isolated-core multiphoton Rydberg-dissociation spectroscopy up to vibrational levels as high as v′ = 29, covering more than 90% of the potential wells. Accurate adiabatic potential-energy functions of the A+ and B+ states, which together form the 3p Rydberg complex, were obtained in a global direct-potential-fit analysis of the present data and the extensive data on the B+ state reported in Paper I [D. Wehrli et al., J. Chem. Phys. 153, 074310 (2020)]. The dissociation energies of the B+ state, the two spin–orbit components of the A+ state, and the X+ state of MgAr+ are obtained with uncertainties (1 cm−1) more than two orders of magnitude smaller than in previous studies. [ABSTRACT FROM AUTHOR]

We observe the formation of long-range Cs2 Rydberg molecules consisting of a Rydberg and a groundstate atom by photoassociation spectroscopy in an ultracold Cs gas near 6s1/2(F = 3,4) → np3/2 resonances (n = 26-34). The spectra reveal two types of molecular states recently predicted by D. A. Anderson, S. A. Miller, and G. Raithel [Phys. Rev. A 90, 062518 (2014)]: states bound purely by triplet 5-wave scattering with binding energies ranging from 400 MHz at n = 26 to 80 MHz at n = 34, and states bound by mixed singlet-triplet 5-wave scattering with smaller and F-dependent binding energies. The experimental observations are accounted for by an effective Hamiltonian including 5-wave scattering pseudopotentials, the hyperfine interaction of the ground-state atom, and the spin-orbit interaction of the Rydberg atom. The analysis enables the characterization of the role of singlet scattering in the formation of long-range Rydberg molecules and the determination of an effective singlet 5-wave scattering length for low-energy-electron-Cs collisions. [ABSTRACT FROM AUTHOR]

Abstract: The electronic spectrum of the C 0+, D 1←X 0+ transitions of KrXe has been studied at high resolution in the vicinity of the Kr(1S0)+Xe 6p[5/2]2 dissociation limit by resonance-enhanced (1+1′) two-photon ionization spectroscopy. The rotational structure of 13 bands, 5 and 8 of which correspond to transitions to levels of 0+ and 1 symmetry, respectively, were observed in the spectra of several isotopomers. The hyperfine structure in the spectrum of the Ω =1 levels of 84Kr129Xe was determined. The five transitions to levels of 0+ symmetry form a regular progression of bands characterized by a regular rotational structure and corresponding to high vibrational levels (with v =16–20) of the C 0+ state. The C 0+ state is found to possess significant X 1/2, A1 3/2 and A2 1/2 ion-core character in combination with an excited electron of 6pσ, 6pπ and 6sσ character, respectively, and to correlate adiabatically to the dissociation limit. The transition to the eight levels of Ω =1 symmetry form a very irregular progression both as far as spectral positions and intensities are concerned. Rotational levels of f-symmetry, accessed via Q-branch transitions, are weakly predissociated by a repulsive level associated with the limit. A local perturbation in the rotational structure of the fourth level of Ω =1 symmetry enabled the identification of a so far unobserved predissociative level of Ω =1 or Ω =2 symmetry with band center near 77318.5cm−1. The determination of the band centers, rotational constants and isotopic shifts of the Ω =1 levels led to the conclusion that the level structure is affected by homogeneous perturbations and that at least two electronic states of Ω =1 symmetry contribute to the spectrum of KrXe in this spectral region. Modelling the observed rovibronic structure using a coupling model involving low vibrational levels of a weakly bound Ω =1 state associated with the dissociation limit and high vibrational levels of a more strongly bound state associated with the Kr(1S0)+Xe 6p[5/2]2 dissociation limit enabled us to reproduce the observed values of the rotational constants, vibronic positions and intensity distribution in a qualitatively satisfactory manner. However, the model failed to account for the observed isotopic shifts. Rotationally resolved photoelectron spectra of the KrXe+ X 1/2←KrXe C 0+ and KrXe+ A1 3/2←KrXe C 0+ ionizing transitions were recorded from selected rotational levels of selected isotopomers of KrXe. The rotational levels of the X 1/2 state of KrXe+ group as pairs of levels of opposite parity and have a spin-rotation coupling constant γ of approximately −2B, as a consequence of pure precession. The rotational level structure of the low-lying electronic states of KrXe+ thus appears to form the same patterns as the corresponding states of ArXe+. [Copyright &y& Elsevier]

Abstract: Rotationally resolved (1+1′) resonance-enhanced two-photon ionization spectra of the band systems of several isotopomers of ArXe were recorded at high resolution in the wave-number range from to using a near-Fourier-transform-limited vacuum-ultraviolet laser system. The rotational structure of the vibrational levels of the ground state of ArXe could be fully resolved and assigned, leading to a new set of spectroscopic parameters. A ‘direct potential fit’ to a combination of these data with microwave spectroscopy results [W. Jäger et al., J. Chem. Phys. 99 (1993) 919–927] and virial coefficients yields an accurate new potential energy function with equilibrium distance and well depth . [ABSTRACT FROM AUTHOR]

The first adiabatic ionization energy and the first singlet–triplet splitting of the amidogen radical (NH 2 ) have been determined by high-level ab initio quantum chemistry based on the coupled-cluster approach (90  041 and 10  319  cm -1 , respectively) and by high-resolution pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy (90 083.8 ± 1.0 and 10 222.0 ± 1.3  cm -1 , respectively). A comparison between the theoretical and experimental values demonstrates the predictive powers of high-level ab initio theory in the derivation of the thermochemical properties of small molecular systems. The absolute accuracy of better than 100  cm -1 alleviates the experimental search for the relevant spectral features. [ABSTRACT FROM AUTHOR]

The term values of the rotational levels of the first excited vibrational state of the electronic ground state of He 2 + with a rotational quantum number N+ ≤ 13 have been determined with an accuracy of 1.2 × 10−3 cm−1 (∼35 MHz) by multichannel-quantum-defect-theory-assisted Rydberg spectroscopy of metastable He2. Comparison of the experimental term values with the most accurate ab initio results for He 2 + available in the literature [W.-C. Tung, M. Pavanello, and L. Adamowicz, J. Chem. Phys. 136, 104309 (2012)] reveals inconsistencies between the theoretical and experimental results that increase with increasing rotational quantum numbers. The fundamental vibrational wavenumber of He 2 + was determined to be 1628.3832(12) cm−1 by fitting effective molecular constants to the obtained term values. [ABSTRACT FROM AUTHOR]

REFERENCES 1 R. Signorell, F. Merkt, Faraday Discuss. Keywords: Ab-initio-Rechnungen; Isomerie; Jahn-Teller-Effekt; Photoelektronenspektroskopie DE Ab-initio-Rechnungen Isomerie Jahn-Teller-Effekt Photoelektronenspektroskopie 8086 8086 1 04/01/21 20210406 NES 210406 In diesem Aufsatz sollen die Legenden von Abbildungen 14 und 16 um Verweise auf früher verwendete Primärdaten ergänzt werden: 1) In der Legende von Abbildung 14: "Die Spektren wurden aus Primärdaten erstellt, die für die Abbildungen 6 a und 8 c von Lit. [1] sowie für die Abbildung 6 von Lit. [2] genutzt wurden." 2) In der Legende von Abbildung 16: "Das Spektrum wurde mit Primärdaten aus der gemeinsamen Arbeit von R. Signorell und F. Merkt erstellt, die für Abbildung 8 d von Lit. [1] (Spektralbereich bis 102060 cm SP -1 sp ) und Abbildung 8.5 von Lit. [3] (Spektralbereich oberhalb 102060 cm SP -1 sp ) sowie für Abbildung 4 von Lit. [4] genutzt wurden.". [Extracted from the article]

Keywords: ab initio calculations; fluxionality; isomerization; Jahn-Teller effect; photoelectron spectroscopy EN ab initio calculations fluxionality isomerization Jahn-Teller effect photoelectron spectroscopy 8009 8009 1 04/01/21 20210406 NES 210406 In this Review article, the captions of Figures 14 and 16 should be amended to include references to primary data used earlier. Corrigendum: Jahn-Teller Effects in Molecular Cations Studied by Photoelectron Spectroscopy and Group Theory Ab initio calculations, fluxionality, isomerization, Jahn-Teller effect, photoelectron spectroscopy. [Extracted from the article]

Partially rotationally resolved pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the three isotopomers (35Cl2, 35Cl37Cl, and 37Cl2) of Cl2 have been recorded in the wavenumber ranges 92 500-96 500 cm-1, corresponding to transitions to the low vibrational levels of the X+ 2Πg (Ω = 3/2, 1/2) ground state of Cl2+, and 106 750-115 500 cm-1, where the a+4Σu-←X1Σg+, A+2Πu←X1Σg+, and B+2Δu←X1Σg+ band systems overlap with transitions to high vibrational levels (v+ > 25) of the X+ state. The observation of Franck-Condon-forbidden transitions to vibrational levels of the X+ state of the cation with v+ >= 25 is rationalized by a mechanism involving vertical excitation of predissociative Rydberg states of mixed singlet-triplet character with an A+ ion core which are coupled to Rydberg states converging to high-v+ levels of the X+ state. The same mechanism is proposed to also be responsible for the observation of Cl+ - Cl- ion pairs and quartet states in the photoionization of Cl2. The potential energy function of the X+ state of Cl2+ was determined in a direct fit to the experimental data. Transitions to vibrational levels of the A+ 2Πu, 3/2 and B+ 2Δu, 3/2 states of Cl2+ could be identified using the results of a recent analysis of the strong perturbation between the A+ 2Πu, 3/2 and B+ 2Δu, 3/2 states of Cl2+ observed in the A+ - X+ band system [Gharaibeh et al., J. Chem. Phys. 137, 194317 (2012)], and transitions to several vibrational levels of the upper spin-orbit component (2Πu, 1/2) of the A+ state were detected in the photoelectron spectrum of Cl2+. The a+4Σu-←X1Σg+ photoelectron band system, which is nominally forbidden by single-photon ionization from the ground state was also observed for the first time and its vibrational and spin-orbit structures were analyzed. The 4Σu- state is split into two spin-orbit components with Ω = 1/2 and Ω = 3/2, separated by 37.5 cm-1. The vibrational energy level structure of both components is regular, which indicates that the splitting results from the interaction with one or more distant ungerade Ω = 1/2 or Ω = 3/2 electronic states. [ABSTRACT FROM AUTHOR]

Rotationally resolved pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the X 1/2, A1 3/2, and A2 1/2 electronic states of the ArXe+ molecular ion have been recorded following resonant (1+1′) two-photon excitation via selected rovibrational levels of the C 1 and D 0+ states of selected isotopomers of the ArXe molecule. Using rovibronic selection and propensity rules for the photoionization out of these intermediate molecular states enabled the determination of the parity of the molecular-ion levels and of the magnitude and sign of the Ω-doubling constants of the coupled X 1/2 (p ≈ 4B) and A2 1/2 (p ≈ -2B) states of ArXe+. The results indicate that these molecular-ion states can be approximately described using Mulliken's second variant of Hund's angular momentum coupling case (c), for which Ja, the total electronic and spin angular momentum of the two atoms, is a good quantum number (semi-united atom). The analysis of the rotational structure enabled the derivation of improved values of the dissociation energies, equilibrium distances, and molecular constants for the X 1/2, A1 3/2, and A2 1/2 states of ArXe+. [ABSTRACT FROM AUTHOR]

We report difference-frequency generation (DFG) of widely tunable, pulsed, Fourier-transform-limited terahertz radiation (bandwidth of ∼10 MHz, peak power up to ∼400 μW) from pulse-amplified near-infrared (λ∼800 nm) laser radiation using a crystal of the organic salt 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST) cut along the b axis to fulfill the phase-matching condition. The broad tunability and narrow bandwidth of the terahertz radiation generated by DFG in DAST are illustrated by a single scan of the terahertz spectrum of H2O from 0.1 to 11 THz, and a recording of the absorption spectrum of a pure rotational transition of HF near 5 THz. [ABSTRACT FROM AUTHOR]

The term values of all rotational levels of the 4He2+ X+ ³Su+ (ν+ = 0) ground vibronic state with rotational quantum number N + ≤ 19 have been determined with an accuracy of 8 × 10-4 cm-1 (~25 MHz) by multichannel-quantum-defect-theory-assisted Rydberg spectroscopy of metastable He2*. Comparison of these term values with term values recently calculated ab initio by Tung et al. [J. Chem. Phys. 136, 104309 (2012)] reveals discrepancies that rapidly increase with increasing rotational quantum number and reach values of 0.07 cm-1 (~2.1 GHz) at N+ = 19. [ABSTRACT FROM AUTHOR]

The reactions between ground-state H 2 + (X 2 Σ g + (v = 0 , J = 0)) and D 2 forming HD 2 + + H and H 2 D + + D were investigated in the range of collision energies E coll between E coll / k B = 0 and 10 K using a merged-beam approach. The reaction rates measured experimentally are compared to those obtained for the reaction between H 2 + and H 2 forming H 3 + + H under similar experimental conditions. Below 1 K, a clear enhancement of the reaction rate coefficient compared to the Langevin rate measured at higher collision energies was observed in both reaction systems. This enhancement is interpreted as originating from the interaction between the charge of H 2 + and the quadrupole of para D 2 and ortho H 2 molecules in the J = 1 rotational level. The enhancement of the reaction with D 2 was found to be significantly less than that of the reaction with H 2 , reflecting the relative population of the J = 1 rotational level of H 2 (75%) and D 2 (33%) in natural samples at low temperatures. Simulations of the experimental results based on the theoretical predictions of the reaction cross sections by Dashevskaya et al. [J. Chem. Phys. 145, 244315 (2016)] reveal agreement within the experimental uncertainties. The branching ratio η of the reaction involving H 2 + and D 2 and forming H 2 D + and D ( η = [ H 2 D + ] [ H 2 D + ] + [ H D 2 + ] ) near E coll = 0 was determined to be 0.341(15). Time-of-flight measurements of the velocity distributions of the reaction products are compatible with an isotropic product emission with an average total kinetic energy of 0.45(5) eV for both channels, representing about 30% of the total energy released by the reaction. [ABSTRACT FROM AUTHOR]

Multichannel quantum-defect theory (MQDT) is used to calculate the electron binding energies of np Rydberg states of H2, HD, and D2 around n = 60 at an accuracy of better than 0.5 MHz. The theory includes the effects of rovibronic channel interactions and the hyperfine structure, and has been extended to the calculation of the asymmetric hyperfine structure of Rydberg states of a heteronuclear diatomic molecule (HD). Starting values for the eigenquantum-defect parameters of MQDT were extracted from ab initio potential-energy functions for the low-lying p Rydberg states of molecular hydrogen and subsequently refined in a global weighted fit to available experimental data on the singlet and triplet Rydberg states of H2 and D2. The electron binding energies of high-np Rydberg states derived in this work represent important quantities for future determinations of the adiabatic ionization energies of H2, HD, and D2 at sub-MHz accuracy. [ABSTRACT FROM AUTHOR]

A laser-based, pulsed, narrow-band source of submillimeter-wave radiation has been developed that is continuously tunable from 0.1 THz to 14.3 THz. The source is based on difference-frequency mixing in the nonlinear crystal trans-4′-(dimethylamino)-N-methyl-4-stilbazolium tosylate. By varying the pulse length, the bandwidth of the submillimeter-wave radiation can be adjusted between 85 MHz and 2.8 MHz. This new radiation source has been integrated in a vacuum-ultraviolet-submillimeter-ware double-resonance spectrometer, with which low-frequency transitions of atoms and molecules in supersonic beams can be detected mass-selectively by photoionization and time-of-flight mass spectrometry. The properties of the radiation source and spectrometer are demonstrated in a study of 33f ← nd Rydberg-Rydberg transitions in Xe with n in the range 16-31. The frequency calibration of the submillimeter-wave radiation was performed with an accuracy of 2.8 MHz. The narrowest lines observed experimentally have a full-width at half-maximum of ∼3 MHz, which is sufficient to fully resolve the hyperfine structure of the Rydberg-Rydberg transitions of 129Xe and 131Xe. A total of 72 transitions were measured in the range between 0.937 THz and 14.245 THz and their frequencies are compared with frequencies calculated by multichannel quantum defect theory. [ABSTRACT FROM AUTHOR]

Vacuum ultraviolet spectra of the C 1 ← X 0+ and D 0+ ← X 0+ band systems of ArXe have been recorded at high resolution. Analysis of the rotational structure of the spectra of several isotopomers, and in the case of Ar129Xe and Ar131Xe also of the hyperfine structure, has led to the derivation of a complete set of spectroscopic parameters for the C 1 and D 0+ states. The rovibrational energy level structure of the C 1 state reveals strong homogeneous perturbations with neighboring Ω = 1 electronic states. The analysis of isotopic shifts led to a reassignment of the vibrational structure of the C 1 state. The observation of electronically excited Xe fragments following excitation to the C state rotational levels of f parity indicates that the C state is predissociated by the electronic state of 0- symmetry associated with the Ar(1S0) + Xe(6s′[1/2]0o) dissociation limit. The observed predissociation dynamics differ both qualitatively and quantitatively from the behavior reported in previous investigations. An adiabatic two-state coupling model has been derived which accounts for the irregularities observed in the rovibronic and hyperfine level structure of the C 1 state. The model predicts the existence of a second state of Ω = 1 symmetry, supporting several tunneling/predissociation resonances located ∼200 cm-1 above the C 1 state. [ABSTRACT FROM AUTHOR]

HI and D2 are coexpanded into a vacuum chamber. The photolysis of HI at 212.8 nm initiates the H+D2 reaction. The HD(v=4,J′=3) velocity distribution is determined by analyzing the time-of-flight profile of HD+ ions produced by delayed pulsed field ionization of long-lived Rydberg states. The angular distribution is deduced using the law of cosines (photoloc technique). © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Although the existence of quasibound rotational levels of the X+²S+g ground state of H+2 was predicted a long time ago, these states have never been observed. Calculated positions and widths of quasibound rotational levels located close to the top of the centrifugal barriers have not been reported either. Given the role that such states play in the recombination of H(1s) and H+ to form H+2, this lack of data may be regarded as one of the largest unknown aspects of this otherwise accurately known fundamental molecular cation. We present measurements of the positions and widths of the lowest-lying quasibound rotational levels of H+2 and compare the experimental results with the positions and widths we calculate using a potential model for the X+ state of H+2 which includes adiabatic, nonadiabatic, relativistic, and radiative corrections to the Born-Oppenheimer approximation. [ABSTRACT FROM AUTHOR]

We report on the experimental observation of the B+2Σ+ state of MgAr+ located below the Mg+(3p²P3/2) + Ar(¹S0) dissociation asymptote. Using the technique of isolated-core multiphoton Rydberg-dissociation spectroscopy, we have recorded rotationally resolved spectra of the B+2Σ+(v′) ← X+ ²Σ+(v″ = 7) transitions, which extend from the vibrational ground state (v′ = 0) to the dissociation continuum above the Mg+(3p²P3/2) + Ar(¹S0) dissociation threshold. The analysis of the rotational structure reveals a transition from Hund’s angular-momentum-coupling case (b) at low v′ values to case (c) at high v′ values caused by the spin–orbit interaction. Measurements of the kinetic-energy release and the angular distribution of the Mg+ fragments detected in the experiments enabled the characterization of the dissociation mechanisms. The vibrational levels of the B+ state above v′ = 6 are subject to predissociation into the Mg+(3p²P1/2) + Ar(¹S0) continuum, and the fragment angular distributions exhibit anisotropy β parameters around 0.5, whereas direct dissociation into the continuum above the Mg+(3p²P3/2) + Ar(¹S0) asymptote is characterized by β parameters approaching 2. Molecular ions excited to the B+ state with v′ = 0–6 efficiently absorb a second photon to the repulsive part of the ²Σ+ state associated with the Mg+(3d ²D3/2,5/2) + Ar(¹S0) continua. The interpretation of the data is validated by the results of ab initio calculations of the low-lying electronic states of MgAr+, which provided initial evidence for the existence of bound vibrational levels of the B+ state and for the photodissociation mechanisms of its low vibrational levels. [ABSTRACT FROM AUTHOR]

This article presents the current status of the knowledge of the rovibronic energy-level structure of CH 4 + obtained by high-resolution photoelectron spectroscopy. The results of previous investigations are summarized and extended by new results obtained by double-resonance experiments involving vacuum-ultraviolet and mid-infrared laser radiation. These experiments have led to assignments of the nuclear-spin symmetry of 303 rovibrational levels of CH 4 + with up to 3575 cm −1 of internal excitation. A two-dimensional model of the pseudorotational motion of CH 4 + is also presented, with which the positions and vibronic symmetry of the low-lying vibronic levels of CH 4 + can be predicted. The model maps the F ⊗ ( f ⊕ e ) Jahn-Teller problem for the e and f 2 modes corresponding to the C H bending motion onto a sphere, including the effects of both linear and quadratic Jahn-Teller coupling terms. The pseudorotation eigenstates are obtained by solving the two-dimensional Schrödinger equation using a basis of spherical harmonics. [ABSTRACT FROM AUTHOR]

We present an overview of our recent investigations of long-range interactions in an ultracold Cs Rydberg gas. These interactions are studied by high-resolution photoassociation spectroscopy, using excitation close to one-photon transitions into np Rydberg states with pulsed and continuous-wave ultraviolet laser radiation, and lead to the formation of long-range Cs molecules. We observe two types of molecular resonances. The first type originates from the correlated excitation of two atoms into Rydberg-atom-pair states interacting at long range via multipole-multipole interactions. The second type results from the interaction of one atom excited to a Rydberg state with one atom in the electronic ground state. Which type of resonances is observed in the experiments depends on the laser intensity and frequency and on the pulse sequences used to prepare the Rydberg states. We obtain insights into both types of molecular resonances by modelling the interaction potentials, using a multipole expansion of the long-range interaction for the first type of resonances and a Fermi-contact pseudo-potential for the second type of resonances. We analyse the relation of these long-range molecular resonances to molecular Rydberg states and ion-pair states, and discuss their decay channels into atomic and molecular ions. In experiments carried out with a two-colour two-photon excitation scheme, we observe a large enhancement of Rydberg-excitation probability, which we interpret as a saturable autocatalytic antiblockade phenomenon. [ABSTRACT FROM AUTHOR]

Optical frequency-comb spectroscopy has proven a very useful tool for high-resolution molecular spectroscopy. Frequency combs based on quantum-cascade lasers (QCL) offer the possibility to easily explore the mid-infrared spectral range (4 µm to 12 µm), but are characterised by large repetition frequencies (∼ 10 GHz) which make them seemingly unsuitable for high-resolution spectroscopy. Here, we present techniques to overcome this limitation. We have employed the combined advantages of high temporal and high spectral resolution to measure the infrared (IR) spectra of CF4 and CHCl2F in pulsed, skimmed supersonic beams. The low rotational temperature of the beams and the narrow expansion cone after the skimmer enabled the recording of spectra of cold samples with high resolution. The spectra cover the range from 1200 cm−1 to 1290 cm−1 and the narrowest lines have a full width at half maximum of 15 MHz, limited by the Doppler effect. The results demonstrate the potential of QCL dual-comb spectroscopy for broadband (> 60 cm−1) acquisition of spectra at high spectral (better than 5·10−4 cm−1, 15 MHz) and temporal (better than 4 µs) resolution and high sensitivity in the mid-infrared range. The power of the new technology is demonstrated by comparison with previous results for these molecules obtained by FTIR and diode laser spectroscopy. [ABSTRACT FROM AUTHOR]

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 ∗ . [ABSTRACT FROM AUTHOR]

Rotationally resolved photoelectron spectra of jet-cooled H2O, HDO and D2O have been recorded near the origin of thephotoionising transition following single-photon ionization using the complementary techniques of mass-analysed threshold-ionization (MATI) and pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy. A gas mixture of H2O, HDO and D2O with Ar was obtained by mixing H2O (ℓ) and D2O (ℓ) in a reservoir and bubbling Ar gas through the mixture. To unambiguously assign the photoelectron bands to H2O, HDO or D2O, the PFI-ZEKE photoelectron spectra of the mixture were compared to MATI spectra and to spectra of H2O. Analysis of the rotational structure of the origin bands (v+1= 0,v2+= 0,v+3= 0) ← (v1= 0,v2= 0,v3= 0) of H2O, HDO and D2O and of the transitions to the (010), (020) and (100) levels of D2O+and the first excited level of the O–D stretching mode of HDO+provided new information on the photoionization dynamics of water and the energy level structure of HDO+and D2O+. [ABSTRACT FROM AUTHOR]

Pulsed-field-ionisation zero-kinetic-energy (PFI-ZEKE) photoelectron spectra of 2-butyne (CH3–CC–CH3) and its fully deuterated isotopomer have been recorded in the region of the origin band of theionising transition. The spectral congestion originating from the combined effects of the internal rotation of the methyl groups, the spin–orbit coupling, and the Jahn–Teller effect prevented the full resolution of the rotational structure of the photoelectron spectra. A tentative analysis of the rotational branch structure of the photoelectron spectra using rovibronic photoionisation selection rules derived in the permutation–inversion spin double group G36(M2) suggests a splitting of ∼10.5 cm−1between the two spin–orbit components E3/2and E1/2of the2E1ground state and an almost free internal rotation of the methyl groups in the cations. Assignments are proposed for several low-lying vibrational levels of the cations. [ABSTRACT FROM AUTHOR]

Abstract: Widely tunable, Fourier-transform-limited pulses of terahertz (THz) radiation have been generated using (i) crystals of the highly nonlinear organic salt 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST), (ii) zinc telluride (ZnTe) crystals, (iii) gallium phosphide (GaP) crystals, and (iv) low-temperature-grown gallium arsenide (LTG-GaAs) photomixers with THz spiral antennas. Outputs from two narrowband (Δν <1 MHz, λ ∼800nm) cw titanium-doped sapphire (Ti:Sa) ring lasers with a well-controlled frequency difference were shaped into pulses using acousto-optic modulators (AOM), coupled into an optical fiber, pulse amplified in Nd:YAG-pumped Ti:Sa crystals and used as optical sources to pump the THz emitters. The THz radiation was detected over a broad frequency range and its bandwidth was determined to be ∼10MHz. The spectroscopic potential of the THz source is illustrated by the absorption spectrum of a pure rotational transition of OCS. [Copyright &y& Elsevier]

Transitions from selected nd Rydberg states of H2 to n'p/f Rydberg series converging on the lowest two (N+ = 0 and 2) rotational levels of the X+ ²Sg+ (v + = 0) ground state of para-H2+ have been measured in the range 1-7.4 THz using a laser-based, pulsed, narrow-band source of submillimeter-wave radiation. The analysis of the spectra by multichannel quantum-defect theory (MQDT) has allowed a complete interpretation of the fine structures of the Rydberg series and their dependence on the principal quantum number. The extrapolation of the series to their limits with MQDT has enabled the determination of the first rotational interval of para-H2+, which is 174.236 71(7) cm-1 (5 223 485.1(2.3) MHz). [ABSTRACT FROM AUTHOR]

Triplet (S = 1) He Rydberg atoms in supersonic beams with an initial velocity of 350 m s−1 have been decelerated to zero velocity and loaded into an off-axis electric trap in the presence and absence of magnetic fields. Comparing the deceleration efficiencies and the radiative decay of the population of trapped He Rydberg atoms to the (1s)1(2s)13S1 metastable level in the two sets of deceleration and trapping experiments revealed that the effects of magnetic fields up to 30 mT are negligible provided that a background dc electric field is maintained in the decelerator. A magnetic quadrupole trap of 30 mT depth corresponds to a He temperature of about 40 mK. The results thus represent an important step towards achieving high densities of cold paramagnetic samples following successive cycles of Rydberg–Stark deceleration, trapping, and radiative decay in overlaid electric and magnetic traps. [ABSTRACT FROM AUTHOR]

A supersonic beam of metastable neon atoms has been decelerated by exploiting the interaction between the magnetic moment of the atoms and time-dependent inhomogeneous magnetic fields in a multistage Zeeman decelerator. Using 91 deceleration solenoids, the atoms were decelerated from an initial velocity of 580 m/s to final velocities as low as 105 m/s, corresponding to a removal of more than 95% of their initial kinetic energy. The phase-space distribution of the cold, decelerated atoms was characterized by time-of-flight and imaging measurements, from which a temperature of 10 mK was obtained in the moving frame of the decelerated sample. In combination with particle-trajectory simulations, these measurements allowed the phase-space acceptance of the decelerator to be quantified. The degree of isotope separation that can be achieved by multistage Zeeman deceleration was also studied by performing experiments with pulse sequences generated for 20Ne and 22Ne. [ABSTRACT FROM AUTHOR]

The adiabatic ionization energy [in units of hc, Ei=124 568.485 81(36) cm-1] and the dissociation energy [D0=36 405.783 66(36) cm-1] of HD have been determined using a hybrid experimental-theoretical method. Experimentally, the wave numbers of the EF(v=0,N=0)→np[X+(v+=0 and 1, N+=0)] and EF(v=0,N=1)→np[X+(v+=0,N+=1)] transitions to singlet Rydberg states were measured by laser spectroscopy and used to validate predictions of the electron binding energies by multichannel quantum defect theory. Adding the transition energies, the electron binding energies and previously reported term energies of the EF state led to a determination of the adiabatic ionization energy of HD and of rovibrational energy spacings in HD+. Combining these measurements with highly accurate theoretical values of the ionization energies of the one-electron systems H, D, and HD+ further enabled a new determination of the dissociation energy of HD. [ABSTRACT FROM AUTHOR]

The transition wave numbers from selected rovibrational levels of the EF 1Σg+(v=0) state to selected np Rydberg states of ortho- and para-D2 located below the adiabatic ionization threshold have been measured at a precision better than 10-3 cm-1. Adding these wave numbers to the previously determined transition wave numbers from the X 1Σg+(v=0, N=0,1) states to the EF 1Σg+(v=0, N=0,1) states of D2 and to the binding energies of the Rydberg states calculated by multichannel quantum defect theory, the ionization energies of ortho- and para-D2 are determined to be 124 745.394 07(58) cm-1 and 124 715.003 77(75) cm-1, respectively. After re-evaluation of the dissociation energy of D2+ and using the known ionization energy of D, the dissociation energy of D2 is determined to be 36 748.362 86(68) cm-1. This result is more precise than previous experimental results by more than one order of magnitude and is in excellent agreement with the most recent theoretical value 36 748.3633(9) cm-1 [K. Piszczatowski, G. Łach, M. Przybytek et al., J. Chem. Theory Comput. 5, 3039 (2009)]. The ortho-para separation of D2, i.e., the energy difference between the N=0 and N=1 rotational levels of the X 1Σg+(v=0) ground state, has been determined to be 59.781 30(95) cm-1. [ABSTRACT FROM AUTHOR]

Pulsed-field-ionization zero-kinetic-energy photoelectron spectra of jet-cooled allyl radical (C3H5) have been recorded following single-photon and resonant multiphoton excitation. Simulations based on an orbital ionization model and rovibronic photoionization selection rules reliably describe the observed intensity distributions in the photoelectron spectra obtained from single-photon excitation from the ground state and resonant multiphoton excitation via the 3s and the 3p Rydberg states. More than 30 transitions to vibrational levels of the cation were identified and assigned on the basis of predictions from ab initio calculations. [ABSTRACT FROM AUTHOR]

The transition wave number from the EF 1Σg+(v=0,N=1) energy level of ortho-H2 to the 54p11(0) Rydberg state below the X+ 2Σg+(v+=0,N+=1) ground state of ortho-H2+ has been measured to be 25 209.997 56±(0.000 22)statistical±(0.000 07)systematic cm-1. Combining this result with previous experimental and theoretical results for other energy level intervals, the ionization and dissociation energies of the hydrogen molecule have been determined to be 124 417.491 13(37) and 36 118.069 62(37) cm-1, respectively, which represents a precision improvement over previous experimental and theoretical results by more than one order of magnitude. The new value of the ionization energy can be regarded as the most precise and accurate experimental result of this quantity, whereas the dissociation energy is a hybrid experimental-theoretical determination. [ABSTRACT FROM AUTHOR]

A supersonic beam of metastable He* atoms and He2* a 3Σu+ molecules has been generated using a pulsed discharge at the exit of a pulsed valve prior to the gas expansion into vacuum. Pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the He2+ X+ 2Σu+ (v+=0-2)←He2* a 3Σu+ (v″=0-2) transitions and photoionization spectra of He2* in the vicinity of the lowest ionization thresholds have been recorded. The energy level structures of 4He2+ X+ 2Σu+ (v+≤=2,N+≤=23) and 3He2+ X+ 2Σu+ (v+=0,N+≤=11) have been determined, and an accurate set of molecular constants for all isotopomers of He2+ has been derived in a global analysis of all spectroscopic data reported to date on the low vibrational levels of He2+. The analysis of the photoionization spectrum by multichannel quantum defect theory has provided a set of parameters describing the threshold photoionization dynamics. [ABSTRACT FROM AUTHOR]

Measuring spin-rotation intervals in molecular cations is challenging, particularly so when the ions do not have electric-dipole-allowed rovibrational transitions. We present a method, based on an angular-momentum basis transformation, to determine the spin-rotational fine structure of molecular ions from the fine structure of high Rydberg states. The method is illustrated by the determination of the so far unknown spin-rotation fine structure of the fundamentally important He2+ ion in the X ²Σ+u state. The fine-structure splittings of the v+=0, N+=1, 3, and 5 levels of He2+ are 7.96(14), 17.91(32), and 28.0(6) MHz, respectively. The experiment relies on the use of single-mode cw radiation to record spectra of high Rydberg states of He2 from the a ³Σ+u metastable state. [ABSTRACT FROM AUTHOR]

The photoelectron spectrum of the X+ 2Π ← X 1Σ+photoionising transition of iododiacetylene, HC4I, has been recorded using pulsed-field-ionisation zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy with partial resolution of the rotational structure. The first adiabatic ionisation energy of HC4I and the spin–orbit splitting of the X+ 2Π state of HC4I+are determined asEadI/(hc) = 74470.7(2) cm−1andcm−1, respectively. Several vibrational levels of the X+ 2Π electronic ground state of the HC4I+cation have been observed. The experimental data are discussed in the realm of a simple three-state charge-transfer model without adjustable parameters which allows for a qualitative description of the electronic structure and spin–orbit coupling in HC4I+and of the change in bond lengths upon ionisation of HC4I. [ABSTRACT FROM AUTHOR]