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Two-color (1 + 1′) zero-electron-kinetic-energy (ZEKE) and photoionization efficiency (PIE) spectra are reported via different levels in the S1 ← S0 ( A ̃ 1 A 2 ← X ̃ 1 A 1 ) one-photon transition of jet-cooled N-methylpyrrole. The laser radiation is produced using two dye lasers, one with an 1800 l/mm grating and one with 2400 l/mm. We report spectra where the excitation and ionization radiation are produced with both combinations of the dye lasers; these spectra differ markedly. This is attributed to Wood's anomalies with the 2400 l/mm grating: one aspect is a loss in light intensity over a range of wavelengths, attributed to a resonance anomaly. Another is the appearance of a "shadow" ZEKE spectrum and PIE curve at apparently higher ionization wavenumbers; under some conditions, a third ZEKE spectrum was observed—these latter observations arise from higher-order dispersion effects, likely caused by a Rayleigh anomaly. We comment on these observations and report more representative ZEKE and PIE spectra than those presented in a recent paper by our group [A. R. Davies, D. J. Kemp, and T. G. Wright, J. Chem. Phys. 154, 224305 (2021)] for four intermediate S1 levels. [ABSTRACT FROM AUTHOR]

The electronic spectrum associated with the S1 ← S0 ( A ̃ 1 A 2 ← X ̃ 1 A 1 ) one-photon transition of jet-cooled N-methylpyrrole is investigated using laser-induced fluorescence (LIF) and (1 + 1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy; in addition, the (2 + 2) REMPI spectrum is considered. Assignment of the observed bands is achieved using a combination of dispersed fluorescence (DF), two-dimensional LIF (2D-LIF), zero-electron-kinetic energy (ZEKE) spectroscopy, and quantum chemical calculations. The spectroscopic studies project the levels of the S1 state onto those of either the S0 state, in DF and 2D-LIF spectroscopy, or the ground state cation (D0+) state, in ZEKE spectroscopy. The assignments of the spectra provide information on the vibrational, vibration-torsion (vibtor), and torsional levels in those states and those of the S1 levels. The spectra are indicative of vibronic (including torsional) interactions between the S1 state and other excited electronic states, deduced both in terms of the vibrational activity observed and shifts from expected vibrational wavenumbers in the S1 state, attributed to the resulting altered shape of the S1 surface. Many of the ZEKE spectra are consistent with the largely Rydberg nature of the S1 state near the Franck–Condon region; however, there is also some activity that is less straightforward to explain. Comments are made regarding the photodynamics of the S1 state. [ABSTRACT FROM AUTHOR]

Zero-electron-kinetic-energy (ZEKE) spectra are presented for m-chlorotoluene (mClT), employing different low-lying torsional and vibration–torsional ("vibtor") levels of the S1 state as intermediates. The adiabatic ionization energy is determined to be 71 319 cm−1 ± 5 cm−1 (8.8424 ± 0.0006 eV). It is found that the activity in the ZEKE spectra varies greatly for different levels and is consistent with the assignments of the S1 levels of m-fluorotoluene (mFT) deduced in the recent fluorescence study of Stewart et al. [J. Chem. Phys. 150, 174303 (2019)] and the ZEKE study from Kemp et al. [J. Chem. Phys. 151, 084311 (2019)]. As with mFT, the intensities in the ZEKE spectra of mClT are consistent with a phase change in the torsional potential upon ionization, allowing a large number of torsions and vibtor levels to be observed for the cation. Vibration-induced modifications of the torsional potential are discussed. Calculated vibrational wavenumbers for the S0, S1, and D0+ states are also presented. [ABSTRACT FROM AUTHOR]

Zero-kinetic-energy (ZEKE) spectra are presented for m-fluorotoluene, employing different low-lying (<350 cm−1) intermediate torsional and vibration-torsional ("vibtor") levels of the S1 state. The adiabatic ionization energy (AIE) is found to be 71 997 ± 5 cm−1 (8.9265 ± 0.0006 eV). It is found that the activity in the ZEKE spectra varies greatly for different levels and is consistent with the assignments of the S1 levels deduced in the recent fluorescence study of Stewart et al. [J. Chem. Phys. 150, 174303 (2019)]. For cation torsional levels, the most intense band corresponds to changes in the torsional quantum number, in line with the known change in the phase of the torsional potential upon ionization. This leads to the observation of an unprecedented number of torsions and vibtor levels, with the pronounced vibtor activity involving out-of-plane vibrations. Interactions between levels involving torsions are discussed, with evidence presented, for the first time it is believed, for modification of a torsional potential induced by a vibration. Also, we discuss the possibility of distortion of the methyl group leading to a change from G6 molecular symmetry to Cs point group symmetry. [ABSTRACT FROM AUTHOR]

The vibrations of the ground state cation (* ²B2g) of para-difluorobenzene (pDFB) have been investigated using zero-electron-kinetic-energy (ZEKE) spectroscopy. Acomprehensive set of ZEKE spectra were recorded via different vibrational levels of the S1 state (<00 + 1300 cm-1). The adiabatic ionization energy for pDFB was measured as 73 869 ± 5 cm-1. Use of different intermediate levels allows different cationic vibrational activity to be obtained via the modification of the Franck-Condon factors for the ionization step, allowing the wavenumbers of different vibrational levels in the cation to be established. In addition, assignment of the vibrational structure in the ZEKE spectra allowed interrogation of the assignments of the S1 ← S0 transition put forward by Knight and Kable [J. Chem. Phys. 89, 7139 (1988)]. Assignment of the vibrational structure has been aided by quantum chemical calculations. In thisway, itwas possible to assign seventeen of the thirty vibrational modes of the ground state pDFB+ cation. Evidence for complex Fermi resonances in the S1 state, i.e., those that involve more than two vibrations, was established. One of these was investigated using picosecond time-resolved photoelectron spectroscopy. In addition, we discuss the appearance of several symmetry-forbidden bands in the ZEKE spectra, attributing their appearance to a Rydberg state variation of an intrachannel vibronic coupling mechanism. [ABSTRACT FROM AUTHOR]

For the first time, a molecular symmetry group (MSG) analysis has been undertaken in the investigation of the electronic spectroscopy of p-xylene (p-dimethylbenzene). Torsional and vibration-torsional (vibtor) levels in the S1 state and ground state of the cation of p-xylene are investigated using resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy (ZEKE) spectroscopy. In the present work, we concentrate on the 0-350 cm-1 region, where there are a number of torsional and vibtor bands and we discuss the assignment of this region. In Paper II [W. D. Tuttle et al., J. Chem. Phys. 146, 124309 (2017)], we examine the 350-600 cm-1 region where vibtor levels are observed as part of a Fermi resonance. The similarity of much of the observed spectral activity to that in the related substituted benzenes, toluene and para-fluorotoluene, is striking, despite the different symmetries. The discussion necessitates a consideration of the MSG of p-xylene, which has been designated G72, but we shall also designate [3,3]D2h and we include the symmetry operations, character table, and direct product table for this. We also discuss the symmetries of the internal rotor (torsional) levels and the selection rules for the particular electronic transition of p-xylene investigated here. [ABSTRACT FROM AUTHOR]

We present high-level ab initio potential energy curves for barium cations and dications interacting with RG atoms (RG=rare gas). These potentials are employed to derive spectroscopic parameters for the Ba+–RG and Ba2+–RG complexes, and also to derive the transport coefficients for Ba+ and Ba2+ moving through a bath of the rare gas. The results are compared to the limited experimental data, which generally show reasonable agreement. We identify a large change in binding energy going from Ba+–He and Ba+–Ne to Ba+–Ar, which is not present in Ba2+–RG, and show that this is due to significant dispersion interactions in Ba+–RG. [ABSTRACT FROM AUTHOR]

High-level ab initio calculations are performed on the coinage metal cations (Cu+, Ag+, and Au+) interacting with each of the rare gases [Rg (Rg=He to Rn)]. The RCCSD(T) procedure is employed, with basis sets being of approximately quintuple-ζ quality, but with the heavier species using relativistic effective core potentials. The interaction potentials are compared to experimental and theoretical data where they exist. In addition, transport coefficients for the mobility and diffusion of the cations in the rare gases are calculated. The latter have involved a rewriting of some of the programs used, and the required modifications are discussed. The mobility results indicate that, rather than being a rare occurrence, mobility minima may be common phenomena. Finally, a new estimate is put forward for the validity of zero-field mobilities in ion mobility spectrometry. [ABSTRACT FROM AUTHOR]

Zero electron kinetic energy (ZEKE) spectroscopy is employed to gain information on the vibrational energy levels of the para-fluorotoluene (pFT) cation. Vibrationally resolved spectra are obtained following excitation through a range of intermediate vibrational energy levels in the S1 state. These spectra allow the observation of different cationic vibrational modes, whose assignment is achieved both from a knowledge of the S1 vibrational states and also by comparison with density functional calculations. In one notable case, clean ZEKE spectra were obtained from two overlapped S1 features. From the authors' data, the adiabatic ionization energy of pFT was derived as 70 946±4 cm-1. The information on the cationic energy levels obtained will be useful in untangling the intramolecular vibrational redistribution dynamics of pFT in the S1 state. [ABSTRACT FROM AUTHOR]

The heavier alkali-metal monosulfides (KS, RbS, CsS, and FrS) have been studied by high-level ab initio calculations. The RCCSD(T) method has been employed, combined with large flexible valence basis sets. All-electron basis sets are used for potassium and sulfur, with effective core potentials being used for the other metals, describing the core electrons. Potential-energy curves are calculated for the lowest two neutral and cationic states: all neutral monosulfide species have a 2Π ground state, in contrast with the alkali-metal monoxide species, which undergo a change in the electronic ground state from 2Π to 2Σ+ as the group is descended. In the cases of KS, RbS, and CsS, spin-orbit curves are also calculated. We also calculate potential-energy curves for the lowest 3Σ- and 3Π states of the cations. From the potential-energy curves, spectroscopic constants are derived, and for KS the spectroscopic results are compared to experimental spectroscopic values. Ionization energies, dissociation energies, and heats of formation are also calculated; for KS, we explore the effects of relativity and basis set extrapolation on these values. [ABSTRACT FROM AUTHOR]

Ab initio calculations employing the coupled-cluster method, with single and double substitutions and accounting for triple excitations noniteratively [CCSD(T)], are used to obtain accurate potential energy curves for the K+·He, K+·Ne, K+·Ar, K+·Kr, K+·Xe, and K+·Rn cationic complexes. From these potentials, rovibrational energy levels and spectroscopic parameters are calculated. In addition, mobilities and diffusion coefficients for K+ cations moving through the six rare gases are calculated, under conditions that match previous experimental determinations. A detailed statistical comparison of the present and previous potentials is made with available experimental data, and critical conclusions are drawn as to the reliability of each set of data. It is concluded that the present ab initio potentials match the accuracy of the best model potentials and the most reliable experimental data. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

High level ab initio calculations are performed on the Ar·NO+ complex to ascertain its geometric structure. The calculations show that the complex is bent, with an Ar–N–O angle of ∼105°, i.e. the argon atom is on the nitrogen side of the complex cation. Calculations are performed with the cc-pVDZ and cc-pVTZ basis sets at the MP2, MP4(SDQ), QCISD, CISD, CCSD(T), and QCISD(T) levels of theory; in addition the hybrid density functionals BLYP, B3LYP, and BPW91 are used. Harmonic vibrational frequencies are also calculated for all except the two highest levels of theory, but it is recognized that the surface is anharmonic and these are not good estimates of the vibrational separations; however, by comparison with the ab initio results, they demonstrate that these density functional methods are not trustworthy for this complex. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The one-color (1+1) resonance-enhanced multiphoton ionization (REMPI) spectrum of the S1(1B2) state of chlorobenzene has been recorded and is similar to that obtained by other workers. Zero-kinetic-energy (ZEKE) photoelectron spectroscopy was then used to probe the vibrational levels in the ground electronic state of the cation (I+0) using a two-color photoionization scheme via the S1 electronic state. By using different intermediate vibrational levels in the S1 state, different vibrations could be accessed in the ion. Vibrational symmetry selection rules for the I+0←S1 ionization appear to hold well. Exciting through different S1 vibrational levels has revealed the probable mixing of the S1 normal coordinates in I+0. A previously-identified Fermi resonance in the S1 state is also confirmed by the ZEKE spectra. The adiabatic ionization energy is measured as 73 170±5 cm-1. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Focuses on the calculation of the intermolecular potential energy surface of the Ar.NO cationic complex using various ab initio methods. Effect of variation of the NO bond length; Recalculation of the intermolecular potential energy surface by point-by-point correction for basis set superposition error; Extraction of inharmonic intermolecular vibrational frequencies.

High-level ab initio calculations are employed to generate potential energy curves for rare gas cations, RG+, interacting with neutral atoms of other rare gases, RG', that are lighter (RG' = Ne-Rn). The calculations employ the RCCSD(T) method, with doubly-augmented basis sets of quintuple-ζ quality. The interaction potential curves, with the full counterpoise correction applied, are calculated point-by-point. Spin-orbit coupling is applied analytically in an atom-based model. The potentials are used to calculate spectroscopic parameters, which are then compared to recent experimental work, and the very limited previous theoretical work. In addition, the potentials are used to calculate ion transport properties and the ion mobilities are compared to the few experimental data available. [ABSTRACT FROM AUTHOR]

Abstract: The KO3 and species are both calculated to have C2v diamond structures, with a trans planar geometry lying higher in energy. For the cation, the energy difference between these two structures is only ∼1 kcalmol−1. We calculate binding energies of the two species and obtain values of 126±1 and 8±1 kcalmol−1, for and K+⋯O3, respectively. The adiabatic ionization energy is determined as 7.0±1 eV and ΔHf(0 K) values of −17±2 and 149±2 kcalmol−1 are obtained for KO3 and , respectively. [Copyright &y& Elsevier]

High level ab initio calculations, up to (R)CCSD(T) as well as B3LYP have been performed on radium diastatide, RaAt2, employing effective core potentials augmented with large, flexible valence basis sets. RaAt2 is found to have a bent equilibrium geometry, with a bond angle of ∼134.5°, but a very low barrier to linearity. In addition, we performed calculations on the lowest cationic states, and calculate the first adiabatic ionization energy to be 7.41 eV, corresponding to a X˜2B2←X˜1A1 process. We also calculate the energies of the lowest neutral states and find that RaAt2 is likely to absorb in the ultraviolet. [Copyright &y& Elsevier]

Interatomic potential energy curves are presented for the Na+-Rg (Rg= He, Ne and Ar) cationic complexes. The curves are calculated at the CCSD(T)/aug-cc-pVQZ level of theory, with correction for basis set superposition error being performed point-by-point. Ninety-six different bond lengths are used in the generation of the curves. From the curves rovibrational energy levels are calculated. These, in turn, are used to calculate the heat of formation of the cationic complexes, both by calculating partition functions under the assumption of a rigid rotor, harmonic oscillator, and also explicitly using the calculated rovibrational energy levels. The long range region of each of the curves is used to derive the D4 and D6 parameters, the former being used to derive the static polarizability alpha1 of each of the Rg atoms and the latter the first dispersion coefficients, C6(Na+-Rg). [ABSTRACT FROM AUTHOR]

The vibrations of pyrrole, N -deuteropyrrole, N -fluoropyrrole, N -aminopyrrole and N -methylpyrrole are studied. The evolution of the vibrational wavenumbers of pyrrole is examined, as the mass of the nitrogen-bonded hydrogen atom is artificially increased. It is found that some vibrations are very sensitive to the mass of the substituent bonded to the nitrogen, and this can be viewed as vibrations mixing as that mass increases; however, these mixings stablilize by the time a mass of 14 m u is reached. A consistent numbering scheme for the ring-localized vibrations of N -substituted pyrroles is then put forward. A discussion of the vibrations of the cations of pyrrole and N -substituted pyrroles is then presented. Calculated vibrational wavenumbers are compared to experimental ones for pyrrole, N -deuteropyrrole and N -methylpyrrole, as well as the pyrrole cation. [ABSTRACT FROM AUTHOR]

• N -methylpyrrole studied by REMPI, ZEKE and 2D-LIF spectroscopy. • Torsional potentials established for S 0 , S 1 and D 0 + states – consistent with optimized geometries. • Rydberg character of S 1 state evident from close-to-diagonal torsional activity. • Vibrations and vibration-torsional levels seen in the S 0 state. Torsional levels in N -methylpyrrole are investigated in the ground (S 0) and first excited (S 1) neutral states using two-dimensional laser-induced fluorescence (2D-LIF), and in the ground state cation (D 0 +) using zero-electron-kinetic-energy (ZEKE) spectroscopy. The ZEKE spectra confirm the largely Rydberg nature of the S 1 state. The activity seen in both the 2D-LIF and ZEKE spectra are indicative of vibronic (including torsional) interactions and torsional potentials in the three electronic states are deduced, and are consistent with calculated geometries. The adiabatic ionization energy of N -methylpyrrole is derived as 64250 ± 5 cm−1. [ABSTRACT FROM AUTHOR]

We present accurate interaction potentials for uranium cations interacting with the rare gases (RG = He-Rn), using effective core potentials that include a description of the 5f electrons in the case of uranium, and justify this approximation in some detail. From these interaction potentials, spectroscopic parameters are derived for the U+-RG complexes. We also employ the potentials to calculate transport coefficients for U+ moving through a bath of each RG. In the case of U+ in He, we are able to compare with previous experimentally determined mobility values, and we make some minor corrections to the previously reported data; this revised data is presented. [ABSTRACT FROM AUTHOR]