Characterisation of the ground X+ 2ΠΩ and first excited A+ 2Σ+ electronic states of MgO+ by high-resolution photoelectron spectroscopy.

Despite the importance of MgO+ for understanding the electronic structure and chemical bonds in alkaline-earth metal oxides and its potential astrophysical relevance, hardly any spectroscopic information is available on this molecular cation. We report on a high-resolution photoelectron spectroscopic study of MgO using a resonant (1 + 1') two-photon excitation scheme in combination with PFI-ZEKE photoelectron spectroscopy. By carrying out the resonant excitation via selected rotational levels of several intermediate states of different electronic configurations, total electronic spins, and internuclear distances, a broad range of vibrational levels of the X+ 2ΠΩ (Ω = 3/2, 1/2) ground and A+ 2Σ+ first excited states of MgO+ were observed for the first time. The new data provide a full characterisation of the rovibronic level structure of MgO+ up to 2 eV (16 000 cm-1) of internal energy. A full set of vibrational, rotational and spin-orbit-coupling molecular constants were extracted for these two electronic states. The adiabatic ionisation energy and the singlet-triplet interval of 24Mg16O were determined to be 64 577.65(20) cm-1 and 2492.4(3) cm-1, respectively.

High-Resolution Photoelectron Spectroscopy of the Ground and First Excited Electronic States of MgXe.

We report on the characterization of the X+ 2Σ+ ground and the A+ 2ΠΩ (Ω = 1/2, 3/2) and B+ 2Σ+ electronically excited states of MgXe+. Rotationally cold MgXe in the a 3Π0(v″ = 0) metastable electronic state was generated in a laser-ablation supersonic-beam source. Following single-photon excitation from the metastable state, the vibrational structure of the X+ state of MgXe+ was measured by pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy, and the adiabatic ionization energy of the X+ ← a ionizing transition was determined to be EI/(hc) = 37,468.3(6) cm-1. Spectra of the A+ ← X+ and B+ ← X+ transitions were recorded by using the method of isolated-core Rydberg-dissociation spectroscopy. The observation of the Mg+(3p) 2P1/2 + Xe 1S0 dissociation limit enabled the determination of the dissociation energies of the X+ [D0(X+) = 2970(7) cm-1] and A+ states [D0(A1/2+) = 9781(7) cm-1 and D0(A3/2+) = 9603(7) cm-1]. We compare these results with those of earlier experimental studies and ab initio quantum-chemical calculations.

Zero-Quantum-Defect Method and the Fundamental Vibrational Interval of H_{2}

The fundamental vibrational interval of H_{2}^{+} has been determined to be ΔG_{1/2}=2191.126 614(17) cm^{-1} by continuous-wave laser spectroscopy of Stark manifolds of Rydberg states of H_{2} with the H_{2}^{+} ion core in the ground and first vibrationally excited states. Extrapolation of the Stark shifts to zero field yields the zero-quantum-defect positions -R_{H_{2}}/n^{2}, from which ionization energies can be determined. Our new result represents a 4-order-of-magnitude improvement compared to earlier measurements. It agrees, within the experimental uncertainty, with the value of 2191.126 626 344(17)(100) cm^{-1} determined in nonrelativistic quantum electrodynamic calculations [V. Korobov, L. Hilico and J.-Ph. Karr, Phys. Rev. Lett. 118, 233001 (2017)PRLTAO0031-900710.1103/PhysRevLett.118.233001].

Determination of the Interval between the Ground States of Para- and Ortho-H_{2}.

Nuclear-spin-symmetry conservation makes the observation of transitions between quantum states of ortho- and para-H_{2} extremely challenging. Consequently, the energy-level structure of H_{2} derived from experiment consists of two disjoint sets of level energies, one for para-H_{2} and the other for ortho-H_{2}. We use a new measurement of the ionization energy of para-H_{2} [E_{I}(H_{2})/(hc)=124 417.491 098(31) cm^{-1}] to determine the energy separation [118.486 770(50) cm^{-1}] between the ground states of para- and ortho-H_{2} and thus link the energy-level structure of the two nuclear-spin isomers of this fundamental molecule. Comparison with recent theoretical results [M. Puchalski et al., Phys. Rev. Lett. 122, 103003 (2019)PRLTAO0031-900710.1103/PhysRevLett.122.103003] enables the derivation of an upper bound of 1.5 MHz for a hypothetical global shift of the energy-level structure of ortho-H_{2} with respect to that of para-H_{2}.

Single-Shot Nondestructive Detection of Rydberg-Atom Ensembles by Transmission Measurement of a Microwave Cavity.

We present an experimental realization of single-shot nondestructive detection of ensembles of helium Rydberg atoms. We use the dispersive frequency shift of a superconducting microwave cavity interacting with the ensemble. By probing the transmission of the cavity, we determine the number of Rydberg atoms or the populations of Rydberg quantum states when the ensemble is prepared in a superposition. At the optimal microwave probe power, determined by the critical photon number, we reach single-shot detection of the atom number with 13% relative precision for ensembles of about 500 Rydberg atoms with a measurement backaction characterized by approximately 2% population transfer.

PFI-ZEKE-photoelectron spectroscopy of N2O using narrow-band VUV laser radiation generated by four-wave mixing in Ar using a KBBF crystal.

A new nonlinear optical scheme relying on sum-frequency mixing in a KBe2BO3F2 crystal has been used to generate intense, broadly tunable, narrow-bandwidth, coherent vacuum-ultraviolet (VUV) radiation beyond 16 eV by resonance-enhanced four-wave mixing in Ar. The VUV radiation was used to record high-resolution pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the N2O+ A+ ← N2O X photoionizing transition in the wave-number range from 132 000 cm-1 to 135 000 cm-1. The rotational structure of almost all vibrational levels of the A+ state with vibrational term values up to 2700 cm-1 could be resolved, and improved values of the first two adiabatic ionization energies of N2O, corresponding to the formation of the X+ 2Π3/2(000) J+ = 3/2 and A+ 2Σ+(000) N+ = 0 levels of N2O+ from the X 1Σ+(000) J″ = 0 ground state [103 969.30(12) cm-1 and 132 197.70(12) cm-1, respectively], were derived. The rotational intensity distributions of the bands were found to depend strongly on the value of the vibrational angular momentum of the ionic levels. The vibrational structure is discussed in terms of previously reported effective-Hamiltonian analyses.

Dissociation Energy of the Hydrogen Molecule at 10

The ionization energy of ortho-H_{2} has been determined to be E_{I}^{o}(H_{2})/(hc)=124 357.238 062(25) cm^{-1} from measurements of the GK(1,1)-X(0,1) interval by Doppler-free, two-photon spectroscopy using a narrow band 179-nm laser source and the ionization energy of the GK(1,1) state by continuous-wave, near-infrared laser spectroscopy. E_{I}^{o}(H_{2}) was used to derive the dissociation energy of H_{2}, D_{0}^{N=1}(H_{2}), at 35 999.582 894(25) cm^{-1} with a precision that is more than one order of magnitude better than all previous results. The new result challenges calculations of this quantity and represents a benchmark value for future relativistic and QED calculations of molecular energies.

Structure and dynamics of the radical cation of ethane arising from the Jahn-Teller and pseudo-Jahn-Teller effects.

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of C2H6 has been recorded in the region of the adiabatic ionization threshold. The partially rotationally resolved spectrum indicates the existence of several vibronic states of C2H6+ with less than 600 cm-1 of internal excitation. The analysis of the rotational structures assisted by ab initio calculations enabled the determination of the adiabatic ionization energy of C2H6 and the investigation of the structure and dynamics of C2H6+ at low energies. The ground state of C2H6+ is found to be a 2Ag state of diborane-like structure with strongly mixed (a1g)-1 and (eg)-1 configurations. The vibrational structure reveals the importance of large-amplitude nuclear motions involving the diborane distortion modes, the C-C stretching motion, and the internal rotation at elongated C-C distances. The spectrum is analyzed in the light of the information obtained in earlier studies of C2H6+ by ab initio quantum chemistry, EPR spectroscopy and photoelectron spectroscopy.

Spin-orbit interaction and Renner-Teller effect in HCCCCH+ studied by high-resolution photoelectron spectroscopy.

The photoelectron spectra of the X+ 2Πg ← X 1Σ photoionizing transition of diacetylene (HCCCCH) and d2-diacetylene (DCCCCD) have been recorded at high resolution using the technique of pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy. The partially resolved rotational structure of the origin band of the spectra of HCCCCH and DCCCCD has enabled the determination of the adiabatic ionization energies of HCCCCH [Ead = 82072.2(5) cm-1] and DCCCCD [Ead = 82090.0(10) cm-1] and of the spin-orbit coupling constant [A = -31.1(4) cm-1] of the ground vibronic state of HCCCCH+, which is smaller than the value of -33.3 cm-1 commonly used since the work of Callomon (J. H. Callomon, Can. J. Phys., 1956, 34, 1046). Several excited vibrational levels of HCCCCH+ and DCCCCD+, including some affected by the Renner-Teller effect and Fermi interactions, have been observed and the fundamental wavenumber of the mode ν9 has been determined in both HCCCCH+ (200.0(10) cm-1) and DCCCCD+ (192.6(20) cm-1). Possible assignments for several of these levels are discussed and deficiencies in the current understanding of the energy-level structure of the radical cation of diacetylene are pointed at.

Vacuum-ultraviolet frequency-modulation spectroscopy.

Frequency-modulation (FM) spectroscopy has been extended to the vacuum-ultraviolet (VUV) range of the electromagnetic spectrum. Coherent VUV laser radiation is produced by resonance-enhanced sum-frequency mixing (νVUV=2νUV+ν2) in Kr and Xe using two near-Fourier-transform-limited laser pulses of frequencies νUV and ν2. Sidebands generated in the output of the second laser (ν2) using an electro-optical modulator operating at the frequency νmod are directly transferred to the VUV and used to record FM spectra. Demodulation is demonstrated both at νmod and 2νmod. The main advantages of the method compared to VUV absorption spectroscopy are its background-free nature, the fact is that its implementation using table-top laser equipment is straightforward and that it can be used to record VUV absorption spectra of cold samples in skimmed supersonic beams simultaneously with laser-induced-fluorescence and photoionization spectra. To illustrate these advantages, we present VUV FM spectra of Ar, Kr, and N2 in selected regions between 105000 cm-1 and 122000 cm-1.

Infrared spectroscopy of molecular ions in selected rotational and spin-orbit states.

First results are presented obtained with an experimental setup developed to record IR spectra of rotationally state-selected ions. The method we use is a state-selective version of a method developed by Schlemmer et al. [Int. J. Mass Spectrom. 185, 589 (1999); J. Chem. Phys. 117, 2068 (2002)] to record IR spectra of ions. Ions are produced in specific rotational levels using mass-analyzed-threshold-ionization spectroscopy. The state-selected ions generated by pulsed-field ionization of Rydberg states of high principal quantum number (n ≈ 200) are extracted toward an octupole ion guide containing a neutral target gas. Prior to entering the octupole, the ions are excited by an IR laser. The target gas is chosen so that only excited ions react to form product ions. These product ions are detected mass selectively as a function of the IR laser wavenumber. To illustrate this method, we present IR spectra of C2H2 (+) in selected rotational levels of the (2)Πu,3/2 and (2)Πu,1/2 spin-orbit components of the vibronic ground state.

Absorption, autoionization, and predissociation in molecular hydrogen: High-resolution spectroscopy and multichannel quantum defect theory.

Absorption and photoionization spectra of H2 have been recorded at a resolution of 0.09 and 0.04 cm(-1), respectively, between 125,600 cm(-1) and 126,000 cm(-1). The observed Rydberg states belong to series (n = 10 - 14) converging on the first vibrationally excited level of the X (2)Σ(g)(+) state of H2(+), and of lower members of series converging on higher vibrational levels. The observed resonances are characterized by the competition between autoionization, predissociation, and fluorescence. The unprecedented resolution of the present experimental data leads to a full characterization of the predissociation/autoionization profiles of many resonances that had not been resolved previously. Multichannel quantum defect theory is used to predict the line positions, widths, shapes, and intensities of the observed spectra and is found to yield quantitative agreement using previously determined quantum defect functions as the unique set of input parameters.

The cyclopropene radical cation: rovibrational level structure at low energies from high-resolution photoelectron spectra.

The cyclopropene radical cation (c-C3H4⁺) is an important but poorly characterized three-membered-ring hydrocarbon. We report on a measurement of the high-resolution photoelectron and photoionization spectra of cyclopropene and several deuterated isotopomers, from which we have determined the rovibrational energy level structure of the X⁺ (2)B2 ground electronic state of c-C3H4⁺ at low energies for the first time. The synthesis of the partially deuterated isotopomers always resulted in mixtures of several isotopomers, differing in their number of D atoms and in the location of these atoms, so that the photoelectron spectra of deuterated samples are superpositions of the spectra of several isotopomers. The rotationally resolved spectra indicate a C(2v)-symmetric R0 structure for the ground electronic state of c-C3H4⁺. Two vibrational modes of c-C3H4⁺ are found to have vibrational wave numbers below 300 cm(-1), which is surprising for such a small cyclic hydrocarbon. The analysis of the isotopic shifts of the vibrational levels enabled the assignment of the lowest-frequency mode (fundamental wave number of ≈110 cm(-1) in c-C3H4⁺) to the CH2 torsional mode (ν8⁺, A2 symmetry) and of the second-lowest-frequency mode (≈210 cm(-1) in c-C3H4⁺) to a mode combining a CH out-of-plane with a CH2 rocking motion (ν15⁺, B2 symmetry). The potential energy along the CH2 torsional coordinate is flat near the equilibrium structure and leads to a pronounced anharmonicity.

High-resolution spectroscopy and quantum-defect model for the gerade triplet np and nf Rydberg states of He2.

Photoionization spectra and Rydberg-state-resolved threshold-ionization spectra of the gerade triplet np Rydberg states of (4)He2 located in the vicinity of the X(+) (2)Σ(u)(+) (ν(+) = 0) ionization threshold were recorded from the 2sσ a (3)Σ(u)(+) metastable state. An accuracy of 0.01 cm(-1) was achieved for the experimental term values of the observed Rydberg states. The data were combined with spectroscopic data on low-lying triplet np and nf Rydberg states from the literature to derive energy- and internuclear-distance-dependent eigenquantum-defect parameters of multichannel quantum-defect theory (MQDT). The MQDT calculations reproduce the experimental data within their experimental uncertainties and enabled the derivation of potential-energy curves for the lowest triplet p Rydberg states (n = 2-5) of He2. The eigenquantum-defect parameters describing the p -f interaction were found to be larger than 0.002 at the energies corresponding to the high-n Rydberg states, so that the p -f interaction plays an important role in the autoionization dynamics of np Rydberg states with v(+) = 0. By extrapolating the experimental term values of triplet np Rydberg states of (4)He2 in the range of principal quantum number n between 87 and 110, the positions of the (v(+) = 0, N(+) = 3) and (v(+) = 0, N(+) = 5) levels of the ground state of (4)He(+)(2) were determined to lie 70.937(3) cm(-1) and 198.369(6) cm(-1), respectively, above the (v(+) = 0, N(+) = 1) ground rotational level.

On the adiabatic ionization energy of the propargyl radical.

The photoionization and pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the propargyl radical have been recorded in the vicinity of the origin of the X(+) (1)A1←X(2)B1 photoionizing transition. An internally cold sample of propargyl with a rotational temperature of ~45 K was produced in a supersonic expansion of 1,3-butadiene in helium. Propargyl was generated by excimer laser (ArF, 193 nm) photolysis of 1,3-butadiene in a quartz capillary mounted at the exit of a pulsed valve. The rotational structure of the origin band of the photoelectron spectrum was partially resolved and an improved value of the adiabatic ionization energy of propargyl (E(I)/hc = 70174.5(20) cm(-1)) was determined.

Precision measurements of ionization and dissociation energies by extrapolation of Rydberg series: from H2 to larger molecules.

Recent experiments are reviewed which have led to the determination of the ionization and dissociation energies of molecular hydrogen with a precision of 0.0007 cm(-)1 (8 mJ/mol or 20 MHz) using a procedure based on high-resolution spectroscopic measurements of high Rydberg states and the extrapolation of the Rydberg series to the ionization thresholds. Molecular hydrogen, with only two protons and two electrons, is the simplest molecule with which all aspects of a chemical bond, including electron correlation effects, can be studied. Highly precise values of its ionization and dissociation energies provide stringent tests of the precision of molecular quantum mechanics and of quantum-electrodynamics calculations in molecules. The comparison of experimental and theoretical values for these quantities enable one to quantify the contributions to a chemical bond that are neglected when making the Born-Oppenheimer approximation, i.e. adiabatic, nonadiabatic, relativistic, and radiative corrections. Ionization energies of a broad range of molecules can now be determined experimentally with high accuracy (i.e. about 0.01 cm(-1)). Calculations at similar accuracies are extremely challenging for systems containing more than two electrons. The combination of precision measurements of molecular ionization energies with highly accurateab initio calculations has the potential to provide, in future, fully reliable sets of thermochemical quantities for gas-phase reactions.

Photoelectron spectroscopic study of the E ⊗ e Jahn-Teller effect in the presence of a tunable spin-orbit interaction. III. Two-state excitonic model accounting for observed trends in the X 2E ground state of CH3X+ (X=F, Cl, Br, I) and CH3Y (Y=O, S).

Open-shell molecules in doubly degenerate (2)E electronic states are subject to the E ⊗ e Jahn-Teller effect and spin-orbit interactions. The rotational structure of the ground vibrational level of the X(+) (2)E ground state of CH(3)F(+) has been observed by high-resolution photoelectron spectroscopy. In contrast to what is observed in other members of the isoelectronic families CH(3)X(+) (X=Cl, Br, I) and CH(3)Y (Y=O, S), the spin-orbit interaction does not lead to a splitting of the ground state of CH(3)F(+). Observed trends in the spectra of the X (2)E ground states of these molecules are summarized. Whereas certain trends, such as the reduction of the observable effects of the Jahn-Teller interactions and the increase of the spin-orbit splitting with increasing nuclear charge of X and Y are easily understood, other trends are more difficult to explain, such as the much reduced spin-orbit splitting in CH(3)F(+) compared to CH(3)O. A simple two-state excitonic model is used to account for the trends observed within the series of the methyl-halide radical cations and also the similarities and differences between CH(3)F(+) and the isoelectronic CH(3)O radical. Within this model, the electron hole in the (2)E ground states of CH(3)X(+) and CH(3)Y is described in terms of contributions from the halogenic (or chalcogenic) p(x, y) orbitals and the pyramidal-methylic (e) orbitals. This model enables a global, semi-quantitative description of the combined effects of the Jahn-Teller and spin-orbit interactions in these molecules and also a simple interpretation of the spin-orbit-coupling reduction factor ζ(e).