Pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy of metastable He2: Ionization potential and rovibrational structure of He2+

Vortex beams of atoms and molecules

[Figure: see text].

Precision Measurements in Few-Electron Molecules: The Ionization Energy of Metastable ^{4}He_{2} and the First Rotational Interval of ^{4}He_{2}^{+}

Molecular helium represents a benchmark system for testing ab initio calculations on few-electron molecules. We report on the determination of the adiabatic ionization energy of the a ^{3}Σ_{u}^{+} state of He_{2}, corresponding to the energy interval between the a ^{3}Σ_{u}^{+} (v^{''}=0, N^{''}=1) state of He_{2} and the X^{+} ^{2}Σ_{u}^{+} (v^{+}=0, N^{+}=1) state of He_{2}^{+}, and of the lowest rotational interval of He_{2}^{+}. These measurements rely on the excitation of metastable He_{2} molecules to high Rydberg states using frequency-comb-calibrated continuous-wave UV radiation in a counterpropagating laser-beam setup. The observed Rydberg states were extrapolated to their series limit using multichannel quantum-defect theory. The ionization energy of He_{2} (a ^{3}Σ_{u}^{+}) and the lowest rotational interval of He_{2}^{+} (X^{+} ^{2}Σ_{u}^{+}) are 34 301.207 002(23)±0.000 037_{syst}  cm^{-1} and 70.937 589(23)±0.000 060_{syst}  cm^{-1}, respectively.

Production of rotationally cold methyl radicals in pulsed supersonic beams

We present a comparison of two technically distinct methods for the generation of rotationally cold, pulsed supersonic beams of methyl radicals (CH₃): a plate discharge source operating in the glow regime and a dielectric barrier discharge source. The results imply that the efficiency of both sources is comparable and that molecular beams with similar translational and rotational temperatures are formed. Methane (CH₄) proved to be the most suitable radical precursor species.

Fundamental vibration frequency and rotational structure of the first excited vibrational level of the molecular helium ion ( He 2 + )

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.

Determination of the Spin-Rotation Fine Structure of He_{2}^{+}

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 He_{2}^{+} ion in the X ^{2}Σ_{u}^{+} state. The fine-structure splittings of the v^{+}=0, N^{+}=1, 3, and 5 levels of He_{2}^{+} 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 He_{2} from the a ^{3}Σ_{u}^{+} metastable state.

Precision measurement of the rotational energy-level structure of the three-electron molecule He2. J Chem Phys 2016;145:204301. [PMID: 27908106 DOI: 10.1063/1.4967256]

The term values of all rotational levels of the ⁴He₂⁺ X^{+  2}Σ_u⁺ (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.

Transport coefficients of He(+) ions in helium

This paper demonstrates that the transport coefficients of (4)He(+) in (4)He can be calculated over wide ranges of E/N, the ratio of the electrostatic field strength to the gas number density, with the same level of precision as can be obtained experimentally if sufficiently accurate potential energy curves are available for the X(2)Σu (+) and A(2)Σg (+) states and one takes into account resonant charge transfer. We start by computing new potential energy curves for these states and testing their accuracy by calculating spectroscopic values for the separate states. It is established that the potentials obtained by extrapolation of results from d-aug-cc-pVXZ (X = 6, 7) basis sets using the CASSCF+MRCISD approach are each in exceptionally close agreement with the best potentials available and with experiment. The potentials are then used in a new computer program to determine the semi-classical phase shifts and the transport cross sections, and from these the gaseous ion transport coefficients are determined. In addition, new experimental values are reported for the mobilities of (4)He(+) in (4)He at 298.7 K, as a function of E/N, where careful consideration is given to minimizing various sources of uncertainty. Comparison with previously measured values establishes that only one set of previous data is reliable. Finally, the experimental and theoretical ion transport coefficients are shown to be in very good to excellent agreement, once corrections are applied to account for quantum-mechanical effects.

Precision Spectroscopy in Cold Molecules: The Lowest Rotational Interval of He_{2}^{+} and Metastable He_{2}

Multistage Zeeman deceleration was used to generate a slow, dense beam of translationally cold He_{2} molecules in the metastable a ^{3}Σ_{u}^{+} state. Precision measurements of the Rydberg spectrum of these molecules at high values of the principal quantum number n have been carried out. The spin-rotational state selectivity of the Zeeman-deceleration process was exploited to reduce the spectral congestion, minimize residual Doppler shifts, resolve the Rydberg series around n=200 and assign their fine structure. The ionization energy of metastable He_{2} and the lowest rotational interval of the X^{+} ^{2}Σ_{u}^{+} (ν^{+}=0) ground state of ^{4}He_{2}^{+} have been determined with unprecedented precision and accuracy by Rydberg-series extrapolation. Comparison with ab initio predictions of the rotational energy level structure of ^{4}He_{2}^{+} [W.-C. Tung, M. Pavanello, and L. Adamowicz, J. Chem. Phys. 136, 104309 (2012)] enabled us to quantify the magnitude of relativistic and quantum-electrodynamics contributions to the fundamental rotational interval of He_{2}^{+}.