Photoelectron Velocity Map Imaging Spectroscopy of the Beryllium Trimer and Tetramer

Computational studies of small beryllium clusters (BeN) predict dramatic, nonmonotonic changes in the bonding mechanisms and per-atom cohesion energies with increasing N. To date, experimental tests of these quantum chemistry models are lacking for all but the Be2 molecule. In the present study, we report spectroscopic data for Be3 and Be4 obtained via anion photodetachment spectroscopy. The trimer is predicted to have D3h symmetric equilibrium structures for both the neutral molecule and the anion. Photodetachment spectra reveal transitions that originate from the X2A2″ ground state and the 12A1' electronically excited state. The state symmetries were assigned on the basis of anisotropic photoelectron angular distributions. The neutral and anionic forms of Be4 are predicted to be tetrahedral. Franck-Condon diagonal photodetachment was observed with a photoelectron angular distribution consistent with the expected Be4-X2A1 → Be4X1A1 transition. The electron affinities of Be3 and Be4 were determined to be 11363 ± 60 and 13052 ± 50 cm-1, respectively.

High resolution electronic spectroscopy of uranium mononitride, UN

The isoelectronic molecules UN and UO+ are known to have Ω = 3.5 and Ω = 4.5 ground states, respectively (where Ω is the unsigned projection of the electronic angular momentum along the internuclear axis). A ligand field theory model has been proposed to account for the difference [Matthew and Morse, J. Chem. Phys. 138, 184303 (2013)]. The ground state of UO+ arises from the U3+(5f3(4I4.5))O2- configuration. Owing to the higher nominal charge of the N3- ligand, the U3+ ion in UN is stabilized by promoting one of the 5f electrons to the more polarizable 7s orbital, reducing the repulsive interaction with the ligand and rendering U3+(5f27s(4H3.5))N3- the lowest energy configuration. In the present work, we have advanced the characterization of the UN ground state through studies of two electronic transitions, [18.35]4.5-X(1)3.5 and [18.63]4.5-X(1)3.5, using sub-Doppler laser excitation techniques with fluorescence detection. Spectra were recorded under field-free conditions and in the presence of static electric or magnetic fields. The ground state electric dipole moment [μ = 4.30(2) D] and magnetic ge-factor [2.160(9)] were determined from these data. These values were both consistent with the 5f27s configurational assignment. Dispersed fluorescence measurements were used to determine vibrational constants for the ground and first electronically excited states. Electric dipole moments and magnetic ge-factors are also reported for the higher-energy electronically excited states.

Kinetics of O3 with Ca+ and Its Higher Oxides CaOn+ (n = 1-3) and Updates to a Model of Meteoric Calcium in the Mesosphere and Lower Thermosphere

The room-temperature rate constants and product branching fractions of CaO_n⁺ (n = 0-3) + O₃ are measured using a selected ion flow tube apparatus. Ca⁺ + O₃ produces CaO⁺ + O₂ with k = 9 ± 4 × 10^-10 cm³ s^-1, within uncertainty equal to the Langevin capture rate constant. This value is significantly larger than several literature values. Most likely, those values were underestimated due to the reformation of Ca⁺ from the sequential chemistry of higher calcium oxide cations with O₃, as explored here. A rate constant of 8 ± 3 × 10^-10 cm³ s^-1 is recommended. Both CaO⁺ and CaO₂⁺ react near the capture rate constant with ozone. The CaO⁺ reaction yields both CaO₂⁺ + O₂ (0.80 ± 0.15 branching) and Ca⁺ + 2O₂. Similarly, the CaO₂⁺ reaction yields both CaO₃⁺ + O₂ (0.85 ± 0.15 branching) and CaO⁺ + 2O₂. CaO₃⁺ + O₃ yield CaO₂⁺ + 2O₂ at 2 ± 1 × 10^-11 cm³ s^-1, about 2% of the capture rate constant. The results are supported using density functional calculations and statistical modeling. In general, CaO_n⁺ + O₃ yield CaO_n+1⁺ + O₂, the expected oxidation. Some fraction of CaO_n+1⁺ is produced with sufficient internal energy to further dissociate to CaO_n-1⁺ + O₂, yielding the same products as the oxidation of O₃ by CaO_n⁺. Mesospheric Ca and Ca⁺ concentrations are modeled as functions of day, latitude, and altitude using the Whole Atmosphere Community Climate Model (WACCM); incorporating the updated rate constants improves agreement with concentrations derived from lidar measurements.

Near-thermo-neutral electron recombination of titanium oxide ions

While the dissociative recombination (DR) of ground-state molecular ions with low-energy free electrons is generally known to be exothermic, it has been predicted to be endothermic for a class of transition-metal oxide ions. To understand this unusual case, the electron recombination of titanium oxide ions (TiO⁺) with electrons has been experimentally investigated using the Cryogenic Storage Ring. In its low radiation field, the TiO⁺ ions relax internally to low rotational excitation (≲100 K). Under controlled collision energies down to ∼2 meV within the merged electron and ion beam configuration, fragment imaging has been applied to determine the kinetic energy released to Ti and O neutral reaction products. Detailed analysis of the fragment imaging data considering the reactant and product excitation channels reveals an endothermicity for the TiO⁺ dissociative electron recombination of (+4 ± 10) meV. This result improves the accuracy of the energy balance by a factor of 7 compared to that found indirectly from hitherto known molecular properties. Conversely, the present endothermicity yields improved dissociation energy values for D₀(TiO) = (6.824 ± 0.010) eV and D₀(TiO⁺) = (6.832 ± 0.010) eV. All thermochemistry values were compared to new coupled-cluster calculations and found to be in good agreement. Moreover, absolute rate coefficients for the electron recombination of rotationally relaxed ions have been measured, yielding an upper limit of 1 × 10^-7 cm³ s^-1 for typical conditions of cold astrophysical media. Strong variation of the DR rate with the TiO⁺ internal excitation is predicted. Furthermore, potential energy curves for TiO⁺ and TiO have been calculated using a multi-reference configuration interaction method to constrain quantum-dynamical paths driving the observed TiO⁺ electron recombination.

Kinetics of Metastable Argon Optical Excitation and Gain in Ar/He Microplasmas

The optically pumped rare-gas metastable laser is capable of high-intensity lasing on a broad range of near-infrared transitions for excited-state rare gas atoms (Ar*, Kr*, Ne*, Xe*) diluted in flowing He. The lasing action is generated by photoexcitation of the metastable atom to an upper state, followed by collisional energy transfer with He to a neighboring state and lasing back to the metastable state. The metastables are generated in a high-efficiency electric discharge at pressures of ∼0.4 to 1 atm. The diode-pumped rare-gas laser (DPRGL) is a chemically inert analogue to diode-pumped alkali laser (DPAL) systems, with similar optical and power scaling characteristics for high-energy laser applications. We used a continuous-wave linear microplasma array in Ar/He mixtures to produce Ar(1s₅) (Paschen notation) metastables at number densities exceeding 10¹³ cm^-3. The gain medium was optically pumped by both a narrow-line 1 W titanium-sapphire laser and a 30 W diode laser. Tunable diode laser absorption and gain spectroscopy determined Ar(1s₅) number densities and small-signal gains up to ∼2.5 cm^-1. Continuous-wave lasing was observed using the diode pump laser. The results were analyzed with a steady-state kinetics model relating the gain and the Ar(1s₅) number density.

Electronic Spectroscopy of Jet-Cooled NdO

Chemi-ionization reactions of the type M + O → MO⁺ + e^- (M = Nd or Sm) are currently being investigated as a method to artificially increase the electron density in the ionosphere for control of micro- and radio wave propagation. Experiments involving the release of atomic Nd into the upper atmosphere have resulted in the production of a cloud that, on excitation by solar radiation, emits green light. It has been assumed that NdO was the carrier of this emission, but the existing spectroscopic data needed for this attribution is lacking. While the electronic spectrum of NdO has been well-characterized at wavelengths greater than 590 nm, relatively little spectroscopic data exist for emission wavelengths in the blue-green spectral range. In this study, spectra for jet-cooled NdO were recorded over the range 15,500-21,000 cm^-1. Rotationally resolved laser induced fluorescence and vibronically resolved dispersed laser-induced fluorescence spectra were recorded, and nine new electronically excited states were identified. The data indicate that the electronic spectrum of NdO has relatively few allowed transitions in the green spectral range, casting doubt on the assignment of the Nd high-altitude release cloud green emission to NdO.

Electronic Configuration Assignments for UO from Electric Dipole Moment Measurements

Diatomic UO has more than 48 bound states within 10000 cm^-1 of the ground state. This electronic state congestion has been attributed to interleaved states from the electronic configurations U²⁺(5f³7s)O^2- and U²⁺(5f²7s²)O^2-, respectively. Ligand field theory predicts that each electronic configuration will exhibit states with distinguishable, characteristic vibrational and rotational constants. However, vibronic state mixing modifies the observed vibration-rotation constants, leading to uncertainty in the configurational assignments. The permanent electric dipole moment (μ_e) of an electronic state should also manifest a value that is characteristic of the parent electronic configuration. μ_e and other electrostatic and magnetostatic properties should be less influenced by the vibronic state mixing, providing more robust indicators for configurational assignments. In the present study, we have measured the μ_e values for four electronic states of UO. The results clearly demonstrate that the ground state (X(1)4) and the first electronically excited state ((2)4) are derived from the U²⁺(5f³7s)O^2- and U²⁺(5f²7s²)O^2- configurations, respectively.

Electronic Spectroscopy of SmO in the 645 to 670 nm Range

The associative ionization reaction Sm + O → SmO⁺ + e^- is being investigated as an electron source that could transiently modify high-altitude electron densities via Sm vapor release. Electronic spectra have been obtained from tests where sounding rockets released Sm vapor, but the interpretation of these results has been hampered by the limited laboratory spectral data available for both SmO and SmO⁺. The present study extends the spectroscopic characterization of SmO in the 645-670 nm range, where the field data show the most prominent molecular emission features. Rotationally resolved excitation spectra, dispersed laser-induced fluorescence spectra, and fluorescence decay lifetimes are reported. The results are consistent with the assignment of a subset of the red-region bands to configurational transitions of the form Sm²⁺(4f⁵6s)O^2- ↔ Sm²⁺(4f⁵5d)O^2-. Analysis of the excited state hyperfine structure supports this configurational description.

Gas-Phase Reactivity of Ozone with Lanthanide Ions (Sm+, Nd+) and Their Higher Oxides

The kinetics of SmO_n⁺ (n = 0-2) and NdO_n⁺ (n = 0-2) with O₃ are measured using a selected-ion flow tube. Reaction of Nd⁺ to yield NdO⁺ + O₂ occurs rapidly, with a rate constant near the capture-controlled limit of ∼8 × 10^-10 cm³ s^-1. NdO⁺ reacts at ∼40% of the capture limit to yield NdO₂⁺ with little temperature dependence from 200 to 400 K. NdO₂⁺ likely reacts very slowly (k ∼ 10^-13 cm³ s^-1) to yield NdO⁺ + 2O₂, does not react to yield NdO₃⁺, and associates slowly (k ∼ 10^-12 cm³ s^-1) to yield NdO₂⁺(O₃)_1-3. Reaction of Sm⁺ also yields SmO⁺ at near the capture limit at all temperatures, but a significant fraction (∼50%) of the SmO⁺ is produced in excited states that are long-lived compared to the millisecond time scale of the experiment. These states are evidently resistant to both radiative and collisional relaxation. The excited-state production is likely due to a spin-conservation constraint on the reaction, despite the large spin-orbit coupling typical for lanthanide-containing species. Ground-state SmO⁺ reacts inefficiently (k = 2 × 10^-11 (T/300)^-2.5 cm³ s^-1) to yield SmO₂⁺ + O₂, while the excited-state SmO^+* reacts at the capture limit, with branching to yield Sm⁺ + 2O₂ (ΔH_r,0K = 148.7 ± 0.4 kJ mol^-1 for ground-state SmO⁺) approximately 60% of the time, the remainder forming SmO₂⁺, which further reacts with O₃ to yield SmO⁺ at about 1% of the collisional value.

Electronic Spectroscopy and Photoionization of LiBe

LiBe has been the subject of several theoretical investigations and one spectroscopic study. Initially, these efforts were motivated by interest in the intermetallic bond. More recent work has explored the potential for producing LiBe and LiBe⁺ at ultracold temperatures. In the present study, we have advanced the spectroscopic characterization of several electronic states of LiBe and the ground state of LiBe⁺. For the neutral molecule, the 1²Π, 2²Σ⁺, 3²Σ⁺, and 4²Π(3d) states were observed for the first time. Data for the 2²Σ⁺-X²Σ⁺ transition support a theoretical prediction that this band system is suitable for direct laser cooling. Photoelectron spectroscopy has been used to determine the ionization energy of LiBe and map the low-energy vibrational levels of LiBe⁺ X¹Σ⁺. Overall, the results validate the predictions of high-level quantum chemistry calculations for both LiBe and LiBe⁺.

Electronic Spectroscopy and Photoionization of LiMg

Dimers consisting of an alkali metal bound to an alkaline earth metal are of interest from the perspectives of their bonding characteristics and their potential for being laser cooled to ultracold temperatures. There have been experimental and theoretical studies of many of these species, but spectroscopic data for LiMg and the LiMg⁺ cation are sparse. In this study, rotationally resolved electronic spectra for LiMg are presented. The ground state is confirmed to be X1²Σ⁺ and observations of low-lying electronically excited states are reported for the first time. Reexamination of transitions in the near-UV spectral range indicates that previous band assignments should be revised. Two-color laser excitation techniques were used to determine an ionization energy of 4.7695(4) eV. This value is 1.2 eV below the previously reported experimental estimate. Vibrationally resolved spectra were obtained for LiMg⁺, yielding molecular constants that were consistent with a substantial strengthening of the bond on ionization.

Dipole-phonon quantum logic with alkaline-earth monoxide and monosulfide cations

Dipole-phonon quantum logic (DPQL) leverages the interaction between polar molecular ions and the motional modes of a trapped-ion Coulomb crystal to provide a potentially scalable route to quantum information science. Here, we study a class of candidate molecular ions for DPQL, the cationic alkaline-earth monoxides and monosulfides, which possess suitable structure for DPQL and can be produced in existing atomic ion experiments with little additional complexity. We present calculations of DPQL operations for one of these molecules, CaO⁺, and discuss progress towards experimental realization. We also further develop the theory of DPQL to include state preparation and measurement and entanglement of multiple molecular ions.

Investigation of dual-wavelength pump schemes for optically pumped rare gas lasers

Optically pumped rare gas lasers (OPRGLs) have shown great potential to generate high energy laser radiation with high beam quality. As an alternative to the diode-pumped alkali vapor lasers (DPALs), they have similar working principles and characteristics, but OPRGLs have the advantage that the gain medium is chemically inert and is appropriate for closed-cycle operation. One of the challenges OPRGLs are faced with is the bottleneck caused by the slow 1s₄-1s₅ collisional relaxations at room temperature. A 1s₄-2p₁₀ dual-wavelength pump method had been proposed to transfer the populations pooled on the 1s₄ level to the lasing cycle using a steady-state laser model. We explored this method further through 1s₄-2p₈ and 1s₄-2p₇ dual-wavelength pump schemes. The enhancement efficiencies at room temperature for a repetitively pulsed discharge, CW dual-wavelength pump system were examined using a dynamic model, and an experiment with a pulsed secondary pump was conducted for qualitative evaluations.

Spectroscopic and theoretical studies of UN and UN. J Chem Phys 2020;152:094302. [PMID: 33480743 DOI: 10.1063/1.5144299]

The low-energy electronic states of UN and UN⁺ have been examined using high-level electronic structure calculations and two-color photoionization techniques. The experimental measurements provided an accurate ionization energy for UN (IE = 50 802 ± 5 cm^-1). Spectra for UN⁺ yielded ro-vibrational constants and established that the ground state has the electronic angular momentum projection Ω = 4. Ab initio calculations were carried out using the spin-orbit state interacting approach with the complete active space second-order perturbation theory method. A series of correlation consistent basis sets were used in conjunction with small-core relativistic pseudopotentials on U to extrapolate to the complete basis set limits. The results for UN correctly obtained an Ω = 3.5 ground state and demonstrated a high density of configurationally related excited states with closely similar ro-vibrational constants. Similar results were obtained for UN⁺, with reduced complexity owing to the smaller number of outer-shell electrons. The calculated IE for UN was in excellent agreement with the measured value. Improved values for the dissociation energies of UN and UN⁺, as well as their heats of formation, were obtained using the Feller-Peterson-Dixon composite thermochemistry method, including corrections up through coupled cluster singles, doubles, triples and quadruples. An analysis of the ab initio results from the perspective of the ligand field theory shows that the patterns of electronic states for both UN and UN⁺ can be understood in terms of the underlying energy level structure of the atomic metal ion.

Characterization of the Ground States of BeC2 and BeC2- via Photoelectron Velocity Map Imaging Spectroscopy

Due to their potentially unique properties, beryllium carbide materials have been the subject of many theoretical studies. However, experimental validation has been lacking due to the difficulties of working with Be. Neutral beryllium dicarbide has been predicted to have a T-shaped equilibrium structure (C_2v), while previous quantum chemistry calculations for the structure of the anion had not yielded consistent results. In this study, we report photoelectron velocity map imaging spectra for the BeC₂^- X ²A₁ → BeC₂ X ¹A₁ transition. These data provide vibrational frequencies and the electron affinity of BeC₂. Ab initio electronic structure calculations, validated against the experimental data, show that both the anion and the neutral form have C_2v equilibrium geometries with polar covalent bonding between Be and the C₂ subunit. Computed vibrational frequencies and the electron affinity, obtained at the CCSD(T) level of theory, were found to be in good agreement with the measurements.

Transversely optically pumped Ar:He laser with a pulsed-periodic discharge

Optically pumped rare gas lasers have the potential for scaling to high-power cw systems with good beam quality. Metastable atoms of heavier rare gases that are the lasing species are produced in an electric discharge at near atmospheric pressure. The key problem for this class of lasers at present is the development of a suitable discharge system. In this paper, we present the results of optimization of a pulsed discharge system with the goal of minimizing cathode sputtering and peak discharge current. The first demonstration of a transversely pumped system and measurements of the optical pumping threshold for the Ar:He laser are also presented.

Computational investigation of energy transfer and line broadening for Ar* + He collisions

Potential energy curves for all states arising from the interaction of He with the 3p⁶, 3p⁵4s, and 3p⁵4p configurations of Ar have been determined using high-level electronic structure calculations. The results have been used to examine collisional energy transfer probabilities and spectral line shape parameters (shifting and broadening rate coefficients). The main focus has been on states and transitions that are of relevance to optically pumped He/Ar^* laser systems. The line shape predictions were found to be in good agreement with experimental data, while there is notable disagreement for the energy transfer probabilities. The experimental data are found to be at variance with the predictions of standard two-state curve crossing models for energy transfer.

Demonstration of a quasi-CW diode-pumped metastable xenon laser

In this work, we present the first demonstration of a quasi-continuous-wave diode-pumped metastable xenon laser at atmospheric pressures. Lasing in metastable noble gas species has received increased attention in the last few years as a possible high-power laser source. This demonstration shows that metastable xenon has a sufficiently broad absorption spectrum to be pumped with a broad-bandwidth diode laser. This implies that a high-power metastable xenon gas laser should be achievable using high-power pump diodes.

Time-dependent simulations of a CW pumped, pulsed DC discharge Ar metastable laser system

Optically pumped rare gas lasers have the potential for scaling to output powers above the kW level. In these devices, electrical discharges through He/Rg mixtures (Rg = Ne, Ar, Kr and Xe) are used to generate metastable Rg atoms in the 1s₅ state. Optical pumping to the 2p₉ level, followed by collisional relaxation to 2p₁₀, is then used to produce lasing on the 2p₁₀-1s₅ transition. Several computational models have been developed to analyze CW systems using steady-state approximations for the discharge excitation, optical pumping and lasing processes. However, recent experiments show that repetitively pulsed discharges have advantages for producing larger volume, high-pressure discharges. Here we present dynamic simulations of a CW laser that uses pulsed-discharge production of Ar metastables. Time-dependent equations are solved for both the discharge and lasing process. Two models are investigated. The first considers the conditions within the lasing medium to be spatially uniform (zero-dimensional model). The second allows for spatial variations along the lasing axis (one-dimensional model). The models were evaluated by simulating the performance characteristics of an experimentally demonstrated system that provides time-averaged output energies in the range of 3-4 W. Time-dependent species densities, laser power and longitudinal spatial distributions are presented.

Ab initio interatomic potentials and transport properties of alkali metal (M = Rb and Cs)-rare gas (Rg = He, Ne, Ar, Kr, and Xe) media

We performed a first principle systematic calculation on the adiabatic potential energy curves (PECs) of alkali metal (M = Rb and Cs) - rare gas (Rg = He, Ne, Ar, Kr, and Xe) van der Waals molecules over a wide range of interatomic distance R. All electron basis sets of triple and quadruple zeta valence quality were used for the He, Ne, Ar and Kr atoms. Scalar relativistic effects were taken into account for the heavy Rb, Cs and Xe atoms by means of Dirac-Fock effective core potentials. The correlated ground state energies have been obtained within the framework of the spin unrestricted open-shell coupled cluster method, with perturbative treatment of triple excitations. The electronic energies were corrected for the basis set superposition error (BSSE) using the counterpoise method. Energies were extrapolated to the complete basis set (CBS) limit using a two-point scheme. The energy convergence towards the CBS limit was monitored by the saturation of the dummy atom basis set that included bond functions centered at the midpoint of the interatomic distance. The ab initio point-wise PEC was followed to small R to the point where the energy was 0.5 Hartree above the dissociation limit. A Morse long-range (MLR, UM Rg(R)) potential possessing the correct asymptotic behavior at R → ∞ was fitted to the single point energies. The resulting set of fully analytical MLR potentials was then used to evaluate classical collision integrals over a wide range of collision energies. By this means, diffusion coefficients (DM Rg(T)) were predicted as functions of the translation temperature T ≤ 3000 K. The reliability of the present ab initio UM Rg(R) and DM Rg(T) functions was accessed through a comparison with previous theoretical and experimental results.

Dative Bonding between Closed-Shell Atoms: The BeF- Anion

Beryllium can exhibit unusually strong attractive interactions under conditions where it is nominally a closed-shell atom. Two prominent examples are the Be₂ dimer and the He-BeO complex. In the present study, we examine the bonding of the closed-shell Be-F^- anion. This molecule preserves the closed-shell character of the individual atoms as the electron affinity of F is high (328.16 kJ mol^-1) while that of Be is negative. Photodetachment spectroscopy was used to determine the vibrational frequency for BeF^- and the electron affinity of BeF (104.2 kJ mol^-1). The latter has been used to determine a lower bound of 343 kJ mol^-1 for the bond energy of BeF^-. Electronic structure calculations yielded predictions that were in good agreement with the observed data. A natural bond orbital analysis shows that BeF^- is primarily bound by a dative interaction.

Demonstration of a CW diode-pumped Ar metastable laser operating at 4 W

Optically pumped rare gas lasers are being investigated as potential high-energy, high beam quality systems. The lasing medium consists of rare gas atoms (Rg=Ne, Ar, Kr, or Xe) that have been electric discharge excited to the metastable np⁵(n+1)s P3₂ state. Following optical excitation, helium (He) at pressures of 200-1000 Torr is used as the energy transfer agent to create a population inversion. The primary technical difficulty for this scheme is the discharge production of sufficient Rg* metastables in the presence of >200  Torr of He. In this Letter, we describe a pulsed discharge that yields >10¹³  cm^-3Ar* in the presence of He at total pressures up to 750 Torr. Using this discharge, a diode-pumped Ar* laser providing 4.1 W has been demonstrated.

O2(b1Σg+) Quenching by O2, CO2, H2O, and N2 at Temperatures of 300-800 K

Rate constants for the removal of O₂(b¹Σ_g⁺) by collisions with O₂, N₂, CO₂, and H₂O have been determined over the temperature range from 297 to 800 K. O₂(b¹Σ_g⁺) was excited by pulses from a tunable dye laser, and the deactivation kinetics were followed by observing the temporal behavior of the b¹Σ_g⁺-X³Σ_g^- fluorescence. The removal rate constants for CO₂, N₂, and H₂O were not strongly dependent on temperature and could be represented by the expressions k_CO2 = (1.18 ± 0.05) × 10^-17 × T^1.5 × exp[Formula: see text], k_N2 = (8 ± 0.3) × 10^-20 × T^1.5 × exp[Formula: see text], and k_H2O = (1.27 ± 0.08) × 10^-16 × T^1.5 × exp[Formula: see text] cm³ molecule^-1 s^-1. Rate constants for O₂(b¹Σ_g⁺) removal by O₂(X), being orders of magnitude lower, demonstrated a sharp increase with temperature, represented by the fitted expression k_O2 = (7.4 ± 0.8) × 10^-17 × T^0.5 × exp[Formula: see text] cm³ molecule^-1 s^-1. All of the rate constants measured at room temperature were found to be in good agreement with previously reported values.

Photoelectron Velocity Map Imaging Spectroscopy of the Beryllium Sulfide Anion, BeS. J Phys Chem A 2017;121:5645-5650. [PMID: 28691819 DOI: 10.1021/acs.jpca.7b04894]

Slow electron velocity map imaging (SEVI) spectroscopy was used to examine the BeS^- anion to neutral ground-state transition, X ²Σ⁺ → X ¹Σ⁺. Rotational constants, vibrational intervals, and the electron binding energy of BeS^- were determined. Partially resolved rotational contours were seen due to the relatively small moment of inertia of beryllium sulfide. Upon analysis of the rotational contours, it was found that changes in the molecular rotational angular momentum, ΔN = -1, -2, -3, and -4, facilitated photodetachment at near-threshold photon energies. The electron affinity of BeS was found to be 2.3346(2) eV. SEVI spectra recorded using photon energies near the threshold for Δv = -1 processes exhibited features that were associated with a dipole-bound state (DBS) of BeS^-. Autodetachment spectroscopy was used to probe this state, and rotationally resolved data were obtained for the DBS ²Σ⁺, v' = 0 - X ²Σ⁺, v″ = 0 transition. Analysis of this structure provided the rotational constants for BeS^- X, v″ = 0, and the electron binding energy of the DBS. Electronic structure calculations, performed at the RCCSD(T) and MRCI levels of theory, gave predictions that were in good agreement with the experimental observations.

Photodetachment spectroscopy of the beryllium oxide anion, BeO. J Chem Phys 2017;146:054301. [PMID: 28178838 DOI: 10.1063/1.4974843]

The X²Σ⁺→X¹Σ⁺ anion to neutral ground state photodetachment of BeO^- has been studied by means of photoelectron velocity-map imaging spectroscopy in a newly constructed apparatus. Vibrational intervals, rotational constants, and the electron detachment threshold of BeO^- were determined for the first time. The small moment of inertia of beryllium oxide allowed for the observation of partially resolved rotational contours. Analyses of these contours provided evidence of several detachment channels resulting from changes in molecular rotational angular momenta of ΔN = 0, ±1, ±2, and ±3. The relative intensities of these detachment channels were found to be a function of the electron kinetic energy. Experimental results are compared to the predictions of high level ab initio calculations.

Removal of Rb(6(2)P) by H(2), CH(4), and C(2)H(6)

The saturated hydrocarbons methane and ethane are often used as collisional energy transfer agents in diode-pumped alkali vapor lasers (DPALs). Problems are encountered because the hydrocarbons eventually react with the optically pumped alkali atoms, resulting in the contamination of the gas lasing medium and damage of the gas cell windows. The reactions require excitation of the more highly excited states of the alkali atoms, which can be generated in DPAL systems by energy pooling processes. Knowledge of the production and loss rates for the higher excited states is needed for a quantitative understanding of the photochemistry. In the present study, we have used experimental and theoretical techniques to characterize the removal of Rb(6P2) by hydrogen, methane, and ethane.

Kinetics of optically pumped Kr metastables

Optically-pumped atomic gas lasers that utilize metastable excited states of rare gas atoms (Rg(*)) have been demonstrated using both pulsed and CW pump sources. These devices resemble diode-pumped alkali vapor lasers, but have the advantage of using a chemically inert lasing medium. Collisional energy transfer is needed to sustain a population inversion, and He is used as the transfer agent. Consequently, values for the Kr(*)+He state-to-state energy transfer rate constants are needed for the analysis and prediction of laser performance characteristics. In the present study, we report He energy transfer rate constants for Kr(*) in the 5p[5/2](2), 5p[5/2](3), 5p[1/2](1), and 5s[3/2](1) states.

Optically pumped microplasma rare gas laser

The optically pumped rare-gas metastable laser is a chemically inert analogue to three-state optically pumped alkali laser systems. The concept requires efficient generation of electronically excited metastable atoms in a continuous-wave (CW) electric discharge in flowing gas mixtures near atmospheric pressure. We have observed CW optical gain and laser oscillation at 912.3 nm using a linear micro-discharge array to generate metastable Ar(4s, 1s(5)) atoms at atmospheric pressure. We observed the optical excitation of the 1s(5) → 2p(9) transition at 811.5 nm and the corresponding fluorescence, optical gain and laser oscillation on the 2p(10) ↔ 1s(5) transition at 912.3 nm, following 2p(9)→2p(10) collisional energy transfer. A steady-state kinetics model indicates efficient collisional coupling within the Ar(4s) manifold.

Kinetics of optically pumped Ar metastables

Optically pumped lasers that use metastable excited states of Ar have been demonstrated using both pulsed and CW excitation. In terms of Paschen labeling of the states of Ar, the laser system uses excitation of the 2p9-1s5 transition, and lases on the 2p10-1s5 line. Collisional transfer of population from 2p9 to 2p10 is achieved using He as the buffer gas. For the purpose of modeling and developing this laser, rate constants for state-to-state transfer in Ar(2p(i))+Ar/He mixtures are needed. As the 2p10 level can radiate down to 1s4, this lower level also plays a significant role in the laser kinetics. Consequently, rate constants for the relaxation of 1s4 by Ar and He are also required. In the present study we have used pulsed laser excitation techniques to measure rate constants of relevance to the optically pumped metastable Ar laser.

Spectroscopy and structure of the simplest actinide bonds

Understanding the influence of electrons in partially filled f- and d-orbitals on bonding and reactivity is a key issue for actinide chemistry. This question can be investigated by using a combination of well-defined experimental measurements and theoretical calculations. Gas phase spectroscopic data are particularly valuable for the evaluation of theoretical models. Consequently, the primary objectives of our research have been to obtain gas phase spectra for small actinide molecules. To complement the experimental effort, we are investigating the potential for using relativistic ab initio calculations and semiempirical models to predict and interpret the electronic energy level patterns for f-element compounds. Multiple resonance spectroscopy and jet cooling techniques have been used to unravel the complex electronic spectra of Th and U compounds. Recent results for fluorides, sulfides, and nitrides are discussed.

Experimental and theoretical characterization of the 2(2)A'-1(2)A' transition of BeOH/D

The hydroxides of Ca, Sr, and Ba are known to be linear molecules, while MgOH is quasilinear. High-level ab initio calculations for BeOH predict a bent equilibrium structure with a bond angle of 140.9°, indicating a significant contribution of covalency to the bonding. However, experimental confirmation of the bent structure is lacking. In the present study, we have used laser excitation techniques to observe the 2(2)A'-1(2)A' transition of BeOH/D in the energy range of 30300-32800 cm(-1). Rotationally resolved spectra were obtained, with sufficient resolution to reveal spin splittings for the electronically excited state. Two-color photoionization was used to determine an ionization energy of 66425(10) cm(-1). Ab initio calculations were used to guide the analysis of the spectroscopic data. Multireference configuration interaction calculations were used to construct potential energy surfaces for the 1(2)A', 2(2)A', and 1(2)A" states. The rovibronic eigenstates supported by these surfaces were determined using the Morse oscillator rigid bender internal dynamics Hamiltonian. The theoretical results were in sufficiently good agreement with the experimental data to permit unambiguous assignment. It was confirmed that the equilibrium geometry of the ground state is bent and that the barrier to linearity lies below the zero-point energies for both BeOH and BeOD.

Demonstration of a diode-pumped metastable Ar laser

Pulsed lasing from optically pumped rare gas metastable atoms (Ne, Ar, Kr, and Xe) has been demonstrated previously. The laser relies on a three-level scheme, which involves the (n+1)p[5/2](3) and (n+1)p[1/2](1) states from the np(5)(n+1)p electronic configuration and the metastable (n+1)s[3/2](2) level of the np(5)(n+1)s configuration (Racah notation). Population inversions were achieved using relaxation from ((n+1)p[5/2](3) to (n+1)p[1/2](1) induced by collisions with helium or argon at pressures near 1 atm. Pulsed lasing was easily achieved using the high instantaneous pump intensities provided by a pulsed optical parametric oscillator excitation laser. In the present study we examine the potential for the development of a continuous wave (CW) optically pumped Ar laser. We report lasing of the 4p[1/2](1)→4s[3/2](2) (912.547 nm) transition following CW diode laser excitation of the 4p[5/2](3)←4s[3/2](2) line (811.754 nm). A pulsed discharge was used to generate Ar 4s[3/2](2), and the time-resolved lasing kinetics provide insights concerning the radiative and collisional relaxation processes.