SrF is a candidate molecule for studies of physics beyond the standard model. Cooling of SrF to ultracold temperatures has been demonstrated, which is essential for the required precision spectroscopic measurements. Interpretation of the experimental data will involve the use of electronic structure parameters derived from high-level theoretical calculations. In the present study, measurements of the ionization energy of SrF and the vibrational constants of SrF+ have been used to evaluate the predictive power of relativistic CCSD(T) calculations for the electronic structure of SrF.

Associative ionization reactions of the type Ln + O → LnO+ + e- (where Ln is a lanthanide) are being investigated as they may be used to transiently modify local electron densities at high altitudes. The most promising elements are those for which the LnO bond dissociation energy (BDE) exceeds the LnO ionization energy (IE). Consequently, accurate measurements of the BDE's and IE's are needed for the identification of exothermic associative ionization reactions. Sounding rocket experiments have been carried out for releases of thermally vaporized Sm. Emission spectroscopy was used as a diagnostic for the Sm + O reaction. It was hoped that the degree of ionization could be deduced from emission spectra, but the analysis has been inconclusive due to the lack of spectroscopic data for SmO+. In the present study we have mapped the low energy states of SmO+ using pulsed field ionization─zero kinetic energy (PFI-ZEKE) photoelectron spectroscopy. The data obtained are suitable for use in the analyses of the overlapped SmO/SmO+ spectra. The Gd + O → GdO+ + e- reaction is known to be exothermic, but a wide range of values for the IE have been reported from both experimental observations and theoretical calculations. In the present study PFI-ZEKE measurements have been used to establish a more accurate IE (6.075 ± 0.001 eV) and the vibrational frequency for the GdO+ ground state.

Rotationally resolved electronic spectra have been recorded for gas-phase UO2. Analysis of the rotational fine structure confirms that the ground state is X3Φ2u, derived from the O2-U4+(5f7s)O2- electronic configuration. All transitions observed below 14500 cm-1 were assigned to the [11.51]3g ↔ X3Φ2u transition, which correlates with the metal-centered 5f7p ↔ 5f7s electron promotion. The symmetric stretch and bending vibrational modes were active in the spectrum. Ground-state vibrational fundamentals of 853(5) and 133(5) cm-1 were determined along with a vibrationally averaged bond length of 1.781 Å. Spectra for the [17.68]4g ↔ X3Φ3u transition exhibited the same lower state vibrational frequencies, confirming that the X3Φ3u state also arises from the 5f7s configuration. Fluorescence decay lifetimes for the excited states were in the range of 1.5-4.7 μs, values that were anomalously long for electric dipole-allowed transitions. The long lifetimes are attributed to extensive mixing with a dense background of vibronic dark states.

Electronic spectra for OThF have been recorded using fluorescence excitation and two-photon resonantly enhanced ionization techniques. Multiple vibronic bands were observed in the 340 – 460 nm range. Dispersed fluorescence spectra provided ground state vibrational constants and evidence of extensive vibronic state mixing at higher excitation energies. Two-photon ionization measurements established an ionization energy for OThF of 6.283(5) eV. To guide the assignment of the OThF spectra, electronic structure calculations were carried out using relativistic equation-of-motion coupled-cluster singles and doubles methods. These calculations indicated that spin-orbit induced mixing of the 3A" and 4A' states was mediated by a seam of potential energy surface intersections.

The polyatomic molecules YbOH and YbOCH3 have been recognized as being of potential value for spectroscopic experiments that explore charge-parity and time-reversal symmetry violation effects. These measurements require very high precision, which, in turn, will necessitate that the molecules be manipulated at ultracold temperatures. Both YbOH and YbOCH3 have electronic transitions that appear suitable for laser cooling ((A) over tilde (2)Pi(1/2)-(X) over tilde (2)Sigma(+) and (A) over tilde E-2(1/2)-(X) over tilde (2)A(1), respectively) but the currently available spectroscopic data are not sufficient to determine the extent to which population leaks may compromise the optical cooling processes. A further complication is that the quantum states of interest for these measurements will need to be selectively populated. The (A) over tilde-(X) over tilde band systems of both YbOH and YbOCH3 show evidence of vibronic perturbations, such that there are unassigned vibronic features at energies that are just above the origin bands. In the present study we have recorded spectra for the (A) over tilde (2)Pi(1/2)-(X) over tilde (2)Sigma(+) transition of jet-cooled YbOD to facilitate the vibronic assignments. In addition, spectra for the (B) over tilde (2)Sigma(+)-(X) over tilde (2)Sigma(+) transition of YbOH were recorded, establishing the origin band at 20473.8 cm(-1). Previously, the reaction of Yb with CH3OH has been used to generate gas-phase YbOCH3. As this reaction also yields YbOH, there have been complications in spectroscopic studies of YbOCH3 due to overlap of the (A) over tilde-(X) over tilde band systems. To identify specific regions of overlap, resonantly enhanced two-photon ionization spectra were recorded using mass-resolved detection of the YbOH+ and YbOCH3+ ions. These data confirmed the overlap of vibronic bands near 17 640 and 17 680 cm(-1). Two-photon ionization spectroscopy also provided accurate ionization energies (IE), IE(YbOH) = 45 788(10) and IE(YbOCH3) = 45 283(10) cm(-1). The IE for YbOH is relevant to problems encountered in previous attempts to determine the bond-dissociation energy of YbOH+.

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.

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.

YbF has been identified as a molecule that can be used to investigate charge-parity symmetry violations that are beyond the Standard Model of particle physics. Cooling to sub-milli-Kelvin is advantageous for experiments that probe manifestations of these symmetry violations. One approach involves laser cooling of YbF via the A2P1/2-X2S+ transition. However, it appears that cooling by means of this transition may be limited by the radiative loss of population from the cooling cycle. YbF has low-energy states that arise from the Yb+(4f136s2)F- configuration. Recent theoretical calculations predict (Zhang et al J. Mol. Spectrsc. 386 11625 (2022)) that radiative decay from A2Π1/2 to the 4f136s2 states occurs with a branching fraction of approximately 10-3. In the present study we have used dispersed laser induced fluorescence spectroscopy to the observe the lowest energy 4f136s2 states. These measurements were carried out using excitation of previously unobserved YbF transitions in the near UV spectral range. An accurate ionization energy (IE) for YbF is also reported. A two-color photoionization technique was used to determine the IE and observe the v+=0-3 vibrational levels of YbF+ X1S+.

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)O2- and U2+(5f27s2)O2-, 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 (μe) of an electronic state should also manifest a value that is characteristic of the parent electronic configuration. μe 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 μe 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)O2- and U2+(5f27s2)O2- configurations, respectively.

YbF has been identified as a molecule that can be used to investigate charge-parity symmetry violations that are beyond the standard model of particle physics. Cooling to sub-milli-Kelvin is advantageous for experiments that probe manifestations of these symmetry violations. One approach involves laser cooling of YbF via the A(2)Pi(1/2)-X-2 Sigma(+) transition. However, it appears that cooling by means of this transition may be limited by the radiative loss of population from the cooling cycle. YbF has low-energy states that arise from the Yb+(4f(13)6s(2))F- that radiative decay from A(2)Pi(1/2) to the 4f(13)6s(2) states occurs with a branching fraction of approximately 10(-3). In the present study we have used dispersed laser induced fluorescence spectroscopy to observe the lowest energy 4f(13)6s(2) states. These measurements were carried out using excitation of previously unobserved YbF transitions in the near UV spectral range. An accurate ionization energy (IE) of 48 703 +/- 5 cm(-1) for YbF is also reported. A two-color photoionization technique was used to determine the IE and observe the v(+) = 0-3 vibrational levels of YbF+ X-1 Sigma(+).

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 1s 4 -1s 5 collisional relaxations at room temperature. A 1s 4 -2p 10 dual-wavelength pump method had been proposed to transfer the populations pooled on the 1s 4 level to the lasing cycle using a steady-state laser model. We explored this method further through 1s 4 -2p 8 and 1s 4 -2p 7 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.

Modeling of 1s 4 -2p 10 , 1s 4 -2p 8 and 1s 4 -2p 7 dual-pump schemes for optically pumped rare gas lasers was conducted. Simulations showed significant enhancement for the laser performance, which was verified by preliminary experimental results.

Optically pumped rare gas lasers have the potential to provide high-power output with excellent beam quality. They lase at wavelengths that are readily transmitted by the atmosphere, and their optical characteristics are closely similar to those of diode-pumped alkali vapor lasers (DPAL’s). As compared to DPAL’s they present two significant advantages. The first is that they use entirely inert reagents that are gases at ambient temperature (Helium plus a heavier rare gas). As there is no chemistry involved, these devices can be operated in a completely closed-cycle mode. A second advantage is derived from the energy level structures of metastable rare gas atoms. While the sp transitions of alkali metals will provide just one laser wavelength for a given element, each rare gas atom can offer a range of different output wavelengths from the first manifold of sp transitions. The primary technical challenge for the optically pumped rare gas laser is the requirement to generate the lasing medium (metastable Rg(3P2) where Rg=Ne, Ar, Kr or Xe) at a density of approximately 1e13 cm-3 in the presence of helium at total pressures in the range 0.5 – 1.0 atm. For this application we have developed a quasi-CW discharge driven by a high repetition rate power supply. The frequency and time duration of the pulses are tailored to exploit the non-steady high field characteristics of the pulsed breakdown, while sustaining a temporally steady argon metastable concentration in the gain medium. This system has been scaled to a 912.3 nm output of ~4 W when pumped by a 20 W diode laser. Recently, the pulsed discharge system has been improved by increasing the upper limit for the voltage available from the power supply. The present system operates at voltages up to 2200 V with a nominal pulse duration of 50 ns and pulse repetition frequency of 100 kHz. This has been used to sustain a discharge in 1 atm of a He/Ar mixture that produces an Ar* metastable density of 2e13 cm-3, a path length of 3 cm and a total volume of 1.2 cm3 (0.64x0.64 cm2 cross section). A diode pumped Ar* laser that employed this discharge has been operated for extended periods with no sign of performance degradation. As expected for a system that uses only inert gases, there was no indication of window damage or chemical activity. Computational models indicate that scaling to the 100 kW level is feasible with a discharge volume of 10 cm3 and discharge power of 200 W.

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.

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.

There have been concerted efforts to develop high-energy diode-pumped alkali vapor lasers (DPAL). These hybrid gas phase / solid-state laser systems offer possibilities for constructing high-powered lasers that have high beam quality. Considerable progress has been made, but there are technical challenges associated with the reactivity of the metal atoms. Rare gas atoms (Rg) excited to the np5 (n+1)s 3P2 configuration are metastable and have spectral properties that are closely similar to those of the alkali metals. Optically pumped lasers have been constructed using excitation of the np5 (n+1)p ← np5 (n+1)s transitions. Pulsed lasing has been observed for Ne*, Ar*, Kr* and Xe*. Helium was used as the collisional energy transfer agent that established population inversions. These systems have the advantage using inert reagents that are gases at room temperature, with excellent potential for closed-cycle, multi-wavelength operation. The primary technical difficulty for the rare gas laser is the discharge production of sufficient Rg* metastables in the presence of >200 Torr of He. We have developed a high frequency pulsed discharge that yields >1013 cm-3 Ar* in the presence of He at pressures up to 730 Torr. Using this discharge, a diode pumped Ar* laser providing 4.1 W of continuous wave output has been demonstrated, with an optical conversion efficiency of 31%. Development of the pulsed discharge system and CW lasing demonstrations with Xe* are reported.

In this communication, we present the results of experiments with optically pumped rare gas laser (OPRGL) with a dielectric barrier discharge (DBD) as the source of metastable Ar atoms and a pulsed OPO system as the optical pump. In a longitudinal pumping scheme, the threshold input intensity to achieve lasing was 3.9 kW cm(-2). Lasing was observed in the pressure range from 100 to 750 Torr. However, lasing was possible only during the limited time of the DBD applied voltage cycle.

Optically pumped all-rare-gas lasers (OPRGL) utilize metastable atoms of the heavier rare gases as lasing species. The required number density of metastables for efficient laser operation is 1012–1013 cm−3 in He buffer gas at pressures in the 400–1000 Torr range. Such metastable densities are easily produced in a nanosecond pulsed discharge, even at pressures larger than atmospheric, but problems appear when one is trying to achieve continuous production. The reason for low production efficiency in many types of continuous discharge at atmospheric pressure is the low value of the E/N parameter (<5–6 Td). In the present work, we have examined the possibility of using a dielectric barrier discharge (DBD) to provide near continuous, high densities of Ar and Xe metastables. Experiments were performed using a 20 kHz DBD in binary Ar and Xe mixtures with He, and in ternary Ar:Xe:He mixtures at pressures up to 1 atmosphere. Concentrations were measured by means of tunable diode laser absorption spectroscopy. Time-averaged [Ar(1s5)] and [Xe(1s5)] number densities on the order of 1012 cm−3 were readily achieved. The temporal behavior of [Xe(1s5)] throughout the DBD cycle was observed. The results demonstrate the feasibility of using DBDs for OPRGL development. Spectral scans over the absorption lines were also used to examine the pressure broadening coefficients for the 912.3 nm Ar line in He and the Xe 904.5 nm line in Ne and He.