Frequency-doubled Nd:YAG MOPA laser system with programmable rectangular pulses up to 200 microseconds.

A compact frequency-doubled diode-pumped Nd:YAG master-oscillator power-amplifier laser system with programmable microsecond pulse length has been developed. Analog pulse shaping of the output from a single-frequency continuous-wave Nd:YAG oscillator, and subsequent amplification, allowed the generation of rectangular pulses with pulse lengths on the order of the Nd:YAG fluorescence lifetime. Temporally flat-top pulses of 1064 nm light with 520 mJ pulse energy, 2.6 kW peak power, and 200 µs duration, with linewidth below 10 kHz, were obtained at a repetition rate of 2 Hz. Second harmonic generation in a LBO crystal yielded pulses of 262 mJ and 1.3 kW peak power at 532 nm. The peak power can be maintained within 2.9% over the duration of the laser pulse, and long-term intensity stability of 1.1% was observed. The spatially flat-top beam at 1064 nm used in the amplifier is converted to a Gaussian beam at 532 nm with beam quality factor M2 = 1.41(14) during the second harmonic generation. This system has potential as a pump source for Ti:sapphire, dye, or optical parametric amplifiers to generate tunable high-power single-frequency radiation for applications in precision measurements and laser slowing.

Benchmarking Theory with an Improved Measurement of the Ionization and Dissociation Energies of H_{2}.

The dissociation energy of H_{2} represents a benchmark quantity to test the accuracy of first-principles calculations. We present a new measurement of the energy interval between the EF ^{1}Σ_{g}^{+}(v=0,N=1) state and the 54p1_{1} Rydberg state of H_{2}. When combined with previously determined intervals, this new measurement leads to an improved value of the dissociation energy D_{0}^{N=1} of ortho-H_{2} that has, for the first time, reached a level of uncertainty that is 3 times smaller than the contribution of about 1 MHz resulting from the finite size of the proton. The new result of 35 999.582 834(11) cm^{-1} is in remarkable agreement with the theoretical result of 35 999.582 820(26) cm^{-1} obtained in calculations including high-order relativistic and quantum-electrodynamics corrections, as reported in the following Letter [M. Puchalski, J. Komasa, P. Czachorowski, and K. Pachucki, Phys. Rev. Lett. 122, 103003 (2019)PRLTAO0031-900710.1103/PhysRevLett.122.103003]. This agreement resolves a recent discrepancy between experiment and theory that had hindered a possible use of the dissociation energy of H_{2} in the context of the current controversy on the charge radius of the proton.

Hyperfine-interaction-induced g/u mixing and its implication on the existence of the first excited vibrational level of the A + Σ u + 2 state of H 2 + and on the scattering length of the H + H+ collision.

Ab initio calculations of the energy level structure of H 2 + that include relativistic and radiative corrections to nonrelativistic energies and the diagonal part of the hyperfine interaction have predicted the existence of four bound rovibrational levels [(v = 0, N = 0 - 2) and (v = 1, N = 0)] of the first electronically excited ( A + Σ u + 2 ) state of H 2 + , the (v = 1, N = 0) level having a calculated binding energy of only E b = 1.082 219 8(4)·10-9 Eh and leading to an extremely large scattering length of 750(5) a0 for the H+ + H collision [J. Carbonell et al., J. Phys. B: At., Mol. Opt. Phys. 37, 2997 (2004)]. We present an investigation of the nonadiabatic coupling between the first two electronic states ( X + Σ g + 2 and A + Σ u + 2 ) of H 2 + induced by the Fermi-contact term of the hyperfine-coupling Hamiltonian. This interaction term, which mixes states of total spin quantum number G = 1/2, is rigorously implemented in a close-coupling approach to solve the spin-rovibronic Schrödinger equation. We show that it mixes states of gerade and ungerade electronic symmetry, that it shifts the positions of all weakly bound rovibrational states of H 2 + , and that it affects both the positions and widths of its shape resonances. The calculations demonstrate that the G = 1/2 hyperfine component of the A+ (v = 1, N = 0) state does not exist and that, for G = 1/2, the s-wave scattering lengths of the H+ + H(1s) collision are -578(6) a0 and -43(4) a0 for the F = 0 and F = 1 hyperfine components of the H(1s) atom, respectively. The binding energy of the G = 3/2 hyperfine component of the A+ (v = 1, N = 0) state is not significantly affected by the hyperfine interaction and the corresponding scattering length for the H+ + H(1s, F = 1) collision is 757(7) a0.

Nonadiabatic effects on the positions and lifetimes of the low-lying rovibrational levels of the GK 1Σ and H 1Σ states of H2.

The term values of rovibrational levels of the GK 1Σ+g and H 1Σ+g states of H2 have been measured with absolute and relative accuracies down to 10-4 cm-1 (≈3 MHz) and 10-6 cm-1 (≈30 kHz), respectively, by measuring transitions to long-lived high-n Rydberg states using single-mode cw laser radiation and a collimated supersonic beam of cold H2 molecules. The term values are compared with those determined in earlier experimental and theoretical investigations and the comparison points at discrepancies and a need for improved theoretical treatments of nonadiabatic, relativistic and radiative corrections for these states. The lifetimes of several low-lying rovibrational levels of the GK, H and I+ 1Πg states have been determined in pump-probe experiments. These lifetimes reveal a significant dependence on the electronic, vibrational and rotational excitation and also deviate from results obtained in earlier experimental and theoretical investigations.

Communication: Heavy-Rydberg states of HD and the electron affinity of the deuterium atom.

The electron affinity of the deuterium atom has been determined to be 6086.81(27) cm-1 from a measurement of the difference between the D+ + H- and H+ + D- ion-pair dissociation energies and a thermochemical cycle involving the electron affinity of H and the ionization energies of H and D. Heavy-Rydberg states and the ion-pair dissociation thresholds of HD were accessed with good efficiency using a three-photon excitation sequence through the B Σu+1 (v = 22, N = 1) and H¯ Σg+1 (v = 9, N = 0) intermediate levels and the threshold positions were determined using the method of threshold-ion-pair-production spectroscopy.

Observation and Calculation of the Quasibound Rovibrational Levels of the Electronic Ground State of H2+.

Although the existence of quasibound rotational levels of the X^{+} ^{2}Σ_{g}^{+} ground state of H_{2}^{+} was predicted a long time ago, these states have never been observed. Calculated positions and widths of quasibound rotational levels located close to the top of the centrifugal barriers have not been reported either. Given the role that such states play in the recombination of H(1s) and H^{+} to form H_{2}^{+}, this lack of data may be regarded as one of the largest unknown aspects of this otherwise accurately known fundamental molecular cation. We present measurements of the positions and widths of the lowest-lying quasibound rotational levels of H_{2}^{+} and compare the experimental results with the positions and widths we calculate using a potential model for the X^{+} state of H_{2}^{+} which includes adiabatic, nonadiabatic, relativistic, and radiative corrections to the Born-Oppenheimer approximation.

The fundamental rotational interval of para-H2 (+) by MQDT-assisted Rydberg spectroscopy of H2.

Transitions from selected nd Rydberg states of H2 to n'p/f Rydberg series converging on the lowest two (N(+) = 0 and 2) rotational levels of the X(+) (2)Σg (+) (v(+) = 0) ground state of para-H2 (+) have been measured in the range 1-7.4 THz using a laser-based, pulsed, narrow-band source of submillimeter-wave radiation. The analysis of the spectra by multichannel quantum-defect theory (MQDT) has allowed a complete interpretation of the fine structures of the Rydberg series and their dependence on the principal quantum number. The extrapolation of the series to their limits with MQDT has enabled the determination of the first rotational interval of para-H2 (+), which is 174.236 71(7) cm(-1) (5 223 485.1(2.3) MHz).