Negative-Ion Photoelectron Spectroscopy, Gas-Phase Acidity, and Thermochemistry of the Peroxyl Radicals CH_3OO and CH_3CH_2OO

Methyl, methyl-d3, and ethyl hydroperoxide anions (CH_3OO-, CD_3OO-, and CH_3CH_2OO-) have been prepared by deprotonation of their respective hydroperoxides in a stream of helium buffer gas. Photodetachment with 364 nm (3.408 eV) radiation was used to measure the adiabatic electron affinities:  EA[CH_3OO, X̃^2A‘‘] = 1.161 ± 0.005 eV, EA[CD_3OO, X̃^2A‘‘] = 1.154 ± 0.004 eV, and EA[CH_3CH_2OO, X̃^2A‘‘] = 1.186 ± 0.004 eV. The photoelectron spectra yield values for the term energies:  ΔE(X̃^2A‘‘−Ã^2A‘)[CH_3OO] = 0.914 ± 0.005 eV, ΔE(X̃^2A‘‘−Ã^2A‘)[CD_3OO] = 0.913 ± 0.004 eV, and ΔE(X̃^2A‘‘−Ã^2A‘)[CH_3CH_2OO] = 0.938 ± 0.004 eV. A localized RO−O stretching mode was observed near 1100 cm^(-1) for the ground state of all three radicals, and low-frequency R−O−O bending modes are also reported. Proton-transfer kinetics of the hydroperoxides have been measured in a tandem flowing afterglow−selected ion flow tube (FA-SIFT) to determine the gas-phase acidity of the parent hydroperoxides: Δ_(acid)G_(298)(CH_3OOH) = 367.6 ± 0.7 kcal mol^(-1), Δ_(acid)G_(298)(CD_3OOH) = 367.9 ± 0.9 kcal mol^(-1), and Δ_(acid)G_(298)(CH_3CH_2OOH) = 363.9 ± 2.0 kcal mol^(-1). From these acidities we have derived the enthalpies of deprotonation: Δ_(acid)H_(298)(CH_3OOH) = 374.6 ± 1.0 kcal mol^(-1), Δ_(acid)H_(298)(CD_3OOH) = 374.9 ± 1.1 kcal mol^(-1), and Δ_(acid)H_(298)(CH_3CH_2OOH) = 371.0 ± 2.2 kcal mol^(-1). Use of the negative-ion acidity/EA cycle provides the ROO−H bond enthalpies: DH_(298)(CH_3OO−H) = 87.8 ± 1.0 kcal mol^(-1), DH_(298)(CD_3OO−H) = 87.9 ± 1.1 kcal mol^(-1), and DH_(298)(CH_3CH_2OO−H) = 84.8 ± 2.2 kcal mol^(-1). We review the thermochemistry of the peroxyl radicals, CH_3OO and CH_3CH_2OO. Using experimental bond enthalpies, DH_(298)(ROO−H), and CBS/APNO ab initio electronic structure calculations for the energies of the corresponding hydroperoxides, we derive the heats of formation of the peroxyl radicals. The “electron affinity/acidity/CBS” cycle yields Δ_fH_(298)[CH_3OO] = 4.8 ± 1.2 kcal mol^(-1) and Δ_fH_(298)[CH_3CH_2OO] = −6.8 ± 2.3 kcal mol^(-1)

Design and Control of Femtosecond Lasers for Optical Clocks and the Synthesis of Low-Noise Optical and Microwave Signals

This paper describes recent advances in the design and control of femtosecond laser combs for their use in optical clocks and in the synthesis of low-noise microwave and optical signals. The authors present a compact and technically simple femtosecond laser that directly emits a broad continuum and shows that it can operate continuously on the timescale of days as the phase-coherent "clockwork" of an optical clock. They further demonstrate phase locking of an octave-spanning frequency comb to an optical frequency standard at the millihertz level. As verified through heterodyne measurements with an independent optical frequency standard, this provides a network of narrow optical modes with linewidths at the level of ~ 150 Hz, presently limited by measurement noise. Finally, they summarize their progress in using the femtosecond laser comb to transfer the stability and low phase-noise optical oscillators to the microwave domain