Pulsed-field-ionization zero-kinetic-energy photoelectron spectra of jet-cooled allyl radical (C3H5) have been recorded following single-photon and resonant multiphoton excitation. Simulations based on an orbital ionization model and rovibronic photoionization selection rules reliably describe the observed intensity distributions in the photoelectron spectra obtained from single-photon excitation from the ground state and resonant multiphoton excitation via the 3s and the 3p Rydberg states. More than 30 transitions to vibrational levels of the cation were identified and assigned on the basis of predictions from ab initio calculations.

We have set up an experiment for the efficient population transfer by a sequential two photon-absorption and stimulated emission-process in a molecular beam to prepare quantum states of well defined parity and their subsequent sensitive detection. This provides a proof of principle for an experiment which would allow for parity selection and measurement of the time evolution of parity in chiral molecules, resulting in a measurement of the parity violating energy difference ΔpvE between enantiomers of chiral molecules. Here, we present first results on a simple achiral molecule demonstrating efficient population transfer (about 80% on the average for each step) and unperturbed persistence of a selected excited parity level over flight times of about 1.3 ms in the beam. In agreement with model calculations with and without including nuclear hyperfine structure, efficient population transfer can be achieved by a rather simple implementation of the rapid adiabatic passage method of Reuss and coworkers and considering also the stimulated Raman adiabatic passage technique of Bergmann and coworkers as an alternative. The preparation step uses two powerful single mode continuous wave optical parametric oscillators of high frequency stability and accuracy. The detection uses a sensitive resonantly enhanced multiphoton ionization method after free flight lengths of up to 0.8 m in the molecular beam. Using this technique, we were able to also resolve the nuclear hyperfine structure in the rovibrational levels of the ν1 and ν3 fundamentals as well as the 2ν4 overtone of (14)NH3, for which no previous data with hyperfine resolution were available. We present our new results on the quadrupole coupling constants for the ν1, ν3, and 2ν4 levels in the context of previously known data for ν2 and its overtone, as well as ν4, and the ground state. Thus, now, (14)N quadrupole coupling constants for all fundamentals and some overtones of (14)NH3 are known and can be used for further theoretical analysis.

A new derivation of an exact finesse F for the description of an optical resonator is reported. The finesse is derived using the superposition principle of plane waves in an ideal Fabry-Perot resonator in combination with the standard definition of the quality factor Q which relates the energy loss of a resonator cycle to the energy stored in the resonator. The derived exact expression of the finesse is compared to equations found in the literature, and it is shown that they are based on approximations. The exact finesse is then used to convert an infinite series of Lorentz functions into the well-known Airy equation in the case of neglected absorption. The derived expression provides a framework for the discussion of the finesse in terms of decay times, free spectral ranges, and resonator line widths. The provided expression for the finesse is valid for any value of the intensity reflectivity and gives insight into the underlying physical principles of resonators.

We report results on nuclear spin symmetry conservation studied by high resolution spectroscopy of relative line intensities for the A and E nuclear spin isomers of symmetric top molecules CHD 3 , CH 3 D, CH 3 F, and CH 3 35 Cl in supersonic jet expansions with He and Ar as carrier gases. Infrared absorption spectra were measured around 3000 cm −1 by an infrared (lead salt) diode laser and a continuous wave IR-OPO (infrared optical parametric oscillator) locked to a frequency comb. A detailed analysis of the R(2)-line intensities of the CH-stretching fundamental shows that nuclear spin symmetry is conserved for CHD 3 , CH 3 F, and CH 3 35 Cl during the expansion. For CH 3 D, a small contribution from nuclear spin symmetry relaxation cannot be excluded completely under our experimental conditions.

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