R. Seiler

Selective field ionization of high Rydberg states: Application to zero-kinetic-energy photoelectron spectroscopy

Sequences of pulsed electric fields have been designed and tested that enable a higher selectivity in the pulsed field ionization of high Rydberg states (n⩾100) than has so far been possible. The enhanced selectivity originates from the permutation of the parabolic quantum numbers n1 and n2 that is induced by a sufficiently rapid inversion of the electric field polarity during a pulse sequence. A reliable procedure, based on numerical simulations of the outcome of pulse field ionization sequences, has been developed to detect and control changes in the parabolic quantum numbers that can occur during a pulse sequence. The procedure can be used to assess under which conditions a clean permutation of the parabolic quantum numbers can be achieved. Unwanted randomization of m, n1 and n2, which reduces the selectivity of the field ionization process, can be avoided by minimizing the time intervals during which the electric field in the pulse sequence is almost zero. The high selectivity reached in the pulsed fi...

High-resolution threshold-ionization spectroscopy of NH3

High-resolution photoionization, zero-kinetic-energy photoelectron and Rydberg-state-resolved threshold-ionization spectra of ammonia and its deuterated isotopomers have been recorded in the region of the lowest vibrational levels (v2+=0,1) of the X+ ground ionic state of NH3+ following single-photon excitation from the ground neutral state using a narrow bandwidth vacuum ultraviolet laser system (bandwidth 0.008 cm−1). The resolution enables the observation of photoionization transitions originating from distinct tunneling components of the ground neutral state and the measurement of the spin-rotational splittings of the ionic energy levels. A new value of the first adiabatic ionization potential of NH3 [I.P.=82 158.751(16) cm−1] has been derived which is more accurate than previous values by almost two orders of magnitude. The photoionization dynamics of NH3 to the lowest vibrational levels of the X+(2A2″) ground state of NH3+ is dominated by the emission of even l photoelectron partial waves, and a s...

Rydberg-state-resolved zero-kinetic-energy photoelectron spectroscopy

A variant of pulsed-field-ionisation zero-kinetic-energy photoelectron spectroscopy (PFI-ZEKE-PES) has been developed to determine the position of ionic energy levels with an accuracy limited by the bandwidth of the tunable photoexcitation source. The development relies on the ability of resolving the transitions to the high Rydberg states (n≃200) which contribute to each line in a PFI-ZEKE photoelectron spectrum using a near-Fourier-transform-limited extreme ultraviolet (XUV) laser system (bandwidth <250 MHz). Thwe lowest ionisation potentials of Ar and N2 have been determined from Rydberg-state-resolved PFI-ZEKE-PE spectra to be (127109.825±0.015)cm−1 and (125667.028±0.015)cm−1, respectively.

Determination of the ionization energy of krypton by Rydberg-state-resolved threshold-ionization spectroscopy

A mass-analysed version of the technique of Rydberg-state-resolved threshold-ionization spectroscopy has been developed, and used to determine the first ionization energy of krypton (IE(86Kr) = 112 914.441 ± 0.016 cm−1) and to derive the isotopic shifts in the ionization energy of several isotopes of Kr. The results imply that the currently accepted value of the ionization energy of krypton must be revised.

Measurement of the hyperfine structure in low-l, high-n Rydberg states of ortho H2 by millimeter wave spectroscopy

Millimeter wave spectroscopy has been used to record high-resolution spectra of high-n (n=51–64), low-l (l=1–3) Rydberg states of ortho H2 located below the N+=1 rotational level of the X 2Σg+(v+=0) ground vibronic state of H2+. The spectral resolution of better than 1 MHz enables the observation of the hyperfine structure in these spectra. A simple procedure, based on the determination of combination differences, is used to reconstruct the energy level structure in np, nd, and nf Rydberg states of H2. The Stark effect is used to distinguish experimentally between p and f Rydberg states. In the weakly penetrating nf series, the hyperfine interaction dominates and the observed hyperfine components are of mixed singlet (S=0) and triplet (S=1) character. In the penetrating np series, the dominant interactions are between the electron orbital and spin angular momenta and the molecular rotation and the observed hyperfine components are characterized by a well-defined total electron spin. The nd Rydberg states ...

High-resolution VUV photoionization spectroscopy of HD between the X (2)Sigma(+)(g) nu(+)=0 and nu(+)=1 thresholds

The photoionization spectrum of HD has been recorded in the region of the first vibrationally excited level (v+ = 1) of the X 2Σ+g ground state of the HD+ ion using a narrow bandwidth vacuum ultraviolet (VUV) laser. Spectral positions, intensities and line widths are reported for all resonances observed between 126 100 and 126 700 cm−1. The np Rydberg series converging to the v+ = 1, N+ = 0 and 2 states of the ion have been observed up to n = 150 and used to determine the corresponding ionization thresholds in a multichannel quantum defect theory analysis. After subtraction of the ionic vibrational energy the adiabatic ionization energy of HD was determined to be 124 568.491 ± 0.017 cm−1. Several low n interloper Rydberg states converging to higher vibrational levels (v+ > 1) of HD+ have also been observed and their interaction with the v+ = 1 channel is discussed.