U. Hollenstein

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...

Ionization from a double bond: Rovibronic photoionization dynamics of ethylene, large amplitude torsional motion and vibronic coupling in the ground state of C2H4+

Rotationally resolved pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the X-->X+ transition in ethylene and ethylene-d4 have been recorded at a resolution of 0.09 cm(-1). The spectra provide new information on the large amplitude torsional motion in the cationic ground state. An effective one-dimensional torsional potential was determined from the experimental data. Both C2H4+ and C2D4+ exhibit a twisted geometry, and the lowest two levels of the torsional potential form a tunneling pair with a tunneling splitting of 83.7(5) cm(-1) in C2H4+ and of 37.1(5) cm(-1) in C2D4+. A model was developed to quantitatively analyze the rotational structure of the photoelectron spectra by generalizing the model of Buckingham, Orr, and Sichel [Philos. Trans. R. Soc. London, Ser. A 268, 147 (1970)] to treat asymmetric top molecules. The quantitative analysis of the rotational intensity distributions of allowed as well as forbidden vibrational bands enabled the identification of strong vibronic mixing between the X+ and A+ states mediated by the torsional mode nu(4) and a weaker mixing between the X+ and B+ states mediated by the symmetric CH2 out-of-plane bending mode nu7. The vibrational intensities could be accounted for quantitatively using a Herzberg-Teller-type model for vibronic intensity borrowing. The adiabatic ionization energies of C2H4 and C2D4 were determined to be 84 790.42(23) cm(-1) and 84 913.3(14) cm(-1), respectively.

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...

High-resolution laser absorption spectroscopy in the extreme ultraviolet

A new apparatus has been developed that enables the measurement of extreme ultraviolet laser absorption spectra of atoms and molecules in cold supersonic expansions at a resolution as high as 0.01 cm−1. These absorption spectra can be measured simultaneously with photoionization spectra and fluorescence excitation spectra. By comparing the photoionization to the absorption spectra, the dissociation yield can be derived. The procedure is illustrated by an investigation of the photochemistry and photophysics of Ar and N2 close to their lowest ionization thresholds. The spectra of N2 between the N2+ X2 Σ g+ (v+ = 0) and N2+ X2 Σ g+ (v+ = 1) ionization thresholds provide new information that helps in the clarification of previous spectral assignments.

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.

Structure of the low-lying electronic states of from rotationally resolved photoelectron spectra

Pulsed-field-ionisation zero-kinetic-energy (PFI-ZEKE) photoelectron spectra of 132Xe2 have been measured in the range of 90100 to 108400 cm−1, where the lowest electronic states of the cation are located. A (1 + 1′) resonance-enhanced two-photon excitation scheme via selected rovibrational levels of the C 0 state of 132Xe2 was used to resolve the rotational structure of several vibrational levels of the I(1/2u), I(3/2g), I(3/2u) and II(1/2u) electronic states. For the ground cationic state even the Ω-doubling was resolved. A vibrational progression belonging to the II(1/2g) electronic state has been observed for the first time proving that the II(1/2g) state is bound. The analysis of the rotational structure, performed in Hunds angular momentum coupling case (c), enabled the determination of the equilibrium internuclear distances and the dissociation energies of the I(1/2u), I(3/2g), I(3/2u) and II(1/2u) states of Xe and the corresponding adiabatic ionisation energies.

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.

PFI-ZEKE photoelectron and high resolution photoionization spectra of ND3 with MQDT simulations

The rotationally resolved PFI-ZEKE (pulsed-field ionization zero-kinetic energy) photoelectron spectrum of ND3 has been recorded at the = 0, 1 and 2 thresholds, by single-photon VUV excitation from the ground-state. An assignment of the ← spectrum is presented, allowing a determination of the adiabatic ionization potential of ND3 (82261.7(15) cm−1) and of the rotational constants and positions of the = 0, 1 and 2 vibrational levels of the ion. These parameters are used in a multichannel quantum defect theory (MQDT) simulation of the high-resolution photoionization spectrum in the range 82100–83300 cm−1 [R. Seiler et al., J. Chem. Phys. 118, 10024 (2003).]. A good simulation and an assignment of around 80% of the lines of the ND3 photoionization spectrum are achieved, verifying the validity of the molecular constants derived from the PFI-ZEKE photoelectron spectrum and also the quantum defect parameters derived from ab initio calculations.

Vacuum-ultraviolet frequency-modulation spectroscopy

Frequency-modulation (FM) spectroscopy has been extended to the vacuum-ultraviolet (VUV) range of the electromagnetic spectrum. Coherent VUV laser radiation is produced by resonance-enhanced sum-frequency mixing (νVUV=2νUV+ν2) in Kr and Xe using two near-Fourier-transform-limited laser pulses of frequencies νUV and ν2. Sidebands generated in the output of the second laser (ν2) using an electro-optical modulator operating at the frequency νmod are directly transferred to the VUV and used to record FM spectra. Demodulation is demonstrated both at νmod and 2νmod. The main advantages of the method compared to VUV absorption spectroscopy are its background-free nature, the fact is that its implementation using table-top laser equipment is straightforward and that it can be used to record VUV absorption spectra of cold samples in skimmed supersonic beams simultaneously with laser-induced-fluorescence and photoionization spectra. To illustrate these advantages, we present VUV FM spectra of Ar, Kr, and N2 in selected regions between 105000 cm-1 and 122000 cm-1.