William A. Chupka

Factors affecting lifetimes and resolution of Rydberg states observed in zero‐electron‐kinetic‐energy spectroscopy

It is shown that under the usual conditions of zero‐electron‐kinetic‐energy, pulsed field ionization (ZEKE–PFI) spectroscopy the lifetimes of very high‐lying Rydberg states are increased by at least approximately the factor n (in addition to the expected factor of n3), the principal quantum number, due to strong l mixing by the Stark effect. Additional factors may increase lifetimes by still another factor of approximately n. Pulsed field ionization under typical conditions is shown as likely to be predominantly diabatic and the effect on resolution is assessed. Factors affecting rotational line intensities are also discussed.

Very high resolution study of photoabsoprtion, photoionization, and predissociation in H2

Relative photoabsorption and photoionization cross sections for H2 (para and ordinary) have been measured at 78°K from 715 to 805 A for para‐H2 and from 745 to 805 A for ordinary H2, with a wavelength resolution of 0.016 A. This resolution represents a factor of 3 improvement over the previous data reported from this laboratory, and in addition, the new data have significantly improved statistics. This enables observation and identification of the R (0) npσ and npπ Rydberg series in para‐H2 to principal quantum numbers of approximately 40 for series converging to H2+(2Σg+, v=1–6). Linewidths and relative intensities were measured for a large number of these levels and the results are compared to calculations using a quantum defect theory (QDT) approach. Two‐channel QDT is used to assign nearly all the prominent structure in the para‐H2 spectrum. The strengths and limitations of the simple two‐channel theory and the necessity for extension to multichannel calculation for a complete understanding of the spe...

Lifetimes of very high Rydberg states of aromatic molecules

Recent experimental results reporting lifetimes of very high Rydberg states of phenanthrene and deuterated phenanthrene together with a theoretical analysis are examined critically. Weaknesses in the theoretical model are pointed out, in particular the proposal that lifetimes of isolated field‐free molecules vary as n6 up to values of n≂100 above which very rapid decay occurs by autoionization. Another model, in which the effects of external fields and collisions result in lifetimes which vary in an ideal statistical limit as n5 at lower values of n and rapid destruction at higher values of n, is shown to explain the experimental results equally well. To the extent that such a statistical limit is not attained, nonexponential decay is expected. Decay processes of high Rydberg states of large polyatomic molecules are discussed. Especially in the case of a fused polynuclear aromatic, the isolated molecule with lower values of n is proposed to decay by an internal conversion mechanism in which the rate‐deter...

Photoionization of molecular Rydberg states: H2, C 1Πu and its doubly excited states

Explanations are offered for some anomalies seen in photoelectron spectra resulting from single‐photon ionization of unperturbed single‐configuration Rydberg states of molecules. Some anomalous behavior may be due to occurrence of Cooper minima and shape resonances. Stronger effects can be due to excitation of the ion core producing doubly excited states whose decay characteristics and their effects on photoelectron spectra are discussed. As an example, anomalous photoelectron spectra produced in resonant multiphoton ionization of H2 via its C state are attributed to the excitation and autoionization of the 1Πg(2pσ)(2pπ) state. Formation of excited atomic hydrogen is predicted. The value of such experiments in investigating doubly excited states in general is discussed as well as the problem of preparation of ions in selected states.

Rydberg–valence interactions in the Πg states of O2

Multiphoton ionization spectroscopy is used to characterize the (3sσ) 3Πg Rydberg state of O2. It is found that the 3Πg Rydberg state is very diffuse and has rotationally resolved structure for the v’=2 level only. The FWHM of rotational lines for both the (3sσ) 3,1Πg states is greater than the excitation bandwidth, indicating strong radiationless decay of the two states. Interactions with the nearby 3Πg and 1Πg valence states are invoked to explain the observed behavior.

Effects of vibronic interaction and autoionization on the photoelectron spectrum of N2O

High‐resolution and high‐sensitivity HeI photoelectron spectra (PES) are reported for the first four valence levels of N2O. The vibrational structure, including many new peaks, is completely assigned for the X 2Π, A 2Σ+, and C 2Σ+ electronic states, and in all three cases excitation of a single quantum of the bending vibration is observed. The bending mode appears as a result of vibronic coupling within and between different electronic states. The Renner–Teller splitting is resolved in the (0,1,0) band of the transition to the X 2Π state. The relative intensities of the vibrational bands in the X 2Π and A 2Σ+ states are compared to the intensities determined from the threshold photoelectron spectra (TPES) of Frey et al. [Chem. Phys. Lett. 54, 411 (1978)]. In both states, autoionization of intermediate neutral states that are nearly degenerate with the molecular ion state to which they decay increases the relative intensities of the higher vibrational bands in the TPES. This resonant autoionization process...

On the mechanism for vibrational autoionization in H2

The cross sections for the reactions of H2+ with He and Ne to form HeH+ and NeH+ are known to depend strongly on the H2+ vibrational level. When the H2+ reactant ions are produced by autoionization of Rydberg states, the measurement of these cross sections provides a means for studying the final vibrational state distribution of H2+ ions resulting from the autoionization process. Relative cross sections for hydride ion formation were determined for approximately 100 autoionizing Rydberg states in para‐H2 between the H2+2Jg+,v=0 and v=6 convergence limits. The relative cross sections for formation of HeH+ and NeH+ (and thus the final vibrational state distributions of H2+) are nearly the same for all the autoionizing Rydberg states between any two H2+ vibrational convergence limits v and v+1 regardless of the initial vibrational level of the autoionizing state. Combining this observation with elementary considerations, we show that vibrational autoionization proceeds predominantly (≳ 95%) via a Δv=1 mechan...

Photoionization of N2 X 1Σ+g, v″=0 and 1 near threshold. Preionization of the Worley–Jenkins Rydberg series

The high resolution relative photoionization cross section for N2 is reported in the wavelength region from the ionization threshold to 650 A, with particular attention given to the region between the N+2 X 2Σ+g, v′=0 and 1 ionization limits. Cross sections from both the X 1Σ+g, v″=0 and 1 vibrational levels were determined in this region, and preionized members of the npπu 1Πu Worley–Jenkins Rydberg series converging to N+2 X 2Σ+g, v′=1 are observed in both spectra. In the spectrum excited from v″=1, the Worley–Jenkins series appears prominently as a result of good Franck–Condon overlap between the N2 X 1Σ+g, v″=1 and the N+2 X 2Σ+g, v′=1 levels; the intensities of the series members decrease approximately as 1/n*3, in accord with simple theoretical predictions. However, in the spectrum excited from v″=0, the Worley–Jenkins series converging to N+2 X 2Σ+g, v′=1 is weak as a result of a poor Franck–Condon overlap with the ground vibrational level; the intensities of the series members show large deviation...

Resonant multiphoton ionization of NO via the A 2Σ+ state: Photoelectron spectra and angular distributions

Photoelectron spectra resulting from the two‐photon resonant, four‐photon ionization of NO via the A 2Σ+ state are reported for several rotational levels in the A, v=0 manifold. The observed final ionic state vibrational distributions are found to be consistent with and confirm spectroscopic studies which have assigned intense features in the four‐photon A state MPI spectrum with accidental resonances in the energy range of the third photon. The upper states in the strong accidental double resonance transitions are assigned to Rydberg vibronic levels which are strongly perturbed by nearby valence states. Photoelectron angular distributions were also measured for two vibronic transitions and these are interpreted on the basis of a simple, one‐electron description of the ionization step.

Rydberg–Rydberg transitions of NO using an optical–optical double resonance multiphoton ionization technique

New Rydberg–Rydberg transitions of 14N 16O have been observed by the technique of optical–optical double resonance multiphoton ionization (OODR‐MPI). The analysis of S 2Σ+–A 2Σ+ (0,0), (1,1) and 4f–A 2Σ+ (1,1) bands are reported. Molecular constants of the S 2Σ+ v=0,1 levels are in good agreement with those derived from absorption data. The structure of the 4f(v=1) level of the 14N 16O isotope can be fully investigated for the first time since the OODR‐MPI spectrum is free from any overlap, in contrast to the corresponding absorption spectral region. No detectable spectral perturbation was observed for levels up to N=10, although the non‐Rydberg G 2Σ−(v=8) level lies at the same energy as the 4f(v=1) level. The 4f(v=1) rotational levels are described by an intermediate case (b)/case (d) coupling scheme and present a similar pattern to that of the 4f(v=0) rotational levels previously studied from absorption data. Rotational constants B1=1.968 cm−1 and αe=0.020 cm−1 have been derived from our analysis and t...