Autoionizing Rydberg States Research Articles

The influence of autoionizing Rydberg states on the H2+ X 2Σg+ v+ = 0, 1, 2 state rotationally resolved photoelectron angular distributions and branching ratios

The effect of vibrational autoionization on the H2+ X 2Σg+ v+=0, 1, 2, N+ state rotationally resolved photoelectron angular distributions and branching ratios has been investigated with a velocity map imaging spectrometer and synchrotron radiation. Measurements have been made in the energy regions close to the v+=0, 1 or 2 ionization thresholds where the photoabsorption spectrum is dominated by structure due to autoionizing Rydberg states. The photoelectron anisotropy parameter associated with the X 1Σg+ v″=0, N″=1→X 2Σg+ v+=0, 1 or 2, N+=1 transition has a high value, characteristic of emission predominantly along the polarization axis of the incident radiation, when ionization occurs directly but in the vicinity of an autoionizing Rydberg state a more isotropic angular distribution is observed. For the v+=1 level, the present experimental data are compared with existing theoretical predictions in the energy range encompassing the R(1) 8pσ v′=2 and the Q(1) 8pπ v′=2 states. Qualitative agreement has been obtained between the measured and the predicted rotationally resolved photoelectron anisotropy parameters. The experimental values of the rotationally unresolved S-branch photoelectron anisotropy parameter are found to lie considerably higher than that (0.2, independent of excitation energy) predicted under the assumption of p-wave emission, and, moreover, exhibit deviations which appear to correlate with autoionizing Rydberg states. These observations suggest that a proper description of the photoionization dynamics requires the inclusion of partial waves higher than l=1. In the neighbourhood of an autoionizing resonance, the variations occurring in the rotationally resolved branching ratios depend upon the rotational level of the Rydberg state.

Far-infrared amplified emission from the v = 1 autoionizing Rydberg states of NO

Far-infrared amplified emission from the v=1 autoionizing levels of the ns (n=9–14), np (n=12–15) and nf (n=8–14) Rydberg states of NO is reported in the gas phase. The dominant emission process for nf is assigned as the nf→(n−1)g transition. Following the excitation of ns (n=11–14), the emission from (n−1)f energetically located above ns is detected. Following the excitation of np (n=12 and 13), the emission from (n−1)f is also detected. The absence of the emission from 11p leads to ∼3ps as a threshold lifetime of the upper state needed for amplification.

The effect of vibrational autoionization on the H2+ X 2Σg+ state rotationally resolved photoionization dynamics

The effect of vibrational autoionization on the H2+ X 2Σg+ v+ = 3, N+ state rotationally resolved photoelectron angular distributions and branching ratios has been investigated with a velocity map imaging spectrometer and synchrotron radiation. In photon excitation regions free from the influence of autoionizing Rydberg states, where direct ionization dominates, the photoelectron anisotropy parameter associated with the X 1Σg+ v″ = 0, N″ = 1 → X 2Σg+ v+ = 3, N+ = 1 transition has a value close to the theoretical maximum. However, in the vicinity of a Rydberg state, vibrational autoionization leads to a substantial reduction in anisotropy. The value of the anisotropy parameter associated with the S-branch of the photoelectron spectrum is found to be considerably higher than that predicted under the assumption that the outgoing electron can be represented solely as a p-wave. This suggests that the f-wave contribution must be taken into account to obtain a proper description of the photoionization dynamics. The observed variations in the rotationally resolved branching ratios, in the vicinity of an autoionizing resonance, depend upon the rotational level of the Rydberg state. The rotationally averaged photoelectron anisotropy parameters have been compared with the corresponding, previously calculated, theoretical results and reasonable agreement has been found. The influence of vibrational autoionization on the H2+ X 2Σg+ v+ = 0, 1, 2, 3 vibrational branching ratios has also been investigated, and the experimental results show that, in energy regions encompassing Rydberg states, these ratios deviate strongly from the Franck–Condon factors for direct ionization.

Linewidth oscillations in the 5d3/2nd autoionizing series of barium

The first autoionization region of barium was studied using a single-color two-photon excitation scheme. The resonance energies and linewidths of barium 5d3/2nd 1D0, 2 series members were obtained by fitting each spectral line with a Beutler-Fano profile function. The quantum defects of the series were calculated using the obtained resonance energies. The behavior of the linewidth and quantum defect of the series members for n = 8 to 30 was then studied to find out about different interactions involving this autoionizing series such as inter-series coupling. Atomic Ba vapor was produced in a hot-pipe oven, and a tunable dye laser pumped by an excimer laser was used to excite autoionizing Rydberg states. We observed spectral linewidth oscillations in the range of the main quantum numbers 8 to 24, and explained this phenomenon based on interconfiguration interaction among autoionizing series of Ba .

Threshold photoelectron spectroscopy of acetaldehyde and acrolein

High-resolution (6meV and 12meV) threshold photoelectron (TPE) spectra of acetaldehyde and acrolein (2-propenal) have been recorded over the valence binding energy region 10–20eV, employing synchrotron radiation and a penetrating-field electron spectrometer. These TPE spectra are presented here for the first time. All of the band structures observed in the TPE spectra replicate those found in their conventional HeI photoelectron (PE) spectra. However, the relative band intensities are found to be dramatically different in the two types of spectra that are attributed to the different dominant operative formation mechanisms. In addition, some band shapes and their vertical ionization potentials are found to differ in the two types of spectra that are associated with the autoionization of Rydberg states in the two molecules.

Communication: A new spectroscopic window on hydroxyl radicals using UV + VUV resonant ionization

A 1 + 1' multiphoton ionization (MPI) detection scheme for OH radicals is presented. The spectroscopic approach combines initial excitation on the well-characterized A(2)Σ(+)-X(2)Π band system with vacuum ultraviolet (VUV) ionization via autoionizing Rydberg states that converge on the OH(+) A(3)Π ion state. Jet-cooled MPI spectra on the (1,0) and (2,0) bands show anomalous rotational line intensities, while initial excitation on the (0,0) band does not lead to detectable OH(+) ions. The onset of ionization with the (1,0) band is attributed to an energetic threshold; the combined UV + VUV photon energies are above the first member of the autoionizing (A(3)Π)nd Rydberg series. Comparison of the OH 1 + 1' MPI signal with that from single photon VUV ionization of NO indicates that the cross section for photoionization from OH A(2)Σ(+), v' = 1 is on the order of 10(-17) cm(2).

Photoautoionization structure in the halogen systems Br2, I2, BrCl, ICl, and IBr

The photoabsorption ion-yield spectra of the titled molecules have been recorded in the threshold ionization region between the two spin-orbit components of the ground states of the molecular ions. All ion-yield spectra display rather simple autoionization structure superimposed on a smoothly rising continuum in the ionization energy region. The structure was analyzed in terms of autoionizing Rydberg states and their vibrational profiles simulated using Rydberg–Klein–Rees (RKR) derived potential energy curves and calculated Franck–Condon factors using Gaussian line-shape functions. All of the structure observed is attributed to spin-orbit autoionization. There seems to be a propensity for autoionization of sσ Rydberg states in all of the interhalogen molecules studied.

Autoionization dynamics of even Ar (3p51/2np′, nf′) resonances: comparison of experiment and theory

The 3p51/2np′J = 0, 1, 2 and 3p51/2nf′J = 2 autoionizing Rydberg states of Ar have been studied by two-step two-color photoionization of ground-state argon atoms via the intermediate levels 3p51/24s′[1/2]1, 3p53/25s[3/2]1 and 3p53/23d[1/2]1, followed by time-of-flight ion detection. Accurate energy positions and widths have been determined for the 15p′[1/2]0, 15p′[3/2]2 and 13f′[5/2]2 resonances. The measured spectra and earlier results obtained for the intermediate levels 5s′[1/2]1,7s′[1/2]1, and 8s[3/2]1 are compared with absolute cross sections, calculated with the configuration interaction Pauli–Fock core-polarization method.

The effect of autoionising Rydberg states on the formation of low energy electrons

A velocity map imaging spectrometer has been used to observe vibrationally resolved photoelectron kinetic energy distributions in the X 2Σ g + state of molecular nitrogen. The results show that autoionisation from Rydberg states significantly affects the energy distributions and can lead to an enhancement in the production of threshold electrons, although this is not always the case. At excitation energies immediately above the ionisation threshold of the v + = 1 level, the photoelectron kinetic energy distribution associated with that vibrational state in the ion, exhibits an unusual broadening. The design of the spectrometer is described briefly and its performance is demonstrated through measurements of the Xe 5p photoelectron angular distributions in the energy range between the Xe + (5p) −1 2P 1/2 and 2P 3/2 limits.

Experimental study of bound and autoionizing Rydberg states of the europium atom

An isolated-core-excitation (ICE) scheme and stepwise excitation are employed to study the highly excited states of the europium atom. The bound europium spectrum with odd parity in a region of 42400–43500 cm−1 is measured, from which spectral information on 38 transitions, such as level position and relative intensity, can be deduced. Combined with information about excitation calibration and the error estimation process, the selection rules enable us to determine the possible values of total angular momentum J for the observed states. The autoionization spectra of atomic europium, belonging to the 4f76pnl (l = 0, 2) configurations, are systematically investigated by using the three-step laser resonance ionization spectroscopy (RIS) approach. With the ICE scheme, all the experimental spectra of the autoionizing states have nearly symmetric profiles whose peak positions and widths can be easily obtained. A comparison between our results and those from the relevant literature shows that our work not only confirms many reported states, but also discovers 14 bound states and 16 autoionizing states.

A study of the valence shell electronic structure of hexafluorobenzene using photoabsorption and photoelectron spectroscopy, and TDDFT calculations

The valence shell electronic structure of hexafluorobenzene (C6F6) has been studied both experimentally and theoretically. The absolute photoabsorption cross section has been measured between 6 and 42 eV, using synchrotron radiation, and is dominated by prominent broad bands associated with intravalence transitions. In contrast, the structure due to Rydberg excitations is weak, but series have been observed converging onto the , , or limits. At photon energies above 13.7 eV the absorption bands become strongly perturbed, possibly as a result of Rydberg/valence state mixing, and Rydberg series can no longer be identified. The TDDFT approach has been used to calculate oscillator strengths and excitation energies for the optically allowed singlet–singlet valence transitions, and also to obtain the excitation energies for electric-dipole forbidden and/or spin forbidden transitions. These theoretical results have allowed many of the experimentally observed bands to be assigned and have proved particularly useful in clarifying the identifications of several of the weak absorption peaks appearing below 7 eV. The valence shell threshold photoelectron spectrum has been recorded to help characterize the vibrational structure and assign autoionizing Rydberg states. Evidence has been found for the participation of the Jahn–Teller active ν+17 and ν+18 modes in the and the state photoelectron bands.

CRDS study of auto-ionizing Rydberg states of argon in a microwave discharge This article is part of a Special Issue on Spectroscopy at the University of New Brunswick in honour of Colan Linton and Ron Lees.

Optical transitions from two microwave discharge excited states of argon have been observed using cavity ring-down spectroscopy. These transitions originate on the high-lying levels, 3d[1/2] 1° and 3d[3/2] 2° , and terminate on the nf ′[5/2] Rydberg (n = 8 to 22) levels, which, except for n = 8, lie between the 2P3/2 and 2P1/2 ionization thresholds. In total, 24 such spectral lines have been observed. The quantum defect for the f ′ series has been measured and is compared to previously measured values. We observe a nearly threefold jump in line width in going from n = 8 to n = 9, below and above the 2P3/2 threshold, respectively. The line widths are broad and increase monotonically with n (above 9), in contrast to the narrowing of line widths usually observed. We cannot attribute this to a single source but conclude that collisional, quasielastic l-mixing of the nf ′[5/2] Rydberg states plays a significant role.

Coster-Kronig electrons from Nq+ (q = 1-3) and Oq+ (q = 1,2) Rydberg states produced in high-energy collisions with He

Coster-Kronig (C-K) electrons ejected through autoionization decay of high-Rydberg states in high-energy collisions of Nq+ (q = 1-3) and O q+ (q = 1,2) with He have been measured with high resolution by using zero-degree electron spectroscopy. Autoionizing lines are observed corresponding to decays from 1s22p(2P)nl Rydberg states for 21 MeV N3+ + He, from 1s22s2p(3P)nl Rydberg states for 21 MeV N2+ + He and from 1s22s2p2(4P)nl Rydberg states for 14 MeV N+ + He, respectively. Angular momentum distributions of these three series of C-K decaying Rydberg states for Nq+ (q = 1-3) projectiles are also discussed, where the highly excited states are formed by single-electron excitation. Similarly, series of the autoionizing Rydberg states are observed for two charge states of incident O ions. The C-K spectra observed can be assigned to decays from 1s22s2p2(4P)nl states for 30 MeV O2+ + He and from 1s22s2p3(5S)nl states for 15 MeV O+ + He.

Ab initioframe-transformation calculations of direct and indirect dissociative recombination rates ofHeH++e−

The $\mathrm{He}{\mathrm{H}}^{+}$ cation undergoes dissociative recombination with a free electron to produce neutral He and H fragments. We present calculations using ab initio quantum defects and Fano's rovibrational frame-transformation technique, employing cation rovibrational wave functions satisfying outgoing wave boundary conditions, to obtain the recombination rate in both the low-energy $(1--300\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$ and high-energy (ca. $0.6\phantom{\rule{0.3em}{0ex}}\text{hartree}$) regions. We obtain very good agreement with experimental results, demonstrating that this relatively simple method is able to reproduce observed rates for both indirect dissociative recombination, driven by rovibrationally autoionizing states in the low-energy region, and direct dissociative recombination, driven by electronically autoionizing Rydberg states attached to higher-energy excited cation channels.

Resonance-enhanced photon excitation spectroscopy of the even-parity autoionizing Rydberg states of Kr

Kr atoms were produced in their metastable states 4p55s [3/2]2 and 4p55s’ [1/2]0 in a pulsed DC discharge in a beam, and subsequently excited to the even-parity autoionizing Rydberg states 4p5 np′ [3/2]1,2, [1/2]1 and 4p5 nf′ [5/2]3 using single photon excitation. The excitation spectra of the even-parity autoionizing resonance series from the metastable Kr were obtained by recording the autoionized Kr+ ions with time-of-flight ion detection in the photon energy range of 29000–40000 cm−1. A wealth of autoionizing resonances were newly observed, from which more precise and more systematic spectroscopic data of the level energy and quantum defects were derived.

Renner–Teller interactions in the vibrational autoionization of polyatomic molecules

Vibrational autoionization induced by the Renner-Teller interaction in linear polyatomic molecules is considered in the context of the three-state electrostatic model developed by Gauyacq and Jungen [Mol. Phys. 41, 383 (1980)]. For small interactions, simple formulas are derived for the quantum defect matrix elements and the autoionization rates in terms of the more common Renner-Teller parameters derived from spectroscopic analyses of low-lying Rydberg states. These formulas should provide guidance for empirical fitting of quantum defect parameters to spectra of high Rydberg states. Consideration of typical values of the Renner-Teller parameters also allows the estimation of vibrational autoionization rates induced by these interactions. These estimates support the validity of the Deltav=-1 propensity rule for vibrational autoionization. Constraints on the vibrational autoionization rates for the symmetric stretching vibration are also discussed. In the following paper, electron capture by polyatomic molecular ions into vibrationally autoionizing Rydberg states is considered from the same perspective, and a simple formula is derived to allow the estimation of the effect of this process on dissociative recombination cross sections.

Spectra of atomic sulfur D1 in transitions to autoionizing Rydberg states in the region of 75 800–89 500 cm−1

We recorded photoionization spectra of sulfur atoms in transitions from state (1)D in the range of 75,800-89,500 cm(-1). Dissociation of CS(2) after photolysis at 193 nm produced these sulfur atoms in a singlet excited state; they were then ionized with synchrotron radiation (NSRRC, beamline U9CGM) at resolution of up to 3 cm(-1) and detected with a quadruple mass filter. Rydberg series 3s(2)3p(3)((2)D(3/2)(0))nd[3/2] and 3s(2)3p(3)((2)D(5/2)(0))ns[5/2] with n extending to 16 and 32, respectively, to limit (2)D(0) are assigned. New Rydberg series 3s(2)3p(3)((2)D(3/2)(0))nd[1/2](1), ((2)D(5/2)(0))nd[5/2], and ((2)D(3/2)(0))nd[5/2] with n from 5-9 for the former two series and 7-13 for the latter are assigned. A new Rydberg line at 85 335 cm(-1) is assigned to 3s(2)3p(3)((2)D(3/2)(0))6d (1)P.

Resonance-Enhanced Photon Excitation Spectroscopy of the Even-Parity Autoionizing Rydberg States of Xe

Xenon atoms were produced in their metastable states 5p56s[3/2]2 and 5p56s[1/2]o in a pulsed DC discharge in a beam, and subsequently excited to the even-parity autoionizing Rydberg states 5p5lp[3/2]1,[1/2]1, and 5p5nf [5/2] 3 using single photon excitation. The excitation spectra of the even-parity autoionizing resonance series from the metastable 129Xe were obtained by recording the autoionized Xe+ with time-of-flight ion detection in the photon energy range of 28000-42000 cm1. A wealth of autoionizing resonances were newly observed, from which more precise and systematic spectroscopic data of the level energies and quantum defects were derived.

Oscillator strength measurements of the3p5(P1∕22)nd′[3∕2]2and[5∕2]2,3autoionizing resonances in argon

We present measurements of oscillator strengths of the $3{p}^{5}n{d}^{\ensuremath{'}}$ $J=2,3$ autoionizing Rydberg states in argon using two-step laser excitation in conjunction with an optogalvanic detection in a dc discharge cell. The $3{p}^{5}4{p}^{\ensuremath{'}}{[3∕2]}_{1,2}$ and ${[1∕2]}_{1}$ intermediate levels were populated via the $3{p}^{5}4s{[3∕2]}_{2}$ metastable state, collisionally populated in the discharge. The photoionization cross section from the $3{p}^{5}4{p}^{\ensuremath{'}}{[3∕2]}_{1,2}$ and ${[1∕2]}_{1}$ intermediate levels has been measured at the $3{p}^{5}\phantom{\rule{0.2em}{0ex}}^{2}P_{1∕2}$ ionization threshold as $34\ifmmode\pm\else\textpm\fi{}5$, $31\ifmmode\pm\else\textpm\fi{}5$, and $28\ifmmode\pm\else\textpm\fi{}4\phantom{\rule{0.3em}{0ex}}\mathrm{Mb}$, respectively, using the saturation technique. The measured absolute cross section values at the $3{p}^{5}\phantom{\rule{0.2em}{0ex}}^{2}P_{1∕2}$ threshold have been used to determine the $f$ values for the $4{p}^{\ensuremath{'}}{[3∕2]}_{1}\ensuremath{\rightarrow}n{d}^{\ensuremath{'}}{[5∕2]}_{2}$, $4{p}^{\ensuremath{'}}{[3∕2]}_{2}\ensuremath{\rightarrow}n{d}^{\ensuremath{'}}{[5∕2]}_{3}$, and $4{p}^{\ensuremath{'}}{[1∕2]}_{1}\ensuremath{\rightarrow}n{d}^{\ensuremath{'}}{[3∕2]}_{2}$ transitions. The extracted $f$ values are corrected by incorporating the contribution of the Fano line shape parameters for the autoionizing resonances.

Autoionizing Rydberg series(np′,nf′)of Ar investigated by stepwise excitations with lasers and synchrotron radiation

1.611 0.011, 1.683 0.013, 1.688 0.010, and 0.016 0.005, respectively. Our values are in excellent agreement with theoretical prediction. The spectral line shapes of autoionizing Rydberg states are analyzed with a Beutler-Fano profile. Reduced autoionization linewidths for the np1 / 20n =1 1–1 6 series vary in the range 2549– 4145 cm �1 , and the nf5 / 22n =9–1 1 series in 186– 247 cm �1 in reasonable agreement with theoret