Autoionizing Rydberg States Research Articles

Computing Decay Widths of Autoionizing Rydberg States with Complex-Variable Coupled-Cluster Theory.

We compute autoionization widths of various Rydberg states of neon and N2 by equation-of-motion coupled-cluster theory combined with complex scaling and complex basis functions. This represents the first time that complex-variable methods are applied to Rydberg states represented in Gaussian basis sets. A new computational protocol based on Kaufmann basis functions is designed to make these methods applicable to atomic and molecular Rydberg states. As a first step, we apply our protocol to the neon atom and compute widths of the 3s, 3p, 4p and 3d Rydberg states. We then proceed to compute the widths of the 3sσg, 3dσg, and 3dπg Rydberg states of N2, which belong to the Hopfield series. Our results demonstrate a decrease in the decay width for increasing angular momentum and principal quantum number within both Rydberg series.

Effects of Autoionizing Resonances on Wave-Packet Dynamics Studied by Time-Resolved Photoelectron Spectroscopy.

We report a combined experimental and theoretical study on the effect of autoionizing resonances in time-resolved photoelectron spectroscopy. The coherent excitation of N_{2} by ∼14.15 eV extreme-ultraviolet photons prepares a superposition of three dominant adjacent vibrational levels (v^{'}=14-16) in the valence b^{'} ^{1}Σ_{u}^{+} state, which are probed by the absorption of two or three near-infrared photons (800nm). The superposition manifests itself as coherent oscillations in the measured photoelectron spectra. A quantum-mechanical simulation confirms that two autoionizing Rydberg states converging to the excited A ^{2}Π_{u} and B ^{2}Σ_{u}^{+} N_{2}^{+} cores are accessed by the resonant absorption of near-infrared photons. We show that these resonances apply different filters to the observation of the vibrational wave packet, which results in different phases and amplitudes of the oscillating photoelectron signal depending on the nature of the autoionizing resonance. This work clarifies the importance of resonances in time-resolved photoelectron spectroscopy and particularly reveals the phase of vibrational quantum beats as a powerful observable for characterizing the properties of such resonances.

Isolated-core quadrupole excitation of highly excited autoionizing Rydberg states

The structure and photoexcitation dynamics of high lying doubly excited states of the strontium atom with high angular momenta are studied in the vicinity of the Sr$^+(N=5)$ threshold. The spectra recorded using resonant multiphoton isolated core excitation are analyzed with calculations based on configuration interaction with exterior complex scaling, which treats the correlated motion of the two valence electrons of Sr from first principles. The results are rationalized with a model based on multichannel quantum-defect theory and transition dipole moments calculated with a perturbative treatment of electron correlations. Together, both approaches reveal that most of the lines observed in the spectra arise from the interaction of a single optically active state, coupled to the initial state by an electric-dipole transition, with entire doubly-excited Rydberg series. The long-range electron correlations responsible for this interaction unexpectedly vanish for identical values of the initial and final principal quantum numbers, a fact related to the quasi hydrogenic nature of the high-$l$ Rydberg electron. This special situation, and in particular the vanishing interaction, leads to the surprising observation of an electric \emph{quadrupole} isolated-core excitation with a similar intensity as the neighboring electric dipole transitions.

Removal of correlated background in a high-order harmonic transient absorption spectra with principal component regression.

We demonstrate a 40x mean noise power reduction (NPR) in core-to-valence extreme ultraviolet (XUV) femtosecond transient absorption spectroscopy with a high harmonic generation (HHG) light source. An adaptive iteratively reweighted principal component regression (airPCR) is used to analyze and suppress spectrally correlated HHG intensity fluctuations. The technique requires significantly less user input and leads to a higher mean NPR than a previously introduced edge-pixel PCR method that relies on the manual identification of signal-free spectral regions. Both techniques are applied in a time-resolved XUV absorption study of the 2snp1Po (n ≥ 2) autoionizing Rydberg states of helium, demonstrating sub-10-3 optical density sensitivity.

Time-resolved shot-by-shot photoelectron spectroscopy of autoionizing Xe+ states by EUV–free-electron-laser and near-IR laser pulses

Ultrafast dynamics of highly excited Rydberg states of ${\mathrm{Xe}}^{+}$ is investigated by time-resolved shot-by-shot photoelectron spectroscopy with EUV--free-electron-laser pump (40.8 eV) and near-infrared probe (1.56 eV) pulses. Sorting the single-shot spectra in the order of relative delay between the two pulses allows for investigating ultrafast decays of intermediate states participating in the double ionization of Xe. Observed photoelectron spectra show a biexponential decay within \ensuremath{\sim}100 fs and \ensuremath{\sim}300 ps in addition to constant signals lasting longer than 1 ns. These ultrafast decays reflect the lifetimes of autoionizing Rydberg states converging to electronically excited states of ${\mathrm{Xe}}^{2+}$. The results demonstrate that time-resolved shot-by-shot EUV--free-electron-laser photoelectron spectroscopywith a synchronized optical laser provides a powerful tool for investigating ultrafast electron emission processes.

The effect of autoionization on the HBr+ X 2Π3/2,1/2 state photoelectron angular distributions

• Photoelectron angular distributions of the HBr + X state have been measured. • Photoelectron anisotropy parameters affected by autoionizing Rydberg states. • X state threshold electron spectrum exhibits partially resolved rotational structure. • Rydberg states observed in ion yield. A double-imaging photoelectron-photoion spectrometer and synchrotron radiation have been used to measure the HBr + X 2 Π 3/2 v + = 0, 1, 2 and the 2 Π 1/2 v + = 0 state photoelectron angular distributions, as characterized by the anisotropy parameter β , the total ion yield, and the threshold photoelectron spectrum. Particular attention has been focussed on the photon energy range between the 2 Π 3/2 and the 2 Π 1/2 spin–orbit components of the ground ionic state. This region encompasses Rydberg states, belonging to series converging onto the upper 2 Π 1/2 ionization limit, which may decay by autoionization into the 2 Π 3/2 ionization continuum. A detailed study has been performed on the effects of autoionization on the 2 Π 3/2 v + = 0 state photoelectron angular distributions. The observed energy dependent variations in the β -values exhibit a regular pattern that correlates with excitation into members of a very broad d-type Rydberg series. Additional rapid variations in the β -parameters, which occur over a narrow energy range, appear to coincide with sharp autoionizing Rydberg states belonging to s, p and d series. The present experimental results for the HBr + X 2 Π 3/2 v + = 0 state photoelectron anisotropy parameter are compared to previously reported theoretical predictions and to earlier studies of the Kr + 4p 5 2 P 3/2 state β -parameter. The threshold photoelectron spectrum of the X 2 Π 3/2 v + = 0 band exhibits partially resolved rotational structure. A simulation of this structure yields an ionization threshold of 11.6673 ± 0.0010 eV, which is consistent with previous measurements.

Selective Resonance Photoionization of Odd Mass Zirconium Isotopes Towards Efficient Separation of Radioactive Waste

We achieve a considerable improvement in proposed schemes for the selective photoionization of odd mass zirconium isotopes. The technique implements intermediate-state alignment for isotope-selective laser excitation by broadband pulsed lasers, which incorporates the spectroscopic selection rules for the absorption of polarized light. The improvement includes newly found intermediate levels, where J = 0 character as a third excited-state intermediate, in cooperation with four-step photoexcitation (J = 2–1–1–0 scheme). Isotope selectivity (separation coefficient for 91Zr: >2400) has been identified in addition to a significant enhancement of ionization efficiency (30×) compared with previous research. A search for suitable third intermediate levels has covered over autoionizing Rydberg states in a singly ionized Zr II region (up to 58 000 cm−1). The measured autoionizing Rydberg states show high photoion yields but are not identified as favoured isotopic selectivity, as observed in the four-step photoionization. Prospects and future directions in laser even/odd-mass isotope separation are discussed.

The ionization energy of the vinyl radical: a Mexican standoff with a happy ending.

The Active Thermochemical Tables (ATcT) yield 8.477 ± 0.007 eV as the ionization energy of the vinyl radical, which agrees with wave function theory results. The photoionization spectrum yielded 8.59 ± 0.03 eV, while the photoelectron spectrum yielded 8.25 ± 0.05 eV for the adiabatic ionization energy. In order to reconcile these contradictory measurements, we produced the vinyl radical by flash pyrolysis of divinyl sulfone, and recorded its photoionization (PI) and threshold photoelectron (TPE) spectrum in the 8.10-10.45 eV photon energy range at the Swiss Light Source for the first time using double imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy. In contrast to the photoelectron spectrum, the ground state TPES band exhibits a weak vibrational structure that correlates with strong peaks in the photoionization spectrum and is assigned to transitions to autoionizing Rydberg states converging to the 3A'' cation state. While, at 8.462 eV, our calculations of the acetylene proton affinity confirm the vinyl adiabatic ionization energy from the ATcT, our experimental data agree with the literature photoelectron and photoionization spectra. The discrepancy is explained by negligible Franck-Condon factors for the origin transition from the bent Y-shaped neutral minimum to the bridge-shaped ionic minimum. We propose that the photoionization signal can instead be assigned in its entirety to three transitions: (i) straight Y-shaped C2H3‡→ bridged C2H3+, (ii) straight Y-shaped C2H3‡→ straight Y-shaped C2H3+ and (iii) bent Y-shaped C2H3→ straight Y-shaped C2H3+, tentatively assigned to the observed photoionization onsets at 8.25, 8.43, and 8.60 eV, respectively.

Highly Charged Rydberg Ions from the Coulomb Explosion of Clusters.

Ion emission from a nanoplasma produced in the interaction of intense optical laser pulses with argon clusters is studied resolving simultaneously charge states and recoil energies. By applying appropriate static electric fields we observe that a significant fraction of the ions Ar^{q+} (q=1-7) has electrons with binding energies lower than 150meV; i.e., n_{Ryd}≥15 levels are populated. Charge state changes observed on a μs time scale can be attributed to electron emission due to autoionizing Rydberg states, indicating that high-ℓ Rydberg levels are populated as well. The experiments support theoretical predictions that a significant fraction of delocalized electrons, which are bound with hundreds of eV to the nanoplasma after the laser exposure, fill up meV bound ion states in the adiabatic expansion. We expect the process to be relevant for the long-term evolution of expanding laser-induced dense plasmas in general.

Autoionization of very-high- n strontium Rydberg states

We study, using a combination of experiment and theory, the excitation and decay of very-high-$n\phantom{\rule{4pt}{0ex}}(n\ensuremath{\sim}280\text{--}430)$ strontium autoionizing Rydberg states formed by near-resonant driving of the $5s\phantom{\rule{0.16em}{0ex}}^{2}S_{1/2}\ensuremath{\rightarrow}5p\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2}$ core-ion transition. The branching ratio between decay through radiative transitions and through autoionization is explored. Autoionization rates are measured as a function of both the $n$ and $\ensuremath{\ell}$ quantum numbers of the Rydberg electron. The nonstationary decay dynamics is studied by creating and manipulating Rydberg wave packets and by varying the laser pulse that drives the core excitation.

Photoelectron angular distributions from rotationally resolved autoionizing states of N2

The single-photon, photoelectron-photoion coincidence spectrum of N2 has been recorded at high (∼1.5 cm-1) resolution in the region between the N2+ X Σg2+, v+ = 0 and 1 ionization thresholds by using a double-imaging spectrometer and intense vacuum-ultraviolet light from the Synchrotron SOLEIL. This approach provides the relative photoionization cross section, the photoelectron energy distribution, and the photoelectron angular distribution as a function of photon energy. The region of interest contains autoionizing valence states, vibrationally autoionizing Rydberg states converging to vibrationally excited levels of the N2+ X Σg2+ ground state, and electronically autoionizing states converging to the N2+A2Π and B 2Σu+ states. The wavelength resolution is sufficient to resolve rotational structure in the autoionizing states, but the electron energy resolution is insufficient to resolve rotational structure in the photoion spectrum. A simplified approach based on multichannel quantum defect theory is used to predict the photoelectron angular distribution parameters, β, and the results are in reasonably good agreement with experiment.

Probing autoionizing states of molecular oxygen with XUV transient absorption: Electronic-symmetry-dependent line shapes and laser-induced modifications

The dynamics of autoionizing Rydberg states of oxygen are studied using attosecond transient absorption technique, where extreme ultraviolet (XUV) initiates molecular polarization and near infrared (NIR) pulse perturbs its evolution. Transient absorption spectra show positive optical density (OD) change in the case of $ns\sigma_g$ and $nd\pi_g$ autoionizing states of oxygen and negative OD change for $nd\sigma_g$ states. Multiconfiguration time-dependent Hartree-Fock (MCTDHF) calculation are used to simulate the transient absorption spectra and their results agree with experimental observations. The time evolution of superexcited states is probed in electronically and vibrationally resolved fashion and we observe the dependence of decay lifetimes on effective quantum number of the Rydberg series. We model the effect of near-infrared (NIR) perturbation on molecular polarization and find that the laser induced phase shift model agrees with the experimental and MCTDHF results, while the laser induced attenuation model does not. We relate the electron state symmetry dependent sign of the OD change to the Fano parameters of the static absorption lineshapes.

Resonant ionization spectroscopy of autoionizing Rydberg states in cobalt and redetermination of its ionization potential**This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to

Multi-step resonance ionization spectroscopy of cobalt has been performed using a hot-cavity laser ion source and three Ti:Sapphire lasers. The photoionization spectra revealed members of five new autoionizing Rydberg series that originate from three different lower levels of 3d74s5s h4F9/2, 3d74s4d f4G11/2, and 3d74s4d f4H13/2 and converge to the first four excited states of singly ionized Co. The analyses of the Rydberg series yield 63 564.689 ± 0.036 cm−1 as the first ionization potential of Co, which is an order of magnitude more accurate than the previous estimation. Using a three-step resonance ionization scheme that employs an autoinizing Rydberg state in the last transition, we obtained an overall ionization efficiency of about 18% for Co.

Vibrational autoionization of state-selective jet-cooled methanethiol (CH3SH) investigated with infrared + vacuum-ultraviolet photoionization.

Vibrational autoionization of Rydberg states provides key information about nonadiabatic processes above an ionization threshold. We employed time-of-flight mass detection of CH3SH+ to record vibrational-state selective photo-ionization efficiency (PIE) spectra of jet-cooled methanethiol (CH3SH) on exciting CH3SH to a specific vibrationally excited state with an infrared (IR) laser, followed by excitation with a tunable laser in the vacuum-ultraviolet (VUV) region for ionization. Autoionizing Rydberg states assigned to the ns, np, nd and nf series are identified. When IR light at 2601 (ν3, SH stretching mode) and 2948 cm-1 (ν2, CH3 symmetric stretching mode) was employed, the Rydberg series converged to the respective vibrationally excited (ν3 and ν2) states of CH3SH+. When IR light at 3014 cm-1 (overlapped ν1/ν9, CH3 antisymmetric stretching and CH2 antisymmetric stretching modes) was employed, Rydberg series converging to two vibrationally excited states (ν1 and ν9) of CH3SH+ were observed. When IR light at 2867 cm-1 (2ν10, overtone of CH3 deformation mode) and 2892 cm-1 (2ν4, overtone of CH2 scissoring mode) was employed, both Δν = -1 and Δν = -2 ionization transitions were observed; there is evidence for direct ionization from the initial state into the CH3SH+ (ν4+ = 1) continuum. In all observed IR-VUV-PIE spectra, the ns and nd series show intensity greater than the other Rydberg series, which is consistent with the fact that the highest-occupied molecular orbital of CH3SH is a p-like lone pair orbital on the S atom. The quantum yields for autoionization of various vibrational excited states are discussed. Values of ν1 = 3035, ν2 = 2884, ν3 = 2514, and ν9 = 2936 cm-1 for CH3SH+ derived from the converged limits agree satisfactorily with values observed for Ar-tagged CH3SH+ at 3026, 2879, 2502, and 2933 cm-1.

Improved efficiency of selective photoionization of palladium isotopes via autoionizing Rydberg states

Odd-mass-selective ionization of palladium for purposes of resource recycling and management of long-lived fission products can be achieved by exploiting transition selection rules in a well-established three-step excitation process. In this conventional scheme, circularly polarized lasers of the same handedness excite isotopes via two intermediate 2D5/2 core states, and a third laser is then used for ionization via autoionizing Rydberg states. We propose an alternative excitation scheme via intermediate 2D3/2 core states before the autoionizing Rydberg state, improving ionization efficiency by over 130 times. We confirm high selectivity and measure odd-mass isotopes of >99.7(3)% of the total ionized product. We have identified and measured the relative ionization efficiency of the series of Rydberg states that converge to upper ionization limit of the 4d9(2D3/2) level, and identify the most efficient excitation is via the Rydberg state at 67668.18(10) cm−1.

Spectroscopic investigation of autoionizing Rydberg states of palladium accessible after odd-mass-selective laser excitation

Excitation of palladium to autoionizing Rydberg states are investigated in order to realize efficient ionization after odd-mass-selective excitation. Wavelength resolution is sufficient to distinguish two autoionizing Rydberg series: 4d9(2D3/2)np and 4d9(2D3/2)np converging to Pd II 4d9(2D3/2) as is expected from transition selection rules. Of all observed transitions, the largest autoionization yield is found to be through the 4d9(2D3/2)11p state at 69055.2 cm−1. From fitting analysis of the observed members of the 4d9(2D3/2)np series, we have obtained the energy of Pd II 4d9(2D3/2) to be 70780.38 ± 0.08 cm−1.

UV + V UV double-resonance studies of autoionizing Rydberg states of the hydroxyl radical.

The hydroxyl radical (OH) is a key oxidant in atmospheric and combustion chemistry. Recently, a sensitive and state-selective ionization method has been developed for detection of the OH radical that utilizes UV excitation on the A(2)Σ(+)-X(2)Π transition followed by fixed 118 nm vacuum ultraviolet (VUV) radiation to access autoionizing Rydberg states [J. M. Beames et al., J. Chem. Phys. 134, 241102 (2011)]. The present study uses tunable VUV radiation generated by four-wave mixing to examine the origin of the enhanced ionization efficiency observed for OH radicals prepared in specific A(2)Σ(+) intermediate levels. The enhancement is shown to arise from resonant excitation to distinct rotational and fine structure levels of two newly identified (2)Π Rydberg states with an A(3)Π cationic core and a 3d electron followed by ionization. Spectroscopic constants are derived and effects due to uncoupling of the Rydberg electron are revealed for the OH (2)Π Rydberg states. The linewidths indicate a Rydberg state lifetime due to autoionization on the order of a picosecond.

Direct Imaging of Transient Fano Resonances inN2Using Time-, Energy-, and Angular-Resolved Photoelectron Spectroscopy

Autoionizing Rydberg states of molecular N_{2} are studied using time-, energy-, and angular-resolved photoelectron spectroscopy. A femtosecond extreme ultraviolet pulse with a photon energy of 17.5eV excites the resonance and a subsequent IR pulse ionizes the molecule before the autoionization takes place. The angular-resolved photoelectron spectra depend on pump-probe time delay and allow for the distinguishing of two electronic states contributing to the resonance. The lifetime of one of the contributions is determined to be 14±1 fs, while the lifetime of the other appears to be significantly shorter than the time resolution of the experiment. These observations suggest that the Rydberg states in this energy region are influenced by the effect of interference stabilization and merge into a complex resonance.

Probing the Dynamics of Rydberg and Valence States of Molecular Nitrogen with Attosecond Transient Absorption Spectroscopy.

An attosecond pulse is used to create a wavepacket in molecular nitrogen composed of multiple bound and autoionizing electronic states of Rydberg and valence character between 12 and 16.7 eV. A time-delayed, few-femtosecond, near-infrared (NIR) laser pulse is used to couple individual states in the wavepacket to multiple neighboring states, resulting in time-dependent modification of the absorption spectrum and revealing both individual quantum beats of the wavepacket and the energy shifts of the excited states in the presence of the strong NIR field. The broad bandwidth of the attosecond pulse and high energy resolution of the extreme ultraviolet spectrometer allow the simultaneous observation of time-dependent dynamics for many individual vibrational levels in each electronic state. Quantum beating with periods from 1.3 to 12 fs and transient line shape changes are observed among vibrational levels of a progression of electronically autoionizing Rydberg states leading to the excited A (2)Πu N2(+) ion core. Vibrational levels in the valence b (1)Πu state exhibit 50 fs oscillation periods, revealing superpositions between individual vibrational levels within this state. Comparisons are made to previous studies of electronic wavepackets in atoms that highlight similarities to atomic behavior yet illustrate unique contributions of the diatomic molecular structure to the wavepacket, including the influence of different electronic potentials and vibrational-level-specific electronic dynamics.

Laser power dependence of the Beutler-Fano profiles for the autoionizing Rydberg states of Ba

The change of the Beutler-Fano profiles of the asymmetric autoionizing lines of barium as a function of the excitation laser power was investigated. Utilizing two-photon absorption, the J = 0 and 2 levels of a few members of the 5d3/2 nd3/2 autoionizing Rydberg series above the first ionization limit of barium were excited in a heat-pipe oven. As the laser power was changed, the transitions with relatively symmetric lineshapes showed the expected power broadening. However, the transitions with asymmetric lineshapes experienced a dramatic profile change while displaying a less significant power broadening. The remarkable profile changes are attributed to the interference between different ionization channels even at the highest excitation laser power.