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An ellipsoidal volume of Rydberg molecules, entrained in a supersonic molecular beam, evolves on a nanosecond timescale to form a strongly coupled ultracold plasma. We present coupled rate-equation simulations that model the underlying kinetic processes and molecular dissociation channels in both a uniformly distributed plasma and under the conditions dictated by our experimental geometry. Simulations predict a fast electron-driven collisional avalanche to plasma followed by slow electron-ion recombination. Within 20 μs, release of Rydberg binding energy raises the electron temperature of a static plasma to T e = 100 K. Providing for a quasi-self-similar expansion, the hot electron gas drives ion radial motion, reducing T e . These simulations provide a classical baseline model from which to consider quantum effects in the evolution of charge gradients and ambipolar forces in an experimental system undergoing responsive avalanche dynamics.

Long-range interparticle interactions are revealed in extremely dilute thermal atomic ensembles using highly sensitive nonlinear femtosecond spectroscopy., Long-range interparticle interactions are revealed in extremely dilute thermal atomic ensembles using highly sensitive nonlinear femtosecond spectroscopy. Delocalized excitons are detected in the atomic systems at particle densities where the mean interatomic distance (>10 μm) is much greater than the laser wavelength and multi-particle coherences should destructively interfere over the ensemble average. With a combined experimental and theoretical analysis, we identify an effective interaction mechanism, presumably of dipolar nature, as the origin of the excitonic signals. Our study implies that even in highly-dilute thermal atom ensembles, significant transition dipole–dipole interaction networks may form that require advanced modeling beyond the nearest neighbor approximation to quantitatively capture the details of their many-body properties.

Adversarial learning is one of the most successful approaches to modelling high-dimensional probability distributions from data. The quantum computing community has recently begun to generalize this idea and to look for potential applications. In this work, we derive an adversarial algorithm for the problem of approximating an unknown quantum pure state. Although this could be done on universal quantum computers, the adversarial formulation enables us to execute the algorithm on near-term quantum computers. Two parametrized circuits are optimized in tandem: One tries to approximate the target state, the other tries to distinguish between target and approximated state. Supported by numerical simulations, we show that resilient backpropagation algorithms perform remarkably well in optimizing the two circuits. We use the bipartite entanglement entropy to design an efficient heuristic for the stopping criterion. Our approach may find application in quantum state tomography., Comment: 14 pages, 6 figures. Minor revisions. As published in New J. Phys

We argue that the quenched ultracold plasma presents an experimental platform for studying quantum many-body physics of disordered systems in the long-time and finite energy-density limits. We consider an experiment that quenches a plasma of nitric oxide to an ultracold system of Rydberg molecules, ions and electrons that exhibits a long-lived state of arrested relaxation. The qualitative features of this state fail to conform with classical models. Here, we develop a microscopic quantum description for the arrested phase based on an effective many-body spin Hamiltonian that includes both dipole-dipole and van der Waals interactions. This effective model appears to offer a way to envision the essential quantum disordered non-equilibrium physics of this system., 6 pages main text + 9 pages supplemental materials, 2 figures main text + 7 figures and 2 tables supplemental materials

The exploration of large-scale many-body phenomena in quantum materials has produced many important experimental discoveries, including novel states of entanglement, topology and quantum order as found for example in quantum spin ices, topological insulators and semimetals, complex magnets, and high-$T_c$ superconductors. Yet, the sheer scale of solid-state systems and the difficulty of exercising exacting control of their quantum mechanical degrees of freedom limit the pace of rational progress in advancing the properties of these and other materials. With extraordinary effort to counteract natural processes of dissipation, precisely engineered ultracold quantum simulators could point the way to exotic new materials. Here, we look instead to the quantum mechanical character of the arrested state formed by a quenched ultracold molecular plasma. This novel class of system arises spontaneously, without a deliberate engineering of interactions, and evolves naturally from state-specified initial conditions, to a long-lived final state of canonical density, in a process that conflicts with classical notions of plasma dissipation and neutral dissociation. We take information from experimental observations to develop a conceptual argument that attempts to explain this state of arrested relaxation in terms of a minimal phenomenological model of randomly interacting dipoles of random energies. This model of the plasma forms a starting point to describe its observed absence of relaxation in terms of many-body localization (MBL). The large number of accessible Rydberg and excitonic states gives rise to an unconventional web of many-body interactions that vastly exceeds the complexity of MBL in a conventional few-level scheme. This experimental platform thus opens an avenue for the coupling of dipoles in disordered environments that will demand the development of new theoretical tools., Comment: 27 pages, 6 figures, 2 tables

Penning ionization initiates the evolution of a dense molecular Rydberg gas to plasma. This process selects for pairs of excited molecules separated by a distance of two Rydberg orbital diameters or less. The deactivated Penning partners predissociate, depleting the leading edge of the distribution of nearest-neighbor distances. For certain density and orbital radii, this sequence of events can form a plasma in which large distances separate a disproportionate fraction of the ions. Experimental results and model calculations suggest that the reduced potential energy of this Penning lattice significantly affects the development of strong coupling in an ultracold plasma.

Triple-resonance excitation and high-resolution photoelectron spectroscopy are combined to characterize the mode selectivity of vibrational autoionization of the high Rydberg states of NO2. Photoelectron spectra and vibrational branching fractions are reported for autoionizing Rydberg states converging to the NO2+ X 1Sigmag +(110) state, that is, with one quantum in the symmetric stretch, nu1, and one quantum in the bending vibration, nu2. These results indicate that autoionization proceeds most efficiently through the loss of one quantum from the symmetric stretch rather than from the bending vibration. The implications of this result are discussed in terms of the autoionization mechanism.

Triple-resonance excitation and high-resolution photoelectron spectroscopy were combined to study the mode dependence of vibrational autoionization in Rydberg states of NO2. Photoselection isolates vibrational autoionization via the symmetric stretching vibration, ν1, and the bending vibration, ν2. The previously characterized Fermi resonance between one quantum of ν1 and two quanta of ν2 [H. Matsui et al., J. Mol. Spectrosc. 175, 203 (1996)] allows the comparison of the vibrational autoionization matrix elements for these two modes. The squared matrix element for vibrational autoionization via the symmetric stretch is found to be approximately 35 times greater than that for the bend, which is also consistent with previous results. The results are discussed in terms of existing theoretical models for the autoionization process.

We report the mass-selected resonance-enhanced multiphoton ionization (REMPI) spectrum of photolytically produced diatomic boron hydride over the spectral range from 368 to 372 nm. Features recorded for time-of-flight-resolved cation masses 12, 11 and 10 amu are assigned to the one-photon transition, A 1 Π (v′=2)– X 1 Σ + (v″=0) , in 11BH and 10BH. Cation production at these wavelengths requires three photons. This signal must therefore reflect resonant one-photon absorption followed by two-photon ionization in a (1+2)-photon process. This work represents the first observation of the well-characterized A 1 Π – X 1 Σ + system in ionization spectroscopy and the first measurement of line positions in the A 1 Π v=2 state for 10BH.

We report the first high-resolution measurement of the adiabatic ionization potential of DCO and the fundamental bending frequency of DCO+. Fixing a first-laser frequency on selected ultraviolet transitions to individual rotational levels in the (000) band of the 3pπ 2Π intermediate Rydberg state of DCO, we scan a second visible laser over the range from 20 000 to 20 300 cm−1 to record double resonance photoionization efficiency (DR/PIE) spectra. Intermediate resonance with this Rydberg state facilitates transitions to the threshold for producing ground-state cations by bridging the Franck–Condon gap between the bent neutral radical and linear cation. By selecting a single rotational state for ionization, double-resonant excitation eliminates thermal congestion. Spectroscopic features for first-photon resonance are identified by reference to a complete assignment of the 3pπ 2Π(000)−X 2A′(000) band system of DCO. Calibration with HCO, for which the adiabatic ionization threshold is accurately known, establ...

Double-resonant ionization-detected absorption experiments extend an investigation of the autoionization dynamics of HCO to include the second overtone of the bend. In these experiments, first-photon transitions to the 3pπ 2Π Rydberg state select single rovibrational levels for second-photon scans of vibrationally autoionizing high Rydberg series that converge to specific rovibrational limits of HCO+. Line shapes reflect coupling widths that join discrete states built on vibrationally excited cores with underlying, vibrationally relaxed continua. Scans of series converging to HCO+ with one and two quanta of bend show narrow linewidths, reflecting relatively long autoionization lifetimes. However, for cores excited to the second harmonic of the bend, certain series abruptly broaden, indicating ultrafast decay. The implications of these results are discussed in terms of a qualitative extension of multichannel quantum defect theory to triatomic molecules.

Formyl radicals, excited to specific rovibrational levels of the strongly bent B 2 A ′ state are photoionized to form vibrationally excited states of the linear cation by the absorption of a tunable pulse of ultraviolet light from a second laser. Strong and essentially continuous photoionization spectra, which appear at intervals lying 7000 and 12 000 cm−1 above the adiabatic threshold, establish that formyl radicals can be efficiently ionized with a large geometry change at both energies. Non-Franck–Condon intensities suggest multiquantum pathways for vibrational autoionization.

Ultraviolet first-photon absorption selects individual rotational levels in the Renner–Teller split (020) vibronic band system in the 3pπ 2Π Rydberg state of HCO. These gateway states serve as originating levels for vertical second-photon transitions to vibrationally autoionizing Rydberg series converging to individual rotational levels associated with the (0200) and (0220) states of HCO+. Linewidths of assigned series convey information on autoionization dynamics. Resonances throughout the (020) autoionization spectrum match sharp profiles seen earlier for series converging to HCO+(010). Linewidths for autoionization via relaxation both of the bending fundamental and its overtone are measurably narrower than resonances built on CO stretch, (001), which indicates that mode-selectivity inhibiting bending autoionization relative to stretch extends to the bending overtone. Features in the (020) spectrum that fall below the threshold energy for decay by autoionization to the (010) continuum appear with dimini...

We report ionization-detected absorption spectra of vibrationally autoionizing high Rydberg states of formyl radical. Steps of uv–visible double resonance with selected rovibrational levels of the 3pπ 2Π Rydberg state of HCO promote Franck–Condon vertical transitions that isolate series converging to (010) (bend) and (001) (CO stretch) excited states of HCO+. Final state energies in these spectra exceed the threshold for production of the cation ground state. Intensities and linewidths of observed resonances convey information on the dynamics of electron ejection driven by the vibronic relaxation of specific normal modes of the linear triatomic core. Many resonances in (010) and (001) autoionization spectra exhibit widths that approach the resolution of our laser. Other resonances in series converging to both limits are noticeably broadened, with linewidths that display an inverse cubic dependence on principal quantum number. Among these broader resonances, those in series that decay by relaxation of CO s...

Rydberg states in polyatomic molecules exhibit dynamics that bear on issues of longstanding importance for theory. Coupling is made complicated, however, by the presence of many rovibrational degrees of freedom and the existence of multiple pathways for intramolecular relaxation. In work on vibrationally autoionizing states of HCO and NO 2 , we have applied multiple resonance excitation strategies to isolate single rovibrational Rydberg–Rydberg transitions, the positions, lineshapes and intensities of which reflect coupling strengths for state–detailed relaxation processes.

Ionization‐detected absorption spectra of autoionizing Rydberg series converging to the (100), (010), (0200), and (0220) vibrational states of NO+2 have been recorded using three‐color triple resonant excitation. Resonances produced by relaxation of the core, excited in one quantum of the bending vibration, (010), are found to be much sharper than those associated with decay by relaxation in symmetric stretch, (100), consistent with earlier findings on mode specificity of vibrational autoionization in this system. In contrast, series excited in the symmetric overtone of the bend, (0200), are found to exhibit dynamics that resemble much more those of series built on one quantum of symmetric stretch, indicating that the bending specificity that characterizes (010) is lost in the overtone. This loss of mode specificity with increasing vibrational energy is ascribed to moderate (100)–(0200) Fermi resonance in the NO+2 core, exemplifying how even small vibrational coupling can affect mode‐specific pathways in ...

We report the first spectroscopic observation of the high Rydberg states of HCO. Individual lines in a system of vibrationally autoionizing Rydberg series converging to the (010) state of HCO+ are rotationally labeled in a double‐resonance excitation scheme that uses resolved levels in the (010) A′ vibronic component of the 3pπ 2Π Rydberg state as intermediates. Observed high‐Rydberg structure extends from the adiabatic ionization threshold—which falls just below the principal quantum number of 12 in the vibrationally excited series—to the (010) vertical threshold. Elements of a single series extending from n=12 to 50, for which the total angular momentumless spin can be assigned as N=1, are extrapolated to obtain a vertical convergence limit with respect to the 3pπ 2Π(010)A′ N′=0 intermediate state of 20 296.9±0.3 cm−1. Referring this transition energy to the ground state, and subtracting the precisely known fundamental bending frequency of the cation, establishes the adiabatic ionization potential corre...

We report the observation of two-pathway coherent control in a bound-free transition for the molecular system, NO. This control is manifested as a modulation of the photoelectron angular distribution produced by coherent one- and two-photon ionization. Photopreparation of an initial state and use of an intermediate-state resonance in the two-photon arm of the interfering interaction helps to reduce the number of possible photoionization channels.

A simple two-channel quantum defect theory approach accounts for resonance positions in the np Rydberg series of (11)BH. The transition from Hund's case (b) to (d) in the interacting levels of this np series represents a fundamental example of electron orbital ⇔ cation core rotational coupling, and frame transformation theory offers a means to connect close-coupled electronically excited-state potentials and l-uncoupled Rydberg positions. This evolving interaction of the np Rydberg electron with the rotational and the vibrational motion of the (11)BH(+) core is formulated in terms of quantum defects, μ(λ)(v(+)).

The (2+1) multiphoton ionization (MPI) spectrum of jet‐cooled CS2 has been recorded in the range 345–370 nm, corresponding to a two‐photon frequency between 54 000 and 58 000 cm−1. The origin bands of the ...2π3g4sσg 1Πg and 3Πg←X 1Σ+g Rydberg transitions have been observed at 351.04 and 355.15 nm (vacuum 56 974 and 56 315 cm−1), respectively. A third, broader and stronger, band is observed at ∼365 nm corresponding to a two‐photon transition at ∼54 700 cm−1, which is suggested to belong to the valence 5σu3πu 1Πg state. The position of the two‐photon band at 351 nm establishes it as the one‐photon electric‐dipole forbidden origin of the 800 cm−1 progression observed in the ultraviolet absorption spectrum. The first‐photon frequency of this transition falls at a level corresponding to the a 3A2 valence state, so that the (2+1) MPI spectrum is supplemented by numerous transitions associated with the (1+2) MPI spectrum of the 3A2←X 1Σ+g transition.

Using state-selected double-resonant excitation, we create a Rydberg gas of NO molecules excited to the principal quantum number n = 50 of the f-series converging to the ion rotational level, N(+) = 2. This gas evolves to form an ultracold plasma, which expands under the thermal pressure of its electrons, and dissipates by electron-ion recombination. Under conditions chosen for this experiment, the observed rates of expansion vary with selected plasma density. Electron temperatures derived from these expansion rates vary from T(e) = 12 K for the highest density up to 16 K at four-fold lower density. Over this range, the apparent electron coupling parameter, defined as Γ(e) = e(2)/4πε(0)ak(B)T(e), falls from nearly three to about one. The decay of charged-particle density fits with a kinetic model that includes parallel paths of direct two-body and stepwise three-body dissociative recombination. The overall recombinative decay follows a second-order rate law, with an observed rate constant that fits with established scattering-theory estimates for elementary two-body dissociative recombination. A small residual increase in this rate constant with decreasing charged-particle density suggests a growing importance of the three-body recombination channel under conditions of decreasing electron correlation.

The ungerade Rydberg states of acetylene in the energy range from 79 400 to 85 000 cm−1 have been investigated by (3+1) ionization‐detected absorption spectroscopy. Bending vibrational transitions induced by Renner–Teller coupling have been discovered and analyzed. The structure observed corresponds closely with analogous bands in C2H2+ recently obtained and assigned to trans‐bending (ν4) by Pratt et al., using electron spectroscopy following resonant two‐photon photoionization via the A 1Au state [J. Chem. Phys. 95, 6238 (1991)]. Based on intensities observed in our higher‐resolution Rydberg spectrum, we offer a reassignment of this bending level structure that suggests an acetylene cation, both isolated and at the core of nonpenetrating Rydberg states, in which trans‐bend is of relatively high ffrequency and strongly coupled by Renner interaction to the cation 2Π electronic degeneracy.

Zero kinetic energy photoelectron spectroscopy is combined with triple-resonant multiphoton absorption to obtain the first experimental measurement of the fundamental vibrational frequencies of isolated NO + 2 . Analysis of observed vibrational structure yields values for the quadratic force constants in both normal coordinate space as well as internal symmetry coordinates. Results are compared with recent ab initio calculations of Lee, with excellent agreement between the two. We obtain values for the fundamental frequencies of 1396.8 ± 1.0, 639.0 ± 1.0, and 2362.0 ± 1.0 cm −1 for the symmetric, bending, and asymmetric vibrations, respectively.

We report the production of radical transients AlH and AlD in a filament‐source pulsed free‐jet expansion. In this source, hydrogen gas from a pulsed valve passes over hot tungsten filament wrapped by a layer of aluminum wire. Liquid aluminum wetting the filament reacts rapidly to form (AlH3)x oligomer. Depending on filament temperature, either AlH or AlH2 sublimes from this oligomeric coating to be entrained in subsequent pulses of H2. The source has been characterized by laser induced fluorescence and ionization‐detected one‐photon absorption with time‐of‐flight mass resolution. Laser‐induced fluorescence measurements focus on the well‐known A–X transition of AlH. Higher energy ionization‐detected absorption spectra resolve the C 1Σ+–X 1Σ+ transition in AlH and AlD, and for the first time the forbidden transition, b 3Σ−–X 1Σ+, in AlH. Spectroscopic constants determined from an analysis of bands assigned to v=0 and v=1 of the b 3Σ− state are Te = 40 524 cm−1, Be = 7.0675 cm−1, De = 6.1 × 10−5 cm−1, αe = ...

Ion rotational distributions for single‐photon VUV photoionization of the 1b_1 orbital of the X̃ ^1A_1 ground state of the jet‐cooled water are reported. These spectra reveal significant type a transitions which are seen to arise from odd angular momentum components of the photoelectron matrix element. The resulting photoionization dynamics are quite nonatomic‐like.

Multiresonant stepwise excitation of NO2 isolates series of individual rovibronic states converging to vibrationally excited levels of NO+2. Resonances detected by third‐photon photoionization of two‐photon photoselected intermediate states show characteristic patterns of intensity and linewidth that convey information on the dynamics of vibrational autoionization for relaxation in specific normal modes of the linear NO+2 core. Earlier studies characterized vibrational‐to‐Rydberg‐electronic energy transfer from symmetric stretching (100) and bending (010) excited states [J. Chem. Phys. 93, 2308, 7731 (1990)]. In the present work, a definitive assignment of double‐resonant rotational structure confirms two‐color selection of 3pσ 2∑+u gateway states that are core excited by one quantum of asymmetric stretch. Ionization‐detected optical‐absorption scans from the double‐resonantly selected N’=2 level of the 3pσ (001) state yield spectra of s, d, and g Rydberg series of vibrationally autoionizing resonances co...

We present a numerical solution for one-parameter linear Jahn-Teller coupling as it affects the full vibronic energy level spectrum of the T⊗τ problem. The Franck-Condon distribution of spectral intensities for the electronic transition from a totally symmetric originating state is characterized as a function of the linear coupling strength.

A rotationally resolved one‐photon threshold photoionization spectrum of jet‐cooled water (H2O and D2O) has been obtained by pulsed field ionization of extremely high‐n Rydberg states. Observed spectral intensities for both vibrationless (0,0,0) and vibrationally excited (1,0,0) water cation show a strong propensity for ΔN=0, ±1 transitions. In contrast to earlier work on O2 and HCl, the lack of large ΔN transitions suggests that ionization occurs with only small angular momentum transfers between the core and photoelectron. The presence of both type A and type C ‘‘symmetric top’’ transitions varies from the conclusions of a recent MQDT analysis of H2O photoionization, which predicts only type C transitions. Rotational analysis of the spectra yields improved ionization potentials for both H2O and D2O. The ionization potential of the (1,0,0) vibrational level provides a direct measurement of the symmetric stretch fundamental in H2O+ which is in excellent agreement with an earlier indirect determination. Ro...

Pulsed field ionization (PFI) has been used in conjunction with a coherent VUV source to obtain high-resolution threshold photoelectron spectra for the (000), (010), (020), and (100) vibrational states of the N2O(+) cation. Simulations for the rotational profiles of each vibronic level were obtained by fitting the Buckingham-Orr-Sichel equations using accurate spectroscopic constants for the ground states of the neutral and the ion. The relative branch intensities are interpreted in terms of the partial waves of the outgoing photoelectron to which the ionic core is coupled and in terms of the angular momentum transferred to the core.

Multiresonant, ionization‐detected absorption spectra of autoionizing ns and nd Rydberg states converging to the (200) and (300) vibrational states of the NO+2 core show unexpectedly broad resonances for many states nominally requiring Δv=−2 and −3 vibrational transitions in the core to energetically achieve electron ejection. The resonances observed do not show evidence of significant predissociation, implying that the autoionization process is primarily vibrational. Perturbed line shapes and the presence of many resonances not clearly associated with the assigned sσ, dσ, and dπ series indicate extensive vibronic coupling with other Rydberg vibrational manifolds. A particularly broad feature at a total energy of 78 720 cm−1 appears in spectra scanned both from the (200) and (300) levels of the 3pσ state. This energy falls just below the threshold for forming NO+2 in the vibrational state (100), and the broad features that appear at this point in the higher vibrational manifolds can be identified as compo...

We explore the problem of an orbitally triple degenerate electronic state coupled with a doubly degenerate vibrational mode, for a molecule within the T d rigid geometrical point group, using exact as well as numerical methods. Franck-Condon overlap integrals are worked out and are displayed as a function of linear coupling parameter. The effect of inclusion of quadratic coupling terms on the eigenvalue and intensity distribution are compared with the linear case.

Ultraviolet two‐photon photoselection, followed by visible one‐photon absorption is applied to HCl to record the first double‐resonant spin–orbit autoionization spectrum of a hydrogen halide, and the first rotationally resolved such spectrum of HCl. The J=2 level of the F 1Δ2(v=0) Rydberg state serves as the intermediate two‐photon resonance. The ionization‐detected absorption spectrum from this initial state, scanned across the 634 cm−1 interval between the lower 2Π3/2 and upper 2Π1/2 thresholds, shows a complex system consisting of hundreds of sharp lines converging to the accessible rotational limits of the upper spin–orbit threshold. The complexity of the spectrum is attributed to the relaxed selection rules associated with dipole transitions from a state in Hund’s case (a) to a manifold approaching Hund’s case (e), in concert with the irregularities expected for angular momentum coupling intermediate between the limits of case (c) and case (e). A simple case (e) fit over the central portion of the sp...

We report the spontaneous formation of a plasma from a gas of cold Rydberg molecules. Double-resonant laser excitation promotes nitric oxide, cooled to 1 K in a seeded supersonic molecular beam, to single Rydberg states extending as deep as 80 cm$^{-1}$ below the lowest ionization threshold. The density of excited molecules in the illuminated volume is as high as 1 x 10$^{13}$ cm$^{-3}$. This population evolves to produce prompt free electrons and a durable cold plasma of electrons and intact NO$^{+}$ ions., 4 pages (two column) 3 figures; smaller figure files, corrected typos

Three‐color, triple‐resonance ionization‐detected absorption spectra of the autoionizing (010) s and d states of NO2 are presented and compared with similar spectra of the (100) and (110) manifolds. The (010) states are well described by Hund’s case (d), and exhibit evidence of vibronic coupling with other vibrational manifolds. The laser limited linewidths of the (010) resonances show autoionization by ν2 to be slower than autoionization via ν1 by a factor of at least 30. The mode dependence of the autoionization rate can be explained by a model established some time ago for diatomics that connects the Rydberg‐electron‐cation‐core coupling dynamics with the time‐dependent core charge distribution associated with each vibrationally excited state. Simple calculations, which focus on the monopolar components of the core‐vibrational dynamic multipole potentials, agree qualitatively with the experimental results. A large signal increase at the (010) vertical threshold evidences significant decay through predissociation. Comparison with the (100) and (110) vertical thresholds suggests that, in addition to coupling less effectively to the ionization continuum, excitation in ν2 promotes dissociation.

Ionization‐detected absorption spectra of the (100) vibrationally autoionizing states of NO2 have been recorded from double‐resonantly prepared N’=1 and N’=7 rotational levels of the 3pσ 2Σ+u (100) Rydberg intermediate state. Photoselection associated with three‐color triple‐resonant absorption spectroscopy resolves single rotational lines in discrete electronic states that lie above the adiabatic ionization threshold at total energies as high as 78 800 cm−1. Most features observed can be assigned to sσ, dσ, and dπ series converging to the (100) vertical threshold. Identified transitions extend over an interval of principal quantum numbers ranging from 9 to more than 40. A fourth short progression of sharp transitions is assigned as a segment of a g series. The complete spectrum of (100) transitions is modulated in intensity by a sequence of dips that form a series of states, recognized as pσ and pπ, converging to the (110) threshold. Evidence is cited for a mirror‐image effect in autoionizing rates, wher...

Optical-optical-optical triple-resonance spectroscopy of (11)BH isolates high Rydberg states that form series converging to rotational state specific ionization potentials in the vibrational levels of (11)BH(+) from nu(+)=0 through 4. Limits defined by a comprehensive fit of these series to state-detailed thresholds yield rovibrational constants describing the X (2)Sigma(+) state of (11)BH(+). The data provide a first determination of the vibrational-rotational interaction parameter alpha(e)=0.4821 cm(-1) and a more accurate estimate of omega(e)=2526.58 cm(-1) together with the higher-order anharmonic terms omega(e)x(e)=61.98 cm(-1) and omega(e)y(e)=-1.989 cm(-1). The deperturbation and global fit of series to state-detailed limits also yield a precise value of the adiabatic ionization potential of (11)BH of 79 120.3+/-0.1 cm(-1), or 9.810 33+/-1x10(-5) eV. High precision is afforded here by the use of graphical analysis techniques, narrow-bandwidth laser systems, and an analysis of newly observed, high principal quantum number Rydberg states that conform well with Hund's case (d) electron-core coupling limit.

We have applied optical-optical-optical triple resonance spectroscopy to resolve a system of high Rydberg states in BH that serves quantitatively to characterize a fundamental example of electron-orbital-cation core rotational coupling. The third-color ionization-detected absorption spectrum originating from the photoselected 3s B 1 Σ + Rydberg state with vibrational and total angular momentum quantum numbers, v' = 1 and N = 0 consists entirely of vibrationally autoionizing resonances for which final N = 1 that converge in series to the BH + v t = 1 rotational limits, N + = 0, 1, and 2. For series with l = 1 converging to N + = 0 and 2, Rydberg orbital and rotational angular momenta couple to systematically perturb level energies and distribute lifetime in a well-isolated two-channel rotronic interaction that spans hundreds of wave numbers.

The dissociation of excited state ${\mathrm{Br}}_{2}$ is probed with the novel technique of ultrafast soft x-ray photoelectron spectroscopy. Excited ${\mathrm{Br}}_{2}$ molecules are prepared in the dissociative $^{1}\ensuremath{\Pi}_{u}$ state with 80 fs, 400 nm pulses, and a series of photoelectron spectra are obtained during dissociation with pulses of soft x-ray light (47 nm, 26.4 eV, 250 fs). The formation of Br atoms is readily detected and the data support an extremely fast dissociation time for ${\mathrm{Br}}_{2}$ on the order of 40 fs. Amplitudes of the pump-probe features reveal that the ionization cross section of atomic Br at 47 nm is $\ensuremath{\sim}40$ times larger than that of ${\mathrm{Br}}_{2}$.

Using optical-optical-optical triple-resonance spectroscopy, we assign rotational levels with N = 0-5 in the vibrationless, lower-n, p Rydberg states of (11)BH. We apply the Hill and Van Vleck formulation for energy levels with l = 1 in a Hund's case intermediate between (b) and (d) to gauge the energy separating (1)Π and (1)Σ(+) states with zero rotation for n = 4-11. This energy difference, A(l, ξ), represents the strength of the coupling, ξ, between the electron orbital angular momentum, l, and the internuclear axis, which determines the Λ-splitting constant, q(0). The np series exhibits a large q(0) that increases monotonically with n to reach a magnitude similar to the rotational constant, B(0), by n = 9. For higher principal quantum numbers, Λ ceases to be a good quantum number, and l-uncoupling becomes virtually complete for n > 10.

State‐selected delayed pulsed‐field threshold photoionizationspectra of HCl and DCl are recorded in double‐resonant transitions through the F ^1Δ, E ^1Σ^+, and g ^3Σ^− states of the 4pπ Rydberg configuration. Comparison of observed rotational line strengths with calculated spectra, as well as with available time‐of‐flight photoelectron spectra, provides useful insight on the influence of spin–orbit and rotational autoionization on delayed pulsed‐field threshold photoionization of HCl. Spin–orbit and rotational autoionization are seen to dramatically reduce the ion rotational intensity associated with the upper spin–orbit level of the ion.

Optical-optical-optical triple resonance spectroscopy isolates transitions to vibrationless Rydberg states of BH with principal quantum numbers from n=7 to 50. Corresponding resonances appear in the excitation spectrum of excited boron atoms produced by the dissociative relaxation of these states. The decay to neutral products occurs on a nanosecond time scale. Yet, corresponding resonances show Fano coupling widths that approach 1 cm-1. Above threshold, spontaneous ionization dominates, but line shapes match for resonances with the same electron orbital quantum numbers built on v+=0 and v+=1 cores. This striking feature-for-feature similarity in predissociation and autoionization line shapes affirms that inelastic electron-cation scattering pathways leading to electron ejection and dissociative recombination proceed through a common continuum.

We re-examine the ionization-detected ultraviolet absorption spectrum of the 3pπ 2Π←X 2A′ transition in HCO and DCO using a high-power visible laser to enhance the observation of first-photon resonant features. This technique, which we term here, assisted REMPI, significantly improves the signal-to-noise ratio of the spectrum, making many weak vibronic sub-bands visible for the first time. A comprehensive fit to the structure evident in a progression of bending levels from (000) to (040) refines the assignment of Song and Cool [X. M. Song and T. A. Cool, J. Chem. Phys. 96, 8664 (1992)] to yield a set of rotational constants that vary with K in relation to v2, together with a higher-order contribution to the Renner–Teller splitting in HCO, which is mirrored in DCO for all levels but (040). The (040) band falls at a frequency that is commensurate with that of CD stretch, and Fermi resonance between 3pπ 2Π(1000)Π and the higher-energy (040) K=1(Π) component gives rise to an added splitting that increases the...

We report the ionization-detected absorption spectra of autoionizing Rydberg states converging to the (010) vibrational level of HCO+. Sharp second-photon resonances appear in transitions from first-photon-prepared originating states that have total angular momentum (less-spin) from N′=0 to 5, selected from the Σ+ and Σ− components of the (010) band of the 3pπ2Π Rydberg state. We systematically compare spectra in order to characterize observed resonances in terms of the good total angular momentum quantum number, N. Rydberg analysis establishes the convergence of series to detailed cation-core rotational quantum numbers, N+. Observed series are found to fit well with simulations employing a limited set of constant quantum defects (δ=1.062, 0.794, 0.606, 0.253, 0.015, 0.002, −0.027 and −0.076). The strengths of observed transitions as a function of initial and final total angular momentum provide a purely experimental indication of the appropriate assignment of the approximately good orbital angular moment...

Polarized two‐photon photodissociation of NO2 in the region of 480 nm yields NO(v‘=0) with an anisotropic distribution of J. Measured polarization ratios are compared to quantum mechanical calculations for a range of expected ratios for the various isolated and mixed branches of the NO X2Π1/2→A 2Σ+ transition. Theoretical results show that main branches and their respective satellites (e.g., R11 and R21 branches) have the same transition moment directionally, though their intensities are in general different, implying that care is needed in interpreting polarization data from the mixed branches, such as (Q21+R11) or (Q11+P21), which measure the Π+ and Π− Λ doublet, respectively. Recognition of this fact is particularly important for properly separating the consideration of electron density distributions of Λ doublets from transition moment directionalities, as this has been a source of confusion in the literature. The measured results indicate that the principal two‐photon photoexcitation pathway in NO2...

We extend recent works of Berry, Simon, and others on the evolution of adiabatic wave functions in parameter spaces with nontrivial global geometry, to show the interesting ways in which wave functions can acquire nonintegrable phase (commonly termed Berry’s phase, geometric phase, or topological phase) upon transport along paths in the parameter space. We emphasize the case of arbitrary paths on the Born–Oppenheimer potential energy surfaces (the parameter space of the electronic states in an isolated molecule) of the linear plus quadratic E⊗e Jahn–Teller system. It is found that these surfaces are degenerate not only at the origin but also at three other, equivalent points, which lie on a radius ρ=2k/g. Here k and g are the linear and quadratic vibronic coupling constants, respectively. This radius is then shown to mark a sharp transition between Jahn–Teller behavior, characterized by half‐odd‐integral vibronic angular momentum, and Renner–Teller behavior, which has integral angular momentum. Finally, we examine the conditions necessary for adiabatic evolution in the E⊗e system, and the observable consequences of the geometric phase.

Ionization-detected UV multiphoton absorption spectroscopy of the excited states of N2O is presented, showing Rydberg structure within 20,000/cm of the first ionization threshold. Despite evidence for strong Rydberg-continuum coupling in the form of broadened bands and Fano line-shapes, the Rydberg structure persists, with atomic-like quantum defects and vibration structure well-matched with that of the ion. In the most clearly resolved spectrum, corresponding to the 3p(delta)1Pi state, Renner-Teller and Herzberg-Teller coupling of electronic and vibrational angular momentum are revealed. It is suggested that these mixings are properties of the N2O(+)Pi ion core.

The two‐photon absorption spectra of the 3sE’ states of cyclopropane‐h6 and cyclopropane‐d6 are obtained by two‐photon resonant three‐photon ionization spectroscopy. Bands are vibronically resolved by virtue of a free‐jet expansion. The hot bands of these spectra suggest that the excited state is distorted in both the carbon ring deformation coordinate and the CH2 wag coordinate. These are the lowest e’ modes, and thus their activity can be interpreted as arising from Jahn–Teller interactions. A satisfactory fit of both line positions and intensities is achieved within this model. The dynamics and structure predicted by the Hamiltonian used are described. It is concluded that, in the ground vibronic state, nonadiabatic interactions are small, but the electronically excited molecule is nevertheless very fluxional, sampling a wide range of geometries. Morever, while only minimal barriers to pseudorotation exist in each coordinate individually, there is some locking of the phases of the two modes, so the pse...

The dynamics of infrared multiphoton excitation and dissociation of SF6 has been investigated under collision‐free conditions by a crossed laser–molecular beam method. In order to understand the excitation mechanism and to elucidate the requirements of laser intensity and energy fluence, a series of experiments have been carried out to measure the dissociation yield dependences on energy fluence, vibrational temperature of SF6, the pulse duration of the CO2 laser, and the frequency in both one and two laser experiments. Translational energy distributions of the primary dissociation product SF5, measured by time‐of‐flight and angular distributions and the dissociation lifetime of excited SF6 as inferred from the observation of secondary dissociation of SF5 into SF4 and F during the laser pulse suggest that the dynamics of dissociation of excited molecules is dominated by complete energy randomization and rapid intramolecular energy transfer and can be adequately described by RRKM theory. An improved phenom...

It is proposed that the two Λ-doublet levels of linear molecules with nonzero electronic orbital angular momentum be labeled Λ(A′) and Λ(A″), e.g., Π(A′) and Π(A″) for Π states, etc., according to the following prescription: All series of levels in which the electronic wave function at high J is symmetric with reflection of the spatial coordinates of the electrons in the plane of rotation will be designated Λ(A′) for all values of J, and all those for which the electronic wave function is antisymmetric with respect to reflection will be denoted Λ(A″).lt is emphasized that this notation is meant to supplement, and not replace, the accepted spectroscopic e/f labeling and the parity quantum number. The utility of the Λ(A′)/Λ(A″) notation is that it is of most relevance in the mechanistic interpretation of reactive or photodissociative processes involving open-shell molecules. © 1988 American Institute of Physics.

The first ionization potentials of benzene and benzene- d 6 have been precisely determined by the extrapolation of three-photon resonant Rydberg states in the four-photon ionization spectrum of the jet-cooled molecule. The convergence of resolved transitions in two Rydberg series for principal quantum numbers as high as 14 (- h 6 ) and 15 (- d 6 ) establish adiabatic thtesholds of 74573.0 ± 2.0 cm −1 , and 74592.5 = ± 1.2 cm −1 , respectively. These results are crucial for the understanding of the many excited states of benzene in terms of quantum defect theory. Precise quantum defects have been obtained for several Rydberg series and their variation with principal quantum number is reported. The results strongly suggest that the R‴ series of Wilkinson is derived from aπ(e 1g )→ n f±1 Rydberg excitation.