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The optical formation of coherent superposition states, a wavepacket, can allow the study of zeroth-order states, the evolution of which exhibit structural and electronic changes as a function of time: this leads to the notion of a molecular movie. Intramolecular vibrational energy redistribution, due to anharmonic coupling between modes, is the molecular movie considered here. There is no guarantee, however, that the formed superposition will behave semi-classically (e.g. Gaussian wavepacket dynamics) or even as an intuitively useful zeroth-order state. Here we present time-resolved photoelectron spectroscopy (TRPES) studies of an electronically excited triatomic molecule wherein the vibrational dynamics must be treated quantum mechanically and the simple picture of population flow between coupled normal modes fails. Specifically, we report on vibronic wavepacket dynamics in the zeroth-order 3pσ2Σu Rydberg state of NO2. This wavepacket exemplifies two general features of excited state dynamics in polyatomic molecules: anharmonic multimodal vibrational coupling (forming polyads); nonadiabatic coupling between nuclear and electronic coordinates, leading to predissociation. The latter suggests that the polyad vibrational states in the zeroth-order 3p Rydberg manifold are quasi-bound and best understood to be scattering resonances. We observed a rapid dephasing of an initially prepared 'bright' valence state into the relatively long-lived 3p Rydberg state whose multimodal vibrational dynamics and decay we monitor as a function of time. Our quantum simulations, based on an effective spectroscopic Hamiltonian, describe the essential features of the multimodal Fermi resonance-driven vibrational dynamics in the 3p state. We also present evidence of polyad-specificity in the state-dependent predissociation rates, leading to free atomic and molecular fragments. We emphasize that a quantum molecular movie is required to visualize wavepacket dynamics in the 3pσ2Σu Rydberg state of NO2.

Process-monitoring laboratories in the pulp and paper industry generally use a combination of wet chemical analyses and physical measurements to certify the fitness of a production pulp for a specific end-use. These laboratory tests require time and the effort of trained personnel, limiting their utility for real-time process control. Here we show that Raman probes of unrefined cellulosic pulps, well-suited to the online measurement of in-process materials, can predict the quality attributes of manufactured papers. The accuracy of prediction improves when the covariance is modelled in a reduced measurement space selected by a data-driven, feature-selection technique referred to as a Template Oriented Genetic Algorithm (TOGA). TOGA, combined with discrete wavelet transform (DWT), isolates functional-group features that correlate best with mechanical properties paper derived from refined pulp. Paper makers refine market pulps to build sheet strength using a beating process that decreases freeness as it increases fibre-fibre bonding. Methods demonstrated here predict manufactured sheet properties obtainable after any specified degree of refining from the Raman spectrum of an unrefined pulp. This analysis capacity will enable both vendors of market pulp and makers of sheet paper to specify in advance the amount of beating required to produce a desired product, thereby saving cost and conserving resources.

The pH at the electrocatalyst surface plays a key role in defining the activity and selectivity of the CO2 reduction reaction (CO2RR). We report here operando Raman measurements of the catalyst sur...

Resonant mm-wave fields drive n0f(2)→(n0±1)g(2) transitions in a state-selected n0f(2) Rydberg gas of NO. This transformation produces a clear signature in the selected field ionisation spectrum an...

We report three-photon continuous-wave laser excitation of nitric oxide to high lying electronic Rydberg states and its conversion into a detectable current in the nA regime using our optogalvanic trace-gas sensor prototype.

Penning ionization releases electrons in a state-selected Rydberg gas of nitric oxide entrained in a supersonic molecular beam. Subsequent processes of electron impact avalanche, bifurcation, and quench form a strongly coupled, spatially correlated, ultracold plasma of ${\mathrm{NO}}^{+}$ ions and electrons that exhibits characteristics of self-organized criticality. This plasma contains a residue of nitric oxide Rydberg molecules. A conventional fluid dynamics of ion-electron-Rydberg quasi-equilibrium predicts rapid decay to neutral atoms. Instead, the NO plasma endures for a millisecond or more, suggesting that quenched disorder creates a state of suppressed electron mobility. Supporting this proposition, a 60-MHz radio frequency field with a peak-to-peak amplitude less than 1 V ${\mathrm{cm}}^{\ensuremath{-}1}$ acts dramatically to mobilize electrons, causing the plasma to dissipate by dissociative recombination and Rydberg predissociation. An evident density dependence shows that this effect relies on collisions, giving weight to the idea of arrested relaxation as a cooperative property of the ensemble.

We present the development of an automated process control technology that employs Raman spectroscopy and chemometrics in the pulp and paper industry, improving efficiency, reducing costs, and providing the opportunity to make product quality guarantees.

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.

Polytetrafluoroethylene (PTFE) flat profiles were extruded using slit dies, which promoted orientation of fibrils in two directions (2-D). Uniaxial tensile experiments were performed on the collected extrudates using the Sentmanat Extensional Rheometer (SER) at different temperatures and Hencky strain rates to determine the mechanical properties of the material. A nonlinear viscoelastic model developed by Matsuoka was found to describe well the transient tensile results using Poisson's ratio as a parameter. Polarized Raman Spectroscopy was also used to gain additional information on the degree of fibril orientation at different locations both along and across the width and length of the extrudates. Results of the Raman spectra were found to be in agreement with the fibril structure/morphology obtained from SEM micrographs. The compressibility of the extrudates upon stretching was studied by measuring the relative density. The results are modeled using a simple equation including the elastic strain recovery coefficient (κ) and Poisson's ratio.

We demonstrate the applicability of 2-photon Rydberg excitations of nitric oxide (NO) at room temperature in a gas mixture with helium (He) as an optogalvanic gas sensor. The charges created initially from succeeding collisions of excited NO Rydberg molecules with free electrons are measured as a current on metallic electrodes inside a glass cell and amplified using a custom-designed highbandwidth transimpedance amplifier attached to the cell. We fnd that this gas sensing method is capable of detecting NO concentrations lower than 10 ppm even at atmospheric pressures, currently only limited by the way we prepare the gas dilutions.

Sudan I is a carcinogenic and mutagenic azo-compound that has been utilized as a common adulterant in spice and spice blends to impart a desirable red color to foods. A novel biosensor combining molecularly imprinted polymers (MIPs), thin layer chromatography (TLC) and surface enhanced Raman spectroscopy (SERS) could determine Sudan I levels in paprika powder to 1 ppm (or 2 ng/spot). Sudan I spiked paprika extracts (spiking levels: 0, 1, 5, 10, 40, 70 and 100 ppm) were prepared. Sudan I imprinted polymers were synthesized by employing the interaction between Sudan I (template) and methacrylic acid (functional monomer), followed by washing to remove Sudan I leaving the Sudan I-binding sites exposed. MIPs were used as a stationary phase for TLC and could selectively retain Sudan I at the original spot with little interference. A gold colloid SERS substrate could enhance Raman intensity for Sudan I in this MIP-TLC system. Principal component analysis plot and partial least squares regression (R(2)=0.978) models were constructed and a linear regression model (R(2)=0.983) correlated spiking levels (5, 10, 40, 70 and 100 ppm) with the peak intensities (721 cm(-1)) of Sudan I SERS spectra. Both separation (30-40s) and detection (1s or 0.1s) were extremely fast by using both commercial bench-top and custom made portable Raman spectrometers. This biosensor can be applied as a rapid, low-cost and reliable tool for screening Sudan I adulteration in foods.

We have developed a silver nanofilm-coated porous anodic aluminum oxide (AAO) as a surface-enhanced Raman scattering (SERS)-active substrate for the detection of trace level of chloramphenicol, a representative antibiotic in food systems. The ordered aluminum template generated during the synthesis of AAO serves as a patterned matrix on which a coated silver film replicates the patterned AAO matrix to form a 2-dimensional ordered nanostructure. We used atomic force microscopy and scanning electron microscopy images to determine the morphology of this nanosubstrate, and characterized its localized surface plasmon resonance by ultraviolet-visible reflection. We gauged the SERS effect of this nanosubstrate by confocal micro-Raman spectroscopy (782-nm laser), finding a satisfactory and consistent performance with enhancement factors of approximately 2 × 10(4) and a limit of detection for chloramphenicol of 7.5 ppb. We applied principal component analysis to determine the limit of quantification for chloramphenicol of 10 ppb. Using electromagnetic field theory, we developed a detailed mathematical model to explain the mechanism of Raman signal enhancement of this nanosubstrate. With simple sample pretreatment and separation steps, this silver nanofilm-coated AAO substrate could detect 50 ppb chloramphenicol in milk, indicating good potential as a reliable SERS-active substrate for rapid detection of chemical contaminants in agricultural and food products.

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

Spontaneous avalanche to plasma splits the core of an ellipsoidal Rydberg gas of nitric oxide. Ambipolar expansion first quenches the electron temperature of this core plasma. Then, long-range, resonant charge transfer from ballistic ions to frozen Rydberg molecules in the wings of the ellipsoid quenches the centre-of-mass ion/Rydberg molecule velocity distribution. This sequence of steps gives rise to a remarkable mechanics of self-assembly, in which the kinetic energy of initially formed hot electrons and ions drives an observed separation of plasma volumes. These dynamics adiabatically sequester energy in a reservoir of mass transport, starting a process that anneals separating volumes to form an apparent glass of strongly coupled ions and electrons. Short-time electron spectroscopy provides experimental evidence for complete ionization. The long lifetime of this system, particularly its stability with respect to recombination and neutral dissociation, suggests that this transformation affords a robust state of arrested relaxation, far from thermal equilibrium.

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

We report the development of a novel hybrid "capture-detection" molecularly imprinted polymers-surface-enhanced Raman spectroscopic (MIPs-SERS) biosensor for the detection and quantification of α-tocopherol (α-Toc) in vegetable oils. α-Toc served as the template for MIPs synthesis. Methacrylic acid formed as the functional monomer. Ethylene glycol dimethacrylate was the cross-linking agent, and 2,2'-azobisisobutyronitrile was used as the initiator. The synthesized MIPs functioned to rapidly and selectively adsorb and separate α-Toc from oil components. We validated a dendritic silver nanostructure synthesized by a displacement reaction to be a suitable SERS substrate for the enhancement of Raman signals. Second-derivative transformations and chemometric models based upon SERS spectral features confirmed the possibility of a rapid and precise detection and quantification of different spiking levels of α-Toc in four different sources of vegetable oils (Mahalanobis distance from 15.93 to 34.01 for PCA model; R > 0.92, RMSE < 0.41 for PLSR model). The MIPs-SERS biosensor had a high sensitivity as well as a good recovery for α-Toc analysis in vegetable oils. The entire analysis required 15 min or less to complete with limited sample preparation.

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.

This paper presents a novel methodology, adaptive multiscale regression (AMR), to adaptively process Raman spectra for quantitative analysis. The proposed methodology aims to construct an optimal calibration model for a Raman spectrum at hand, regardless of its structural characteristics, thus facilitating the application of Raman spectroscopy as a general tool for analytical chemistry. AMR firstly splits the spectra in a calibration set into frequency components at different scales using adaptive wavelet transform (AWT). Parallel member models constructed at different scales are then fused into a final prediction. The contributions of member models to a fusion model are straightforwardly estimated by a partial least square (PLS) model that emerges from a cross-validation results matrix (X) and reference values (Y). This procedure avoids information leakage by fully utilizing the multiscale nature of the input Raman spectra instead of arbitrarily removing some part of the spectral information by calibrating to selected features. Theoretically, we establish that AMR represents an automatic data-driven strategy that captures the Raman spectral structures adaptively and accurately. Our work tests and refines the AMR method by drawing upon the systematic analysis of spectra formulated to yield challenges representative of those encountered in common Raman analyses. AMR compares favorably with other popular preprocessing methods. Satisfactory calibration results suggest that AMR has the capacity to improve robustness and reliability of Raman spectral analysis, and may well extend to other spectroscopic techniques.

The application of Raman spectroscopic techniques combined with multivariate chemometrics signal processing promise new means for the rapid multidimensional analysis of metabolites non-destructively, with little or no sample preparation and little sensitivity to water. However, Rayleigh scattering, fluorescence and uncontrolled variance present substantial challenges for the accurate quantitative analysis of metabolites at physiological levels in bio- logically varying samples. Effective strategies include the application of chemometrics pretreatments for reducing Raman spectral interference. However, the arbitrary application of individual or combined pretreatment procedures can significantly alter the outcome of a measurement, thereby complicating spectral analysis. This paper evaluates and compares six signal pretreatment methods for correcting the baseline variances, together with three variable se- lection methods for eliminating uninformative variables, all within the context of multivariate calibration models based on partial least squares （PLS） regression. Raman spectra of 90 artificial bio-fluid samples with eight urine metabolites at near-physiological concentrations were used to test these models. The combination of multiplicative scatter correction （MSC）, continuous wavelet transform （CWT）, randomization test （RT） and PLS modeling pre- sented the best performance for all the metabolites. The correlation coefficient （R） between predicted and prepared concentration reached as high as 0.96.

We have applied triple-resonant optical photoionization spectroscopy to characterize the lineshapes of autoionizing Rydberg resonances in 11BH converging to the X2Σ+ ground state of 11BH+ in vibrational levels from v + = 0 through 4. Third photon transitions originating from the B1Σ+ v′ = 0, 1, 2 or 3 states of 11BH reach high Rydberg resonances in Δv = 0 (below vertical threshold) and Δv = 1 (above vertical threshold) transitions. Observed spectra display contributions from discrete–continuum and discrete–discrete transition moments, where intensities are governed by Franck–Condon factors and the discrete–continuum mixing of final states. The spectra show Fano line profiles attributable to the phase interference between competing photoionization/photodissociation transition moments. Two fitting parameters, q and Γ, which characterize asymmetric spectral lineshapes in terms of an interaction between a single discrete state and single continuum, serve well to model the photoionization cross-section. Quanti...

Formation of a secondary plasma of ${\mathrm{NO}}^{+}$ ions and electrons within an ultracold plasma produces an observable change in the hydrodynamics of the system. Direct photoionization adds energetic electrons, which increases the rate of expansion. The introduction of a secondary Rydberg gas has the opposite effect. In both cases, the added ions create an inertial drag that acts initially to retard the expansion of the electron gas. A cold-ion hydrodynamic shell model, which accounts well for the effect of energy added by photoionization electrons, predicts the formation of collisionless shock waves.

We describe a novel instrument combining interferometric scattering microscopy (iSCAT) with confocal Raman microscopy. This system images the refractive index morphology of a complex material. Co-axial Raman sampling provides chemical information with a high degree of reproducibility.

We report new results suggesting the feasibility of Raman spectrometry as a tool by which to examine the variability of tablet coatings. Our experiments feature a probe that can operate with a revolving laser focus to average content and coating non-uniformity. Raman spectral changes are correlated with tablet exposure times in a pan coater by means of partial least squares (PLS) multivariate analysis. Statistical models are found to be improved by pre-processing schemes that emphasize spectral changes while minimizing the effects of background light scattering and fluorescence. These pre-processing techniques include multiplicative scatter correction (MSC) and standard normal variate (SNV) transformation, used in concert with Savitzky-Golay second derivative smoothing (SGSD). The two approaches give comparable results yielding R2 values for PLS calibration and cross-calibrated prediction variance regression of 0.999 and 0.997, respectively. Correlation results and model residual values demonstrate that Raman spectroscopy serves sensitively to reflect the coating thickness of the tablets studied.

We report optical-optical-optical triple-resonant spectroscopic structure for BH that corresponds with scattering resonances formed by high-lying Rydberg states that couple strongly to decay channels for electron loss and boron-hydrogen bond cleavage to neutral atoms. Lineshapes and intensities provide information on state-to-state relaxation dynamics and interference between bound-bound and bound-continuum transition moments.

The analysis of precious artefacts and antiquities demands care in order to minimise the risk of accidental damage during measurement. Mobile fibre-optic-based Raman instruments offer a means to avoid destructive sampling and eliminate the need to transport artefacts for spectrochemical analysis. In this work we present a new mobile instrument developed and optimised for the in situ Raman investigation of objects of art and antiquities. The instrument is controlled by a portable computer. Selected mounts cover many types of artefacts. Newly written software routines organise spectra together with measurement parameters and facilitate calibration of the instrument. The present paper describes this new Raman instrument and discusses some challenges in the transition from a laboratory environment to in situ investigations in museums.

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 introduce a fast computational method for feature selection that facilitates the accurate spectral analysis of a chemical species of interest in the presence of overlapping uncorrelated variance. Using a genetic algorithm in a data-driven approach, our method assigns predictors according to a template determined to minimize prediction variance in a calibration space. This template-oriented genetic algorithm (TOGA) efficiently establishes features of greatest significance and determines their optimal combination. We demonstrate the efficacy of TOGA using an elementary model system in which we seek to quantify a target monosaccharide in mixtures containing other sugars added in random amounts. The results establish TOGA as an effective and reliable technique for isolating signature spectra of targeted substances in complex mixtures.

A novel biosensor combining molecularly imprinted polymers and surface-enhanced Raman spectroscopy (MIPs-SERS) determines melamine in whole milk. MIPs were synthesized by bulk polymerization of melamine (template), methacrylic acid (functional monomer), ethylene glycol dimethacrylate (cross-linking agent) and 2,2'-azobisisobutyronitrile (initiator). Static and kinetic adsorption tests validated the use of MIPs to efficiently separate and enrich melamine from whole milk. Silver dendrite nanostructure served as SERS-active substrate for signal collection. Principal component analysis and hierarchical cluster analysis segregated Raman signatures of whole milk samples with different melamine concentrations. Regression models showed a good linear relationship (R(2)=0.93) between the height of melamine SERS band (at 703cm(-1)) and melamine concentration in the range from 0.005mmolL(-1) to 0.05mmolL(-1). The limit of detection and limit of quantification were 0.012mmolL(-1) and 0.039mmolL(-1), confirming the high sensitivity of this biosensor to accurately determine melamine in whole milk. Simple sample pretreatment reduced full analysis time to determine melamine in whole milk to less than 20min.

Analyses of high Rydberg series of HCO converging to the (030) vibrational state of the cation establish rovibrational state-detailed thresholds for HCO(+). UV-visible laser double resonance isolates series for assignment. Strongly vertical Rydberg-Rydberg transitions from photoselected N' = 0 and N' = 2 rotational levels of the Sigma(-) Renner-Teller vibronic component of the 3ppi (2)Pi (030) complex define individual series converging to rotational levels, N(+) = 1 through 5 and 3 through 5 of the HCO(+) vibrational states (03(1)0) and (03(3)0), respectively. Extrapolation of autoionizing series locates the positions of these rovibrational states to within +/-0.01 cm(-1). The use of this information combined with precise ionization limits for lower vibrational states determined from earlier Rydberg extrapolations and spectroscopic information available from infrared absorption measurements enables an estimate of the force-field parameters for HCO(+) bending. These parameters include the harmonic bending frequency, omega(2) = 842.57 cm(-1), the vibrational angular momentum splitting constant, g(22) = 3.26 cm(-1), and the diagonal bending anharmonicity, x(22) = -2.53 cm(-1), separated from the off-diagonal contribution, x(12), by reference to ab initio calculations. Results of experiment on the higher vibrationally excited states of HCO(+) are compared with recent theoretical predictions. Copyright 2000 Academic Press.

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.

A number of reasons for the appearance of anomalous intensities in zero-kinetic-energy (ZEKE) spectroscopy are briefly reviewed and extended. Particular attention is paid to a recent paper of Matsui, Behm, and Grant in which the authors detect, in the ZEKE spectrum of NO2, a number of Franck−Condon forbidden transitions of significant intensity which they attribute in large part to channel interaction between allowed continua and Franck−Condon forbidden discrete Rydberg series. A quantitative approximate analysis of their data is given here which supports this claim. Implications of these results for final state interactions in photoelectron spectroscopy are discussed.

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...

Photoionizing transitions that change NO2+−core vibrational quantum numbers are examined by means of zero-kinetic-energy (ZEKE) threshold photoelectron spectroscopy. Observable intensity is found in Franck−Condon-off-diagonal transitions from the (000) vibrational ground state of the 3pσ 2Σu+ Rydberg state of NO2 to long-lived resonances at thresholds corresponding to (010), (0200), and (100) vibrationally excited states of NO2+. Analyses of the rovibrational structure of the originating Rydberg state and the cation establish that NO2+−core potential energy surfaces are in each case substantially parallel and thus predict zero Franck−Condon factors for transitions that change vibrational quantum numbers. The appearance of nonvertical transitions is attributed to the mixing of long-lived high-principal-quantum-number discrete Rydberg states that converge to each excited threshold with the optically accessible ground-state free-electron continuum. The vibrationally inelastic interaction that gives rise to t...

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.

By means of zero-electron kinetic energy (ZEKE) threshold photoelectron spectroscopy, we examine bend-stretch Fermi resonance in NO2+. Accurate fundamental frequencies are obtained by using the (000) 3pσ Rydberg state as a reference state for all ionic vibrational ZEKE spectra. The degree of (100)–(02°0) Fermi resonance observed in NO2+ is comparable to that found in the NO2 3pσ Rydberg state, and results for both systems agree well with recent CCSD(T) and MRD-CI ab initio calculations. Comparison of the vibrational frequencies of the 3pσ Rydberg state and the cation suggests that the presence of the Rydberg electron slightly perturbs the harmonic potential and its second-order anharmonicities, but has little effect on the cubic coupling term associated with core Fermi resonance.

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...

This paper offers a toolbox for characterizing the initial conditions and predicting the evolution of the ultracold plasma that forms after resonant laser preparation of a Rydberg gas entrained in a differentially pumped supersonic molecular beam. The conditions afforded by a skimmed free-jet expansion combined with the geometry of laser excitation, determines the phase-space volume of the excited gas. A hydrodynamic shell model, that accounts for the ellipsoidal spatial distribution of this excitation volume in concert with the deforming effects of dissociative recombination, serves to simulate the ambipolar expansion of this molecular ultracold plasma.

Double-resonant transitions excite nitric oxide in a seeded supersonic molecular beam, yielding a state-selected Rydberg gas that evolves to form an ultracold plasma. This plasma propagates in z with the molecular beam over a variable distance as great as 600 mm to strike an imaging detector, which records the charge distribution in the dimensions, x and y. The laser-crossed molecular beam excitation geometry convolutes an axial Gaussian distribution of NO about z with the Gaussian intensity distribution of the laser beam about x to create an ellipsoidal volume of Rydberg gas. Plasma images provide evidence for the relaxation of this Rydberg gas volume in an electron impact avalanche that breaks the ellipsoidal symmetry in x to form repelling plasma volumes. We find that the energy deposited in the recoil velocity of mass transport, V x depends systematically on the initially selected Rydberg gas principal quantum number, n 0, and the initial density of the Rydberg gas, ρ 0. These quantities combine to determine ρ e, the initial density of electrons formed by the prompt Penning ionization of closely spaced pairs of Rydberg molecules. Above a threshold density of Penning electrons, we find that V x depends linearly on ρ e. We argue that this bifurcation occurs as a consequence of the initial geometry of the Rydberg gas. Ambipolar electron expansion accelerates initially formed core ions. Resonant charge transfer redistributes this ion energy to the column of Rydberg molecules on the long axis of the ellipsoid. The equalized velocities in each direction give rise to a ±x streaming motion that concentrates density in opposing plasma volumes, causing the symmetric gas volume to split like a rotating liquid drop. Significantly, these dynamics reduce electron temperature with little decrease in the ion density or increase in the ion temperature. This appears to facilitate the formation of a strongly coupled plasma.

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(+)).

Gas-phase samples of Cr(CO) 5 (C 2 H 4 ) are prepared in situ by laser irradiation of quantitative mixtures of Cr(CO) 6 , CO, and C 2 H 4 . In the presence of CO and C 2 H 4 , Cr(CO) 5 (C 2 H 4 ) decays thermally to re-form Cr(CO) 6 by the mechanism of dissociative substitution. Systematic study of the rate of this reaction as a function of partial pressures of CO and C 2 H 4 yields the elementary high-pressure limiting thermal rate constant for unimolecular dissociation of Cr(CO) 5 (C 2 H 4 ) (k 1 ) and the relative rate constant for recombination of Cr(CO) 5 with CO and C 2 H 4 (k 2 /k 3 )

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.

Double-resonant photoexcitation of nitric oxide in a molecular beam creates a dense ensemble of 50f(2) Rydberg states, which evolves to form a plasma of free electrons trapped in the potential well of an NO+ space charge. The plasma travels at the velocity of the molecular beam and, on passing through a grounded grid, yields an electron time-of-flight signal that gauges the plasma size and quantity of trapped electrons. This plasma expands at a rate that fits with an electron temperature as low as 5 K, colder than typically observed for atomic ultracold plasmas. The recombination of molecular NO + cations with electrons forms neutral molecules excited by more than twice the energy of the NO chemical bond, and the question arises whether neutral fragmentation plays a role in shaping the redistribution of energy and particle density that directs the short-time evolution from Rydberg gas to plasma. To explore this question, we adapt a coupled rate-equations model established for atomic ultracold plasmas to describe the energy-grained avalanche of electron-Rydberg and electron-ion collisions in our system. Adding channels of Rydberg predissociation and two-body, electron-cation dissociative recombination to the atomic formalism, we investigate the kinetics by which this relaxation distributes particle density and energy over Rydberg states, free electrons and neutral fragments. The results of this investigation point to conditions under which such processes can effect the steady-state temperature of plasma electrons. © 2011 IOP Publishing Ltd.

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.

In the threshold photoionization spectrum of CH 3 I obtained by one-color two-photon excitation, intermediate resonance with the CH 3 +I dissociative continuum manifests itself in an expanded Franck-Condon envelope of accessible cation vibrational states together with the production and accompanying near-threshold two-photon photoionization of atomic iodine. The spectrum of the atomic photoproduct shows a new system of iodine autoionizing resonances converging to the 3 P 1 state of I + . Two series are identified, [ 3 P 1 ]np and [ 3 P 1 ]nf

Transitions from individual rovibrational levels of the 3pσ 2 Σ u + state, selected by double resonance, yield rotationally resolved threshold photoionization spectra of NO 2 . Observed photoionization cross sections conform with simple angular-momentum selection rules for transitions from a well-defined Hund's case b intermediate state to the case d Rydberg states that lie just below each successive threshold, except for the appearance of anomalous lines corresponding to large negative changes in ion-core rotational angular momentum

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...