Superexcited States Research Articles

Atomic autoionization in the photo-dissociation of super-excited deuterated water molecules fragmenting into D+ + O+ + D.

We present the relaxation dynamics of deuterated water molecules via autoionization, initiated by the absorption of a 61 eV photon, producing the very rare D+ + O+ + D breakup channel. We employ the COLd target recoil ion momentum spectroscopy method to measure the 3D momenta of the ionic fragments and emitted electrons from the dissociating molecule in coincidence. We interpret the results using the potential energy surfaces extracted from multi-reference configuration interaction calculations. The measured particle energy distributions can be related to a super-excited monocationic state located above the double ionization threshold of D2O. The autoionized electron energy shows a sharp distribution centered around 0.5 eV, which is a signature of the atomic oxygen autoionization occurring in the direct and sequential dissociation processes of D2O+* at a large internuclear distance. In this way, an O+ radical fragment and a low-energy electron are created, both of which can trigger secondary reactions in their environment.

Photoexcited state of the organic electron acceptor TCNQ molecular anion in the condensed state

The first and second excited states of 7,7,8,8-tetracyanoquinodimethane (TCNQ) anions in acetonitrile were studied employing femtosecond time-resolved photoluminescence and transient absorption spectroscopy. It was found that the second excited state, which is a superexcited state, undergoes ultrafast decay due to spontaneous electron emission with a time constant of 0.16 ps. The rationale for the fast electron emission in acetonitrile is discussed in terms of the interactions with the surrounding polar solvent molecules.

Reactive collisions between electrons and BeT[formula omitted]: Complete set of thermal rate coefficients up to 5000 K

Rate coefficients for the dissociative recombination, vibrational excitation and vibrational de-excitation of the BeT+ ion for all vibrational levels of its ground electronic state (X1Σ+,vi+=0,…,27) are reported, including in the calculation the contribution of super-excited states of the BeT complex pertaining to three electronic symmetries - 2Π, 2Σ+, and 2Δ. These data are suitable for the kinetic modeling of beryllium and tritium containing plasma, as encountered in magnetic fusion devices with beryllium walls (JET, ITER). In the present study we restrict ourselves to incident electron energies from 10−3 up to 2.7eV, and to electron temperatures between 100 and 5000 K, respectively. Together with our earlier and closely related studies on the BeH+ and BeD+ systems, this present work completes the isotopic coverage for the beryllium monohydride ions. The vibrational energy (rather than the vibrational quantum state) is identified as a proper isotopic similarity parameter, e.g., for reduced but still isotopically correct plasma chemistry models.

Electronically Excited OH Super-rotors from Water Photodissociation by Using Vacuum Ultraviolet Free-Electron Laser Pulses.

The fragmentation dynamics of water in a superexcited state play an important role in the ionosphere of the planets and in the photodissociation region (PDR) of the planetary nebula. In this Letter, we experimentally study the fragmentation dynamics of H2O with the energy above its ionization potential initiated by vacuum ultraviolet free-electron laser pulses. The experimental results indicate that the binary fragmentation channels H + OH and the triple channels O + 2H both present at 96.4 nm photolysis. Electronically excited OH super-rotors (v = 0, N ≥ 36, or v = 1, N ≥ 34), with the internal energy just above the OH (A) dissociation energy, are observed for the first time, which are only supported by the large centrifugal barriers. An absolute cross section of these super-rotors is estimated to be 0.7(±0.3) × 10-18 cm2. The tunnelling rates of these extremely rotationally excited states are also analyzed. This work shows a spectacular example of energy transfer from a photon to fragment rotation through photodissociation.

Inverted Vibrational and Rotational Excitation of CN(B2Σ+) Produced through Superexcited Ion-Pair States of MCN (M = Na, K, Rb).

This paper reports an energy-partition mechanism in dissociative excitation of alkali cyanide molecules, MCN (M = Rb, K, Na), to produce CN(B2Σ+) and M(ns2S) (n = 5, 4, and 3 for Rb, K, and Na, respectively) in collision with Kr metastable atoms, Krm(3P2,0). Both the vibrational and rotational distributions of CN(B2Σ+) produced in the reactions of RbCN and KCN were inverted as being peaked at v' = 1 and N' = 35, respectively, where v' and N' are the vibrational and the rotational quantum numbers of CN(B2Σ+), respectively. According to a state crossing model, it was derived that CN(B2Σ+) is produced by predissociation through a superexcited ion-pair state, CN-(31Σ+)·M+(1S), followed by an adiabatic transition to a repulsive state correlating to the dissociation limit of CN(B2Σ+) + M(ns2S). The inverted distributions are driven by structural changes during the excitation and the adiabatic transition. The maximum vibrational population at v' = 1 originates from a large Franck-Condon overlap between the vibrational wavefunctions of CN-(31Σ+) and CN(B2Σ+) at v' = 1. The rotational excitation of the CN(B2Σ+) product is explained with changing from a T-shape geometry of MCN in the ground state to a linear one in the superexcited ion-pair state.

Distribution of dipole oscillator strengths for the formation of excited hydrogen atom from C2H2

SynopsisDipole oscillator strengths for the formation of excited hydrogen atom from C2H2 were measured to investigate formation and decay of molecular superexcited states. It seems that non-adiabatic transition plays an important role in the formations of the hot hydrogen atom.

Solvated electron formation from the conduction band of liquid methanol: Transformation from a shallow to deep trap state

We report solvated electron (esolv -) formation dynamics from the conduction band of liquid methanol studied using femtosecond time-resolved photoelectron spectroscopy. Liquid methanol is excited with vacuum UV (9.3 eV) pump pulses, and the subsequent electron dynamics are probed with UV pulses. The photoelectron signal exhibits a short-lived component (τ = 85 fs) without spectral evolution followed by a long-lived component with continuous spectral evolution over tens of picoseconds. We ascribe the former to a superexcited state, most likely the Wannier exciton, and the latter to the ground electronic state of esolv -. In order to extract accurate energetics from the observed photoelectron spectra, we employ a spectral retrieval method to account for spectral broadening and shifting due to inelastic scattering of photoelectrons in the liquid. The electron binding energy (eBE) of the initial trap state of an electron is determined to be about 1.5 eV, and its biexponential increase up to 3.4 eV is observed with time constants of 2 and 31 ps, which are greater than 0.27 and 13 ps observed for esolv - created by the charge-transfer-to-solvent reaction from CH3O- to liquid methanol. The solvation dynamics of esolv - created by the electron trapping exhibit a pseudoisosbestic point at a pump-probe delay time of around 15 ps, and the peak energy of the eBE distribution rapidly changes around that time. These results indicate that there exist two trap states, both of which exhibit increasing eBE with time; however, the eBE of the shallow trap state increases only up to 2.1 eV, and transformation to a deep trap state at 25 ps occurs to reach an eBE of 3.4 eV.

Multiphoton Rydberg and valence dynamics of CH3Br probed by mass spectrometry and slice imaging.

The multiphoton dynamics of CH3Br were probed by Mass Resolved MultiPhoton Ionization (MR-MPI), Slice Imaging and Photoelectron Imaging in the two-photon excitation region of 66 000 to 80 000 cm-1. Slice images of the CH3+ and Br+ photoproducts of ten two-photon resonant transitions to np and nd Rydberg states of the parent molecule were recorded. CH3+ ions dominate the mass spectra. Kinetic energy release spectra (KERs) were derived from slice and photoelectron images and anisotropy parameters were extracted from the angular distributions of the ions to identify the processes and the dynamics involved. At all wavelengths we observe three-photon excitations, via the two-photon resonant transitions to molecular Rydberg states, forming metastable, superexcited (CH3Br#) states which dissociate to form CH3 Rydberg states (CH3**) along with Br/Br*. A correlation between the parent Rydberg states excited and CH3** formed is evident. For the three highest excitation energies used, the CH3Br# metastable states also generate high kinetic energy fragments of CH3(X) and Br/Br*. In addition for two out of these three wavelengths we also measure one-photon photolysis of CH3Br in the A band forming CH3(X) in various vibrational modes and bromine atoms in the ground (Br) and spin-orbit excited (Br*) states.

Getting super-excited with modified dispersion relations

We demonstrate that in some regions of parameter space, modified dispersion relations can lead to highly populated excited states, which we dub as "super-excited" states. In order to prepare such super-excited states, we invoke dispersion relations that have negative slope in an interim sub-horizon phase at high momenta. This behaviour of quantum fluctuations can lead to large corrections relative to the Bunch-Davies power spectrum, which mimics highly excited initial conditions. We identify the Bogolyubov coefficients that can yield these power spectra. In the course of this computation, we also point out the shortcomings of the gluing method for evaluating the power spectrum and the Bogolyubov coefficients. As we discuss, there are other regions of parameter space, where the power spectrum does not get modified. Therefore, modified dispersion relations can also lead to so-called "calm excited states". We conclude by commenting on the possibility of obtaining these modified dispersion relations within the Effective Field Theory of Inflation.

Dissociative recombination and vibrational excitation of BF+ in low energy electron collisions

The latest molecular data—potential energy curves and Rydberg-valence interactions—characterising the super-excited electronic states of BF are reviewed in order to provide the input for the study of their fragmentation dynamics. Starting from this input, the main paths and mechanisms of BF+ dissociative recombination and vibrational excitation are analysed. Their cross sections are computed for the first time using a method based on the multichannel quantum defect theory (MQDT), and Maxwellian rate-coefficients are calculated and displayed in ready-to-be-used format for low temperature plasma kinetics simulations.

Dissociative recombination and vibrational excitation of CO+: model calculations and comparison with experiment

The latest molecular data—potential energy curves and Rydberg/valence interactions—characterizing the super-excited electronic states of CO are reviewed, in order to provide inputs for the study of their fragmentation dynamics. Starting from this input, the main paths and mechanisms for CO+ dissociative recombination are analyzed; its cross sections are computed using a method based on multichannel quantum defect theory. Convoluted cross sections, giving both isotropic and anisotropic Maxwellian rate coefficients, are compared with merged-beam and storage-ring experimental results. The calculated cross sections underestimate the measured ones by a factor of two, but display a very similar resonant shape. These facts confirm the quality of our approach for the dynamics, and call for more accurate and more extensive molecular structure calculations.

Hydrogen migration in formation of NH(A³Π) radicals via superexcited states in photodissociation of isoxazole molecules.

Formation of the excited NH(A(3)Π) free radicals in the photodissociation of isoxazole (C3H3NO) molecules has been studied over the 14-22 eV energy range using photon-induced fluorescence spectroscopy. The NH(A(3)Π) is produced through excitation of the isoxazole molecules into higher-lying superexcited states. Observation of the NH radical, which is not a structural unit of the isoxazole molecule, corroborates the hydrogen atom (or proton) migration within the molecule prior to dissociation. The vertical excitation energies of the superexcited states were determined and the dissociation mechanisms of isoxazole are discussed. The density functional and ab initio quantum chemical calculations have been performed to study the mechanism of the NH formation.

Decay pathways of superexcited states of nitrous oxide

The ionization and ionic dissociation of the superexcited state of N2O are studied by using electron energy loss spectroscopy and positive ion time-of-flight mass spectroscopy at different momentum transfers; that is, 0 and 0.23 a.u. (atomic unit). The transitions at 13.8 eV and 14.0 eV are reassigned as 3pπ(000) and 3pσ(000) converging to A2Σ+, respectively. The competition between the main decay pathways of superexcited states at different momentum transfers is revealed. It is found that 3dσ converging to C2Σ+ mainly decays into N2O+ while 4dσ can decay into both N2O+ and NO+.

Formation of CN (B2Σ+) radicals in the vacuum-ultraviolet photodissociation of pyridine and pyrimidine molecules

Formation of the excited CN(B2Σ+) free radicals in the photodissociation of pyridine (C5H5N) and pyrimidine (C4H4N2) molecules was investigated over the energy ranges 16–27 and 14.7–25 eV, respectively. Photon-induced fluorescence spectroscopy was applied to detect the vibrationally and rotationally excited CN radicals by recording the B2Σ+→X2Σ+ emission bands (violet system). The measured dissociation yield curves demonstrate that the CN(B2Σ+) formation occurs via excitation of pyridine and pyrimidine molecules into higher-lying superexcited states. This is followed by rearrangement and isomerization of the excited molecules before dissociation. The vertical excitation energies of the superexcited states were determined and the probable dissociation mechanisms of both molecules are discussed.

Two-Photon Excited State Dynamics of Dark Valence, Rydberg, and Superexcited States in 1,3-Butadiene.

Two-photon absorption in systems with parity permits access to states that cannot be prepared by one-photon absorption. Here we present the first time-resolved photoelectron spectroscopy study using this technique, applied to 1,3-butadiene, in which we investigated the dynamics of its dark valence, Rydberg, and superexcited states. The dark valence state dynamics are accessed via the Rydberg manifold, excited by two photons of 400 nm. We find that the 'dark' 2(1)Ag state populated in this manner has a much longer lifetime than when accesses via the 1(1)Bu 'bright' valence state when populated by one photon of 200 nm. In addition, we compared the dynamics of the 3sπ- and 3dπ-Rydberg states. These Rydberg states relax to the valence manifold on a subpicosecond time scale, with the 3sπ-Rydberg state decay rate being larger due to a stronger valence-Rydberg mixing. Finally, we investigated superexcited valence states that fragment or autoionize within 200 fs, likely without involving Rydberg states.

Controlling neutral fragmentation yields of methane by femtosecond laser pulses

We demonstrate the control of neutral fragmentation of methane (CH4) induced by a Ti:sapphire intense laser pulse (800 nm, 40 fs) by using a pump-probe spectroscopy. Enhancement of the fluorescence emission from the neutral radical CH (A2? ? X2?) induced by the intense laser field (~1014 W/cm2) is observed when the wavelength of the probe laser pulse is tuned to 400 nm. The phenomena are explained based on excitation enhancement of the super-excited state of the parent molecule resulting in an increase in neutral dissociation of the methane molecules.

Cross sections for the formation of H(n = 2) atom via superexcited states in photoexcitation of methane and ammonia

The absolute cross sections for the formation of the H(2s) and H(2p) atoms, σ2s and σ2p, respectively, in photoexcitation of CH4 and NH3 were measured in the range of the incident photon energy 15-48 eV for studying superexcited states of the molecules. The same superexcited states were found to contribute to the σ2s and σ2p cross sections. It was concluded that the non-adiabatic transitions play a significant role during the dissociation of the superexcited states and ionic states.

The absolute cross section of the Fluorescence, Dissociation, and Ionization of H2 super excited states

Absolute cross sections of three competing decay channels, namely ionization, dissociation and fluorescence, were determined after excitation of superexcited states of the hydrogen molecule.

Ultrafast Dynamics of Neutral Superexcited Oxygen: A Direct Measurement of the Competition between Autoionization and Predissociation

Using ultrafast extreme ultraviolet pulses, we performed a direct measurement of the relaxation dynamics of neutral superexcited states corresponding to the nlσ(g)(c(4)Σ(u)(-)) Rydberg series of O(2). An extreme ultraviolet attosecond pulse train was used to create a temporally localized Rydberg wave packet and the ensuing electronic and nuclear dynamics were probed using a time delayed femtosecond near-infrared pulse. We investigated the competing predissociation and autoionization mechanisms in superexcited oxygen molecules and found that autoionization is dominant for the low n Rydberg states. We measured an autoionization lifetime of 92±6 fs and 180±10 fs for the (5s,4d)σ(g) and (6s,5d)σ(g) Rydberg state groups, respectively. We also determine that the disputed neutral dissociation lifetime for the ν=0 vibrational level of the Rydberg series is 1100±100 fs.

Ultrafast decay of superexcited $c{\ }\sideset{^{4}}{_{u}^{-}}{\Sigma}\ nl\sigma _{g}\ v=0,1$cΣu−4nlσgv=,1 states of O2 probed with femtosecond photoelectron spectroscopy

Neutral superexcited states in molecular oxygen converging to the O(2)(+) c (4)Σ(u)(-) ion state are excited and probed with femtosecond time-resolved photoelectron spectroscopy to investigate predissociation and autoionization relaxation channels as the superexcited states decay. The c (4)Σ(u)(-) 4sσ(g) v=0, c (4)Σ(u)(-) 4sσ(g) v=1, and c (4)Σ(u)(-) 3dσ(g) v=1 superexcited states are prepared with pulsed high-harmonic radiation centered at 23.10 eV. A time-delayed 805 nm laser pulse is used to probe the excited molecular states and neutral atomic fragments by ionization; the ejected photoelectrons from these states are spectrally resolved with a velocity map imaging spectrometer. Three excited neutral O* atom products are identified in the photoelectron spectrum as 4d(1) (3)D(J)°, 4p(1) (5)P(J)° and 3d(1) (3)D(J)° fragments. Additionally, several features in the photoelectron spectrum are assigned to photoionization of the transiently populated superexcited states. Using principles of the ion core dissociation model, the atomic fragments measured are correlated with the molecular superexcited states from which they originate. The 4d(1) (3)D(J)° fragment is observed to be formed on a timescale of 65 ± 5 fs and is likely a photoproduct of the 4sσ(g) v = 1 state. The 4p(1) (5)P(J)° fragment is formed on a timescale of 427 ± 75 fs and correlated with the neutral predissociation of the 4sσ(g) v = 0 state. The timescales represent the sum of predissociation and autoionization decay rates for the respective superexcited state. The production of the 3d(1) (3)D(J)° fragment is not unambiguously resolved in time due to an overlapping decay of a v = 1 superexcited state photoelectron signal. The observed 65 fs timescale is in good agreement with previous experiments and theory on the predissociation lifetimes of the v = 1 ion state, suggesting that predissociation may dominate the decay dynamics from the v = 1 superexcited states. An unidentified molecular state is inferred by the detection of a long-lived depletion signal (reduction in autoionization) associated with the B (2)Σ(g)(-) ion state that persists up to time delays of 105 ps.