Superexcited States Research Articles

Multiphoton dissociative ionization of tert-pentyl bromide near 265 nm

We report on the photodissociation dynamics of tert-pentyl bromide near 265 nm investigated by time-sliced velocity map imaging. The speed and angular distributions have been analyzed for both the ground-state Br((2)P(3∕2)) atom (denoted Br) and the spin-orbit excited-state Br((2)P(1∕2)) atom (denoted Br*). The speed distributions of Br and Br* atoms are all found to consist of three Gaussian components, which correlate to three independent dissociation pathways on the excited potential energy surfaces: (1) the high translational energy (E(T)) component from the prompt dissociation along the C-Br stretching mode, (2) the middle E(T) component from the repulsive mode along the C-Br stretching coupled with some bending motions, and (3) the low E(T) component from the repulsive mode along the C-Br stretching coupled with more bending motions. More interestingly, we have also observed the tert-C(5)H(11)(+) ions in 263-267 nm. The near-zero kinetic energy distributions extracted from the three tert-C(5)H(11)(+) images near 265 nm show the typical characteristics that are attributable to multiphoton dissociative ionization, suggesting the existence of a neutral superexcited state of the parent tert-pentyl bromide molecule. The contribution of bromine atoms formed in this dissociative ionization channel adds in the total relative distribution of low E(T) component in the Br*(Br) formation channel, which reasonably explains the abnormal distributions observed in between the middle and low E(T) components in the Br*(Br) formation channel.

Application of multichannel quantum defect theory to unveil quantum interferences in dissociation of superexcitedF2

We report experimental measurements of a high-precision photoreaction spectroscopy in combination with the velocity map imaging method. With such high-precision experimental measurements, we can ``observe'' detailed photodissociation processes of a specific superexcited state of our choosing. Based on multichannel quantum defect theory, an interesting quantum interference mechanism of dissociation of superexcited ${\mathrm{F}}_{2}$ into (${\mathrm{F}}^{+}+{\mathrm{F}}^{\ensuremath{-}}$) ion pairs has been unveiled.

Neutral Dissociation of Superexcited Nitric Oxide Induced by Intense Laser Fields

Superexcited states of NO molecule and their neutral dissociation processes have been studied both experimentally and theoretically. Neutral excited N* and O* atoms are detected by fluorescence spectroscopy for the NO molecule upon interaction with 800 nm intense laser radiation of duration 60 fs and intensity 0.2 PW/cm2. Intense laser pulse causes neutral dissociation of superexcited NO molecule by way of multiphoton excitation, which is equivalent to single photon excitation in the extreme-ultraviolet region by synchrotron radiation. Potential energy curves (PECs) are also built using the calculated superexcited state of NO+. In light of the PECs, direct dissociation and pre-dissociation mechanisms are proposed respectively for the neutral dissociation leading to excited fragments N* and O*.

Neutral Dissociation of Superexcited Oxygen Molecules in Intense Laser Fields

Superexcited states (SESs) of oxygen molecules and their neutral dissociation processes have been studied both experimentally and theoretically using intense femtosecond laser. We find that at the laser intensity of approximately 2 x 10(14) W/cm(2), ultrashort laser pulse causes neutral dissociation of oxygen molecule by way of SESs. The dissociation products are the excited neutral oxygen atoms, which are observed through fluorescence spectroscopy. Laser power dependence of the fluorescence intensity shows that each molecule effectively absorbs an average of ten laser photons. The total energy absorbed is sufficient to stimulate the molecule to many of the SESs. The effect is equivalent to single photon excitation in the extreme-ultraviolet (XUV) region by synchrotron radiation (SR). Morse potential energy curves (PECs) are constructed for the SESs of O(2) molecules. In light of the PECs, predissociation mechanism is proposed for the neutral dissociation. Quasi-classical trajectory (QCT) calculations show that the predissociation time is as short as 100 fs, which is consistent with our experimental measurement using ultrafast pump-probe technique.

Electronic feshbach resonances created in soft x-ray-induced 02 dissociation

When an atom or molecule is ionized by an x-ray, super-excited states can be created that then decay, or autoionize, by ejecting a second electron from the ion. Here we find that autoionization following soft x-ray photoionization of molecular oxygen follows a complex, multi-step, process[1]. By interrupting the autoionization process using a short laser pulse, we show that autoionization cannot occur until the internuclear separation of the fragments is greater than 30 Ångstroms. As the ion and excited neutral atom separate, we directly observe the birth of a new state. We follow the transformation of electronically bound states of the molecular ion into Feshbach resonances of the neutral oxygen atom that are characterized by both positive and negative binding energies. States with negative binding energies have not been predicted or observed in neutral atoms previously.

Ionization and Fragmentation of Alkylphenols by 0.8−1.5 μm Femtosecond Laser Pulses

Ionization and fragmentation were studied on alkylphenols with long alkyl chains (p-(C6H4)(OH)(C(n)H(2n+1)), n = 1,3,5,8,9) and, for reference, on alkylbenzenes ((C6H5)(C(n)H(2n+1)), n = 1,3,5,7,9) by intense femtosecond laser pulses, typically with 43 fs duration at 0.8 microm and 140 fs at 1.3 microm in an intensity range of 10(14) W cm(-2). The major products were the corresponding molecular and C7 fragment ions from the alkylphenols and alkylbenzenes. The molecular ion yields decreased from nearly 1 (n = 1) to 0.3-0.5 (n = 9) when the carbon number in the alkyl chain increased for both excitation wavelengths. Higher yields of the molecular ions were observed at a longer wavelength of 1.3 microm. The long wavelengths in the range of 1.3-1.5 microm were used to determine whether or not -OH absorption had any increase in fragment ions. No effect was observed by vibrational overtone excitation of the -OH group in this wavelength range. Direct dissociation by cation absorption is the most plausible explanation of the present fragmentation results. Other possible mechanisms were discussed, including a statistical model, an effect of electron rescattering, a multiactive electron model, and dissociation from the superexcited state. In the case of cyclohexane, nonresonant wavelength excitation with a pulse of 1.3 microm (150 fs) effectively suppressed fragmentation more than excitation by a resonant but short-duration pulse (0.8 microm, 15 fs).

Rotational structure of a super-excited state of the NO molecule revealed by OODR-multiphoton laser spectroscopy.

The optical-optical double resonance time of flight (OODR-TOF) spectroscopy technique was employed to examine the 65,000-66,500 cm(-1) region of the nitric oxide spectrum. In this region, we detected the following three electronic states: E (2)Sigma(+) (nu = 2) (Rydberg state), B (2)Pi (nu = 23) (valence state), and L (2)Pi (nu = 4) (valence state). The rotational structure analysis of an unexpected band in the red part of the spectra revealed the presence of a new super-excited (2)Sigma(+) Rydberg state at approximately 13.3 eV, which was populated through a three-photon transition from the intermediate A (2)Sigma(+) (nu = 0) state. This super-excited state converges to the NO (a(3)Sigma(+)) ionic state with electronic configuration (1sigma)(2)(2sigma)(2)(3sigma)(2)(4sigma)(2)(5sigma)(2)(1pi)(3)(2pi)(1)(3ssigma)(1).

Dissociation of molecules in intense laser beam

Dissociation of molecules in the strong laser beam at the intensity of 1013–1014 W/cm2 is investigated. Experimentally, the fragmentation of neutral molecules, CH4, C2H4, C4H8; and the disintegration of molecular ions, CH4+ are studied by fluorescence spectroscopy and mass spectroscopy respectively. Some new phenomena, the strong dependence on the laser intensity, the universal dissociation, and the thorough fragmentation, are found in the fragmentation, and cannot be explained by the existing theories, as Coulomb Explosion theory or Re-scattering theory. We have suggested two new theories. The Super-excited State (SES) theory interprets the neutral fragmentation of molecules, which is stimulated to the SES by intense laser pulse. The Morse potential energy surface of the SES shows that either direct dissociation or pre-dissociation can take place in the SESs. Another theory, the theory of Field-assisted Dissociation (FAD) interprets the fragmentation of ionic molecules. According to this theory, the electric field of the laser pulse is involved directly to the dissociation process. QCT calculations for the trajectories moving on the dressed PES are performed. The result shows that the chemical bond which is parallel to the laser field vector undergo dissociation spontaneously. The dissociation takes place around 100 fs, which is in agreement with the ultrafast measurement in the pump-probe experiment.

Relaxation Dynamics of Naphthalene and 1-Aminonaphthalene in Superexcited States

The relaxation dynamics of naphthalene (NPH) and 1-aminonaphthalene (AMN) in superexcited (SE) states has been investigated by pump-probe femtosecond ionization. SE states were produced by two-photon resonant absorption via S1 and S2 states and their time evolution probed by ionization with 800 nm light pulses. A fine-tuning of the pump and probe laser intensities permits us to separate the dynamics of the SE states from that of the S1/S2 intermediate states. SE state relaxation pathways were investigated in the channels of the parent and the major fragment ions. The lifetimes of two relaxation processes were derived from the exponential fit and found to be in the femtosecond and the picosecond time scales. The fast component is attributed to internal conversion (IC) from the SE states to the Rydberg states, whereas the slow one, only observed in the fragment transients, is assigned to fragmentation of the relaxed neutral molecules. The study sheds light on the significant role of SE states dynamics in pump-probe ionization experiments.

Theoretical study of the molecular super-excited states: assignment and theoretical relative intensities of the photofragment yield spectra for F- from F2 ion-pair production

In the framework of quantum defect theory, the photofragment yield spectra for F- from F2 ion-pair production have been studied. The multiple-scattering self-consistent field method and the theoretical time-dependent wave-packet dynamics method are employed. Based on our calculated results, the vibrational resolved assignment and the calculated relative intensity are provided.

Direct observation of super-excited states in methane created by a femtosecond intense laser field

We report, for the first time, a direct observation of the super-excited states of CH4 in femtosecond intense laser fields using a pump (800 nm)–probe (1338 nm) technique. An unambiguous depletion of the CH (A2Δ → X2Π) fluorescence signal as a function of the delay time is attributed to the de-excitation of the super-excited states by the probe laser pulse. The lifetime of the super-excited state is measured to be about 160 fs.

The influence of Rydberg states on the photodissociation of nitrous oxide

The absolute photoabsorption cross section and the photoionization quantum efficiency of nitrous oxide have been measured from the ionization threshold to 21 eV, using a double ion chamber and monochromated synchrotron radiation, and these have enabled the absolute photodissociation cross section to be deduced. Variations in the photoionization quantum efficiency in the vicinity of a Rydberg state have allowed the competition between predissociation and autoionization in the decay of the super-excited state to be investigated. The photodissociation cross section exhibits prominent structure which correlates with known Rydberg series. Dispersed fluorescence techniques have been employed to identify the electronically excited neutral molecular species (N2 B 3Πg and NO B 2Π) formed through photodissociation of N2O. Fluorescence excitation spectra due to the N2 B 3Πg → A 3Σ+u and the NO B 2Π → X 2Π transitions following excitation of N2O have been recorded and the features in these spectra have been compared with the corresponding structure in the photodissociation spectrum. The N2 B 3Πg fragments are produced directly through photodissociation of the parent molecule. However, at low excitation energies the NO B 2Π fragments are formed only through secondary reactions. The five Rydberg series converging onto the N2O+ C 2Σ+g limit are considered in detail because each series exhibits a specific decay mechanism.

Neutral dissociation of methane in the ultra-fast laser pulse

Neutral fragments of methane were performed using femetosecond laser at an intensity of 1013–14 W/cm2. A new mechanism of neutral dissociation is proposed in this work. The methane molecule is excited to super-excited states, in which it would dissociate into neutral fragments. We made Morse type potential energy surfaces for the super-excited molecules. Furthermore, we investigated the dissociation dynamics of the super-excited states by using quasi-classical trajectory (QCT) method. The results thus interpret the neutral dissociation of the methane molecule in the ultra-fast laser pulse.

Super-excited states: Change the channel

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A Theoretical Study of Super-Excited States of F2

In the framework of quantum defect theory, we study super-excited statesof F2 molecules which can dissociate into F+(3P2,1,0) andF−(1S0) ion-pair. Based on our calculation, we present avibrational resolved assignment of the high precision photofragmentyield spectra for F− from the F2 ion-pair production.

Multiphoton dissociation dynamics of BrCl and the BrCl+ cation

Ion imaging methods have enabled identification of three mechanisms by which (79)Br(+) and (35)Cl(+) fragment ions are formed following one-color multiphoton excitation of BrCl molecules in the wavelength range 324.6 > lambda > 311.7 nm. Two-photon excitation within this range populates selected vibrational levels (v'= 0-5) of the [X (2)Pi(1/2)]5ssigma Rydberg state. Absorption of a third photon results in branching between (i) photoionization (i.e. removal of the Rydberg electron-a traditional 2 + 1 REMPI process) and (ii)pi*<--pi excitation within the core, resulting in formation of one or more super-excited states with Omega= 1 and configuration [A (2)Pi(1/2)]5ssigma. The fate of the latter states involves a further branching. They can autoionize (yielding BrCl(+)(X (2)Pi) ions in a wider range of v(+) states than formed by direct 2 + 1 REMPI). Further, one-photon absorption by the parent ions resulting from direct ionization or autoionization leads to formation of Br(+) and (energy permitting) Cl(+) fragment ions. Alternatively, the super-excited molecules can fragment to neutral atoms, one of which is in a Rydberg state. Complementary ab initio calculations lead to the conclusion that the observed [Cl**[(3)P(J)]4s + Br/Br*] products result from direct dissociation of the photo-prepared super-excited states, whereas [Br**[(3)P(J)]5p + Cl/Cl*] product formation involves interaction between the [A (2)Pi(1/2)]5ssigma and [X (2)Pi(1/2)]5psigma Rydberg potentials at extended Br-Cl bond lengths. Absorption of one further photon by the resulting Br** and Cl** Rydberg atoms leads to their ionization, and thus their appearance in the Br(+) and Cl(+) fragment ion images.

Explosive photodissociation of methane induced by ultrafast intense laser

A new type of molecular fragmentation induced by femtosecond intense laser at the intensity of 2 x 10(14) W/cm2 is reported. For the parent molecule of methane, ethylene, n-butane, and 1-butene, fluorescence from H (n = 3-->2), CH (A 2Delta, B 2Sigma-, and C 2Sigma+-->X 2Pi), or C2 (d 3Pi g-->a 3Pi u) is observed in the spectrum. It shows that the fragmentation is a universal property of neutral molecule in the intense laser field. Unlike breaking only one or two chemical bonds in conventional UV photodissociation, the fragmentation caused by the intense laser undergoes vigorous changes, breaking most of the bonds in the molecule, like an explosion. The fragments are neutral species and cannot be produced through Coulomb explosion of multiply charged ion. The laser power dependence of CH (A-->X) emission of methane on a log-log scale has a slope of 10 +/- 1. The fragmentation is thus explained as multiple channel dissociation of the superexcited state of parent molecule, which is created by multiphoton excitation.

ION PAIR DISSOCIATION: Spectroscopy and Dynamics

Ion pair dissociation processes may be studied using coherent vacuum ultraviolet laser sources in a manner entirely analogous to photoelectron spectroscopy, albeit with the anion playing the role of a heavy electron. If the excitation energy is above the dissociation energy and the kinetic energy of the fragment is measured using ion imaging, this approach is termed ion pair imaging spectroscopy (IPIS) and is related to conventional photoelectron spectroscopy. If the excitation energy is just below the dissociation energy and pulsed-field dissociation is employed, this approach is analogous to mass analyzed threshold ionization (MATI) spectroscopy and is termed threshold ion pair production spectroscopy (TIPPS). These approaches provide a novel means of investigating ion thermochemistry and spectroscopy and superexcited state decay dynamics at high resolution.

Photoelectron imaging following 2 + 1 multiphoton excitation of HBr

The photodissociation and photoionization dynamics of HBr via low-n Rydberg and ion-pair states was studied by using 2 + 1 REMPI spectroscopy and velocity map imaging of photoelectrons. Two-photon excitation at about 9.4-10 eV was used to prepare rotationally selected excited states. Following absorption of the third photon the unperturbed F (1)Delta(2) and i (3)Delta(2) states ionize directly into the ground vibrational state of the molecular ion according to the Franck-Condon principle and upon preservation of the ion core. In case of the V (1)Sigma(+)(0(+)) ion-pair state and the perturbed E (1)Sigma(+)(0(+)), g (3)Sigma(-)(0(+)), and H (1)Sigma(+)(0(+)) Rydberg states the absorption of the third photon additionally results in a long vibrational progression of HBr(+) in the X (2)Pi state as well as formation of electronically excited atomic photofragments. The vibrational excitation of the molecular ion is explained by autoionization of repulsive superexcited states into the ground state of the molecular ion. In contrast to HCl, the perturbed Rydberg states of HBr show strong participation of the direct ionization process, with ionic core preservation.

Low-Energy Electron-Stimulated Luminescence of Thin H2O and D2O Layers on Pt(111)

The electron-stimulated luminescence (ESL) from amorphous solid water and crystalline ice films deposited on Pt(111) at 100 K is investigated as a function of the film thickness, incident electron energy (5-1000 eV), isotopic composition, and film structure. The ESL emission spectrum has a characteristic double-peaked shape that has been attributed to a transition between a superexcited state (C) and the dissociative, first excited state (A) in water: C --> A. Comparing the electron-stimulated luminescence and O2 electron-stimulated desorption (ESD) yields versus incident electron energy, we find the ESL threshold is approximately 3 eV higher than the O2 ESD threshold, which is close to the center of the emission spectrum near 400 nm and supports the C --> A assignment for the ESL. For thin films, radiative and nonradiative interactions with the substrate tend to quench the luminescence. The luminescence yield increases with coverage since the interactions with the substrate become less important. The ESL yield from D2O is approximately 4-5 times higher than that from H2O. With use of layered films of H2O and D2O, this sizable isotopic effect on the ESL is exploited to spatially profile the luminescence emission within the ASW films. These experiments show that most of the luminescence is emitted from within the penetration depth of the incident electron. However, the results depend on the order of the isotopes in the film and can be modeled by assuming some migration of the electronically excited states within the film. The ESL is very sensitive to defects and structural changes in solid water, and the emission yield is significantly higher from amorphous films than from crystalline ice.