Degree Of Vibrational Excitation Research Articles

Using Surface-Enhanced Raman Spectroscopy to Probe Surface-Localized Nonthermal Plasma Activation.

Low-temperature, nonthermal plasmas generate a complex environment even when operated in nonreactive gases. Plasma-produced species impinge on exposed surfaces, and their thermalization is highly localized at the surface. Here we present a Raman thermometry approach to quantifying the resulting degree of surface heating. A nanostructured silver substrate is used to enhance the Raman signal and make it easily distinguishable from the background radiation from the plasma. Phenyl phosphonic acid is used as a molecular probe. Even under moderate plasma power and density, we measure a significant degree of vibrational excitation for the phenyl group, corresponding to an increase in surface temperature of ∼80 °C at a plasma density of 2 × 1010 cm-3. This work confirms that surface-localized thermal effects can be quantified in low-temperature plasma processes. Their characterization is needed to improve our understanding of the plasma-induced activation of surface reactions, which is highly relevant for a broad range of plasma-driven processes.

Stark control of multiphoton ionization through Freeman resonances in alkyl iodides.

Multiphoton ionization (MPI) of alkyl iodides (RI, R = CnH2n+1, n = 1-4) has been investigated with femtosecond laser pulses centered at 800 and 400nm along with photoelectron imaging detection. In addition, the ultraviolet (UV)-vacuum ultraviolet (VUV) absorption spectra of gas-phase RIs have been measured in the photon energy range of 5-11eV using the VUV Fourier transform spectrometer at the VUV DESIRS beamline of the synchrotron SOLEIL facility. The use of high-laser-field strengths in matter-radiation interaction generates highly non-linear phenomena, such as the Stark shift effect, which distorts the potential energy surfaces of molecules by varying both the energy of electronic and rovibrational states and their ionization energies. The Stark shift can then generate resonances between intermediate states and an integer number of laser photons of a given wavelength, which are commonly known as Freeman resonances. Here, we study how the molecular structure of linear and branched alkyl iodides affects the UV-VUV absorption spectrum, the MPI process, and the generation of Freeman resonances. The obtained results reveal a dominant resonance in the experiments at 800nm, which counter-intuitively appears at the same photoelectron kinetic energy in the whole alkyl iodide series. The ionization pathways of this resonance strongly involve the 6p(2E3/2) Rydberg state with different degrees of vibrational excitation, revealing an energy compensation effect as the R-chain complexity increases.

Криоспектроскопическое исследование резонансных мультиплетов ν-=SUB=-s-=/SUB=- ~2ν-=SUB=-b-=/SUB=- в молекуле CHF-=SUB=-3-=/SUB=-

The sequences of Fermi resonances vs~2vb in the IR spectrum of a solution of fluoroform (CHF3) in liquefied krypton are investigated. Here vs is the CH stretching vibration, vb is the bending vibration. It is shown that for a correct description of resonance multiplets (polyads) at a high degree of vibrational excitation, it is necessary to use an extended set of spectroscopic parameters. In particular, it is necessary to take into account the dependence of the anharmonic interaction constant asbb on the vibrational quantum numbers. The conclusions are generalized for the arbitrary case of the CH-chromophore CHX3.

FTIR spectroscopy of three rovibrational bands of CO confined in silica aerogel: Molecule-surface collisions in nanoscale volumes

The absorption spectra of carbon monoxide confined in aerogel were recorded using Bruker IFS 125HR Fourier transform spectrometer within 2000–6500 cm−1 spectral range. The influence of degree of vibrational excitation on the line half-width for 1-0, 2-0 and 3-0 bands of CO confined in the same aerogel sample was studied for the first time. It was found that the character of obtained rotational dependencies is identical for all three bands and vibrational excitation insignificantly affects line HWHM.

Conformational landscape and inertial defect of methoxyphenol isomers studied by mm-wave spectroscopy and quantum chemistry calculations.

Because methoxyphenols (MP) are emitted in significant quantities during biomass fires and contribute to the secondary organic aerosols formation which impacts the climate, their gas phase monitoring in the atmosphere is crucial and requires accurate rovibrational cross sections determined with a good knowledge of their ground state (GS) and vibrationally excited state (ES) molecular parameters. Therefore, the rotational spectra of the two isomers, 2-MP (guaïacol) and 4-MP (mequinol), have been measured in absorption and in emission at room temperature using a frequency multiplication chain and a mm-wave Fourier transform chirped-pulse spectrometer, respectively. Guided by quantum chemistry calculations, the conformational landscape has been characterised and the observation of only one rotamer in the spectra of 2-MP and 4-MP has been explained. For 2-MP, the most stable conformation is justified by an intramolecular O-H⋯OCH3 hydrogen-bond which has been characterised by a topology analysis of the electron density. In a global fit including more than 30 000 line assignments, rotational and quartic centrifugal constants of the GS and the three lowest energy ES have been determined allowing to reproduce the millimeter-wave spectra at the experimental accuracy. The same work has been performed on the cis-rotamer of 4-MP highlighting some perturbations marring the fit quality for two vibrationally ES. Finally, the isomeric dependence of the negative inertial defect ΔI agrees with that of the lowest energy out of plane mode ν45, and the variation of ΔI with the degree of vibrational excitation allows a fine estimation of v45 = 1 vibrational wavenumber.

Kinetics of highly vibrationally excited O2(X) molecules in inductively-coupled oxygen plasmas

The high degree of vibrational excitation of O2 ground state molecules recently observed in inductively coupled plasma discharges is investigated experimentally in more detail and interpreted using a detailed self-consistent 0D global kinetic model for oxygen plasmas. Additional experimental results are presented and used to validate the model. The vibrational kinetics considers vibrational levels up to v = 41 and accounts for electron impact excitation and de-excitation (e-V), vibration-to-translation relaxation (V-T) in collisions with O2 molecules and O atoms, vibration-to-vibration energy exchanges (V-V), excitation of electronically excited states, dissociative electron attachment, and electron impact dissociation. Measurements were performed at pressures of 10–80 mTorr (1.33 and 10.67 Pa) and radio frequency (13.56 MHz) powers up to 500 W. The simulation results are compared with the absolute densities in each O2 vibrational level obtained by high sensitivity absorption spectroscopy measurements of the Schumann–Runge bands for O2(X, v = 4–18), O(3P) atom density measurements by two-photon absorption laser induced fluorescence (TALIF) calibrated against Xe, and laser photodetachment measurements of the O− negative ions. The highly excited O2(X, v) distribution exhibits a shape similar to a Treanor-Gordiets distribution, but its origin lies in electron impact e-V collisions and not in V-V up-pumping, in contrast to what happens in all other molecular gases known to date. The relaxation of vibrational quanta is mainly due to V-T energy-transfer collisions with O atoms and to electron impact dissociation of vibrationally excited molecules, e+O2(X, v)→O(3P)+O(3P).

Direct observation of ring-opening dynamics in strong-field ionized selenophene using femtosecond inner-shell absorption spectroscopy.

Femtosecond extreme ultraviolet transient absorption spectroscopy is used to explore strong-field ionization induced dynamics in selenophene (C4H4Se). The dynamics are monitored in real-time from the viewpoint of the Se atom by recording the temporal evolution of element-specific spectral features near the Se 3d inner-shell absorption edge (∼58 eV). The interpretation of the experimental results is supported by first-principles time-dependent density functional theory calculations. The experiments simultaneously capture the instantaneous population of stable molecular ions, the emergence and decay of excited cation states, and the appearance of atomic fragments. The experiments reveal, in particular, insight into the strong-field induced ring-opening dynamics in the selenophene cation, which are traced by the emergence of non-cyclic molecules as well as the liberation of Se+ ions within an overall time scale of approximately 170 fs. We propose that both products may be associated with dynamics on the same electronic surfaces but with different degrees of vibrational excitation. The time-dependent inner-shell absorption features provide direct evidence for a complex relaxation mechanism that may be approximated by a two-step model, whereby the initially prepared, excited cyclic cation decays within τ1 = 80 ± 30 fs into a transient molecular species, which then gives rise to the emergence of bare Se+ and ring-open cations within an additional τ2 = 80 ± 30 fs. The combined experimental and theoretical results suggest a close relationship between σ* excited cation states and the observed ring-opening reactions. The findings demonstrate that the combination of femtosecond time-resolved core-level spectroscopy with ab initio estimates of spectroscopic signatures provide new insights into complex, ultrafast photochemical reactions such as ring-opening dynamics in organic molecules in real-time and with simultaneous sensitivity for electronic and structural rearrangements.

High sensitivity ultra-broad-band absorption spectroscopy of inductively coupled chlorine plasma

We propose a method to measure the densities of vibrationally excited Cl2(v) molecules in levels up to v = 3 in pure chlorine inductively coupled plasmas (ICPs). The absorption continuum of Cl2 in the 250–450 nm spectral range is deconvoluted into the individual components originating from the different vibrational levels of the ground state, using a set of ab initio absorption cross sections. It is shown that gas heating at constant pressure is the major depletion mechanism of the Cl2 feedstock in the plasma. In these line-integrated absorption measurements, the absorption by the hot (and therefore rarefied) Cl2 gas in the reactor centre is masked by the cooler (and therefore denser) Cl2 near the walls. These radial gradients in temperature and density make it difficult to assess the degree of vibrational excitation in the centre of the reactor. The observed line-averaged vibrational distributions, when analyzed taking into account the radial temperature gradient, suggest that vibrational and translational degrees of freedom in the plasma are close to local equilibrium. This can be explained by efficient vibrational-translational (VT) relaxation between Cl2 and Cl atoms. Besides the Cl2(v) absorption band, a weak continuum absorption is observed at shorter wavelengths, and is attributed to photodetachment of Cl− negative ions. Thus, line-integrated densities of negative ions in chlorine plasmas can be directly measured using broad-band absorption spectroscopy.

Interfacing the Ab initio multiple spawning method with electronic structure methods in GAMESS: Photodecay of trans-azomethane.

This work presents a nonadiabatic molecular dynamics study of the nonradiative decay of photoexcited trans-azomethane, using the ab initio multiple spawning (AIMS) program that has been interfaced with the General Atomic and Molecular Electronic Structure System (GAMESS) quantum chemistry package for on-the-fly electronic structure evaluation. The interface strategy is discussed, and the capabilities of the combined programs are demonstrated with a nonadiabatic molecular dynamics study of the nonradiative decay of photoexcited trans-azomethane. Energies, gradients, and nonadiabatic coupling matrix elements were obtained with the state-averaged complete active space self-consistent field method, as implemented in GAMESS. The influence of initial vibrational excitation on the outcome of the photoinduced isomerization is explored. Increased vibrational excitation in the CNNC torsional mode shortens the excited state lifetime. Depending on the degree of vibrational excitation, the excited state lifetime varies from ∼60-200 fs. These short lifetimes are in agreement with time-resolved photoionization mass spectroscopy experiments.

Photofragmentation Dynamics in Solution Probed by Transient IR Absorption Spectroscopy: πσ*-Mediated Bond Cleavage in p-Methylthiophenol and p-Methylthioanisole.

The 267 nm photodissociation dynamics of p-methylthiophenol (p-MePhSH) and p-methylthioanisole (p-MePhSMe) dissolved in CD3CN have been probed by subpicosecond time-resolved broadband infrared spectroscopy. Prompt (τ < 1 ps) S-H bond fission in p-MePhSH is confirmed by monitoring the time-evolution of the parent (S0) bleach and the transient absorption of the p-MePhS products. Vibrational relaxation of the latter occurs on a ∼8.5 ps time scale, and ∼40% of the total radical population undergoes geminate recombination over a ∼150 ps time scale, yielding (mainly) the p-MePhSH(S0) parent. S-Me bond fission following photoexcitation to the S1 state of p-MePhSMe occurs over a much longer timescale, with a rate that is very dependent on the degree of vibrational excitation within S1. The various findings are compared and contrasted with results from complementary gas-phase photofragmentation studies of both molecules, which are shown to provide a valuable starting point for describing the solution-phase dynamics.

Surface vibrational relaxation of N2 studied by CO2 titration with time-resolved quantum cascade laser absorption spectroscopy

A new method for determination of the wall de-excitation probability of vibrationally excited N2 on different surfaces exposed to low-pressure plasmas has been developed. A short dc discharge pulse of only a few milliseconds was applied to a mixture containing 0.05–1% of CO2 in N2 at a pressure of 133 Pa. Due to a nearly resonant fast vibrational transfer between N2(v) and the asymmetric ν3 mode of CO2 the vibrational excitation of these titrating molecules is an image of the degree of vibrational excitation of N2. In the afterglow, the vibrational relaxation of CO2 was monitored in situ using quantum cascade laser absorption spectroscopy. The experimental results were interpreted in terms of a numerical model of non-equilibrium vibrational kinetics in CO2–N2 mixtures. Heterogeneous relaxation was the main quenching process of N2(v) under the conditions of this study, which allowed determination of the value of from the best agreement between the experiment and the model. The new method is suitable for determination in a single plasma pulse with the discharge tube surface pretreated by a low-pressure plasma. The relaxation probability of the first vibrational level of nitrogen γ1 = (1.1 ± 0.15) × 10−3 found for Pyrex and silica is in reasonable agreement with the literature data. Using the new technique the N2(v = 1) quenching probability was measured on TiO2 surface, γ1 = (9 ± 1) × 10−3. A linear enhancement of the N2(v) wall deactivation probability with an increase in the admixture of CO2 was observed for all studied materials. In order to explain this effect, a vibrational energy transfer mechanism between N2(v) and adsorbed CO2 is proposed.

Roaming Dynamics in Formaldehyde-d2 Dissociation

State-resolved photodissociation dynamics of formaldehyde-d(2), i.e., D(2)CO, at energies slightly above the deuterium atom elimination channel have been studied both experimentally and theoretically. The results showed a clear bimodal distribution of energy into molecular photofragments. Substantial translational excitation of products at high rotational levels of CO was observed together with the D(2) cofragment in moderately excited vibrational levels, whereas rather small translational energy release of CO in low rotational levels was matched by a large degree of vibrational excitation in the D(2) molecule. An analogous distribution of energy in two distinct channels has been recently observed under similar conditions in H(2)CO photolysis and attributed to two different dissociation pathways, namely, a pathway via the conventional transition state geometry and the previously unobserved pathway, deemed "roaming". Our experimental and theoretical data indicated that the same two dissociation pathways were responsible for the bimodal energy distribution into the molecular fragments resulting from the photolysis of D(2)CO. Energy partitioning into molecular products was compared between photolysis of H(2)CO and D(2)CO at energies slightly above the H/D atom abstraction threshold.

Formation of the CH fragment in the 193nm photodissociation of CHCl

The CH fragment from the 193 nm photodissociation of CHCl is observed in a molecular beam experiment. This fragment is formed in the higher-energy dissociation pathway, the lower pathway involving formation of CCl. Both the CHCl parent molecule and the CH fragment were detected by laser-induced fluorescence. The 193 nm CHCl absorption cross section was estimated from the reduction of the CHCl signal as a function of the photolysis laser fluence. The CH internal state distribution was derived from the analysis of laser-induced fluorescence spectra of the A-X Deltav=0 sequence. A modest degree of rotational excitation was found in the CH fragment; the most probable rotational level is N=1, but the distribution has a tail extending to N>25. Also observed is a slight preference for formation of Lambda-doublets of A(") symmetry, which appears to increase with increasing rotational angular momentum N. Vibrationally excited CH was observed, and the degree of vibrational excitation was found to be low. The energy available to the photofragments is predominantly released as translational excitation. The preferential formation of A(") Lambda-doublets suggests that dissociation occurs through a nonlinear excited state.

Investigation of H− production by surface interaction of the plasma generated in “Camembert III” reactor via distributed electron cyclotron resonance at 2.45GHz (abstract)

When considering the state-of-the-art on H− ion sources, ions can be produced either by plasma-surface interaction and/or inside the plasma volume. For the production of negative ions by surface ionization, a low work function material is required. For this purpose, cesium has been used in many cases at LBNL, JAEA, KEK, and in other facilities [M. Bacal, Nucl. FusionNUFUAU0029-5515 46, 250 (2006)]. Despite an enhancement in the negative ion production (by a factor of 2.5 in JAEA source), the use of cesium could lead to many drawbacks in the plasma functioning of ITER, for example. An alternative material to cesium could lead to an important improvement for negative ion source.For this purpose, both theoretical and experimental studies must be undertaken. Surface mechanisms have to be taken into account both for creation and loss mechanisms: (i)By recycling the atomic hydrogen into highly vibrationally excited molecular hydrogen via recombinative desorption on specific surfaces (fresh tantalum on surface increases the negative ion density [M. Bacal, A. A. Ivanov, Jr. et al., Rev. Sci. Instrum.RSINAK0034-674810.1063/1.1699456 75, 1699 (2004)] by more than 60%). It has been shown for a rigid substrate model that both the recombination cross section and the degree of vibrational excitation are highly sensitive to the nature of the surface [B. Jackson and D. Lemoine, J. Chem. Phys.JCPSA60021-960610.1063/1.1328041 114, 474 (2001)].(ii)By surface passivation, which could lead to a substantial decrease in H2 (X,v″) wall losses.In order to understand the fundamental mechanisms of surface production and losses, “Camembert III” experimental setup, recently settled in the LPSC laboratory (Grenoble, France) is used. In this experimental structure, hydrogen multidipolar plasma sustained by microwaves (2.45GHz) presents the potential advantage to operate either in a metallic or a conductive chamber. The inner walls could be then frequently coated, by sputtering or chemical vapor deposition techniques, with no opening of the chamber with various materials. First experiments of H− surface production will be performed on target material. Hence, perfectly knowing the target material (in terms of composition and structural and physical properties), H− ion density production near the target surface will be monitored by laser photodetachment [M. Bacal, Rev. Sci. Instrum.RSINAK0034-674810.1063/1.1310362 71, 3981 (2000)]. Even if this diagnostic gives no direct information on high rovibrationally excited level of the H2 molecule, its implementation and use are far less complicated than vuv LIF.

Quantum Effects in an Exoergic, Barrierless Reaction at High Collision Energies

The exoergic Eley-Rideal hydrogen recombination on graphite surfaces is known to produce vibrationally hot product molecules. Realistic quantum scattering calculations at normal incidence over a wide range of collision energies show that the degree of vibrational excitation of the reaction product is a steep decreasing function of the collision energy. This results in selective population of the lower-lying vibrational levels and gives rise to an oscillating structure in the total reaction cross-sections at high energies, which cannot be reproduced by classical and quasi-classical trajectory calculations. An analogous quantum structure appears in the total collision-induced desorption cross-sections and is assigned to selective population of the metastable states of the transient molecular hydrogen.

Effect of the vibrational excitation of CO molecules on the parameters of an RF discharge

A one-dimensional model of an RF discharge in CO-containing gas mixtures is developed. The model takes into account the effect of the degree of vibrational excitation of CO molecules on the structure of the discharge and on its parameters. Experimental data are presented from measurements of the voltage-power characteristics of RF discharges in gas mixtures with different CO contents in the pressure range of 10–100 torr. The model developed is used to calculate the dependence of the root-mean-square discharge voltage on the specific power deposition in an RF discharge under our experimental conditions. The experimental data are compared to the results of numerical simulations. For working gas pressures of about 100 torr, which are typical of the operation of slab CO lasers, the calculated voltage-power characteristics of an RF discharge agree satisfactorily with those obtained experimentally. The theoretical model predicts that the vibrational excitation of CO molecules leads to a redistribution of the RF field in the discharge gap and to an increase in the laser efficiency.

Distinguishing and Quantifying Peptides and Proteins Containingd-Amino Acids by Tandem Mass Spectrometry

Tandem mass spectrometry (MS/MS) utilizing both electron capture dissociation (ECD) and collisionally activated dissociation (CAD) was used to develop a qualitative and quantitative analytical method for chiral analysis of individual amino acid residues in polypeptides. ECD produced a more distinct chiral recognition than CAD, which is attributed to the smaller degree of vibrational excitation in ECD. Several peptide and protein model systems were used in this study, including the smallest known protein, tryptophan cage, a lactoferrin peptide, and the biologically relevant opioid peptide, dermorphin. An adaptation of the kinetic method was used to quantify the degree of separation between fragmentation patterns of stereoisomeric peptides as a function of fragment ion abundances. The obtained calibration scale for relative abundances of d-amino acids in diastereomeric peptide mixtures was accurate to 1% for ECD and to 3-5% for CAD. It was found that separation and quantification of stereoisomers could be advantageously performed by nanoflow reversed-phase liquid chromatography, with the objective of on-line MS/MS limited to stereoisomer identification. This technique shows promise for the analysis of chiral substitution in peptides and proteins, broadening the application area for tandem mass spectrometry.

Molecular beam investigation of hydrogen dissociation on Si(001) and Si(111) surfaces.

The influence of molecular vibrations on the reaction dynamics of H2 on Si(001) as well as isotopic effects have been investigated by means of optical second-harmonic generation and molecular beam techniques. Enhanced dissociation of vibrationally excited H2 on Si(001)2 x 1 has been found corresponding to a reduction of the mean adsorption barrier to 390 meV and 180 meV for nu=1 and nu=2, respectively. The adsorption dynamics of the isotopes H2 and D2 show only small differences in the accessible range of beam energies between 50 meV and 350 meV. They are traced back to different degrees of vibrational excitation and do not point to an important influence of quantum tunneling in crossing the adsorption barrier. The sticking probability of H2 on the 7 x 7-reconstructed Si(111) surface was found to be activated both by H2 kinetic energy and surface temperature in a qualitatively similar fashion as H2/Si(001)2 x 1. Quantitatively, the overall sticking probabilities of H2 on the Si(111) surface are about one order of magnitude lower than on Si(001), the influence of surface temperature is generally stronger.

Dephasing of vibrationally excited molecules at surfaces: CO/Ru(0 0 1)

The v=0→1 fundamental and the v=1→2 hot-band transitions of the C–O stretch vibration of CO chemisorbed on Ru(0 0 1) are investigated as a function of temperature using broadband-IR sum-frequency generation spectroscopy. Although the linewidths are markedly different for the two transitions, we demonstrate that the temperature-dependent contribution to the linewidths from anharmonic coupling to the low-frequency frustrated translation mode (giving rise to pure dephasing) is identical for both transitions. This implies that the anharmonic coupling between the high-frequency and the low-frequency mode is independent of the degree of vibrational excitation.

Energy disposal in the two-photon laser-assisted reaction in xenon and chlorine gas mixtures

The two-photon laser assisted reaction (LAR) in low pressure xenon and chlorine gas mixtures has been studied over a broad range of excitation wavelengths in order to characterize the role of the entrance channel in determining the vibrational state distribution of the reaction products. We measure a high degree of vibrational excitation in the XeCl* product, confirming observations from previous studies of the LAR of Xe+Cl2 collision pairs [Ku et al., J. Phys. Chem. 87, 2989 (1983)], and in distinct contrast with the vibrationally cold excimer observed following laser excitation in molecular beam experiments [Boivineau et al., Chem. Phys. Lett. 128, 528 (1986)]. The mean vibrational energy in the XeCl excimer depends strongly on laser wavelength, increasing with decreasing wavelength. Moreover, an increasing fraction of the available energy from the reaction is observed as vibration in the XeCl* product as the laser is tuned toward shorter wavelengths. The reaction outcomes are interpreted in terms of a selectivity of initial conditions on the ionic potential surface that mediates the reactive collision.