Vibrational Band Structure Research Articles

Photo-acoustic measurements of gas and aerosol absorption with diode lasers

The results of designing multipurpose high-sensitive photo-acoustic (PA) detectors and their application to high-resolution diode laser spectroscopy of molecular gases, gas analysis, and aerosol absorption measurements are summarized in this paper. The hardware and software of the diode laser spectrometer with a Helmholtz resonant PA detector providing an absorption sensitivity limit of better than 10 −7 W m −1 Hz −1/2 are described. A procedure is proposed for an experiment involving the measurements of the rotational structure of hot vibrational bands of molecules. The results of the application of the nonresonant PA cell with temporal resolution of signals to measurements of weak nonresonant absorption of gases and soot aerosols are presented, and the possibility of creating a broad-band PA laser diode aerosol-meter is discussed.

Adsorption and diffusion on a stepped surface: atomic hydrogen on Pt(211).

We present density functional theory calculations for atomic hydrogen interacting with a stepped surface, the Pt(211) surface. The calculations have been performed at the generalized gradient approximation level, using a slab representation of the surface. This is the state-of-the-art method for calculating the interaction of atoms or molecules with metal surfaces, nevertheless only few studies have used it to study atoms or molecules interacting with stepped surfaces, and none, to the best of our knowledge, have considered hydrogen interacting with stepped platinum surfaces. Our goal has been to initiate a systematic study of this topic. We have calculated the full three-dimensional potential energy surface (PES) for the H/Pt(211) system together with the vibrational band structure and vibrational eigenfunctions of H. A deep global minimum of the PES is found for bridge-bonded hydrogen on the step edge, in agreement with experimental results for the similar H/Pt(533) system. All the local vibrational excitations at the global minimum have been identified, and this will serve as a helpful guide to the interpretation of future experiments on this (or similar) system(s). Furthermore, from the calculated PES and vibrational band structure, we identify a number of consequences for the interpretation or modelling of diffusion experiments studying the coverage and directional dependence of atomic hydrogen diffusion on stepped platinum surfaces.

Determination of the rotational temperature and uncertainties

Currently there exist many methods how to determine the plasma parameters from molecular spectra. The most widely used method is concerned with the determination of the neutral gas temperature by the pyrometric curves. In case at least one branch in the vibrational band is well distinguished, the Knaus and Caye method is possible to apply. The Janin and Eyraurde method can be used when the vibrational band structure is closed into the simple singlet structure. The statistic and the accuracy of rotational temperature determination must not be omitted. At times this procedure could be difficult since we must know all the measuring method details and be conversant of all the aspects affecting the experimental data. This article describes the methods of rotational temperature determination mentioned above and introduces the analysis sources of uncertainties during the measuring and data processing. The couplings coming from the various sources and their combinations are also presented. The interest of this article lies with their usage in the spectroscopy pointing out that systematic analysis is only one way to correctly interpret the measured data.

A monomers-in-dimers model for carboxylic acid dimers

The OH stretching fundamental band system of carboxylic acid dimers is studied using acetic acid and its isotopomers as a model system. Comparing experimental jet spectra with multidimensional quantum mechanical calculations the origin of the extremely broad vibrational band structure (Δν̃≈800 cm−1) is found in strong anharmonic resonances involving the OH stretching vibration. Within an adiabatic picture of hydrogen bonding a new monomers-in-dimers model allows to analyze the observed vibrational band structure in terms of the anharmonic quantum dynamics of the CO2H functional group. The results are discussed in terms of the time-dependent population dynamics and its implications for the mode-specificity of the vibrational predissociation of the hydrogen bonds. On a subpicosecond time scale the intramolecular vibrational energy redistribution of the dimer remains effectively localized within the six-dimensional manifold of the internal vibrations of the carboxyl group, conserving its local CS symmetry.

The Real Time Diagnostics of Combustion Characteristics of Solid Propellant by Remote Sensing FTIR System

The flame infrared emission spectra between 4700 and 800 cm−1 of nitroguanidine solid propellant combustion were recorded using a real time, remote sensing Fourier transform infrared (FTIR) spectrometer with a spectral resolution of 2 cm−1. The combustion flame temperature of this high intensity IR propellant combustion was studied via the fine structure of the rotational vibrational emission bands of HF (4400∼3950 cm−1) and HCl (2900∼2400 cm−1). This study yielded the flame temperatures of nitroguanidine solid propellant at different burning times. The combustion products, such as HF and HCl, were determined simultaneously and quantitatively as well. Results showed that remote sensing FTIR emission spectroscopy is a very reliable, sensitive, and rapid method for measuring propellant combustion flame temperature and product concentrations, and it could be used in combustion diagnostics for a variety of applications.

Calculating Seebeck Coefficients for Arbitrary Temperature Gradients

ABSTRACTA novel computational scheme has been used to predict the electric potentials generated by arbitrary temperature gradients in semiconductor materials. Written in object-oriented code, the Discrete State Simulation (DSS) is a coupled cellular automata simulator that builds upon the objects and rules of quantum mechanics. The DSS represents global non-equilibrium processes as patterns that emerge through an ensemble of scattering events that are localized at vibronic nodes. By tracking the energy-momentum-position coordinates of the individual particles that define the vibronic state at a node, the DSS undercuts equilibrium concepts such as temperature. Consequently, the DSS can represent physical systems that are described by more than one temperature or that contain physical features that defy definitions of temperature. Using modified bootstrap sampling algorithms, the DSS depicted (1) shifts in distribution functions induced by external fields and temperature gradients, (2) field-dependent transitions from linear mobility to non-linear mobility, (3) saturation velocities, (4) non-exponential decay functions generated by multiple phonon scattering modes, and (5) charge separations and electric potentials generated by temperature gradients. Ensemble averages were sensitive to the structure of dispersion relations, to the energy of the system, and to quantum coupling strengths. Seebeck coefficients were sensitive to the features of the electronic and the vibrational band structures, and their associated coupling coefficients.

Collective nuclear structure of neutron rich 104–108Mo

The recently extended level structure of 104–108Mo isotopes, with g-, γ- and the possible Kπ = 4+ 2γ phonon band structures have been studied in the interacting boson model-1. The unique features of these Mo isotopes with large quadrupole deformation β2, and the γ-soft vibrational band structures as reflected in the γ–g B(E2) ratios have been reproduced. The evidence for the 2γ-phonon bands in 104,106Mo is discussed.

Resonant-enhanced two photon ionization and mass-analyzed threshold ionization spectroscopy of jet-cooled 2-aminopyridines (2AP–NH2,–NHD,–NDH,–ND2)

Resonantly-enhanced two photon ionization and mass-analyzed threshold ionization (MATI) spectra of 2-aminopyridine (2AP–NH2) and its deuterated analogs have been obtained using two-photon (1+1′) excitation process via S1 intermediate states for ionization. Ionization energies of 2AP–NH2 and 2AP–ND2 are both precisely and accurately determined to be 8.1086±0.0005 and 8.1027±0.0005 eV, respectively. Two geometrical isomers, 2AP–NHD or 2AP–NDH, of which H or D is hydrogen-bonded to the nitrogen atom on the pyridine ring, respectively, are spectroscopically well isolated using the hole-burning spectroscopy in the S1 states. Corresponding ionization energies are thus separately determined to be 8.1067±0.0005 or 8.1048±0.0005 eV for 2AP–NHD or 2AP–NDH, respectively. Vibrational bands of 2-aminopyridine ions associated with various aromatic ring-skeletal modes are identified in the MATI spectra and appropriately assigned with the aid of ab initio calculation. All of the ring-skeletal vibrational frequencies observed in this work become slightly higher than those in the S1 states when the molecules are ionized, consistent with the fact that the S1–S0 excitation is due to π*–π transition. According to ab initio calculation, the amino group is in the molecular plane both in the S1 and D0 states, while it is slightly distorted in the ground state of 2-aminopyridine. Inversion modes in 2AP–ND2 and 2AP–NDH in S1 states are split into two bands due to their strong coupling with the other mode, which is most probably due to torsional motion of the amino group. Strong mode couplings are clearly manifested in interferencelike patterns observed in vibrational band structures of MATI spectra taken via those two bands in S1 states as intermediate states. A new spectroscopic scheme, in which MATI signals are used for obtaining mode-resolved spectra for the intermediate state is introduced. The vibrational band at 911 cm−1 from the S1–S0 origin that has been previously assigned as the inversion mode of 2AP–NH2 is found to actually consist of two closely-spaced different modes giving two clearly-resolved different Franck–Condon active modes in corresponding MATI spectra.

Mean-field analysis of interacting boson models with random interactions

We investigate the origin of the regular features observed in numerical studies of the interacting boson model with random interactions, in particular the dominance of L=0 ground states and the occurrence of vibrational and rotational band structures. It is shown that all of these properties can be interpreted and explained in terms of a Hartree-Bose mean-field analysis, in which different regions of the parameter space are associated with geometric shapes. The same conclusions hold for the vibron model.

The infrared spectrum of the O⋯H⋯O fragment of H5O2+: Ab initio classical molecular dynamics and quantum 4D model calculations

The gas phase IR spectrum of the O⋯H⋯O fragment of H5O2+ and its deuterated analogue are calculated using ab initio classical molecular dynamics based on a MP2 potential energy surface. The assignment of the bands is made in terms of the quantum four-dimensional model calculations of anharmonic frequencies and intensities. Comparing low and high kinetic temperature simulations the importance of anharmonicities of the potential energy surface for understanding the vibrational band structure is highlighted. It is shown that any reasonable simulation of IR spectra of systems with very strong hydrogen bonds has to account for the dipole moment function beyond the linear approximation.

The gas phase L2,3VV Auger electron spectra of chlorine in XCl (X=H, D, Li, Na, K) molecules

The Auger electron spectra (AES) following the chlorine 2p ionization in the gas phase alkali-chlorides as well as in the HCl and DCl molecules were studied theoretically and experimentally. Nonrelativistic ab initio calculations based on quantum chemical methods and the one-center approximation were used to compute the Cl L2,3VV AES. The vibrational band structure in the AES was simulated by full life time vibrational interference (LVI) theory and a more approximate moment method. Calculations were compared with the corresponding experimental electron or photon impact excited spectra. Overall features and changes in the series of the experimental AES are correctly predicted by the theory. For the most intense transitions in these spectra a qualitative explanation of the energies is given on the basis of a model that includes electrostatic interactions, Pauli repulsion, and polarization. This explains that the substantial changes of the spectra with X are due to a R−3 dependence of the electrostatic interaction on the X–Cl bond length. A comparison of the two theoretical methods shows that the LVI vibrational band breadths are well reproduced by the moment method in the AES of HCl, DCl, and LiCl, whereas the moment breadths are underestimated in the AES of NaCl and KCl. The LVI band breadths for HCl, DCl, and KCl are in good agreement with experimental data. In contrast, the experimental spectra for LiCl and NaCl show almost two times broader vibrational bands than predicted by the LVI theory. This contradiction indicates that the LiCl and NaCl vapors are contaminated by the dimer form of these substances. A large contribution of Li2Cl2 (74%) and Na2Cl2 (29%) was measured in the LiCl and NaCl vapors by time-of-flight mass spectroscopy. The Auger peaks in the spectra of the dimers lie very close to the monomer peaks, and thus make it very difficult to distinguish dimer and monomer contributions.

Collective states in nuclei and many-body random interactions

Low-lying collective states in nuclei are investigated in the framework of the interacting boson model using an ensemble of random many-body interactions. It is shown that whenever the number of bosons is sufficiently large compared to the rank of the interactions, the spectral properties are characterized by a dominance of L(P)=0(+) ground states and the occurrence of both vibrational and rotational band structures. This indicates that these features represent a general and robust property of the collective model space.

Band structure from random interactions

The anharmonic vibrator and rotor regions in nuclei are investigated in the framework of the interacting boson model using an ensemble of random one- and two-body interactions. We find a predominance of L(P) = 0(+) ground states, as well as strong evidence for the occurrence of both vibrational and rotational band structures. This remarkable result suggests that such band structures represent a far more general (robust) property of the collective model space than is generally thought.

The Cd 2 and Zn 2 van der Waals dimers revisited. Correction for some molecular potential parameters

The excitation spectrum of Cd 2 van der Waals complexes produced in a free-jet supersonic expansion beam was studied using Ar as carrier gas and a pulsed dye-laser as light source. A well resolved structure of vibrational bands arising from transitions between X0 g +( 1Σ g +) and 0 u +( 3Π u) states were observed. Owing to the properties of the molecular beam we were able to obtain spectra with different progressions originating from higher (v′′≥1) vibrational levels of the ground state. The ‘hot band’ frequencies made possible—for the first time—a direct determination of the ground state parameters of the molecule. We verified our earlier results of Cd 2 molecular spectra that were recently questioned by Helmi et al. [1]. We have also critically revised the previously reported, estimated bond-lengths of Cd 2 and Zn 2, demonstrating an excellent experiment-theory agreement of results of Refs. [2] and [3] and Refs. [4] and [5], respectively.

The N2 + O+ charge‐exchange reaction and the dayglow N2+ emission

Dayglow spectra were recorded by the Arizona Airglow Experiment from the payload bay of the shuttle, STS‐74. These spectra are used to reexamine the role of the prominent N2+ first negative emission from the dayglow thermosphere. Many reports of the N2+ emissions identify problems in validating the intensity of the emission. Also, an extended vibrational and rotational structure of the bands remains unexplained in the historical analysis. These anomalies appear to be due to the charge‐exchange reaction, N2+O+(2D,2P)→N2++O, which is the dominant source of N2+ ions in the sunlit atmosphere at high altitudes. In the present work the N2+ emission was considered to originate from two separate ion sources. First are those emissions originating from ions produced by photoionizaton and electron bombardment; these emissions can be modeled. Second are the emissions originating from ions produced by the charge‐exchange reaction; these emissions cannot be modeled. Synthetic emission profiles due to the first ion source were modeled and subtracted from the observed spectrum, leaving emission profiles resulting from the charge‐exchange ion source. These residual vibrational and rotational profiles were analyzed to retrieve resonance scattering rates for these ions. These scattering rates can be used to estimate the N2+ first negative emission rate expected from the thermosphere with a model of the atmosphere. It is suggested that measurements of the N2+ emission rate can be used to determine the daytime concentration of the oxygen ion, O+(2D,2P). Although the present work appears to resolve the question of the extended vibrational and rotational band structure, it does not help the excess intensity problem significantly. It does point out that O+ must play an important role in intensity problems.

UV absorption cross sections for SO3

Room temperature gas phase absorption cross sections for SO3 have been measured over the wavelength range 195 to 330 nm using a diode array spectrometer. The SO3 spectrum is continuous with weak diffuse vibrational band structure in the 225 to 295 nm region. Atmospheric photolysis rate calculations show that photolysis of SO3 in the 190 to 230 nm actinic window is important in determining the sulfur partitioning in the upper stratosphere.

Excitation Wavelength Dependence of Bacterial Reaction Center Photochemistry. 2. Low-Temperature Measurements and Spectroscopy of Charge Separation

Excitation with spectrally narrow (5 nm), temporally short duration (200 fs) laser pulses at a variety of wavelengths between 848 and 903 nm results in substantial excitation wavelength dependent differences in the evolution of the bacterial reaction center absorbance spectrum both before and after charge separation occurs. The transient holes in the initial electron donor band showed a more resolved vibrational band structure at 20 K, when compared to those of earlier room temperature transient hole-burning experiments (Peloquin, J. M.; Lin, S.; Taguchi, A. K. W.; Woodbury, N. W. J. Phys. Chem. 1995, 99, 1349). The dominant vibrational band observed is at 120 cm-1, in agreement with dynamic measurements of coherent oscillations on this time scale (Vos, M. H.; Rappaport, F.; Lambry, J.-C.; Breton, J.; Martin, J.-L. Nature 1993, 363, 320). At both room temperature and low temperature, there is a distribution of P to P* transition energies due to a distribution of the protein conformations in the ground state. At 20 K, one can also see a distribution of P* to P stimulated emission energies. As might be expected, the barriers to conformational interconversion are more easily crossed at room temperature, resulting in a smaller difference between the mean transition energies of the photoselectable subpopulations on the picosecond time scale relative to those at low temperature. At 20 K, much of this conformational interconversion is apparently lost when exciting near the 0−0 transition wavelength of P. More conformational interconversion appears to take place at low temperature when higher energy excitation is used, implying that P* is vibrationally hot for at least hundreds of femtoseconds following excitation on the blue side of its QY band. Of particular interest is the insensitivity of the overall charge separation kinetics to the selection of ground state conformational subpopulations by different excitation wavelengths. There appears to be very little coupling between the nuclear motion that defines the ground state transition distribution and the charge separation reaction itself. Given the apparently slow rate of vibrational relaxation of some of the excited state modes most strongly coupled to the P to P* transition, the lack of excitation wavelength dependence of the charge separation rate also suggests that these modes are not strongly coupled to the charge separation reaction. What the ground state (and possibly excited state) conformational distributions and interconversions do affect is the kinetic complexity of the absorbance changes in the 800 nm region. Specifically, the extent of involvement of fast, multiexponential decay components in this region depends strongly on the wavelength of excitation.

A two-color (1+1′)+1 multiphoton ionization study of CS2 in the 61 000–65 600 cm−1 energy region

The (1+1′)+1 resonance enhanced multiphoton ionization (REMPI) spectrum of jet-cooled CS2 has been recorded in the 61 000–65 600 cm−1 excitation energy range. Four prominent band groups are observed that can be assigned to Δν2=−1 and Δν2=1 sequences of the two-photon electronically forbidden 4p 1,3Δu←X̃ 1Σ+g transitions. Weak bands to higher energy appear to be associated with the 310, 230 and 110210 bands and corresponding sequence bands. The results show that the upper states are not 3d Rydberg states as has been previously supposed, and are consistent with a recent reinvestigation of the corresponding (3+1) REMPI spectrum. Further experimental information is obtained on the anomalous vibrational band structure of transitions to the 3Δu state. The 201 and 212 bands of the 4p 1Πu←X̃ 1Σ+g transition are also observed, but are much weaker, suggesting that vibronic interactions are less important in this state compared to the 4p 1,3Δu states.

Ionic Association of Lithium Triflate in Glymes, Model Solvents, and High Molecular Weight Poly(Ethylene Oxide)

AbstractIonic association in lithium trifluoromethanesulfonate (triflate) dissolved in glymes, glycols, model solvents, and high molecular weight poly(ethylene oxide) is studied by analysis of vibrational band structure. Surprisingly, the CF3 symmetric deformation mode is shown to be the most accurate measure of ionic association and the reasons for this are discussed. The nature of various associated lithium triflate species as deduced from ab initio calculations is supported by spectral data presented here. A weak interaction of the triflate anion with certain solvent molecules containing an alcohol OH group is observed. Finally the temperature dependence of ionic association in a complex of lithium triflate and high molecular weight poly(ethylene oxide) is described.

Non-linear Raman spectroscopy in gases

Recent progress in high-resolution non-linear coherent Raman spectroscopy in the gas phase is reviewed. An instrumental spectral resolution of less than 100 MHz can be routinely achieved. It is limited by the convoluted linewidths of the lasers used for excitation and is of particular advantage in the gas phase, where the rotational structure of vibrational bands is to be resolved. It also allows the measurement of linewidths and line positions with an accuracy of 60 MHz when an appropriate wavemeter is used for calibration. After a short overview of the various techniques being used, examples of recent results of the evaluation of measurements on nitrogen, oxygen, carbon dioxide and methane will be presented.