Absorption Band System Research Articles

Observation of the electronic band system 23Σg--a3Πu of C2 in the vacuum ultraviolet region.

A systematic spectroscopic study of the dicarbon molecule C2 has important applications in various research fields, such as astrochemistry and combustion. In the short vacuum ultraviolet (VUV) wavelength region, recent theoretical calculations have predicted many absorption band systems of C2, but only few of them have been verified experimentally yet. In this work, we employed a tunable VUV laser radiation source based on the two-photon resonance-enhanced four-wave mixing method and a time-of-flight mass spectrometer to investigate the absorption bands of C2 in the VUV range of 64 000-66 000cm-1. The electronic transition 23Σg-(v')-a3Πu(v″) of C2 has been observed and identified experimentally for the first time. The term value Te for the 23Σg- state is determined to be 66 389.9 ± 0.5cm-1 above the ground state X1Σg+, and the vibrational and rotational constants are also determined. The experimentally measured spectroscopic parameters in this study are in excellent agreement with the theoretical results based on high-level abinitio calculations.

Reinvestigation of the Herzberg-Lagerqvist-Malmberg transitions of C2 in vacuum ultraviolet region and the implications for astronomical observations.

The dicarbon radical, C2, is one of the most abundant molecules in the universe, and has been widely observed in various energetic environments. Even though numerous experimental and theoretical investigations on C2 have been done during the last two centuries, spectroscopic study of C2 in vacuum ultraviolet (VUV) region has been rare. The only three known absorption band systems in VUV region were identified by Herzberg and co-workers in 1969 by VUV spectrograph, namely the electronic transitions F1Πu(v')-X1Σg +(v″), f3Σg -(v')-a3Πu(v″) and g3Δg(v')-a3Πu(v″) (Herzberg-Lagerqvist-Malmberg transitions). In this study, we employ a two-photon resonance-enhanced four-wave mixing based VUV laser source and a time-of-flight mass spectrometer for reinvestigating the above three electronic transitions of C2 through a resonant (1VUV + 1'UV) photoionization scheme. Besides those vibronic transitions as identified by Herzberg and co-workers, many more absorption bands belonging to the electronic transitions f3Σg -(v')-a3Πu(v″) and g3Δg(v')-a3Πu(v″) are observed with their spectroscopic parameters determined. The rather astrophysically important F1Πu state is not observed here by the resonant (1VUV + 1'UV) photoionization scheme, which must be due to its fast predissociation process. Instead, our study shows that the vibronic band g3Δg(v' = 2)-a3Πu(v″ = 0) exactly overlaps with F1Πu(v' = 0)-X1Σg +(v″ = 0), which was not realized in previous studies. The potential implications of these findings to astronomical observations are discussed.

Electronic spectroscopy and photophysics of calix[4]azulene

Calix[4]azulene is a non-alternant aromatic calixarene composed of four azulene chromophores linked by methylene groups. Its photochemical stability, photophysical properties and an analysis of its electronic spectra are reported using monomeric azulene as a known reference standard. The molecule is stable when excited in its visible and near uv absorptions and, unlike azulene, produces no measurable “anti-Kasha” fluorescence when excited to its second excited singlet state, S2. This lack of fluorescence places the lifetime of the initially excited, photochemically stable S2 species at less than 1 picosecond. A significant, bathochromic shift of the S2 absorption band system in the calixarene, and the appearance of an additional weak, broad absorption immediately to the red signals significant intramolecular chromophore interaction. Femtosecond transient absorption spectroscopy using excitation in this red-shifted tail of the S2 band system reveals a very weak transient signal most of which decays within one ps, but with suggestions of a slightly longer-lived underlying component. No longer-lived T1 triplet transient is observed. A complete analysis of the data using monomeric azulene as a reference suggests, following elimination of several alternate mechanisms, that the initially excited S2 species may be relaxing via a novel singlet-singlet fission process.

Cavity ring down spectroscopy of cold neutral phenanthrene and phenanthridine in supersonic jets

Here we study the effect of nitrogen insertion on the electronic spectra of polycyclic aromatic hydrocarbon (PAH) molecules measured under laboratory conditions that are relevant for comparison with astronomical data. We examine the case of the substitution of carbon atoms by nitrogen atoms in the benzenoid skeleton of phenanthrene (C14H10), a prototype non-compact PAH. The vibronic transitions of the 21A1(S1)⟵X1A1(S0) electronic absorption band system of neutral phenanthrene (C14H10) and phenanthridine (C13H9N) molecules seeded in a supersonic free jet expansion of argon gas are measured in the 315-345 nm region using the cavity ring down spectroscopy (CRDS) technique. Additional measurements of the absorption spectra of the phenanthrene, phenanthridine, and 1,10-phenanthroline (C12H8N2) molecules isolated in 10-K solid argon matrices are also presented. The results obtained confirm that laboratory spectra of cold, isolated molecules obtained under astrophysically relevant conditions are required for a direct, unambiguous comparison with astronomical observations as well as for the understanding of the physical evolution of the interstellar medium.

SO<sub>2</sub> photolysis as a source for sulfur mass-independent isotope signatures in stratospehric aerosols

Abstract. Signatures of sulfur isotope mass-independent fractionation (S-MIF) have been observed in stratospheric sulfate aerosols deposited in polar ice. The S-MIF signatures are thought to be associated with stratospheric photochemistry following stratospheric volcanic eruptions, but the exact mechanism responsible for the production and preservation of these signatures is debated. In order to identify the origin and the mechanism of preservation for these signatures, a series of laboratory photochemical experiments were carried out to investigate the effect of temperature and added O2 on the S-MIF produced by two absorption band systems of SO2: photolysis in the 190 to 220 nm region and photoexcitation in the 250 to 350 nm region. The SO2 photolysis (SO2 + hν → SO + O) experiments showed S-MIF signals with large 34S/34S fractionations, which increases with decreasing temperature. The overall S-MIF pattern observed for photolysis experiments, including high 34S/34S fractionations, positive mass-independent anomalies in 33S, and negative anomalies in 36S, is consistent with a major contribution from optical isotopologue screening effects and data for stratospheric sulfate aerosols. In contrast, SO2 photoexcitation produced products with positive S-MIF anomalies in both 33S and 36S, which is different from stratospheric sulfate aerosols. SO2 photolysis in the presence of O2 produced SO3 with S-MIF signals, suggesting the transfer of the S-MIF anomalies from SO to SO3 by the SO + O2 + M → SO3 + M reaction. This is supported with energy calculations of stationary points on the SO3 potential energy surfaces, which indicate that this reaction occurs slowly on a single adiabatic surface, but that it can occur more rapidly through intersystem crossing. Based on our experimental results, we estimate a termolecular rate constant on the order of 10−37 cm6 molecule−2 s−1. This rate can explain the preservation of mass independent isotope signatures in stratospheric sulfate aerosols and provides a minor, but important, oxidation pathway for stratospheric SO2. The production and preservation of S-MIF signals requires a high SO2 column density to allow for optical isotopologue screening effects to occur and to generate a large enough signature that it can be preserved. In addition, the SO2 plume must reach an altitude of around 20 to 25 km, where SO2 photolysis becomes a dominant process. These experiments are the first step towards understanding the origin of the sulfur isotope anomalies in stratospheric sulfate aerosols.

2-Methoxyphenyl isocyanate and 2-Methoxyphenyl isothiocyanate: Conformers, vibration structure and multiplet Fermi resonance

IR and Raman spectral measurements in the region 3500–400/50cm−1 have been made for the liquid samples of 2-Methoxyphenyl isocyanate and 2-Methoxyphenyl isothiocyanate. A complete assignment of the measured bands has been proposed as aided by conformational and vibration analyses at B3LYP/6-311++G** level of calculations. Three conformers for 2-Methoxyphenyl isocyanate and two for 2-Methoxyphenyl isothiocyanate have been determined. The tilt of the isocyanate (NCO) and isothiocyanate (NCS) moieties with respect to phenyl ring are in broad agreement with their parents. Stretching mode frequencies of methyl group (–OCH3) in 2-Methoxyphenyl isocyanate have been lowered in the 2900–2800cm−1; deformation asymmetric modes are IR strong and symmetric one Raman strong. In 2-Methoxyphenyl isothiocyanate, a similar pattern is true for stretching modes but deformation asymmetric modes are IR strong and symmetric mode has not been observed. Multiplet absorption band system near 2200cm−1 in 2-Methoxyphenyl isocyanate has been interpreted to be caused by Fermi resonance. A similar pattern in absorption near 2100cm−1 in 2-Methoxyphenyl isothiocyanate but more complex Raman band pattern has also been explained through Fermi resonance from heuristic stand-point. Many Raman modes in 1300–1100cm−1 are intensified apparently owing to isothiocyanate than isocyanate moiety. Phenyl ring breathing mode is shifted to 1040cm−1 as strong Raman; the symmetric stretching mode of O–CH3 near 1023cm−1 as strong absorption.

The near-IR spectrum of Titan modeled with an improved methane line list

We have obtained spatially resolved spectra of Titan in the near-infrared J, H and K bands at a resolving power of ∼5000 using the near-infrared integral field spectrometer (NIFS) on the Gemini North 8 m telescope. Using recent data from the Cassini/Huygens mission on the atmospheric composition and surface and aerosol properties, we develop a multiple-scattering radiative transfer model for the Titan atmosphere. The Titan spectrum at these wavelengths is dominated by absorption due to methane with a series of strong absorption band systems separated by window regions where the surface of Titan can be seen. We use a line-by-line approach to derive the methane absorption coefficients. The methane spectrum is only accurately represented in standard line lists down to ∼2.1 μm. However, by making use of recent laboratory data and modeling of the methane spectrum we are able to construct a new line list that can be used down to 1.3 μm. The new line list allows us to generate spectra that are a good match to the observations at all wavelengths longer than 1.3 μm and allow us to model regions, such as the 1.55 μm window that could not be studied usefully with previous line lists such as HITRAN 2008. We point out the importance of the far-wing line shape of strong methane lines in determining the shape of the methane windows. Line shapes with Lorentzian, and sub-Lorentzian regions are needed to match the shape of the windows, but different shape parameters are needed for the 1.55 μm and 2 μm windows. After the methane lines are modeled our observations are sensitive to additional absorptions, and we use the data in the 1.55 μm region to determine a D/ H ratio of 1.77 ± 0.20 × 10 −4, and a CO mixing ratio of 50 ± 11 ppmv. In the 2 μm window we detect absorption features that can be identified with the ν 5 + 3 ν 6 and 2 ν 3 + 2 ν 6 bands of CH 3D.

Electronic Spectroscopy of UO2Cl2 Isolated in Solid Ar

Laser-induced fluorescence spectra have been recorded for uranyl chloride isolated in a solid Ar matrix. Pulsed excitation was examined using a XeCl excimer laser (308 nm) and a dye laser operating in the 19 500−27 500 cm−1 range. Several absorption and emission band systems were observed. The emission spectra were characterized by a nearly harmonic vibrational progression with a frequency of 840 cm−1 starting at 20 323 cm−1. The electronic absorption spectra were dominated by five harmonic vibrational progressions with frequencies of approximately 710 cm−1. Comparisons with theoretical calculations indicate that all of the transitions observed were associated with the UO2+2 subunit. They involved the promotion of an electron from a bonding orbital to the metal-centered 5fδ and 5fϕ orbitals. Band origins and vibrational constants for five excited states were obtained. Fluorescence was observed from the lowest-energy excited state alone, regardless of the excitation wavelength. The decay curve was found to be biexponential, with characteristic decay lifetimes of 50 and 260 μs.

Vacuum Ultraviolet Absorption Spectrum of Difluoromethane Reinvestigated

ABSTRACT The vacuum ultraviolet (VUV) absorption spectrum of difluoromethane (CH2F2) was studied using synchrotron radiation from the storage ring Indus-1, Indore, India. Spectra were recorded in the spectral region 1050–1500 Å (∼8.3–11.8 eV) at a resolution of 1.5 Å. Three absorption band systems were observed in this region. Overall features observed are in good agreement with previously published work. Some discrepancies in assignments of the observed vibronic bands carried out by previous workers have been resolved. The observed bands have been classified in terms of Rydberg series.

Electronic spectrum of the hydrocarbon cation

A high-resolution gas-phase electronic spectrum has been detected in the 408 nm region using a cw cavity ring-down spectrometer. The absorption band system is observed using a pulsed slit nozzle discharge with a mixture of acetylene in argon. The rotationally resolved spectrum obtained is that of a symmetric top molecule. The determined constants are ν = 24491.0500 (108) and B′′ = 0.14165 (44) cm−1. Consideration of the latter value and ab initio calculations leads to the assignment 3E ← X 3E transition of the linear carbon chain species in the triplet ground electronic state with C3v symmetry.

Theoretical study on the photoabsorption in the Herzberg I band system of the O 2 molecule

The Herzberg I band system of the oxygen molecule is electric-dipole forbidden and its absorption strength has been explained by intensity borrowing models which include the spin–orbit (SO) and L-uncoupling (RO) interactions as perturbations. We employed three different levels of theoretical models to evaluate these two interactions, and obtained the rotational and vibronic absorption strengths using the ab initio method. The first model calculates the transition moments induced by the SO interaction variationally with the SO configuration interaction method (SOCI), and uses the first-order perturbation theory for the RO interaction, and is called SOCI. The second is based on the first-order perturbation theory for both the SO and RO interactions, and is called Pert(Full). The last is a limited version of Pert(Full), in that the first-order perturbation wavefunction for the initial and final state is represented by only one dominant basis, namely the 1 3Π g and B 3 Σ u - state, respectively, as originally used by England et al. [Can. J. Phys. 74 (1996) 185], and is called Pert(England). The vibronic oscillator strengths calculated by these three models were in good agreement with the experimental values. As for the integrated rotational linestrengths, the SOCI and Pert(Full) models reproduced the experimental results very well, however the Pert(England) model did not give satisfactory results. Since the Pert(England) model takes only the 1 3Π g and B 3 Σ u - states into consideration, it cannot contain the complicated configuration interactions with highly excited states induced by the SO and RO interaction, which plays an important role for calculating the delicate integrated rotational linestrength. This result suggests that the configuration interaction with highly excited states due to some perturbations cannot be neglected in the case of very weak absorption band systems.

Solid-state absorption and luminescence spectroscopy of nitronyl nitroxide radicals

Low-temperature absorption and luminescence spectra of three crystalline nitronyl nitroxides are presented. For 2-(2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide and 2-(2-benzimidazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide, luminescence with resolved vibronic structure is observed between 700 nm and 1100 nm. The modes forming the resolved structure have frequencies of approximately 600 cm−1 and 1400 cm−1. They are characterized by Raman spectroscopy and electronic structure calculations. The intensity distribution within the vibronic progressions indicates small structural changes between the ground and emitting states. The lowest-energy absorption bands observed between 450 nm and 700 nm of the two luminescent compounds and of 2-cyano-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide also show resolved structure, but the overall width of these absorption band systems is larger by a factor of 2 than that of the luminescence spectra. This difference is rationalized in terms of overlapping electronic transitions to at least two excited states arising from the (SOMO − 1)1(SOMO)2 and (SOMO)0(SOMO + 1)1 electron configurations, supported by transition energies and oscillator strengths from density functional calculations.

Vacuum Ultraviolet Spectroscopy of the Carbon Molecule C3 in Matrix Isolated State: Experiment and Theory

Carbon molecules were produced by evaporation of graphite and matrix-isolated in solid neon and argon. Using synchrotron radiation, the absorption spectra of the carbon clusters were recorded from 1100 to 5600 A, and the fluorescence or phosphorescence spectra were recorded from 1200 to 9000 A. We observed an intense, broad absorption band system centered at around 1600 A in neon and 1700 A in argon. By measuring the excitation spectrum of the a 3Πu → X 1Σ+g phosphorescence and checking the intensity correlation with the known 1Πu ← X 1Σ+g absorption band, we could show that the observed VUV band system is the allowed 1Σ+u ← X 1Σ+g electronic transition of the C3 molecule. At the blue and the red side of the band system, distinct progressions can be observed which most likely correspond to a symmetric stretch of about ν1 ≈ 1100 cm-1 and a bending mode of about ν3 ≈ 550 cm-1, respectively. In the band center, however, a complicated superposition of several vibrational progressions appears indicating that b...

High resolution analysis of the complex symmetric CF3 stretching chromophore absorption in CF3I

We report a detailed high resolution rovibrational analysis of the ν1 fundamental absorption band system of CF3I, which appears as a very complex resonance polyad. Pulsed slit jet diode laser spectra (instrumental bandwidth ⩽25 MHz) of CF3I (neat and seeded in He) at rotational temperatures of about 7, 30, and 70 K have been obtained. The low J, K part of the observed transitions can be fitted by a model including the anharmonically coupled levels ν1(ν̃0=1076.0551 cm−1), 2ν50(ν̃0=1079.6732 cm−1) and ν3+3ν6±3(ν̃0=1072.3884 cm−1), and the local Coriolis perturber ν3+3ν6+1(ν̃0=1074.1845 cm−1). The level ν3+3ν6+3 was observed and included for the first time, and an additional anharmonic coupling between the levels ν3+3ν6+3 and ν3+3ν6−3 proved relevant. In order to analyze the higher J, K regions, perturbations arising from 2ν5±2 turned out to be important. In certain regions of the spectra, effects due to nuclear quadrupole interaction are clearly visible. We report new sets of parameters for a low J, K model and for an extended model including the perturber 2ν5±2. The results are discussed in relation to infrared multiphoton excitation of CF3I, infrared absorption in the atmospheric window and intramolecular vibrational redistribution.

The Near Ultraviolet Band System of Singlet Methylene

In a classic paper by G. Herzberg and J. W. C. Johns entitled “The Spectrum and Structure of Singlet CH2” (Proc. Roy. Soc. A295, 107–128 (1966)) the analysis of the b̃1B1←ã1A1 red absorption band system of CH2 is discussed in detail for the first time. In addition to that band system the observation of a fragment of a weak near ultraviolet absorption band system is reported. The three observed bands of the system could not be vibrationally assigned or rotationally analyzed but it was pointed out that they probably involve absorption into the second excited singlet state, c̃1A1. We show this supposition to be true here by simulation. In order to simulate the spectrum we have calculated ab initio the c̃–ã and c̃–b̃ transition moment surfaces and used the MORBID and RENNER program systems with previously determined potential energy surfaces for the ã, b̃, and c̃ states in a calculation of the energy levels and wavefunctions. We find that the three bands seen by Herzberg and Johns are part of the c̃←(ã/b̃) system but that all of the bands of the system above about 31 000 cm−1 are missing as a result of c̃ state predissociation. We vibrationally assign the bands but the weakness of the spectrum, and the presence of perturbations, make it impossible for us to analyze the rotational structure fully. Further experimental and theoretical studies are suggested.

On the Correlation between CO Absorption and Far‐Ultraviolet Nonlinear Extinction toward Galactic OB Stars

A sample of 59 sight lines to reddened Galactic OB stars was examined for correlations of the strength of the CO Fourth Positive (A 1 � − X 1 � + ) absorption band system with the ultraviolet interstellar extinction curve parameters. We used archival high-dispersion NEWSIPS IUE spectra to measure the CO absorption for comparison to parametric fits of the extinction curves from the literature. A strong correlation with the non-linear far-UV curvature term was found with greater absorption, normalized to E(B − V ), being associated with more curvature. A weaker trend with the linear extinction term was also found. Mechanisms for enhancing CO in dust environments exhibiting high non-linear curvature are discussed. Subject headings: Dust, extinction — ISM: abundances — ISM: molecules — ultraviolet: ISM

Spectroscopy and photophysics of 1-phenylisatin and oxindole

The absorption and photoluminescence characteristics of 1-phenylisatin (PI) and oxindole (OI) were studied in different solvents at 300 K. In the basic solution a new absorption band system found for PI has been attributed to a structural change leading to formation of new species. Dual fluorescence for PI and OI in different solvents was explained as normal 1B 2u → 1A 1g transition and the other well-structured band as an anomalous band. This large Stokes shifted anomalous band of PI in water and alcohol was explained as fluorescence emission from a state formed due to solute–solvent relaxation producing a sort of critical inversion. Studies in different solvents including a β-cyclodextrin solution revealed that in suitable active medium PI has all the properties to be potential laser dye.

Quantitative IR spectroscopic determination of the component H2O in staurolite

Staurolite exhibits a complex IR absorption band system in the range of 3750 to 3250 cm (super -1) due to OH stretching vibrations. The bands show a strong pleochroism with nearly no absorbance in spectra with E//b and strong (intermediate) absorbance in spectra with E//a (E//c). The total integrated absorbance was determined from spectra of (010) staurolite plates. These spectra were polarized with E//a and E//c. The total integrated molar absorption coefficient (epsilon (sub i.tot) = 83000+ or -5000 l.mol H 2 O-1.cm (super -2) ) was calculated using H 2 O contents as determined from independent physico-chemical measurements (Karl-Fischer titration). Based on Lambert-Beer9s law, the content of the component H 2 O in staurolite is obtained with a relative standard deviation of 9% (1sigma ) using this epsilon (sub i.tot) value and by measuring the total integrated absorbance of the whole band system from polarized spectra of a (010) staurolite plate. The quantitative IR data on staurolite are in good agreement with the IR absorption calibrations for water in minerals by Paterson (1982) and Libowitzky & Rossman (1997).

Remarkable substituent effects on the photophysics of Pt(4′-X-trpy)Cl + systems (trpy = 2,2′; 6′,2″-terpyridine)

In view of the interest in probing the binding interactions that occur between platinum complexes and biological macromolecules, the aim of this work has been to develop systems that exhibit enhanced excited-state lifetimes and emission yields in fluid solution. The investigation focuses on a series of complexes of the type Pt(4′-X-T)Cl + where 4′-X-T denotes a 4′-substituted derivative of 2,2′; 6′,2″-terpyridine. In all cases the counterion is the non-coordinating ion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate. The substituents employed include electron-withdrawing groups like CN and SO 2Me as well as electron-donating groups like SMe and NMe 2. Within the series of complexes, the first reduction wave ranges over about 0.7 V in DMF. Although the process probably entails ligand reduction, the acceptor orbital appears to have some platinum 6p z character. Even though electron-donating substituents destabilize the reduced form of the ligand, all substituents induce a red-shift in the charge-transfer (CT) absorption band system that occurs around 400 nm. Furthermore, there is generally an increase in the CT absorption intensity, the emission lifetime and the emission quantum yield in methylene chloride. Thus, at room temperature, the complex with the terpyridine ligand itself is a very poor emitter with an emission lifetime of 10 ns or less, while the Pt(4′-SMe-T)Cl + and Pt(4′-NMe 2-T)Cl + systems exhibit lifetimes of 140 ns and 1.9 μs, respectively. With the electron-donating substituents in particular, the lifetime enhancement reflects a configuration interaction between the original CT state and an intraligand charge-transfer excited state. Substituents also influence a thermally activated pathway to radiationless decay.

Study of the electronic and vibrational spectra of 2-chloro-6-methoxypyridine

The ultraviolet absorption spectrum in the region 340–190 nm in the vapour state as well as in solutions and the fluorescence and phosphorescence spectra in ethanol solution at 77 K have been reported for 2-chloro-6-methoxypyridine. Two systems of absorption bands corresponding to π→ π* transitions II and III are identified and their oscillator strengths have been calculated. The dipole moments in the excited states associated with these transitions have been determined. The laser Raman spectrum of the molecule in the liquid state and the infrared absorption spectrum in the region 4000–200 cm −1 in the liquid and solution phases are also measured, and a vibrational assignment of the observed frequencies is given. A correlation of the excited state fundamental frequencies observed in the UV absorption spectrum with the Raman or infrared frequencies is obtained.