NH stretching vibrations of pyrrole clusters studied by infrared cavity ringdown spectroscopy

The IR spectra for various sizes of pyrrole clusters were measured in the NH stretching vibration region by infrared cavity ringdown spectroscopy. The hydrogen-bonded structures and normal modes of the pyrrole clusters were analyzed by a density functional theory calculation of the B3LYP/6-311+G(d,p) level. Two types of pulsed nozzles, a slit and a large pinhole, were used to generate different cluster size distributions in a supersonic jet. A rotational contour analysis of the NH stretching vibration for the monomer revealed that the slit nozzle provides a warmer jet condition than the pinhole one. The IR spectra, measured under the warmer condition, showed the intense bands at 3444, 3392, and 3382 cm(-1), which were assigned to hydrogen-bonded NH stretching vibrations due to the dimer, the trimer, and the tetramer, respectively. On the other hand, the IR spectra measured under a lower temperature condition by a pinhole nozzle showed a broad absorption feature in addition to sharp bands. This broad absorption was reproduced by the sum of two Gaussians peaks at 3400 and 3372 cm(-1) with widths of 30 and 50 cm(-1) (FWHM), respectively. Compared with the spectra of the condensed phase, two bands at 3400 and 3372 cm(-1) were assigned to hydrogen-bonded NH stretching vibrations of larger clusters having liquid-like and solid-like structures, respectively.     

Non-linear effects by continuous wave cavity ringdown spectroscopy in jet-cooled NO2

A new method of high resolution cavity ringdown spectroscopy (CRDS) was recently developed in our laboratory, where a narrow line, continuous wave (CW) single-frequency laser is used instead of a pulsed laser. Here, we will first discuss the main differences between the `traditional' pulsed CRDS and CW-CRDS. Then, we will describe our results exploiting the high intracavity power that can be achieved with CW-CRDS. Using a single-mode Ti:Sa laser, we obtained CRDS spectra where the excitation power of a single cavity mode is close to 20 W. In the virtually collisionless regime of a supersonic slit jet, we observed saturation in some of the weak rovibronic transitions of NO₂ around 796 nm, as evidenced by loss of absorption intensity and formation of Doppler-free Lamb dips. In addition, in coincidence with absorption by these near infrared transitions, an appreciable fluorescence signal was detected in the visible range. According to our interpretation, this fluorescence is from NO₂ levels excited by two photons in a stepwise incoherent process, with a strongly allowed second step. Since the fluorescence spectrum has the same lineshapes as the CRDS absorption spectrum, it seems that the first transition step is the one limiting the overall two-step process. In addition, we also observed very narrow fluorescence features, not coincident with any absorption feature. These must be coherent (non-stepwise), Doppler-free, two-photon transitions. Interesting new questions arise from these preliminary data, and we believe that more measurements of this kind will provide new information about the rovibronic states of NO₂ in the dissociation region.     

Gas phase decomposition and isomerization reactions of 2-pentoxy radicals

The rate of decomposition of 2-pentoxy radical to acetaldehyde and n-propyl radical has been studied in the presence of NO in competition with nitrite formation at and above 200 kPa pressure over the temperature range of 363-413 K. The rate coefficient for the decomposition is given as log(k_la/s^?1) = (14.2 ± 0.4) - (13.8 ± 0.8) kcal mol^?1/RT ln 10. Isomerization of 2-pentoxy radical by 1,5-hydrogen shift has been investigated in the range 279–385 K in competition with the decomposition in a static system, with methyl radicals present in high concentration to ensure trapping of the isomerized free radicals. The rate coefficient for isomerization is given as log(k₃/s^?1) = (11.1 ± 0.7) - (9.5 ± 1.1) kcal mol^?1/RT ln 10. The implications of the results for atmospheric chemistry are discussed.     

Spatial and temporal probing of a laser-induced plasma plume by cavity ringdown spectroscopy

The laser-induced plasma plume of a Ti target in vacuum is probed by the technique of cavity ringdown spectroscopy. A model is developed to perform a forward convolution of atomic absorption line profile measurements. The model accounts for laser-induced plasma characteristics such as anisotropy of the plume and velocity distributions of the ablated particles as well as of the cavity ringdown features such as geometry and time selectivity. The absorption lineshapes of atomic transitions are calculated and discussed for given sets of parameters. Calculated line profiles are fitted to experimental line profiles obtained from nanosecond-laser ablation of the target and provide data about the plume dynamics.     

Nonlinear effects in pulsed cavity ringdown spectroscopy of lithium vapour

We report cavity ringdown spectra of lithium vapour generated in the heat-pipe oven. Evaluation of ringdown decay curves in early and late time windows gives the change of absorption peak values as well as in the line shape profiles. Pronounced dips in the line center occur depending on molecular densities, injected laser pulse energies and chosen time window.     

Study of the morphology of a laser-produced aerosol plume by cavity ringdown laser absorption spectroscopy

Cavity ring-down laser absorption spectroscopy (CRLAS) was applied for the first time to detection and characterization of laser breakdown generated aerosols. The method provided time-resolved morphological information on the aerosol plume, which is of importance in laser ablation (LA) and deposition, in laser-induced breakdown spectroscopy (LIBS) analysis, and in laser ablation inductively coupled plasma (LA-ICP) methods. This method provides sensitive detection of a variety of aerosols produced under ambient conditions. The morphological investigation revealed that the aerosol density has a reproducible pattern as a function of distance from the surface, although its details depend on time, on geometrical parameters and on the surface characteristics.     

Time-resolved infrared spectroscopy of thiopeptide isomerization and hydrogen-bond breaking

The photoisomerization of the protected tetrathioxopeptide Boc-Ala-Gly(=S)-Ala-Aib-OMe was followed using time-resolved infrared spectroscopy in the amide I region in combination with isotope labeling. In acetonitrile at room temperature, approximately half of the molecules are found in a loop conformation, restrained by an intramolecular hydrogen bond, while the other half adopts more extended conformations. UV-excitation of the thioxopeptide unit immediately weakens the intramolecular hydrogen bond. After the molecules have relaxed to the electronic ground state with a 130 ps time-constant, a delayed re-formation of the intramolecular hydrogen bond is observed for molecules returning to the initial trans conformation of the thioamide bond, while the loop structure is permanently broken when the molecules isomerize to the cis conformation.     

Determination of OH number densities outside of a platinum catalyst using cavity ringdown spectroscopy

It is demonstrated that cavity ringdown spectroscopy (CRDS) can be used to probe reaction intermediates desorbing from the surface during a heterogeneous catalytic reaction and provide information valuable in understanding the reaction kinetics. During water formation from H2 and O2, desorbed OH molecules outside of a polycrystalline platinum catalyst were quantified as a function of the relative hydrogen concentration, alphaH2 using CRDS. The temperature of the catalyst was 1500 K, the total pressure was 26 Pa, and the flow was set to 100 sccm. At a distance of 6.5 mm from the Pt catalyst, the maximum OH concentration was found to be 1.5+/-0.2x10(12) cm(-3) at an alphaH2 value of 10%, and the rotational temperature was determined to be 775+/-24 K. The desorbed OH molecules were also probed using laser-induced fluorescence (LIF), and the alphaH2-dependent OH abundance was compared with the CRDS results. The relative concentration of OH probed with LIF appeared to be lower at alphaH2=30-50% compared to what was determined by CRDS. The observed discrepancy is suggested to be due to electronic quenching, as was indicated by a shorter fluorescence lifetime at alphaH2=30% compared to at alphaH2=10%.     

Near-IR cavity ringdown spectroscopy and kinetics of the isomers and conformers of the butyl peroxy radical

Cavity ringdown spectra of butyl peroxy radicals have been obtained for their A-X electronic transition in the near-IR. The radicals were produced by two independent chemical methods, allowing unambiguous assignment of the spectra of the four butyl peroxy isomers with probable conformer assignments also possible for a number of cases. Using the analyzed spectra semiquantatively, isomer specific rate constants for butyl peroxy self-reaction were measured, as was the relative reactivity of the various sorts of H atoms in butane to Cl atom attack.     

The vibrationless A<--X transition of the jet-cooled deuterated methyl peroxy radical CD3O2 by cavity ringdown spectroscopy

The nearly rotationally resolved spectrum of the A (2)A(')<--X (2)A(") 0(0)(0) transition of perdeutero methyl peroxy near 1.35 microm has been studied via pulsed cavity ringdown spectroscopy. Albeit, this is a weak transition, it is possible to observe the spectrum under jet-cooled conditions (approximately 15 K) by combining a source of narrow-bandwidth radiation (approximately 250 MHz) with a supersonic slit-jet expansion incorporating an electric discharge. The near infrared radiation was obtained by using stimulated Raman scattering and a pulsed, nearly Fourier-transform-limited Ti:sapphire amplifier seeded by a scanable cw Ti:sapphire ring laser. The experimental spectrum has been fitted using a model Hamiltonian that includes the rigid body rotation of an asymmetric top and the spin-rotation interaction. An excellent quality fit was obtained resulting in the determination of 15 molecular parameters characterizing the A and X states. Other results reported for CD(3)O(2) include an estimate of the radical concentration and the vibronic transition dipole from the observed absorption intensities. Details about the spectral linewidths are also discussed.     

Gold atoms and dimers on amorphous SiO(2): calculation of optical properties and cavity ringdown spectroscopy measurements

We report on the optical absorption spectra of gold atoms and dimers deposited on amorphous silica in size-selected fashion. Experimental spectra were obtained by cavity ringdown spectroscopy. Issues on soft-landing, fragmentation, and thermal diffusion are discussed on the basis of the experimental results. In parallel, cluster and periodic supercell density functional theory (DFT) calculations were performed to model atoms and dimers trapped on various defect sites of amorphous silica. Optically allowed electronic transitions were calculated, and comparisons with the experimental spectra show that silicon dangling bonds [[triple bond]Si(.-)], nonbridging oxygen [[triple bond]Si-O(.-)], and the silanolate group [[triple bond]Si-O(-)] act as trapping centers for the gold particles. The results are not only important for understanding the chemical bonding of atoms and clusters on oxide surfaces, but they will also be of fundamental interest for photochemical studies of size-selected clusters on surfaces.     

Investigation of ethyl peroxy radical conformers via cavity ringdown spectroscopy of the A-X electronic transition

Both stable conformers, trans (T) and gauche (G), of the ethyl peroxy radical and its perdeutero analogue have been observed via cavity ringdown spectroscopy (CRDS) of the A2A'-X2A' ' electronic transition in the near-IR. Assignments of specific spectral lines to the electronic transition origin (T00), to observed vibrational hot bands, and to the COO bend and the O-O stretch vibrations are given with the help of equation of motion (EOMIP) quantum chemical calculations. In particular, spectral information for the previously unknown/unassigned T conformer of ethyl peroxy is given in this study for the first time and compared to the data for the previously observed G conformer. The conformer assignment is confirmed by an analysis of the partially resolved rotational structures. The electronic origins for the T and G conformers of C2H5O2 are located at 7362(1) and 7592(1) cm-1, respectively.     

Hydrogen-bonded structures for self-aggregates of 2,5-dimethylpyrrole and its binary clusters with pyrrole studied by IR cavity ringdown spectroscopy

N-H···π hydrogen-bonded (H-bonded) structures were studied by applying vibrational spectroscopy to self-aggregate clusters of 2,5-dimethylpyrrole (DMPy) and its binary clusters with pyrrole (Py). The NH stretching vibrations of jet-cooled clusters were observed by IR cavity ringdown spectroscopy. A combination of experiments and density functional theory calculations revealed the stable structures, intermolecular binding energies, and harmonic vibrational frequencies. The IR spectrum of the DMPy self-aggregate clusters was very similar in spectral features to that of the Py clusters in a previous work. The observed NH stretching vibrations at 3505, 3420, 3371, and 3353 cm(-1) are simultaneously red-shifted by ～25 cm(-1) from the Py monomer, dimer, trimer, and tetramer, respectively. Based on a spectral analogy of DMPy with Py, and a consistency of the calculated harmonic frequencies with experiments, the H-bonded structures of the DMPy clusters were determined to be of a T-shape for a dimer and a cyclic for a trimer and a tetramer. For the DMPy-Py binary clusters, we discussed the stability and geometry of the N-H···π interactions in the T-shaped dimer and the cyclic trimer. The binary dimer showed the only single NH stretch at 3419 cm(-1) in the IR spectrum. A vibrational analysis of the H-bonded NH stretches as well as the calculated stabilization energies deduced that only the binary dimer by DMPy as an acceptor and Py as a donor can exist in a supersonic jet. For binary trimers, NH stretches were observed due to both (DMPy)(2)-(Py)(1) and (DMPy)(1)-(Py)(2). They were found to have different vibrational patterns from each other; the former showed three dispersed NH stretches, and the other had two quasi-degenerate NH stretches. Throughout this study, we also considered the intermolecular geometries, such as the H-bond distance and the angle in terms of the methyl group substitution effect.     

Absorption and scattering characterization of airborne microparticulates by a cavity ringdown technique

Nonresonant cavity ringdown laser absorption spectroscopy (CRLAS) was applied for detection and characterization of airborne particulates. Sensitive detection of a variety of aerosols under ambient conditions was achieved. The method provides, for the first time, time-resolved absolute aerosol concentration, with spatial resolution (along a line). The first report on absorption spectroscopy of monodispersed aerosols (in the size range 100–200 nm) is provided, and comparisons are made with the bulk data. The results indicate the possibility of applying CRLAS for selective analysis of aerosols. A new method for estimating the aerosol refraction index is also obtained from the ringdown data.     

Transient 2D-IR spectroscopy of thiopeptide isomerization

Transient 2D-IR spectra have been recorded during the photoreaction of the thioxopeptide Boc-Ala-Gly(S)-Ala-Aib-OMe. We demonstrate the potential of transient 2D-IR spectroscopy to resolve vibrational bands hidden in conventional pump-probe spectra. Different types of transient cross-peak signals are observed, and their information content is discussed by comparison with model spectra.     

Observation of methyl iodide clusters in gas phase by infrared cavity ring-down spectroscopy

Infrared spectra of methyl iodide clusters produced in a supersonic jet have been observed in the C–H stretching region by cavity ring-down spectroscopy. The dependence of the spectra on the mixing ratio of CH₃I versus He and on the stagnation pressure has led to a tentative assignment of the absorption peaks to trimer up to pentamer, based on our previous study with matrix isolation technique (Chem. Phys. Lett. 343 (2001) 185). Ab initio calculations at the MP2 level for the trimer and tetramer have shown that two stable isomers exist for the tetramer whereas only one isomer is found to be stable for the trimer. The tentative assignment of the observed spectra has been in qualitative agreement with the results of the calculations. The structure of each isomer and its photochemical relevance are discussed.     

Complete quality analysis of commercial surface-active products by Fourier-transform near infrared spectroscopy

Using proper calibration data Fourier-transform near infrared spectroscopy is used for developing multivariate calibrations for different analytical determinations routinely used in the surfactants industry. Four products were studied: oleyl-cetyl alcohol polyethoxylated, cocamidopropyl betaine (CAPB), sodium lauryl sulfate (SLS) and nonylphenol polyethoxylated (NPEO). Calibrations for major as well as very low concentrated compounds were achieved and every model was validated through linearity, bias, accuracy and precision tests, showing good results and the viability of NIR spectroscopy as a full quality control method for this products. Duplicate and complete analysis on a single sample takes at most 3 min, requiring neither sample preparation nor the use of reagents. The analytical reference procedures involved in this work represent the typical ones used in the industry and the NIR method shows good results in the analysis of components with weight concentrations less than 1%.     

Observation of the A-x electronic transition of the isomers and conformers of pentyl peroxy radical using cavity ringdown spectroscopy

Cavity ringdown spectra of the A-X electronic transition of all eight isomers of the pentyl peroxy radical are reported. Using the corresponding assignments from previously studied smaller alkyl peroxy radicals, assignments of origin bands are made for the pentyl peroxy isomers including some conformer-specific assignments for bands of a given isomer. Ab initio calculations also were performed to aid in the spectral assignments for neopentyl, t-butyl, and t-pentyl peroxies. In addition to the origins, vibrational bands have also been assigned for some species. Using the analyzed spectra, the relative reactivity of the primary, secondary, and tertiary hydrogen atoms in isopentane could be determined semiquantitatively.     

Observation of the A-X electronic transition of the 1-C3H7O2 and 2-C3H7O2 radicals using cavity ringdown spectroscopy

Cavity ringdown spectra of the A-X electronic transition of the 1-propyl and 2-propyl peroxy radicals are reported. Spectroscopic assignments are facilitated by implementing several production mechanisms, either isomer-specific or not. Assignments of specific spectral lines to particular conformers of a given isomer are suggested. Observations on the temporal decay of the various species are reported.