Time-resolved FTIR Emission Spectroscopy Research Articles

Time-resolved FTIR emission spectroscopy of Cu in the 1800–3800 cm−1 region: transitions involving f and g states and oscillator strengths

Time-resolved Fourier-transform spectroscopy was applied to the study of the emission spectra of Cu vapours in a vacuum (10−2 Torr) produced in ablation of a Cu metal target by a high-repetition rate (1.0 kHz) pulsed nanosecond ArF laser (λ = 193 nm, output energy of 15 mJ). The time-resolved infrared emission spectrum of Cu was recorded in the 1800–3800 cm−1 spectral region with a resolution of 0.017 cm−1. The time profiles of the measured lines have maxima at 18–20 µs after a laser shot and display non-exponential decay with a decay time of 5–15 µs. This study reports 17 lines (uncertainty 0.0003–0.018 cm−1) of Cu I not previously observed. This results in seven newly-found levels and revised energy values for 11 known levels (uncertainty 0.01–0.03 cm−1). We also calculate transition probabilities and oscillator strengths for several transitions involving the reported Cu levels.

The Reactions of O(3P) with Terminal Alkenes: The H2CO Channel via 3,2 H-Atom Shift

The step-scan time-resolved FTIR emission spectroscopy is used to characterize systematically the H(2)CO channel for the reactions of O((3)P) with various alkenes. IR emission bands due to the products of CO, CO(2), and H(2)CO have been observed in the spectra. H(2)CO is identified to be the primary reaction product whereas CO and CO(2) are secondary reaction products of O((3)P) with alkenes. A general trend is observed in which the fraction yield of the H(2)CO product increases substantially as the reactant alkene varies from C(2)H(4), C(3)H(6), 1-C(4)H(8), iso-C(4)H(8), to 1-C(5)H(10). The formation mechanism of the H(2)CO is therefore elucidated to arise from a 3,2 H-atom shift followed by breaking of the C(1)-C(2) bond in the initially formed energized diradical RCH(2)CHCH(2)O*. The 3,2 H-atom shift may become the dominant process with the more rapid delocalization of the energy when the hydrocarbon chain of the alkene molecule is lengthened.

Reaction of CH2Cl with O2

The elementary reaction of CH2Cl+O2 in gas phase was investigated by time-resolved FTIR emission spectroscopy. Vibrationally excited products CO (v 4), and CO2 (3, v 7) were observed. The yield ratio of CO/CO2(3) was 72.27. The reaction pathways were studied theoretically at QCISD//UB3LYP/6-311++G(d,p) level. In the beginning of the reaction, CH2Cl radical associated with O2 to form CH2ClOO, followed by removal of the Cl atom to yield another intermediate dioxirane CH2OO. Subsequently, a series of isomerization and decomposition of the CH2OO took place, yielding the final products of CO and CO2. The calculated result was in consistent with the experimental observation.

Time-Resolved FTIR Emission Spectroscopy of Transient Radicals

Fourier Transform spectroscopy with 10−8 second time resolution for recording IR emission spectra has been developed as an efficient means for detecting previously unknown vibrational modes of transient radicals. 193 nm photodissociation of a precursor molecule is used to generate vibrationally excited radicals, from which IR emission is recorded with time and spectral resolution. Assignment of the spectra is performed using information obtained through multiple precursors, isotopic substitution, time dependence of emission intensity, theoretical calculations, and 2-dimensional cross-spectra correlation analysis. The radicals vinyl, cyanovinyl, and OCCN have been studied with many vibrational modes identified.

Infrared Fluorescence, a Versatile Probe in Chemical Dynamics

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Infrared fluorescence, a versatile probe in chemical dynamics

The study of reactive and inelastic energy transfer collision events forms a cornerstone of modern chemical dynamics and kinetics. Infrared fluorescence (IRF) was one of the earliest techniques applied to detailed measurements of molecular states involved in reactions. Results from our laboratory are presented in which state-of-the-art IRF is used to study a broad spectrum of problems in chemical dynamics. The examples include laser-initiated chemical reactions involving molecular elimination of diatomics, competition between reactive and inelastic collision events and intermolecular vibrational energy redistribution dynamics. The prospects for future innovations in the study of chemical dynamics using time-resolved FT-IR emission spectroscopy are discussed.

The Studies of Free Radical Reactions by Using Time-resolved FTIR Emission Spectroscopy

In the past fifteen years，free radical reactions in the gaseous phase have been widely studied by timeresolved fourier transform spectroscopy. Sloan et al. first investigated O（1D）reactions and then Leone and Hancock groups studied O（3P）reactions. Later，Kong，Zhu et al. investigated small radical，including CH，CH2，CH3，C2H3，C2H5，C2H，C3H3 and C3H5，reacting with the molecules of O2，NO，N2O，NO2 . For each reaction，nascent products and primary channels are observed in the IR spectrum. Combining with ab initio theoretical investigations，the reaction mechanisms are almost understood.

Time-resolved Fourier transform infrared study of the 193 nm photolysis of SO 2

The 193 nm photolysis of SO 2 has been studied by monitoring the infrared emission of the vibrationally excited species using time-resolved FTIR emission spectroscopy. Evidence for a direct electronic transition from the C state to the ground state of SO 2 was found in addition to photodissociation following the 193 nm excitation. Relaxation rate constants for the observed vibrationally excited species, SO, SO 2( ν 1), and SO 2( ν 3), were calculated from the experimental decays.

Reaction of vinyl radical with O2 studied by time-resolved infrared emission spectroscopy

Nascent products of C2H3+O2 reaction have been studied by time-resolved FTIR emission spectroscopy. Vibrational excitation of reaction products H2CO, HCO, CO, and CO2 was observed, providing the evidence of existing two exothermic reaction channels: HCO+H2CO and CH3+CO2 for the reaction. The latter channel was firstly observed in the experiment. Simulation of the spectrum of the products reveals that the highest excitation of CO and CO2(ν3) at 5 μs is v=4 and 6, respectively; the branching ratio of the channel HCO+H2CO to the channel CH3+CO2 is estimated to be 17.

Reaction of C 2H with NO and O 2 studied by TR-FTIR emission spectroscopy

Vibrationally excited CO ( v), HCO ( ν 1, ν 2), HNC ( ν 3) and HCN ( ν 3) were observed as products of the reaction of C 2H with NO by using time-resolved FTIR emission spectroscopy. Three exothermic reaction channels leading to HCN+CO, HNC+CO and CN+HCO are identified, verifying an association-elimination reaction mechanism. The nascent products of CO ( v⩽10) and CO 2 ( ν 3, v⩽3) were observed for the reaction of C 2H with O 2. A yield ratio of CO/CO 2 is estimated as 9. The experimental observations suggest that reaction is rapid and forms CO and HCO.

Collisional Deactivation of Highly Vibrationally Excited SO2: A Time-Resolved FTIR Emission Spectroscopy Study

Time-resolved Fourier transform IR emission spectroscopy, capable of 10

UV Photolysis of Oxalyl Chloride and 2,3-butanedione Studied by Time-resolved FTIR

the photodissociation of oxalyl chloride anti 2,3-butanedione at 193nm and 248nm have been studied by time-resolved ftir emission spectroscopy, the ir fluorescence emission was observed and the rotational and vibrational populations ai the co product were obtained. it is suggested that the photolysis mechanism is two rco radicals formed via a fission of c-c bond, followed by a decomposition of the rco intermediate to vibrationally excited co(v) and r radical.

Nanosecond time-resolved FTIR emission spectroscopy: Monitoring the energy distribution of highly vibrationally excited molecules during collisional deactivation

The 10−8 second time resolution in infrared emission spectroscopy has been demonstrated using a Fourier Transform spectrometer paired with a fast HgCdTe detector. The rapid time response of this system has enabled us to measure, with subcollisional period time resolution, the emission spectrum of highly vibrationally excited NO2 molecules during collisional deactivation by room temperature NO2. The greatly improved time resolution of the spectra allows the determination of N(E,t), the instantaneous energy distribution of the ensemble of excited molecules, with virtually no distortion due to collisional averaging. In addition, an improved procedure for extracting optimized N(E,t) from the spectral data makes no prior assumptions about the shape of the energy distribution. It is found that the distribution is well approximated as the sum of a Gaussian function at high vibrational energies and a population at low energies resulting from V–V transfer to bath NO2 molecules. The observation of a Gaussian-like function for the highly excited molecules is consistent with the widely invoked assumption that the step-size function of energy transfer per collision is exponential.

Infrared Emission from the CF3+ NO2Reaction

The reaction of the CF3 radical with NO2 has been investigated by using time-resolved FTIR emission spectroscopy. Strong infrared emission has been attributed to products, CF2O and FNO, excited in the ν2 and ν1 modes, respectively. The direct one-step production pathway 1a is suggested as a major reaction channel: CF3 + NO2 → CF2O + FNO, ΔH°298 = −267 kJ mol-1 (1a); → CF2O + F + NO, ΔH°298 = −31 kJ mol-1 (1b). The rate constant for reaction 1 was measured to be (2.4 ± 0.5) × 10-11 cm3 molecule-1 s-1. The F atom formation pathway 1b is the minor channel: the relative branching ratio of reactions 1a and 1b was estimated as 1 : 0.015. The formation of the FON isomer formed via a five-center intermediate is discussed as a possible carrier of an unidentified emission band near 1880 cm-1. The CF3O + NO reaction was investigated by the same technique. Lower emission intensities from the same reaction products were observed, with proportionally less vibrational excitation in the CF2O product.

A stop-scan interferometer used for time-resolved FTIR emission spectroscopy

The authors describe how a low-cost teaching interferometer has been modified for use as a time-resolved instrument to study the infrared vibrational chemiluminescence of the products from atom-radical reactions and molecular photodissociations. This technique allows the entire temporal evolution of the emission spectrum to be obtained from a single interferometric scan, with spectral and temporal resolutions of 2 cm-1 and 3 mu s respectively. Illustrative results are given for discharge-flow studies of laser initiated atom-radical reactions, where product emission from highly vibrationally excited CO2 and HF is observed, and for emission from vibrationally excited products of the infrared multiple-photon dissociation of halocarbon molecules.

Photofragmentation of acetone at 193 nm: rotational- and vibrational-state distributions of the carbon monoxide fragment by time-resolved FTIR emission spectroscopy

Photofragmentation of acetone at 193 nm is known to produce predominantly two methyl radicals and a CO molecule. Infrared emission is collected for the CO fragment at time delays of 6-30 ..mu..s after the exciting laser pulse by a time-resolved Fourier transform spectrometer. High-resolution vibration-rotation spectra are obtained both under collision-free and rotationally relaxed conditions. The observed vibrational distribution is, for v = 1/2/3, 0.73 /plus minus/ 0.04/0.23 /plus minus/ 0.02/0.04 /plus minus/ 0.01. This distribution fits a temperature of /approximately/ 2030/sub /minus/430//sup +730/ K, in approximate agreement with previous low-resolution results from this laboratory. The observed rotational distribution is highly excited, approximating a temperature of 3360/sub /minus/800//sup +1540/ K. Both the vibrational and rotational distributions can be successfully modeled by using a pure impulsive mechanism for the fragmentation of an initial bent acetyl fragment. Thus, the observed distribution supports previous arguments that the dissociation occurs by a two-step fragmentation mechanism following single-photon excitation.

Rotationally resolved product states of polyatomic photofragmentation by time-resolved FTIR emission: HF elimination from 1,1-CH2CClF at 193 mm

Time-resolved FTIR emission spectroscopy is used to obtain collision-free rotational distributions of the products in a photofragmentation process for the first time. A detailed description of the excimer laser photolysis-FTIR emission method is presented, and results are derived for the internal state distribution of nascent HF eliminated from 1,1-CH2CClF following photoactivation at 193 nm. The HF product is born rotationally excited, with Jmax at least ∼J=15. The vibrational distribution is also highly excited and fits a Boltzmann distribution with a vibrational temperature of 28 000 K. Even though a large barrier to elimination exists, a statistical model for the energy partitioning reproduces the vibrational distribution accurately. In the model used, the full exoergicity is assumed, however, the HF vibrational frequency is chosen to be the value in the transition state, which is one-half of the frequency for free HF. Agreement between the statistical model and the observed rotational distributions is less satisfactory, especially for low J states. The results suggest that some modification of the rotational distribution may occur as the HF and HCCCl products separate during the photofragmentation.

Time-resolved FTIR photofragment emission spectroscopy: HCl vibrational distributions from the 193 nm photolysis of chloroethylenes

With minor modifications to commercial instrumentation, time-resolved FTIR emission spectroscopy is demonstrated for the determination of photofragment internal state distributions. Vinyl chloride and dichloroethylene photolysis at 193 nm serve as test cases and exhibit high signal-to-noise. The vibrational distributions in the first four levels of the HCl photo-product of vinyl chloride and 1,2-trans-dichloroethylene are in good agreement with previous determinations.