Time-resolved Fourier-transform Emission Spectroscopy Research Articles

Time-Resolved Fourier Transform Emission Spectroscopy of CF3Br and CF3CFHCF3 in a Pulsed Electrical Discharge

The environmentally important decomposition of halogenated species CF3Br and CF3CHFCF3 in helium discharge plasma was investigated by time-resolved high-resolution Fourier transform infrared emission spectroscopy. Contrary to classical pyrolysis, a deeper fragmentation of precursors up to atoms and lower molecular species was observed. Excited molecular products CF, CF2 and CF4 achieved the maximal concentration in the afterglow. The high concentration of all these species is in agreement with a kinetic model based on radical chemistry. The non-detectable concentration of CF3 can be connected to its high reactivity and the formation of more stable products, CF4 and CF2, by addition or release of a fluorine atom, respectively. Other products included HF, HBr, CO and cyano compounds that were produced by secondary reactions with traces of water vapor, atmospheric oxygen and nitrogen present in original industrial samples as impurities.

Time-resolved Fourier transform emission spectroscopy of A 2Π–X 2Σ + infrared transition of the CN radical

The A 2Π–X 2Σ +Δ v = −3 bands of the 12C 14N radical have been observed by time-resolved Fourier transform spectroscopy in the 1850–3100 cm −1 region with a wavenumber resolution of 0.025 cm −1. The radical was produced in a pulsed positive column discharge in a cyanogene and helium mixture. Seven bands of v = 0–3, 1–4, 2–5, 3–6, 4–7, 5–8, and 6–9 were analyzed to give the molecular constants of each state by least-squares fitting of 801 lines. The pulsed discharge was found to be efficient for production of CN in the excited A 2Π state. The vibrational excitation temperature was determined to be 6680 ± 835 K and 6757 ± 534 K for the A 2Π and X 2Σ + states, respectively. The population of the A 2Π was found to be 4% of that of the X 2Σ + state in the time after turning off the discharge.

Ultraviolet photodissociation of vinyl iodide: understanding the halogen dependence of photodissociation mechanisms in vinyl halides

The photodissociation of vinyl iodide has been investigated at several wavelengths between 193 and 266 nm using three techniques: time-resolved Fourier transform emission spectroscopy, multiple pass laser absorption spectroscopy, and velocity-mapped ion imaging. The only dissociation channel observed is C-I bond cleavage to produce C2H3 (nu, N) + I (2P(J)) at all wavelengths investigated. Unlike photodissociation of other vinyl halides (C2H3X, X = F, Cl, Br), in which the HX product channel is significant, no HI elimination is observed. The angular and translational energy distributions of I atoms indicate that atomic products arise solely from dissociation on excited states with negligible contribution from internal conversion to the ground state. We derive an upper limit on the C-I bond strength of D0(C2H3-I) < or = 65 kcal mol(-1). The ground-state potential-energy surface of vinyl iodide is explored by ab initio calculations. We present a model in which the highest occupied molecular orbital in vinyl halides has increasing X(np) non-bonding character with increasing halogen mass. This change leads to reduced torsional force around the C-C bond in the excited state. Because the ground-state energy is highest when the CH2 plane is perpendicular to the CHX plane, a reduced torsional force in the excited state correlates with a lower rate for internal conversion compared to excited-state C-X bond fission. This model explains the gradual change in photodissociation mechanisms of vinyl halides from the dominance of internal conversion in vinyl fluoride to the dominance of excited-state dissociation in vinyl iodide.

The Vinyl + NO Reaction: Determining the Products with Time-Resolved Fourier Transform Spectroscopy

We have studied the vinyl + NO reaction using time-resolved Fourier transform emission spectroscopy, complemented by electronic structure and microcanonical RRKM rate coefficient calculations. To unambiguously determine the reaction products, three precursors are used to produce the vinyl radical by laser photolysis: vinyl bromide, methyl vinyl ketone, and vinyl iodide. The emission spectra and theoretical calculations indicate that HCN + CH2O is the only significant product channel for the C2H3 + NO reaction near room temperature, in contradiction to several reports in the literature. Although CO emission is observed when vinyl bromide is used as the precursor, it arises from the reaction of NO with photofragments other than vinyl. This conclusion is supported by the absence of CO emission when vinyl iodide or methyl vinyl ketone is used. Prompt emission from vibrationally excited NO is evidence of the competition between back dissociation and isomerization of the initially formed nitrosoethylene adduct, consistent with previous work on the pressure dependence of this reaction. Our calculations indicate that production of products is dominated by the low energy portion of the energy distribution. The calculation also predicts an upper bound of 0.19% for the branching ratio of the H2CNH + CO channel, which is consistent with our experimental results.

The ro-vibrational spectra of CO molecule resulting from the photolysis of acetone

A time-resolved Fourier-transform emission spectroscopy was used to detect the IR emission of the fragments resulting from the photolysis of acetone by 193 nm excimer laser; a double peak vibrational spectrum in the range of 1950–2250 cm −1 corresponding to the P and R branches of the CO stretching vibration. At 0.3 cm −1 resolution; the ro-vibrational lines appeared for the first three vibrational transitions. The experimental spectra match greatly the simulated one. Using the fitting methods, the populations were found for the first four vibrational levels. The vibrational spectrum and populations of the CO resulting from the photolysis of acetone in helium differ from acetone alone due to the relaxation effect of helium.

Infrared emission accompanying the gas phase recombination of alkyl radicalsElectronic supplementary information (ESI) available: Appendix: Model for emission from the C2H6 recombination product of CH3 radicals. See http://www.rsc.org/suppdata/cp/b3/b303780k/

Infrared emission from nascent products of the methyl radical recombination reaction has been detected using time-resolved Fourier transform emission spectroscopy. Methyl radicals were generated by 193 nm photolysis of acetone, and at low pressures emission from nascent photofragments (CH3 and CO) is found to dominate. At higher pressures an additional emission band is seen in the C–H stretching region, the time dependence of which shows a rapid rise and a slow decay, the latter being dependent on the laser intensity and accurately described by a second-order fit. The emission is assigned to vibrationally excited ethane formed by recombination of methyl radicals, with the decay identified as the recombination rate and the rapid rise as quenching of C2H6 to levels which have no excitation in the emitting mode (the ν5 C–H stretch). The recombination rate constants were determined from fits to the decays together with measurements of the laser fluence, and agree well with previous determinations. The wavelength dependence of the C2H6 emission spectrum is invariant with time, and is always red-shifted with respect to a room-temperature C2H6 absorption spectrum. This can be rationalised in a model which recognises that formation of vibrationally excited C2H6 is considerably slower than quenching, and from which it is found that the average level of excitation per excited ethane molecule reaches a steady-state value after a short induction period. Similar spectra and kinetics were observed with other methyl radical sources, and emission from the products of the ethyl radical recombination/disproportionation reaction was also observed when diethyl ketone was photolysed at 193 nm.

State-to-state rate constants for collision induced energy transfer of electronically excited NH2 with NH3

Collisional energy transfer of NH2 in its electronically excited state à 2A1 is investigated with time-resolved Fourier transform emission spectroscopy. NH2 is produced by photodissociation of NH3 and relaxed to low rotational levels before excitation into the electronically excited state. Originating from collisions with NH3, rate constants for total collisional removal and state-to-state rate constants for rotational energy transfer within v2=4, Ka=1 with collision induced changes of |ΔKc|⩽3 are determined. The latter rate constants are fitted with several scaling laws. Among these, those based on the energy corrected sudden approximation work best. An approximate potential curve for the anisotropic part of the interaction potential is derived and verified with cross sections obtained with straight line trajectories. The rotational energy transfer originates primarily from collisions with small impact parameters. The observed rate constants for total collisional removal are in accordance with the collision complex model.

Observation of collision induced state-to-state energy transfer in electronically and highly rotationally excited NH2

State-to-state energy transfer of NH2 in its excited state à 2A1 is investigated with time-resolved Fourier transform emission spectroscopy. Originating from collisions with NH3, rovibrational energy transfer in NH2(à 2A1) with energy separations |ΔE|&amp;lt;260 cm−1 and in multiples of ΔE≈−1050 cm−1 is observed. Based on the experimental determination of relative transition probabilities, absolute state-to-state rate constants are derived. Collisional changes in the rotational quantum number range from −3 to +4. The analysis of the time dependence of the levels populated by rovibrational energy transfer shows that this variety is not the result of secondary collisions.

Multi-photon dissociation of CHBr 3 at 248 and 193 nm: observation of the electronically excited CH( [formula omitted]) product

The CH(A 2 Δ ) multi-photon dissociation channel of CHBr 3 following the excitation at 193 and 248 nm has been studied. Two experimental techniques are applied: time-resolved Fourier transform emission spectroscopy (TR-FTES) and two-laser experiments. The rovibrational state distribution of the nascent CH(A 2 Δ ) fragments generated by multi-photon dissociation of CHBr 3 in a supersonic jet is determined through an analysis of the CH(A 2 Δ –X 2 Π ) emission spectrum obtained by TR-FTES with resolutions between 0.5 and 4.0 cm −1. From the power dependence of the fluorescence the order of the multi-photon dissociation is determined to be two for 193 nm and three for 248 nm radiation. At both wavelengths the rovibrational distributions are power dependent but the effect is most pronounced for 193 nm excitation. At the lowest power investigated, the rotational temperatures are T=1450(70) K for 193 nm and T=2090(90) K for 248 nm multi-photon dissociation. While only v′=0 is observed in the two-photon dissociation at 193 nm, the vibrational temperature at low laser energy is 2400(200) K for three-photon dissociation at 248 nm. In experiments with two spatially separated volumes of the supersonic jet excited by two delayed laser pulses, the dissociation mechanism is shown to be a sequential fragmentation for 193 nm and to involve at least one intermediate species for 248 nm multi-photon dissociation. The possible dissociation channels for the production of CH(A 2 Δ ) by multi-step dissociation of CHBr 3 are discussed with respect to the observed rovibrational distributions.

Collisional deactivation of highly vibrationally excited NO2 monitored by time-resolved Fourier transform infrared emission spectroscopy

Infrared emission from highly vibrationally excited NO2, prepared by collision induced internal conversion, can be detected with 1 cm−1 spectral and 0.5 μs time resolution over the 800–10 000 cm−1 range by time-resolved Fourier transform emission spectroscopy. The energy distribution of vibrationally excited NO2 during collisional deactivation can be extracted from the emission spectra and shows that the energy loss per collision increases dramatically from &amp;lt;50 cm−1 below 13 000 cm−1 energy to 1300 cm−1 at 20 000 cm−1 energy.

State-to-state rotational energy transfer and reaction with ketene of highly vibrationally excited b̃ 1B1 CH2 by time-resolved Fourier transform emission spectroscopy

Dispersed fluorescence spectra from the CH2 b̃ 1B1→ã 1A1 band were recorded with time-resolution by Fourier transform emission spectroscopy after pulsed excitation of a single rotational level of the b̃ 1B1 (0,160,0) state. Fluorescence observed from the initially excited level and from levels populated by rotational energy changing collisions with the bath gas (ketene) was used to deduce the state-to-state rate constants for rotational energy transfer and the state-resolved rate constants for total collisional removal of b̃ 1B1 CH2. The observed propensity rules for rotational energy transfer—ΔJ=±2, ΔKa=0, and ΔKc=±2—are consistent with a quadrupole–dipole interaction between b̃ 1B1 (0,160,0) CH2 and ketene. The existence of a quadrupole in the intermolecular interaction suggests that the structure of CH2 in the b̃ 1B1 (0,160,0) state, averaged over the time of a collision, must be linear. The state-to-state rotational energy transfer rate constants range from approximately equal to the hard sphere gas kinetic rate to four times the gas kinetic rate, with the largest rate constants between rotational levels with the smallest energy gaps. Examination of fluorescence spectra recorded with polarization analysis shows that rotationally elastic (ΔJ=0)M changing collisions are negligible. State-resolved rate constants for reactive collisions between b̃ 1B1 CH2 and ketene were obtained by subtracting the rotational energy transfer contribution from the total rate constants for collisional removal of b̃ 1B1 CH2 (obtained from a Stern–Volmer analysis). These rate constants vary from one to five times the hard sphere gas kinetic rate, and increase with rotational energy for the levels studied. Their magnitudes show that CH2 is about two times as reactive in its b̃ 1B1 state than its ã 1A1 state.