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The kinetics of the reaction of hydroxyl radical (OH) with trimethyl phosphate (CH3O)3PO (TMP) (reaction (1)) OH + TMP → products (1) was studied at the bath gas (He) pressure of 1 bar over the 273–837 K temperature range. Hydroxyl radicals were produced in fast reactions of electronically excited oxygen atoms O(1D) with either H2O or H2. Excited oxygen atoms O(1D) were produced by photolysis of ozone, O3, at 266 nm (4th harmonic of Nd:YAG laser) over the 273–470 K temperature range and by photolysis of N2O at 193 nm (ArF excimer laser) over the whole temperature range including the elevated temperature range 470–837 K. The reaction rate constant exhibits a V-shaped temperature dependence, negative in the low temperature range, 273–470 K (the rate constant decreases with temperature), and positive in the elevated temperature range, 470–837 K (the rate constant increases with temperature), with a turning point at 471 K. The rate constant could be fairly well fitted with the three parameter modified Arrhenius expression, k1 = 7.52 × 10−18 (T/298)9 exp(34 367 J mol−1/RT) cm3 per molecule per s (273–837 K). Previously, only one indirect experimental measurement at a single (ambient) temperature was available. The temperature dependence over an extended temperature range obtained in this study together with the peculiar V-shaped temperature dependence will have an impact on the modelling of the flame inhibition by phosphates as well on the further understanding of the mechanisms of elementary chemical reactions.

International audience; The rate constant of the disproportionation channel 1a of the self-reaction of hydroxyl radicals OH + OH → H2O + O (1a) was measured at ambient temperature as well as over an extended temperature range to resolve the discrepancy between the IUPAC recommended value (k1a = 1.48 × 10–12 cm3 molecule–1 s–1, discharge flow system, Bedjanian et al. J. Phys. Chem. A 1999, 103, 7017) and a factor of ca. 1.8 higher value by pulsed laser photolysis (2.7 × 10–12 cm3 molecule–1 s–1, Bahng et al. J. Phys. Chem. A 2007, 111, 3850, and 2.52 × 10–12 cm3 molecule–1 s–1, Altinay et al. J. Phys. Chem. A 2014, 118, 38). To resolve this discrepancy, the rate constant of the title reaction was remeasured in three laboratories using two different experimental techniques, namely, laser-pulsed photolysis–transient UV absorption and fast discharge flow system coupled with mass spectrometry. Two different precursors were used to generate OH radicals in the laser-pulsed photolysis experiments. The experiments confirmed the low value of the rate constant at ambient temperature (k1a = (1.4 ± 0.2) × 10–12 cm3 molecule–1 s–1 at 295 K) as well as the V-shaped temperature dependence, negative at low temperatures and positive at high temperatures, with a turning point at 427 K: k1a = 8.38 × 10–14 × (T/300)1.99 × exp(855/T) cm3 molecule–1 s–1 (220–950 K). Recommended expression over the 220–2384 K temperature range: k1a = 2.68 × 10–14 × (T/300)2.75 × exp(1165/T) cm3 molecule–1 s–1 (220–2384 K)

The effect of an external longitudinal magnetic field on the optical free induction decay from a free radical is observed for the first time. The experiments were performed on a rotational transition of the hydroxyl radical, OH (2Π3/2(J = 1) ← 2Π3/2(J = 0) at 83.8 cm-1), using a terahertz free electron laser. In contrast to the results of the experiments with a stable paramagnetic molecule, NO, the observed effect of an external magnetic field on the free induction decay from hydroxyl radicals is more complicated. A longitudinal magnetic field leads to the rotation of the polarization plane of the FID radiation and to an additional modulation of the signal intensity. The angle of the rotation of the plane of polarization is large, in agreement with the theoretical predictions. The observed FID kinetics in the time domain are in semi-quantitative agreement with the modeling. This observation opens an opportunity for the selective detection of weak signals of short-lived reactive paramagnetic free radicals from overwhelming signals that originate from stable non-paramagnetic species by polarization discrimination.

The OH radical is a short-lived intermediate component in many important chemical reactions. The Novosibirsk free electron laser (NovoFEL) makes it possible to measure ultrafast spectral cinema of dynamics of this radical. In this paper, methods of increasing the sensitivity of such measurements by using a weak magnetic field and a heterodyne scheme are considered.

Effect of external longitudinal magnetic field on the optical Free Induction Decay (FID) from a free radical was observed for the first time. The experiments were performed on the rotational transition of hydroxyl radical OH (2Π3/2(J=1) ← 2Π3/2(J=0) at 83.8 cm-1) using the Terahertz Free Electron Laser. In contrast to the results of the experiments with a stable paramagnetic molecule NO, the observed effect of external magnetic field on the Free Induction Decay from hydroxyl radicals is more complicated. A longitudinal magnetic field leads to rotation of the polarization plane of the FID radiation, as well as to additional modulation of the signal intensity. The angle of rotation of the polarization plane is large, in agreement with the theoretical predictions. The observed FID kinetics in the time domain are in semi-quantitative agreement with the modeling. This observation opens an opportunity for selective detection by polarization discrimination of weak signals of short-lived reactive paramagnetic free radicals from overwhelming signals that originate from stable non-paramagnetic species.

This work demonstrates the possibility of switching the phase of optical free induction decay (FID) in time-domain experiments. This possibility arises in experiments with paramagnetic molecules (free radicals) in a magnetic field. Changing the direction of the magnetic field alters the direction of rotation of the FID polarization plane and inverts the phase of one of the components of FID. This effect will be useful for heterodyne detection of the optical FID of the free radicals in the terahertz region.

In this study, we accurately measured the breakdown thresholds in four atmospheric gases using the high-power terahertz radiation of the Novosibirsk free electron laser (NovoFEL). The breakdown intensities of the 130-μm radiation in a form of 74-ps pulses were equal to 1.1–1.4 GW/cm2. These data have been compared with calculations based on a phenomenological criterion for a breakdown and the classical theory of stochastic microwave heating of electrons. Presented are threshold conditions for a quasi-continuous plasma discharge that is maintained by means of a sequence of pulses of the NovoFEL. At a pulse repetition rate of 5.6 MHz, the discharge quenching thresholds are 15–60% lower than the breakdown thresholds depending on the amount of plasma emerging.

The method of detection of molecular gas in the terahertz region using the free induction decay (FID) signal was tested. FID signal was recorded in real time after the pulse of free electron laser. To achieve linearity of the measured signal on the gas concentration, mixing with monochromatic reference FID was used. The beats of measured FID and reference FID were separated by a band-pass filter. It was shown that the energy of the beat signal can be equal to the absorbed energy. The method was tested for detecting methyl fluoride CH3F, hydrogen bromide HBr and methanol molecules. Hydrogen cyanide HCN was used as a source of reference FID.

Infrared multiphoton dissociation of Ge(OCH3)4 molecules by irradiation with a pulsed TEA CO2 laser has been studied. The basic characteristics of the process have been experimentally investigated. The spectral characterization of the dissociation has been made. The product composition of the dissociation has been analyzed, and possible dissociation pathways have been proposed.

Ultrafast dynamics of OH radical was investigated by its Free Induction Decay (FID) signal. In our experiments doublet rotational line of the radical was excited by pulse of free-electron laser. Effect of a magnetic field on polarization of the FID was studied by numerical simulation. Experimental investigations of the magnetic field effect is in progress now.

The effect of magnetic field on the free induction in NO2 was studied. The experiments were performed using the Free Electron Laser in the terahertz frequency domain. A magnetic field affects the free induction decay in two ways. First, it reduces the integral intensity of the free induction radiation. Second, application of the magnetic field results in the rotation of the polarization of the free induction radiation. Both effects were observed experimentally as well as simulated numerically using a theoretical model. A large angle of the polarization plane rotation was observed upon application of magnetic field of 120 G.

New procedures for recording flame using the terahertz radiation of the Novosibirsk free electron laser (NFEL) are described that allow us to investigate the kinetics of nonstationary waves of combustion and detonation in a gas mixture. Schemes of experiments and the results from recording a propagating front of combustion produced by a hydrogen–oxygen mixture are presented. Experiments are performed in the 115 to 200 μm range of wavelengths. Investigations are conducted at the absorption lines of water and OH radical and with these lines tuned out. Recording in the 0.4 to 0.9 μm optical range of wavelengths is done simultaneously. Waves of combustion from a stoichiometric 2H2 +O2 mixture are investigated at the initial atmospheric pressure in a channel 40 mm in diameter. The measured velocities of the combustion wave lie in the range of 10 to 1500 m s–1.

Effect of rotation of the polarization plane of free induction decay in magnetic field is observed. Experiments were performed using short pulse of terahertz Free Electron Laser in the region of pure rotation transitions of NO molecule. Rotation of polarization was observed in real time using ultra-fast Schottky diode detectors. Angle of rotation depends on time after the laser pulse and exceeds 180° in field

The possibility of measuring the concentration of H2O molecules in flames based on the absorption of terahertz free-electron laser radiation was studied. These measurements were performed using the 177.32 cm−1 absorption line in the rotational spectrum of H2O. This line has a low intensity at room temperature, and at about 1000 K, its intensity is comparable to that of the strongest lines. The temperature dependence of the radiation absorption coefficient at a frequency of 77.32 cm−1 was studied theoretically and experimentally. It is shown that the method can be used for measurements in a sooty C2H4/O2/Ar flame, which strongly scatters visible and UV radiation.

A chemical reaction between C2H3SiF3 and NO2 induced by radiation from a pulse CO2 laser was studied by mass spectrometry and IR spectroscopy. The composition of gaseous products was determined. A macrokinetic approach was developed to study bimolecular reactions initiated by pulsed IR radiation. The procedure developed allowed us to answer the question of whether the test reaction occurs under conditions when the molecules of only one reactant are vibrationally excited or under conditions of equal vibrational temperatures. The use of this approach to the reaction between C2H3SiF3 and NO2 demonstrated that its acceleration was due to the equilibrium heating of the system.

Infrared multiphoton absorption and dissociation of chloromethyltrifluorosilane molecules under the action of pulsed transversely excited atmospheric pressure CO2 laser were experimentally studied. Dissociation products were analyzed. The dissociation proceeds via chlorine atom transfer from carbon to silicone. High degrees of silicon isotope separation were achieved. The presence of alpha-chlorine atom in a silicon organic compound brings about a significant improvement in multiple photon dissociation characteristics and an essential increase in isotopic selectivity.

Infrared multiphoton absorption and dissociation of 1,2-dichloroethyltrifluorosilane molecules under the action of pulsed TEA CO2 laser were experimentally studied. The composition of dissociation products was analyzed. The only products of dissociation have been found to be stable molecules: chloroethylene and trifluorochlorosilane. Dissociation proceeds via chlorine atom transfer from carbon to silicon. The silicon isotope-selective infrared multiphoton dissociation was performed at different wavelengths of the CO2-laser radiation. High degrees of silicon isotope separation have been achieved.

ISSN 0012-5016, Doklady Physical Chemistry, 2007, Vol. 415, Part 2, pp. 221–224. © Pleiades Publishing, Ltd., 2007.Original Russian Text © P.V. Koshlyakov, S.R. Gorelik, E.N. Chesnokov, A.S. Rakhymzhan, A.V. Vorob’ev, A.K. Petrov, 2007, publi shed in Doklady Akademii Nauk, 2007, Vol. 415,No. 5, pp. 635–638.

Threshold conditions of quasicontinuous terahertz optical discharge in five atmospheric gases were measured exactly by the Novosibirsk terahertz free-electron laser (THz NovoFEL) at wavelength of 130 um (2.3 THz). Comparison of our breakdown data with the data in other experiments and calculations based on a classical theory of microwave electron heating was made. Accuracy of our experiment (± 20%) was much better than one of previous experiments in terahertz range.

The IR spectrum of methyltrifluorosilane is measured in the range 220–3500 cm−1. Both the equilibrium configuration of the molecule and its vibrational frequencies are calculated quantum chemically. Using the rotational and Coriolis constants obtained in these calculations, the contours of the rotational bands in the IR spectrum are numerically modeled. A comparison between the calculated and the experimental spectra made it possible to refine the frequencies of the fundamental vibrations of the molecule and determine the isotope shifts of the fundamental Si-F vibration with respect to the stable isotopes of silicon.

In nine polyatomic molecules, we have studied the intramolecular redistribution of vibrational energy from chromophore C═O group excited by a resonant femtosecond IR laser radiation at a wavelength of ∼5 μm. All experiments have been performed in the gas phase using the IR-IR pump-probe technique in combination with the spectral analysis of the probe radiation. For molecules with one C═O end group, characteristic times of intramolecular vibrational redistribution (IVR) lie in the range between 2.4 and 20 ps and correlate with the density of four-frequency Fermi resonances. The IVR times in metal carbonyl molecules are anomalously long, being ∼1.0 ns for Fe(CO)5 and ∼1.5 ns for Cr(CO)6. In the CH3(C═O)OC2H5 and H2CCH(C═O)OC2H5 molecules, it has been observed that there are two characteristic IVR times, which differ by an order of magnitude from each other; this was interpreted in terms of the developed model of "accumulating states". For the ICF2COF molecule, it has been revealed that the IVR time decreases with increasing level of the vibrational excitation of the C═O bond of the molecule.

The IR laser induced unimolecular decomposition of SiF3CHClCH2Cl reveals a high efficiency and a high silicon isotope selectivity.

Development and applications of one-pulse time-domain spectroscopy on terahertz Novosibirsk free-electron laser is described. Spectrometer with Fabry-Perot etalon in reference branch is used for NovoFEL pulse diagnostics with measuring time of 0.4-0.5 ns. More long HCN-gas reference was applied for high resolution (38 MHz; 1.8·10-5) gas spectroscopy. Spectral resolution of the method as expected to be directly proportional to a measuring time.

Ultrafast time-domain spectroscopy based on free induction decay radiation of molecules excited by NovoFEL THz pulses is described. The radiation was detected by ultrafast Schottky diode detectors with a rise time shorter than 20 ps. The highest spectral resolution of the method is limited by the lowest repetition frequency of NovoFEL pulses, which constitutes 5.6 MHz. Routine spectral measurements can be made for a time of one nanosecond. The technique can be used to study ultrafast nonrepeatable phenomena. Another application is a spectral “cinema” with a picture frequency of up to 22.5 MHz.

Using the pulses of terahertz free electron laser and ultra-fast Schottky diode detectors, we observed the coherent transients within a free induction decay of gaseous nitrogen dioxide NO2. The laser excited different sub-bands of rotation spectra of NO2 containing about 50–70 lines. The free induction signal continued more than 30 ns and consisted of many echo-like bursts duration about 0.2 ns. Unlike the similar effect observed previously for linear and symmetric top molecules, the sequence of echo bursts is not periodic. The values for delay of individual echo are stable, and the set of these delays can be considered as a “molecular fingerprint” in the time domain.

Optical free induction decay (FID) in the region of 60–75 cm−1 was detected using 120 ps pulses of free electron laser. Signals were detected in real time using ultra-fast Schottky diode detectors. The oscillations corresponding to the splitting of absorption lines in deuterated water vapor (Δf = 0.15 cm−1) and hydrogen bromide (Δf = 0.02 cm−1) were detected. At high optical density, we observed the oscillations arising from “top-hat” shape of absorption lines. Free induction decay signals could be detected in a single shot. This observation allowed obtaining a spectrum in one laser pulse, which facilitates studies of very fast processes.