Time-resolved Cavity Ring-down Spectroscopy Research Articles

Ternary Association Reaction of Vibrationally Cold N2+ Ions in Ambient Helium.

The rate coefficients for the ternary association reaction of vibrationally cold N2+ ions with N2 and He were measured in an afterglow plasma in the temperature range of 140-250 K: kN4+ = (1.02 ± 0.39)(300/T)(1.28±0.08) × 10-29 cm6 s-1. The rotational and vibrational state populations of N2+ ions were probed in situ by time-resolved cavity ring-down spectroscopy. The rate coefficient for Penning ionization of N2 by helium metastable atoms was also determined: km = [(2.70 ± 0.17) + (84.3 ± 5.9)e-(877±36)/T] × 10-11 cm3 s-1.

Erosion sensor using time-resolved cavity ring-down spectroscopy for Hall thrusters.

A high-sensitivity sensor to measure titanium atom density based on time-resolved cavity ring-down spectroscopy (CRDS) was developed to monitor the wall erosion and predict the lifetime of Hall thrusters. The minimum detection limit for the sensor was dependent on the discharge current oscillation in the Hall thruster. A Volterra engine management system was employed for time-resolved measurements to develop the time-resolved CRDS system, which was synchronized to the discharge current oscillation. The results confirmed that the path-integrated number density of sputtered titanium atoms was synchronized with the discharge current oscillation. The minimum detection limit was decreased by ∼30% from 2 × 1012 to 6 × 1011 m-2.

Experimental estimation of the atmospheric lifetimes of CF2HI, CF3CH2I, CF3(CH2)2I and CF3(CH2)3I with removal via the sunlight photolysis and the reactions with NO3

Hydroiodofluorocarbons (HIFCs) have recently attracted attention as new candidates for replacements for chlorofluorocarbons and bromofluorocarbons. Thus, it is very important for the practical and industrial use of HIFCs to know the atmospheric removal processes of HIFCs emitted into the atmosphere. In this study, the absorption spectra of difluoroiodomethane (CF2HI), 1,1,1-trifluoro-2-iodoethane (CF3CH2I), 1,1,1-trifluoro-3-iodopropane [CF3(CH2)2I] and 1,1,1-trifluoro-4-iodobutane [CF3(CH2)3I] were experimentally determined to estimate the lifetimes of the removals from the atmosphere by the sunlight photolysis. The absorption maxima for these HIFCs are at ~262 nm and their peak absorption cross sections range from 6.8 × 10−19 to 8.5 × 10−19 cm2 molecule−1. These results suggest that the lifetimes of the studied HIFCs by solar photolysis are ranged from 7.2 to 14 h. The reactions of NO3 with the four HIFCs were also investigated using time-resolved cavity ring-down spectroscopy. The rate constant values of (4.3 ± 1.1), (5.5 ± 2.7), (6.6 ± 3.7), (9.5 ± 3.1) × 10−14 cm3 molecule−1 s−1 have been determined for the reactions of NO3 with CF2HI, CF3CH2I, CF3(CH2)2I and CF3(CH2)3I at 298 K and 100 Torr of total pressure (the errors designate 2σ statistical uncertainty). The determined values of the rate constants suggest that the lifetimes of the studied four HIFCs with respect to reactions with NO3 range from 12 to 26 h.

Kinetic measurements of the C2H5O2 radical using time-resolved cavity ring-down spectroscopy with a continuous source

We report on the design of a time-resolved, high duty-factor cavity ring-down apparatus utilizing a continuous laser and detail a technique for the accurate and precise measurement of effective reaction rate constants with it. This report complements an earlier paper concerning the measurement of the absolute absorption cross-sections, σP, of reactive intermediates. To demonstrate the performance of the new technique, we have measured the decay rate of ethyl peroxy radicals by monitoring the Ã←X̃ origin band of the G-conformer of these species. A measured value kobs∕σP = 1.827(45) × 10(7) cm/s was determined and it, along with the previously measured value of σP, was used to derive the value of kobs = 9.66(44)×10(-14) cm(3)/s, for the effective rate constant for ethyl peroxy self-reaction (all uncertainties are 1 σ). The present value of kobs is compared to those previously reported, and sources of systematic errors and their impact are discussed.

Determination of the rate constant of the reaction of NO3 with C2H5I using time-resolved cavity ring-down spectroscopy

The reaction of NO3 with C2H5I was studied using time-resolved cavity ring-down spectroscopy. The rate constant of NO3+C2H5I reaction was determined to be (2.4±0.4)×10−13cm3molecule−1s−1 at 298K and showed no pressure dependence in the range of 25–100Torr of N2 diluent. The determined value of the rate constant suggests that the reaction of NO3 with C2H5I would play an important role in the iodine cycle at night.

Study of the pressure dependence of the rate constant of the reaction of NO 3 with NO 2

We have applied time resolved cavity ring-down spectroscopy to a kinetic study of the reaction of NO 3 with NO 2 in 20–700 Torr of N 2 diluent at 298 and 283 K. By fitting the data to Troe’s expression with F c = 0.6, the low-pressure and high-pressure limiting rate constants of this reaction were determined to be k 1 low (298 K) = (3.1 ± 0.8) × 10 −30 cm 6 molecule −2 s −1 and k 1 high (298 K) = (1.6 ± 0.3) × 10 −12 cm 3 molecule −1 s −1. These rate constants thus determined were 10–40% larger than those recommend in the NASA/JPL database.

Formation of SiH3 Radicals and Nanoparticles in SiH4–H2 Plasmas Observed by Time-Resolved Cavity Ringdown Spectroscopy

The growth of nanoparticles between electrodes in a pulsed very high frequency (VHF) silane–hydrogen (SiH4–H2) plasma under hydrogenated microcrystalline silicon (µc-Si:H) growth conditions has been studied by time-resolved cavity ringdown (τ-CRD) spectroscopy. The light absorption of silyl (SiH3) radicals and the light scattering and absorption of nanoparticles have been measured in the UV spectral range (220 and 280 nm). The contribution of the SiH3 radicals and nanoparticles to the measured cavity loss could be distinguished by 1) varying the wavelength of the probe laser pulse, 2) using time-resolved information of the SiH3 radicals and the nanoparticle density, 3) the measured spatial distribution of the species between the electrodes, and 4) the dependence of these distributions on the electrode temperature. From the time evolution of cavity loss related to the nanoparticles growing in plasma, we can determine whether the light losses of the nanoparticles are in the Rayleigh or Mie regime. Subsequently, these measurements also provide information on typical particle size. Additional scanning electron microscopy (SEM) analyses, which reveal the evolution of the nanoparticle size distribution with time, corroborate the results obtained by τ-CRD. Finally, the spatial distribution of the SiH3 radical density and the electrode temperature dependence of the nanoparticles between the electrodes have been studied. These results are discussed in terms of the dominant forces acting on nanoparticles in plasma and the nanoparticle growth mechanism.

Time-Resolved Cavity Ringdown Measurements and Kinetic Modeling of the Pressure Dependences of the Recombination Reactions of SiH2 with the Alkenes C2H4, C3H6, and t-C4H8

Room-temperature rate constants for the pressure-dependent reactions SiH2 + ethene, propene, and t-butene have been determined at total pressures of 3.3 mbar < or = p < or = 300 mbar with Ar as buffer gas. SiH2 was detected by means of time-resolved cavity ringdown spectroscopy (CRDS), and the deconvolution of ringdown, kinetics, and laser bandwidth effect was accomplished with the extended simultaneous kinetics and ringdown model (eSKaR). In this way, pseudofirst-order rate constants could be extracted from nonexponential ringdown profiles. The recombination reactions, including the reaction SiH2 + i-butene, have been modeled based on the simplified statistical adiabatic channel model (SACM) and weak collision energy-grained master equation (ME) simulations. The influence of an interfering fast isomerization channel was investigated based on the Rice, Ramsperger, Kassel, Marcus theory (RRKM) and was found to be only important for the C2H4 reaction. Using ab initio energies (G3) and structures (MP2/6-311G(d,p)) as input parameters for the kinetic models, a consistent description of the pressure and temperature dependences of all four reactions was possible. In the temperature range 295 K < or = T < or = 600 K, the extrapolated limiting high-pressure rate constants, k(infinity)(C2H4)/(cm(3) x mol(-1) x s(-1)) = 1.9 x 10(14) (T/K)(-0.065), k(infinity)(C3H6)/(cm(3) x mol(-1) x s(-1)) = 1.3 x 10(14) (T/K)(0.075), k(infinity)(i-C4H8) = 1.8 x 10(14) cm(3) x mol(-1) x s(-1), and k(infinity)(t-C4H8)/(cm(3) x mol(-1) x s(-1)) = 4.6 x 10(13) (T/K)(0.21), are close to the collision number and are more or less temperature independent. In the case of ethene, probably due to the approximate treatment of rotational effects and/or the interfering isomerization process, the applied model slightly underestimates the falloff and thus yields too high extrapolated rate constants at p < 10 mbar.

Time-resolved cavity ringdown spectroscopy on nanoparticle generation in a SiH4–H2 VHF plasma

Time-resolved cavity ringdown (τ-CRD) spectroscopy has been applied to monitor the silyl (SiH3) radicals and nanoparticles in a pulsed very high frequency (VHF) silane-hydrogen plasma under microcrystalline silicon (μc-Si:H) deposition conditions. The measured cavity loss reveals four time intervals (I up to VI) in the first 4s of the plasma pulse. By variation of the laser wavelength, it is demonstrated that the small cavity loss at 220nm reflects the SiH3 absorption in interval I. In intervals II and III, an additional cavity loss appears. This additional cavity loss corresponds to Rayleigh and Mie scattering by growing nanoparticles. Interval IV reflects the loss of nanoparticles between the electrodes during the afterglow of the plasma pulse. The evolution of the nanoparticle generation determined from the τ-CRD measurements are further confirmed by additional scanning electron microscopy analyses on the nanoparticles created in the plasma pulse.

Time-resolved Cavity Ringdown Spectroscopy as a Monitoring Technique of Nanoparticles in Pulsed VHF Plasmas

AbstractTime-resolved cavity ringdown (τ-CRD) spectroscopy has been applied to monitor the sylil (SiH3) radicals and nano-particles in pulsed very high frequency (VHF) silane (SiH4)/hydrogen (H2) plasmas under microcrystalline silicon (μc-Si:H) deposition conditions. After the plasma ignition, a small constant cavity loss (~100 ppm) on timescales smaller than ~1 s has been observed, whereas on time scales larger than ~1 s after plasma ignition, an additional cavity loss is observed. By variation of the wavelength of the CRD laser pulse, we demonstrate that the cavity loss on time scales smaller than ~1 s reflects the SiH3 absorption. On time scales larger than ~1 s, the additional cavity loss corresponds to the loss of light due to mainly scattering at the nano-particles. Under the conditions studied, the light scattering at nano-particles can be described by Rayleigh scattering during its initial growth. After ~ 2.5 s, the cavity loss reflects the transition of the scattering mechanism from dominant Rayleigh to dominant Mie-scattering. These results are discussed in terms of nano-particles growing in time and further confirmed by additional scanning electron microscopy analyses on the nano-particles created in the plasma pulse.

Time-resolved cavity ringdown study of the Si and SiH3 surface reaction probability during plasma deposition of a-Si:H at different substrate temperatures

Time-resolved cavity ringdown spectroscopy (τ-CRDS) has been applied to determine the surface reaction probability β of Si and SiH3 radicals during plasma deposition of hydrogenated amorphous silicon (a-Si:H). In an innovative approach, our remote Ar-H2-SiH4 plasma is modulated by applying pulsed rf power to the substrate and the resulting time-dependent radical densities are monitored to yield the radical loss rates. It is demonstrated that the loss rates obtained with this τ-CRDS technique equal the loss rates in the undisturbed plasma and the determination of the gas phase reaction rates of Si and SiH3 as well as their surface reaction probability β is discussed in detail. It is shown that Si is mainly lost in the gas phase to SiH4 [reaction rate kr=(3.0±0.6)×10−16m3s−1], while the probability for Si to react at an a-Si:H surface is 0.95&amp;lt;βSi&amp;lt;1 for a substrate temperature of 200°C. SiH3 is only lost in reactions with the surface and measurements of β of SiH3 for substrate temperatures in the range of 50–450°C show that βSiH3=(0.30±0.03), independent of the substrate temperature. The implications for a-Si:H film growth are discussed.

Time-resolved cavity ring-down spectroscopic study of the gas phase and surface loss rates of Si and SiH 3 plasma radicals

Time-resolved cavity ring-down spectroscopy (CRDS) has been applied to determine gas phase and surface loss rates of Si and SiH 3 radicals during plasma deposition of hydrogenated amorphous silicon. This has been done by monitoring the temporal decay of the radicals densities as initiated by a minor periodic modulation applied to a remote SiH 4 plasma. From pressure dependence, it is shown that Si is reactive with SiH 4 [(1.4±0.6)×10 −16 m −3 s −1 reaction rate constant], while SiH 3 is unreactive in the gas phase. A surface reaction probability β of 0.9< β⩽1 and β=0.30±0.05 has been obtained for Si and SiH 3, respectively.