NH Radicals Research Articles

N, NH, and NH2 radical densities in a remote Ar–NH3–SiH4 plasma and their role in silicon nitride deposition

The densities of N, NH, and NH2 radicals in a remote Ar–NH3–SiH4 plasma used for high-rate silicon nitride deposition were investigated for different gas mixtures and plasma settings using cavity ringdown absorption spectroscopy and threshold ionization mass spectrometry. For typical deposition conditions, the N, NH, and NH2 radical densities are on the order of 1012cm−3 and the trends with NH3 flow, SiH4 flow, and plasma source current are reported. We present a feasible reaction pathway for the production and loss of the NHx radicals that is consistent with the experimental results. Furthermore, mass spectrometry revealed that the consumption of NH3 was typically 40%, while it was over 80% for SiH4. On the basis of the measured N densities we deduced the recombination and sticking coefficient for N radicals on a silicon nitride film. Using this sticking coefficient and reported surface reaction probabilities of NH and NH2 radicals, we conclude that N and NH2 radicals are mainly responsible for the N incorporation in the silicon nitride film, while Si atoms are most likely brought to the surface in the form of SiHx radicals.

Production and deceleration of a pulsed beam of metastable NH (a1Δ) radicals

We report on the production of a pulsed molecular beam of metastable NH (a1Δ) radicals and present first results on the Stark deceleration of the NH (a1Δ, J = 2, MΩ = −4) radicals from 550 m s−1 to 330 m s−1. The decelerated molecules are excited on the spin-forbidden A3Π ← a1Δ transition, and detected via their subsequent spontaneous fluorescence to the X3Σ−, v = 0 ground state. These experiments demonstrate the feasibility of our recently proposed scheme (2001 Phys. Rev. A 64 041401) to accumulate ground-state NH radicals in a magnetic trap. In addition, we propose to transfer the NH radicals from the a1Δ state to the X3Σ− state using a cw-laser to allow cooling of the beam during trap loading.

Discovery and characterization of optically pumped far-infrared laser emissions using laser magnetic resonance spectroscopy

A laser magnetic resonance spectrometer has been used to discover and subsequently measure a far-infrared laser emission: the 166.6-micron line of CH2F2, optically pumped by the 9P24 CO2 laser. By recording spectra for the NH radical, the frequency of this laser emission has been determined to be 1799950±13 MHz. Spectra for the NH radical were also recorded with two other far-infrared laser emissions: the 160.4-micron line of N2H4 (9P46 CO2 pump) and the 328.6-micron line of 13CH3OH (9P12 CO2 pump). From the NH spectra, a discrepancy of 2.1 GHz with the previously measured laser frequency was identified for the 160.4-micron line. A three-laser heterodyne system was then used to remeasure the frequency to be 1868475.5±0.5 MHz. The NH spectra were also used to determine the frequency for the 328.6-micron line to be 912366±7 MHz, in agreement with the value previously calculated from the Rydberg–Ritz combination principle.

Control of Nitrogen Depth Profile near Silicon Oxynitride/Si(100) Interface Formed by Radical Nitridation

Depth profiles of composition and chemical structures in radical nitrided silicon oxynitride films formed with Ar/N2, Xe/N2, or Ar/NH3 plasma excited by microwave have been investigated by X-ray photoelectron spectroscopy combined with step etching in HF solution. The relationship between the intensities of emission from N2+ radical in these plasmas and the concentration of nitrogen atoms forming Si3≡N configuration near the silicon oxynitride film/Si substrate interface nitrided using these plasmas was studied. The emission intensities from N2+ radical generated in Xe/N2 or Ar/NH3 plasma are a quarter or one-sixth of that from N2+ radical generated in Ar/N2 plasma respectively. However, the emission from NH radical is also detected in Ar/NH3 plasma. Although the nitrogen concentration of Xe/N2 plasma is smaller than that of Ar/N2 plasma at the film/substrate interface, that of Ar/NH3 plasma is larger than that of Ar/N2 plasma at the interface. It is important for the reduction of the nitrogen concentration near the film/substrate interface to use Xe/N2 plasma in which both of the generation efficiencies of N2+ and NH radicals are low.

Downstream ion and radical densities in an Ar–NH3 plasma generated by the expanding thermal plasma technique

A full characterization of the Ar–NH3 expanding thermal plasma source applied in industrial processes for high-rate silicon nitride deposition is presented in terms of absolute densities of reactive species such as ions and radicals. The ion composition of the plasma was identified by mass spectrometry, while absolute ion density information was obtained by Langmuir probe measurements. N radicals were detected by threshold ionization mass spectrometry, whereas NH and NH2 radicals were measured by cavity ringdown spectroscopy. It was found that the ion density decreases from 1013 cm−3 in a pure Ar plasma to 1010–1011 cm−3 when NH3 is injected, with ArH+, , , and being the most abundant ions. The densities of N and NH both saturate at a level of 3 × 1012 cm−3 at NH3 flows above 3 sccs while the density of NH2 increases linearly with the NH3 flow and reaches a level of 4 × 1012 cm−3. When the plasma source current is increased, the densities of N and NH increase linearly, while the NH2 density remains approximately constant. Furthermore, it is revealed that most of the consumed NH3 is converted into N2 and H2 in the plasma.

Impurity effects on solitons in conjugated polymer linking model hamiltonians and ab initio method descriptions

Conducting polymers present an enormous gamma of technological applications. Polyacetylene is the best studied conducting polymer and the presence of topological defects in the bonding conjugation called solitons is known to be associated with the principal properties of the material. The charge and bond length structure of a single polyacetylene chain with soliton trapped at impurities is investigated. As general model Hamiltonian we use the Su–Schrieffer–Heeger model combined with the Pariser–Parr–Pople model, with site type impurities. AM1 and PM3 semi-empirical and ab initio methods calculation as well as the B3LYP density functional calculation are performed with various radical molecules bonded to the polymer chain with a soliton. Actually, we have performed the calculations using the radicals NO, CH 3, CN, I 3, I, NF 2, NH 2, NHCH 3, NO 2, NCH 2, IH 2, K, Li, and Na. The charge density distributions associated with the soliton in the quantum chemistry calculations and the model calculation are presented. We found what radical molecules actually correspond to the charge distribution and bond length variations obtained with model calculations. Therefore, it is shown, within the present results, what impurities are being represented in model calculations.

Reactivity of the 4d Transition Metals toward N Hydrogenation and NH Dissociation: A DFT-Based HSAB Analysis

The density functional theory (DFT) based hard-soft acid-base (HSAB) reactivity indices, including the electrophilicity index, have been successfully applied to many areas of molecular chemistry. In this work we test the applicability of such an approach to fundamental surface chemistry. We have considered, as prototypical surface reactions, both the hydrogenation of atomic nitrogen and the dissociative adsorption of the NH molecular radical. By use of a DFT methodology, the minimum energy reaction pathways, and corresponding reaction barriers, of the above reactions over Zr(001), Nb(110), Mo(110), Tc(001), Ru(001), Rh(111), and Pd(111) have been determined. By consideration of the chemical potential and chemical hardness of the surface metal atoms, and the principle of electronegativity equalization, it is found that the charge transferred to the NH radical during the process of dissociative adsorption correlates very well with that determined by Mulliken population analysis. Furthermore, it is found that the stability of the NH/surface transition state complex relates directly to this charge transfer and that the trend in transition state stability predicted by a HSAB treatment correlates very strongly with that determined by DFT calculations. With regards to N hydrogenation, we find that during the course of the reaction, H loses cohesion to the surface, as it must migrate from a 3-fold hollow site to either a bridge or top site, to react with N. Partial density of states (PDOS) and Mulliken population analysis reveal that this loss of bonding is accompanied by charge transfer from H to the surface metal atoms. Moreover, by simple modeling, we show that the reaction barriers are directly proportional to this mandatory charge transfer. Indeed, it is found that the reaction barriers correlate very well with the electrophilicity index of the metal atoms.

Kinetics of the NH reaction with H 2 and reassessment of HNO formation from NH + CO 2, H 2O

The reaction of ground-state NH with H 2 has been studied in a high-temperature photochemistry (HTP) reactor. The NH ( X 3 Σ ) radicals were generated by the 2-photon 193 nm photolysis of NH 3, following the decay of the originally produced NH ( A 3 Π ) radicals. Laser-induced fluorescence on the NH ( A 3 Π – X 3 Σ 0 , 0 ) transition at 336 nm was used to monitor the progress of the reaction. We obtained k (833–1432 K) = 3.5 × 10 −11 exp ( − 7758 K / T ) cm 3 molecule −1 s −1 , with ± 2 σ precision limits varying from 12 to 33% and corresponding accuracy levels from 23 to 39%. This result is in excellent agreement with that of Rohrig and Wagner [Proc. Combust. Inst. 25 (1994) 975] and the data sets can be combined to yield k (833–1685 K) = 4.4 × 10 −11 exp ( − 8142 K / T ) . Starting with this agreement, it is argued that their rate coefficients for NH + CO 2 could not be significantly in error [Proc. Combust. Inst. 25 (1994) 975]. This, combined with models of several combustion systems, indicates that HNO + CO cannot be the products, contrary to their suggestion [Proc. Combust. Inst. 25 (1994) 975]. Ab initio calculations have been performed which confirm this conclusion by showing the barriers leading to these products to be too high compared to the measured activation energies. The calculations indicate the likelihood of formation of adducts, of low stability. These then may undergo further reactions. The NH + H 2O reaction is briefly discussed and it is similarly argued that HNO + H 2 cannot be the products, as had been previously suggested.

Radical-surface interactions during film deposition: A sticky situation?

Abstract Our imaging of radicals interacting with surfaces (IRIS) method was used to investigate radical-surface reactions during low-temperature plasma-enhanced chemical vapor deposition (PECVD) processes. Special emphasis was placed on the analysis of surface reactivities for CH, SiH, CN, NH, NH2, CF2, and SiCl2 radicals during film growth. The effects of plasma parameters, such as radio frequency (rf) power and gas composition, substrate temperature, and substrate bias on radical-surface reactivity were analyzed. Different radicals exhibit different behavior at the surface of a depositing film. Specifically, CH, SiH, and CN are "sticky", with high surface reactivities. In contrast, other species such as NH, CF2, and SiCl2 do not stick to the surface of growing films and, in some cases are actually generated at the surface of the depositing film. Different plasma systems and parameters can have an effect on the stickiness of some of these species. Our IRIS measurements indicate a molecule's surface sticking probability may also be related to the molecule's electronic configuration and stability, with the most reactive species being molecules with a doublet electron configuration. In contrast, the singlet species examined here tend to be generated at the surface during film deposition. Our results also indicate that when a molecule scatters with greater than 100 % probability, it is likely to be strongly affected by energetic ion bombardment of the film surface.

Structure and potential energy function of CH, NH and OH free radical ground and low-lying states

Structure and properties of the ground states and low-lying excited electronic states of CH, NH and OH radicals are reported in this paper using CCSD(T) method and aug-cc-pVTZ basis set.Their analytic potential energy functions are in good agreement with the Murrell-Sorbie function, and the ground states are X2Π for CH, X3Σ- for NH and X2Π for OH radicals, and the adiabatic excitation energies of a4Σ- and 6Σ- for CH, a1Δ and 5Σ- for NH, a4Σ- and 6Σ- for OH are 0.705, 7.669, 1.895, 3.492, 4.535 and 14.041eV, respectively.

Reactivity induced at 25 K by low-energy electron irradiation of condensed NH3–CH3COOD (1 : 1) mixture

Chemical reactivity is observed following electron irradiation of a binary mixture of ammonia (NH(3)) and acetic acid (CH(3)COOD) at 25 K, without any subsequent thermal activation, as evidenced by vibrational high resolution electron energy loss spectroscopy (HREELS). Analysis of the HREEL spectra and comparison with infrared and Raman data of different molecules are compatible with glycine formation in its zwitterionic form. The onset for electron induced reaction is found to be at about approximately 13 eV. The mechanisms may involve NH radicals interaction with CH(3)COOD molecules. Then glycine formation does not imply any displacement of reactants, so that it involves only NH(3) and CH(3)COOD neighboring molecules.

Quantum Chemical Study of the Mechanism of Reaction between NH (X 3Σ-) and H2, H2O, and CO2 under Combustion Conditions

Reactions of ground-state NH (3sigma-) radicals with H2, H2O, and CO2 have been investigated quantum chemically, whereby the stationary points of the appropriate reaction potential energy surfaces, that is, reactants, products, intermediates, and transition states, have been identified at the G3//B3LYP level of theory. Reaction between NH and H2 takes place via a simple abstraction transition state, and the rate coefficient for this reaction as derived from the quantum chemical calculations, k(NH + H2) = (1.1 x 10(14)) exp(-20.9 kcal mol(-1)/RT) cm3 mol(-1) s(-1) between 1000 and 2000 K, is found to be in good agreement with experiment. For reaction between triplet NH and H2O, no stable intermediates were located on the triplet reaction surface although several stable species were found on the singlet surface. No intersystem crossing seam between triplet NH + H2O and singlet HNO + H2 (the products of lowest energy) was found; hence there is no evidence to support the existence of a low-energy pathway to these products. A rate coefficient of k(NH + H2O) = (6.1 x 10(13)) exp(-32.8 kcal mol(-1)/RT) cm3 mol(-1) s(-1) between 1000 and 2000 K for the reaction NH (3sigma-) + H2O --> NH2 (2B) + OH (2pi) was derived from the quantum chemical results. The reverse rate coefficient, calculated via the equilibrium constant, is in agreement with values used in modeling the thermal de-NO(x) process. For the reaction between triplet NH and CO2, several stable intermediates on both triplet and singlet reaction surfaces were located. Although a pathway from triplet NH + CO2 to singlet HNO + CO involving intersystem crossing in an HN-CO2 adduct was discovered, no pathway of sufficiently low activation energy was discovered to compare with that found in an earlier experiment [Rohrig, M.; Wagner, H. G. Proc. Combust. Inst. 1994, 25, 993.].

Density and production of NH and NH2 in an Ar–NH3 expanding plasma jet

The densities of NH and NH2 radicals in an Ar–NH3 plasma jet created by the expanding thermal plasma source were investigated for various source-operating conditions such as plasma current and NH3 flow. The radicals were measured by cavity ringdown absorption spectroscopy using the (0,0) band of the AΠ3←XΣ−3 transition for NH and the (0,9,0)-(0,0,0) band of the ÃA12←X̃B12 transition for NH2. For NH, a kinetic gas temperature and rotational temperature of 1750±100 and 1920±100K were found, respectively. The measurements revealed typical densities of 2.5×1012cm−3 for the NH radical and 3.5×1012cm−3 for the NH2 radical. From the combination of the data with ion density and NH3 consumption measurements in the plasma as well as from a simple one-dimensional plug down model, the key production reactions for NH and NH2 are discussed.

Experimental and theoretical investigations of the reactions NH(XΣ−3)+D(S2)→ND(XΣ−3)+H(S2) and NH(XΣ−3)+D(S2)→N(S4)+HD(XΣg+1)

The rate coefficient of the reaction NH(XΣ−3)+D(S2)→k1products (1) is determined in a quasistatic laser-flash photolysis, laser-induced fluorescence system at low pressures. The NH(X) radicals are produced by quenching of NH(aΔ1) (obtained in the photolysis of HN3) with Xe and the D atoms are generated in a D2/He microwave discharge. The NH(X) concentration profile is measured in the presence of a large excess of D atoms. The room-temperature rate coefficient is determined to be k1=(3.9±1.5)×1013cm3mol−1s−1. The rate coefficient k1 is the sum of the two rate coefficients, k1a and k1b, which correspond to the reactions NH(XΣ−3)+D(S2)→k1aND(XΣ−3)+H(S2) (1a) and NH(XΣ−3)+D(S2)→k1bN(S4)+HD(XΣg+1) (1b), respectively. The first reaction proceeds via the A″2 ground state of NH2 whereas the second one proceeds in the A″4 state. A global potential energy surface is constructed for the A″2 state using the internally contracted multireference configuration interaction method and the augmented correlation consistent polarized valence quadrupte zeta atomic basis. This potential energy surface is used in classical trajectory calculations to determine k1a. Similar trajectory calculations are performed for reaction (1b) employing a previously calculated potential for the A″4 state. The calculated room-temperature rate coefficient is k1=4.1×1013cm3mol−1s−1 with k1a=4.0×1013cm3mol−1s−1 and k1b=9.1×1011cm3mol−1s−1. The theoretically determined k1 shows a very weak positive temperature dependence in the range 250⩽T∕K⩽1000. Despite the deep potential well, the exchange reaction on the A″2 ground-state potential energy surface is not statistical.

Plasma Chemistry in an Atmospheric Pressure Ar/NH3 Dielectric Barrier Discharge

AbstractSummary: An atmospheric pressure dielectric barrier discharge (DBD) in Ar/NH3 (0.1–10%) mixtures with a parallel plate electrode geometry was studied. The plasma was investigated by emission and absorption spectroscopy in the UV spectral range. Discharge current and voltage were measured as well. UV absorption spectroscopy was also employed for the detection of stable products in the exhaust gas. To clarify the different processes for ammonia decomposition, N2 (2–10%) was added to the plasma. Modeling of the chemical kinetics in an Ar/NH3 plasma was performed as well. The dominant stable products of an atmospheric pressure Ar/NH3 DBD are H2, N2 and N2H4. The hydrazine (N2H4) concentration in the plasma and in the exhaust gases at various ammonia concentrations and different discharge powers was measured. Thermal N2H4 decomposition into NH2 radicals may be used for NOx reduction processes.The differences in the optical emission spectra of NH radicals formed in Ar/NH3 plasma with various ammonia concentrations (0.7, 2, and 10%) were associated with different excitation mechanisms of NH radicals.magnified imageThe differences in the optical emission spectra of NH radicals formed in Ar/NH3 plasma with various ammonia concentrations (0.7, 2, and 10%) were associated with different excitation mechanisms of NH radicals.

Experimental and theoretical investigation of the reaction NH(XΣ−3)+H(S2)→N(S4)+H2(XΣg+1)

The rate coefficient of the reaction NH(XΣ−3)+H(S2)→k1aN(S4)+H2(XΣg+1) is determined in a quasistatic laser-flash photolysis, laser-induced fluorescence system at low pressures (2mbar⩽p⩽10mbar). The NH(X) radicals are produced via the quenching of NH(aΔ1) (obtained by photolyzing HN3) with Xe whereas the H atoms are generated in a H2∕He microwave discharge. The NH(X) concentration profile is measured under pseudo-first-order condition, i.e., in the presence of a large excess of H atoms. The room temperature rate coefficient is determined to be k1a=(1.9±0.5)×1012cm3mol−1s−1. It is found to be independent of the pressure in the range considered in the present experiment. A global potential energy surface for the A″4 state is calculated with the internally contracted multireference configuration interaction method and the augmented correlation consistent polarized valence quadruple zeta atomic basis. The title reaction is investigated by classical trajectory calculations on this surface. The theoretical room temperature rate coefficient is k1a=0.92×1012cm3mol−1s−1. Using the thermodynamical data for the atoms and molecules involved, the rate coefficient for the reverse reaction, k−1a, is also calculated. At high temperatures it agrees well with the measured k−1a.

Production of ammonia-derived radicals in a dielectric barrier discharge and their injection for denitrification

Atmospheric pressure dielectric barrier discharges (DBDs) have been widely studied for nitric oxide (NO) reduction in flue gases. In particular applying the DBD to generate activated species externally and mix them with the flue gas in a second step is favoured due to its potential energy efficiency and no generation of corrosive acids.In the present work ammonia-derived radicals were generated using an atmospheric pressure Ar/NH3 DBD and subsequently injected into an exhaust chamber where a synthetic flue gas of an NO/N2 mixture was fed for demonstration of NO reduction. Optical emission and laser diode absorption spectroscopy was employed for detection of NH and NH2 in the discharge respectively, while ultraviolet absorption and Fourier transform infrared spectroscopy was used for detection of nitrogen oxides, ammonia, ammonia-derived radicals, and other products after mixing the plasma activated gas with the synthetic flue gas.Although NH and NH2 radicals were observed in the discharge, due to their short lifetimes it is unlikely that they would be simply transported, mixed with the flue gas and react with NO to form N2. On the other hand, hydrazine (N2H4), which is a stable ammonia-derived radical, was observed in the exhaust gas from the Ar/NH3 DBD. It is indicated that the hydrazine is transported into the exhaust chamber, thermally decomposed to NH2, and the efficient NO reduction can be carried out at temperatures of more than 800 K.

Cross-sections for electron–imidogen radical (NH) collisions

In this work, we report a theoretical study on electron collisions with NH radicals in the low and intermediate energy ranges. Calculated elastic differential, integral and momentum-transfer cross-sections as well as grand-total (elastic+inelastic) and total absorption cross-sections for electron–NH collisions are reported in the (1–500)-eV range. A complex optical potential composed by static, exchange, correlation–polarization plus absorption contributions, derived from a fully molecular wave function, is used to describe the interaction dynamics. The Schwinger variational iterative method combined with the distorted-wave approximation is applied to calculate scattering amplitudes. Present calculated results are compared with the existing data for electron–NH scattering in the literature. Also, comparison made between our calculated cross-sections for elastic scattering with the theoretical and experimental results for electron–NH 3 collisions has revealed remarkable similarity even at incident energies as low as 1 eV.

Low Temperature NH(X 3Σ-) Radical Reactions with NO, Saturated, and Unsaturated Hydrocarbons Studied in a Pulsed Supersonic Laval Nozzle Flow Reactor between 53 and 188 K

The reactions of ground-state imidogen radicals (NH(X 3sigma-)) with NO and select saturated and unsaturated hydrocarbons have been measured in a pulsed supersonic expansion Laval nozzle flow reactor in the temperature range 53-188 K. The rate coefficients for the NH + NO system display negative temperature dependence in the temperature regime currently investigated and a global temperature-dependent fit is best represented in a modified power law functional form, with k1(NH + NO) = (4.11 +/- 0.31) x 10(-11) x (T/300)(-0.30+/-0.17) x exp(77+/-21/T) cm3/s. The reactions of NH with ethylene, acetylene, propene, and diacetylene were measured over the temperature range 53-135 K. In addition, the reactions of NH with methane and ethane were also measured at 53 K, for reasons discussed later. The temperature dependence of the reactions of NH with the unsaturated hydrocarbons are fit using power law expressions, k(T) = A(T/300)(-n), and are as follows: k4 = (2.3 +/- 1.2) x 10(-12) x (T/300)(-1.09+/-0.33) cm3/s, k5 = (4.5 +/- 0.3) x 10(-12) x (T/300)(-1.07+/-0.04) cm3/s, k6 = (5.6 +/- 1.9) x 10(-12) x (T/300)(-1.23+/-0.21) cm3/s, and k7 = (7.4 +/- 1.8) x 10(-12) x (T/300)(-1.23+/-0.15) cm3/s for ethylene, acetylene, propene, and diacetylene, respectively. The rate for NH + ethane at 53 K is measured to be k3 = (6.8 +/- 1.7) x 10(-12) cm3/s, while that for methane at the same temperature represents an upper bound of k2 < or = (1.1 +/- 4.3) x 10(-12) cm3/s, as this is at the limits of measurement with our current technique. The behavior of these systems throughout the temperature range explored indicates that these reactions occur over a potential energy surface without an appreciable barrier through a complex formation mechanism. Implications for chemistry in low temperature environments where these species are found are briefly discussed.

A Molecular Line Survey of Orion KL in the 350 Micron Band

With the Caltech Submillimeter Observatory, we have carried out an unbiased spectral line survey of Orion KL throughout the 350 μm band (from 795 to 903 GHz). This is the first systematic study of molecular radiation in this frequency range. A total of 541 features, resulting from 929 transitions from a total of 26 species, have been detected. High-excitation transitions from CH3OH, CH3CN, H2CO, HNCO, and C2H5CN indicate the presence of a very hot (~250 K) component at the systemic velocity characteristic of the Hot Core. Physical parameters (column density and rotational temperature) relative to a number of species have been estimated by fitting, in the LTE approximation, the whole 100 GHz spectrum at once, thus taking line blending and optical depth effects properly into account. We also report the tentative detection, for the first time outside the Galactic center region, of the radical NH2, one of the building blocks of the chemistry of ammonia.