Ground State NH Research Articles

An exploratory modelling study of chemiluminescence in ammonia-fuelled flames. Part 1

The present exploratory study is aimed at the modelling of the chemiluminescence signature of premixed laminar NH3+H2+air flames. The detailed kinetic mechanism of the author was extended by reactions describing the formation and consumption of excited NO2*, NO(A), NH*, and NH2* mostly relying on previous analyses of chemiluminescence processes from the literature. The lowest-lying excited state of molecular nitrogen, N2(A) is also invoked to analyse its possible impact on the formation of these emitting species. In the present model, the formation of NO(A) and NH* in NH3+H2+air flames is governed by the energy transfer from N2(A) to the ground state NO and NH, respectively. The new model predictions are compared with available experimental data obtained by Zhu et al. in laminar counterflow NH3+H2+air flames (PROCI 39 (2023)). Good agreement with the measurements was demonstrated for NO(A), OH*, and NH*, both in terms of their variation with equivalence ratio and the amounts of ammonia in the fuel. Moreover, the model qualitatively captures the ratios of chemiluminescence intensity of OH*/NO(A), NH*/OH* and NH*/NO(A). Further analysis revealed that the bimodal behaviour of chemiluminescence in the spectral regions identified by Zhu et al. as “blue”, “green”, “yellow”, “orange”, and “red”, can be explained by the interplay of emissions from NO2* and NH2*, while emission in the “violet” spectral band could be assigned to excited H2O*.

Multistage Zeeman deceleration of NH XΣ−3 radicals

We report on the Zeeman deceleration of ground-state NH radicals, using a decelerator that consists of 100 pulsed solenoids and 100 permanent hexapoles. Packets of state-selected NH ($X\,^3\Sigma^-$, N=0, J=1) radicals are produced with final velocities ranging between 510 m/s and 150 m/s. The velocity distributions of the packets of NH exiting the Zeeman decelerator are probed using velocity map imaging detection. We present a new 1+2' resonance-enhanced multiphoton ionization scheme for NH, that allows for velocity map imaging detection under ion recoil-free conditions. The packets of Zeeman-decelerated NH radicals, in combination with the new detection scheme, offer interesting prospects for the use of this important radical in high-resolution crossed-beam scattering experiments.

Note: Improved line strengths of rovibrational and rotational transitions within the X(3)Σ(-) ground state of NH.

Recently, a line list including positions and transition strengths was published for the NH X(3)Σ(-) rovibrational and rotational transitions. The calculation of the transition strengths requires a conversion of transition matrix elements from Hund's case (b) to (a). The method of this conversion has recently been improved during other work on the OH X(2)Π rovibrational transitions, by removing an approximation that was present previously. The adjusted method has been applied to the NH line list, resulting in more accurate transition strengths. An updated line list is presented that contains all possible transitions with v' and v″ up to 6, and J up to between 25 and 44, depending on the band.

Line strengths of rovibrational and rotational transitions within the X³Σ⁻ ground state of NH.

A new line list for rovibrational and rotational transitions, including fine structure, within the NH X³Σ⁻ ground state has been created. It contains line intensities in the form of Einstein A and f-values, for all possible bands up to v' = 6, and for J up to between 25 and 44. The intensities are based on a new dipole moment function (DMF), which has been calculated using the internally contracted multi-reference configuration interaction method with an aug-cc-pV6Z basis set. The programs RKR1, LEVEL, and PGOPHER were used to calculate line positions and intensities using the most recent spectroscopic line position observations and the new DMF, including the rotational dependence on the matrix elements. The Hund's case (b) matrix elements from the LEVEL output (available as Supplement 1 of the supplementary material) have been transformed to the case (a) form required by PGOPHER. New relative intensities for the (1,0) band have been measured, and the calculated and observed Herman-Wallis effects are compared, showing good agreement. The line list (see Supplement 5 of the supplementary material) will be useful for the study of NH in astronomy, cold and ultracold molecular systems, and in the nitrogen chemistry of combustion.

Variation of radiative lifetimes of NH2(Ã2A1) with rotational levels in the (0, 8, 0) and (0, 9, 0) vibration bands

Radiative lifetimes from the first electronically excited state of the amidogen free radical, NH(2)(Ã(2)A(1)), are reported for rotational states in selected vibrational levels ν(2)' using laser-induced fluorescence. Thermal collision of argon, Ar(∗)((3)P(0), (3)P(2)) metastable atoms in a microwave discharge-flow system with ammonia (NH(3)) molecules produced ground state NH(2)(X̃(2)B(1)). The radiative lifetimes for the deactivation of NH(2)(Ã(2)A(1)) were determined by measuring the decay profiles of NH(2)(Ã(2)A(1) → X̃(2)B(1)). In addition to the Fermi resonances with the ground state that lengthen the radiative lifetimes, a systematic increase in the radiative lifetimes with rotational quantum number was observed. Furthermore, the average radiative lifetimes of the (0, 9, 0) Γ, τ(1) = 18.65 ± 0.47 μs and (0, 8, 0) Φ, τ(2) = 23.72 ± 0.65 μs levels were much longer than those of the (0, 9, 0) Σ, τ(3) = 10.62 ± 0.47 μs, and (0, 8, 0) Π, τ(4) = 13.55 ± 0.55 μs states suggesting increased mixing of the first electronic excited and the ground states.

Refining to near spectroscopic accuracy the double many-body expansion potential energy surface for ground-state NH 2

The single-sheeted double many-body expansion potential energy surface for the ground-state of NH 2 is refined to attain near spectroscopic accuracy from a multiproperty fit to accurate ab initio energies and experimental vibrational levels, including the Renner–Teller effect for total angular momentum (excluding spin) of N = 0. Quasiclassical trajectory calculations on both the original and newly reported potential energy surfaces suggest that the dynamical properties of the original form remain essentially unaltered as aimed.

Accumulation of Stark-decelerated NH molecules in a magnetic trap

Here we report on the accumulation of ground-state NH molecules in a static magnetic trap. A pulsed supersonic beam of NH (a1Δ) radicals is produced and brought to a near standstill at the center of a quadrupole magnetic trap using a Stark decelerator. There, optical pumping of the metastable NH radicals to the X3Σ− ground state is performed by driving the spin-forbidden A3Π ← a1Δ transition, followed by spontaneous A → X emission. The resulting population in the various rotational levels of the ground state is monitored via laser induced fluorescence detection. A substantial fraction of the ground-state NH molecules stays confined in the several milliKelvin deep magnetic trap. The loading scheme allows one to increase the phase-space density of trapped molecules by accumulating packets from consecutive deceleration cycles in the trap. In the present experiment, accumulation of six packets is demonstrated to result in an overall increase of only slightly over a factor of two, limited by the trap-loss and reloading rates.

Theoretical study of the quenching of NH(Δ1)molecules via collisions with Rb atoms

We examine the quenching reaction $\text{Rb}(^{2}S)+\text{NH}(^{1}\ensuremath{\Delta})\ensuremath{\rightarrow}\text{Rb}({^{2}P}_{1/2})+\text{NH}(X\text{ }{^{3}\ensuremath{\Sigma}}^{\ensuremath{-}})$. This reaction may be utilized to produce ground-state NH molecules for studies of ultracold physics or for other purposes and is interesting in that it involves initial and final states that are nearly degenerate. This near degeneracy is expected to lead to a large reaction rate. We examine this system using ab initio quantum chemistry calculations and scattering calculations, which include spin-orbit effects, and find that the reaction rate is large and, in fact, approaches the quantum-mechanical unitarity limit. We discuss the prospects for an experimental examination of this system.

Hückel transformation theory of the hybridization problem in NH 3

Hückel transformation theory is applied to the study of the hybridization problem in ground state NH 3. Under assumptions similar to those recently introduced for H 2O, minimization of the model Hückel energy at the valence level yields a trigonometric relation between bond angle and hybridization parameter showing that hybridization opens the interbond angle beyond 90°. A further orthogonal transformation mixing in AOs belonging to A 1 and E symmetry yields four orthogonal sp 3 hybrids of C 3v symmetry on nitrogen that can be engaged in the axially symmetric lone pair and three equivalent straight N–H bonds making the experimental interbond angle.

Magnetic Trapping and Zeeman Relaxation of NH (X Σ−3)

NH radicals are magnetically trapped and their Zeeman relaxation and energy transport collision cross sections with helium are measured. Continuous buffer-gas loading of the trap is direct from a room-temperature molecular beam. The Zeeman relaxation (inelastic) cross section of magnetically trapped electronic, vibrational, and rotational ground state NH molecules in collisions with 3He is measured to be 3.8+/-1.1 x 10(-19) cm(2) at 710 mK. The NH-He energy transport cross section is also measured, indicating a ratio of diffusive to inelastic cross sections of gamma=7 x 10(4), in agreement with recent theory [R. V. Krems, H. R. Sadeghpour, A. Dalgarno, D. Zgid, J. Kłos, and G. Chałasiński, Phys. Rev. A 68, 051401 (2003)10.1103/PhysRevA.68.051401].

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.

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.

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.].

Magneto-Optical Investigations of Imidogen in Inert-Gas Matrices

Magnetic circular dichroism (MCD) and absorption spectra are reported for the A 3Πi ← X 3Σ- transition of imidogen (NH) isolated in Kr, Xe, and N2 matrices at cryogenic temperatures (∼1.4−20 K) and over a range of magnetic field strengths (0−5 T). The results are analyzed by the method of moments, and parameters are extracted by fitting the experimental data to a model in which the A 3Πi term is split by spin−orbit (SO) coupling interactions and the X 3Σ- term is split by spin−spin and higher-order SO coupling. The analysis indicates that the ground-state NH radicals behave essentially as free rotors in the noble-gas matrices but as hindered rotors in solid N2. Trends in excited-state SO coupling constants are attributed to the external heavy-atom effect.

Competition between Adiabatic and Nonadiabatic Pathways in the Photodissociation of Vibrationally Excited Ammonia

Vibrationally mediated photodissociation combined with Doppler spectroscopy and time-of-flight detection of H-atoms provides information on the photofragmentation dynamics from selected rovibrational states of A 1 A 2 -state ammonia. The competition between adiabatic dissociation forming excited-state NH 2 ( 2 A 1 ) + H and nonadiabatic dissociation leading to ground-state NH 2 ( 2 B 1 ) + H products changes drastically for dissociation from different parent levels prepared by double-resonance excitation. The H-atom speed distributions suggest that the nonadiabatic dissociation channel is the major pathway except for dissociation from the antisymmetric N-H stretching (3') parent level, which forms exclusively NH 2 ( 2 A 1 ) + H. The energy disposal depends strongly on the parent state with as little as 2% of the available energy channeled into translational energy for dissociation from the 31 state.

State-resolved radiative lifetimes of the NH(A 3Π,v=0) radical and rate constants for the quenching by NH3

A reinvestigation of the radiative lifetimes and the quenching rate constants of single excited rotational states (N,Fj,e/f ) (j=1–3) of the NH(A 3Π,v=0) radical has been performed. The quenching species was the parent molecule NH3. The electronically excited NH(A 3Π) radicals are generated by first forming ground state NH(X 3Σ−) in the ArF laser photolysis of NH3; after a delay, NH(X 3Σ−) is pumped by a dye laser to selected fine structure levels of the A 3Π state. Both the radiative lifetimes and quenching rate constants were found to be dependent on the rotational level but not on the spin and Λ-doublet component. The formation of a blast wave was observed under certain conditions leading to time dependent changes of the number density and temperature in the system. In case of a blast wave, kinetic measurements have to be evaluated with great care.

State-to-state energy transfer of NH(X 3Σ−,v=0,J,N) in collisions with He and N2

State-to-state rotational energy transfer of ground state NH(X 3Σ−,v=0,J,N) in collisions with He and N2 is studied. A complete inversion between the metastable NH(a 1Δ) state and the NH(X 3Σ−) state is generated via the photodissociation of hydrazoic acid at a wavelength of 266 nm. Single state NH(X 3Σ−,v=0,J,N) is generated by applying the stimulated emission pumping technique using the strongly forbidden NH(a 1Δ→X 3Σ−) intercombination transition around 794 nm. The ground state NH(X 3Σ−,v=0,J,N) distribution is probed with respect to all quantum states using laser induced fluorescence varying delay times and pressures. The collision induced energy transfer between the different rotational and spin levels is extensively studied and two comprehensive sets of rate constants for vibrationally elastic and rotationally inelastic collisions with He and N2 as collision partners are given which include the effect of multiple collisions. We find propensities for (ΔN=0,Δi=±1) and (ΔN=±1,Δi=0) transitions where N represents the quantum state for nuclear rotation and i represents the index of the spin component Fi. The rotational relaxation for N2 as a collision partner occurs on the average three times faster than the rotational relaxation with He as a collision partner. The energy dependence of the transition efficiency for only the nuclear rotational quantum number N obeys an energy-gap law for both He and N2.

State-to-state studies of ground state NH(X 3Σ−,v=0,J,N)+Ne

State-to-state rotational energy transfer of ground state NH(X 3Σ,v=0,J,N) in collisions with Ne is examined. NH is exclusively generated in the metastable NH(a 1Δ) state via photodissociation of hydrazoic acid at a wavelength of 266 nm. The strongly forbidden NH(a 1Δ→X 3Σ−) intercombination transition around 794 nm is used to generate single state NH(X 3Σ−,v=0,J,N) applying the stimulated emission pumping technique. The ground state radicals are detected after a certain delay time with laser induced fluorescence (LIF) using the intense NH(A 3Π←X 3Σ−) transition around 336 nm with respect to all quantum states. The collision induced energy flux between the different rotation and spin levels is studied in detail and a comprehensive set of state-to-state rate constants for inelastic collisions of NH(X 3Σ−,v=0,J,N) with Ne up to N=7 which include the effect of multiple collisions is given. The state-to-state rate constants are obtained by the use of an iterative integrated profiles method. We find a propensity for (ΔN=0, Δi=±1) and (ΔN=±1, Δi=0) transitions where N represents the quantum state for nuclear rotation and i represents the index of the spin component Fi. In most cases the energy transfer which changes the spin component and conserves the nuclear rotation quantum number N (ΔN=0, Δi=±1), is the most effective energy transfer in collisions with Ne. The energy dependence of the transition efficiency concerning only the nuclear rotation quantum number N obeys an energy-gap law (EGL).

Reaction of NH (X 3Σ-) with NO in Xenon Matrix: Infrared Detection of the HNNO Intermediate

The reaction of ground-state NH with NO has been investigated in solid xenon matrix. NH radicals are formed in good yield in Xe matrix after UV photolysis of HN3 and are reacted with NO in the matrix by annealing the solid to 40 K. A photolabile product is formed and identified as trans-HNNO by infrared spectroscopy, including isotope labeling and comparison with published frequencies from quantum chemical calculations. A minor product with similar formation and photodestruction behavior is assigned to the cis-HNNO isomer. Cis−trans interconversion was also observed. The reaction to form HNNO is essentially activationless. Upon photolysis at wavelengths up to 600 nm, HNNO is destroyed and converted to N2O. This is the first detection of this intermediate, which plays a role in ammonia combustion and NOx removal schemes.

Single state NH(X 3Σ−,v=0,J,N) preparation for state-to-state studies

A new method is presented to examine state-to-state rotational energy transfer in ground state NH(X 3Σ−,v=0,J,N). NH(X 3Σ−) is generated via state selective stimulated emission pumping using the strongly forbidden NH(a 1Δ→X 3Σ−) intercombination transition around 794 nm after foregoing photodissociation of HN3 at a wavelength of 266 nm. Products are detected by laser induced fluorescence (LIF). Chemically relevant collision dynamics including spatial processes can be studied for the first time in v=0 of the electronic ground state. State-to-state rate constants for inelastic collisions of NH(X 3Σ−,v=0,J=3,N=3) with Ne are presented.