NH in the Electronic Ground State

Early experiments to produce NH, either by thermal decomposition of various precursors (NH3 [15], NH2CI [16, 17], N2H4 [18 to 20], HNCO [21], HN3 [22 to 25]), or by photolysis of these compounds (NH3 [26], N2H4 [27], HNCO [28 to 32]), were not state-specific. In particular, in discharges of different compounds or mixtures of compounds (N2-H2 [33, 34], HN3 [35]), NH was not produced and detected in a specific state, since the existence of NH was either concluded only from the final products or NH was detected indirectly, with a long time delay between the production and detection, so that quenching could have occurred. In several experiments NH was detected even after trapping processes. On the other hand, if emission of NH out of a specific state is observed, it is not at all obvious that most of the NH radicals are in this state. 

The controlled formation of NH radicals in their electronic ground state is not an easy task. The thermal decomposition of HN3 was recommended as an NH radical source in the early 30 s [22]. This, however, is true only at sufficiently high temperatures - well above 800 K. The thermal decomposition of HN3 at lower temperatures (530 T/K 750), however, was not recommended as an NH(X) source due to secondary reactions of NH with HN3, which proceed rapidly compared to the formation reaction at these low temperatures [36]. 

In the thermal decomposition of N2H4 in shock waves, NH(X) is formed in secondary reactions [20]. In shock-heated NH3-noble gas mixtures at high temperatures (T 3000 K) [37 to 39] and in a high-temperature plasma (T = 3200 K), emission from NH(A) was observed [40]. At lower temperatures in shock waves NH(X) was observed [41]. HN3 and HNCO can be pyrolyzed at significantly lower temperatures (T 1200 K) under these conditions mainly NH(X) is formed [44, 45]. 

At very short wavelengths ( i=121.6nm) HN3 can be photolyzed via HN3-l-hv- NH(X S) + N2(B Ilg) [78]. The reactions of the electronically excited nitrogen molecules with HN3, however, will preclude this photodecomposition process, or the equivalent photoreaction of HNCO, being used as an NH(X) source. 

The two-photon laser photolysis of NH3, N2H4, and CH3NH2, with NH2 as an intermediate state, does not lead to the formation of NH in the electronic ground state but to electronically excited NH radicals [79 to 82]. The multiphoton dissociation of NH3 at A,= 193 nm, however, produces mainly highly excited electronic ground-state NH [83]. 

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In the vibrational overtone excitation in HN3(Vi =5, 6), NH in the electronic ground state can be produced in a spin-forbidden dissociation [89 to 91]. 

To detect NH in the electronic ground state, the infrared absorption was applied in particular in solid matrices [21, 69] but also in the gas phase using a color center laser [70] and different frequency laser systems [71]. On the other hand, also the IR emission of NH(X v) was investigated experimentally and theoretically [72]. IR emission of the vibrational states of the X electronic state were also observed with Fourier transform emission spectroscopy [73] with low [74] and high resolution [75]. 

Matrix-isolated NH, ND, and molecules formed by photolysis of normal and isotopically substituted HN3 in solid nitrogen or argon (T = 4 to 20 K) were identified by their fundamental vibrations and isotopic shifts observed in the IR spectrum. Since the observed IR frequencies correspond reasonably well with the gas-phase values for the electronic ground state and spin-conservation rules, on the other hand, require NH to be initially produced in an upper singlet state, a collisional deactivation in the matrix has been suspected. The following wavenumbers (in cm" T=15 K) were assigned to the fundamental vibrations in the electronic ground state X ... 

The NH in flames is mainly in the electronic ground state and can be observed by LiF in, e.g., CH4-air flames [14] and CH4-NO2-O2 flames [15]. But also electronically excited NH can be present in flames as was observed by emission [16, 17]. The NH can be formed initially in an electronically excited state in reactions like CH(X H) + NO(X) NH(A H) + CO(X) [18]. At high temperatures, around 2000 K, the contribution of electronically excited NH has to be taken into consideration [3]. in special combustion systems, like NF3-H2 flames, NH(A H-X Z ) has been observed [19]. In general, however, the modeling of nitrogen chemistry in combustion can be achieved with NH(X) radical reactions [6, 12]. 

The lower J transitions of the fundamental vibration-rotation spectra of NH" and NH", both in the electronic ground state X ITj, show well-resolved A-type doubling the following constants p and q (in cm" ) for the vibrational levels v = 0 and 1 were derived ... 

Detailed investigations were carried out on the dissociation of hydrazoic acid from the first excited state, A A", at wavelengths above - 220 nm. The upper potential energy surfaces (PES) of HN3 and DN3 probably are very similar. The dynamic features of the dissociation are essentially independent of parent rotation [26]. The PES exhibits gradients in different molecular coordinates based on vector correlations of the products. The molecular motion before dissociation is influenced by forces which lead to in-plane as well as out-ofplane bending motions which are similar to those of the fundamentals Vg and Vg in the electronic ground state. Vector correlations at low rotational quantum numbers J of NH indicate a planar dissociation geometry [23, 27, 42, 43]. This movement in the molecular... 

Ammonia (NH ) is pyramidal like PH and in its electronic ground state there are two versions of the numbered equilibrium structure exactly as shown for PH in figure Al.4.5. The potential barrier between the... 

In principal it is possible to generate NH(X, A,... a, b, c,...) in two consecutive steps first, to produce the species NH, and, second, to bring it into the desired electronic state by excitation or deactivation. In many experiments, however, a single step mechanism is used to produce NH directly in the desired quantum state. In thermal systems at temperatures below about 1000 K, the electronic ground state is the dominant state. At very high temperatures or in nonthermal systems (photolytic, radiolytic systems), higher electronic states can be populated. Thus the NH sources are divided into those for NH(X) and those for NH(a, b,. A,...), i.e., NH in higher electronic states. 

The lowest triplet state above the electronic ground state, A n, is used for the LIF detection of NH(X), and is thus of interest in that system (see p. 25). Since the A-X transition is allowed, NH(A H) can be formed by exciting the electronic ground state with light at > = 336 nm [68]. Besides the two-step mechanism (formation of NH(X) followed by excitation), it can be obtained directly by NH3 photolysis. There are, however, also ways to produce NH in the A state directly from stable precursor molecules. 

For the electronic ground state X of NH and ND, the spin-rotation and spin-spin coupling constants were derived from the far-IR absorption [1] and LMR [2] spectra, from IR absorption [3] and emission [4] spectra in the region of the fundamental band, from IR emission spectra in the region of the Av=1 (v" = 0 to 4) sequence [5], and from the A absorption and emission transitions with v and v" up to 2 [6 to 15]. Results... 

NH radicals in their electronic ground state are mainly studied with respect to their chemical behavior, whereas for NH in electronically excited states, the quenching processes also have to be taken into account thus NH(X) and NH are treated in separate paragraphs. 

Reactions of NH radicals in their electronic ground state were studied in the liquid phase. When NH(X) radicals were produced by y radiation in liquid ammonia a slow reaction, NH(X) + NH3N2H4, was observed [1]. NH(X) reactions in the solid state were assumed when NH4C104(s) was decomposed by an excimer laser [2]. NH(X) reactions have been observed on a surface [3]. NH reactions with H atoms absorbed on zeolite [4] also have to be mentioned. 

Hollow Cathode Discharges. Discharges through flowing NH3 or NH3-He mixtures produce NH ions in the electronic ground and various excited states. This technique has been applied for spectroscopic studies of NH, as there are observations of the A B A, and C 2 - X emission band systems in the visible and UV [19 to 23] (cf. pp. 146/7), the v = 1 (-0 vibration-rotation transitions in the X and a 2 states, and transitions between the X and a states [24, 25] (cf. p. 146). 

The primary photodissociation probably occurs from high vibrational levels of the electronic ground state reached by rapid intersystem crossing directly from the originally excited Bg state. No evidence was found for collisional deexcitation, and the dissociation via reaction (4) occurred with a quantum yield close to unity. Some molecular dissociation Into N2 and H2 can not be excluded, because this process Is symmetry-allowed for C/S-N2H2 [1, 12]. The successive fission of the NH bonds is predicted from ab Initio SCF Cl studies to be the preferred pathway in the photolysis of C/S-N2H2 [13, 14]. The photolytic decomposition into N2 and H2 was already observed earlier in a few qualitative experiments at very low pressures (< 0.1 Torr) [9]. 

The following channels have been observed [484, 485] from the reaction 0 ( D) + NH3, giving OH(X) + NH2(X), OH(X) + NH (A) and NH (a) + HzO. No evidence was found for the reaction channel forming HNO + H2 [485]. A bimodal rotational distribution for the OH(z = 0) is suggested to result from the electronic branching associated with the formation of NH2 in either its ground state (2J51) or its excited state (2At). The lack of a bimodal rotational distribution for OH formed... 

NH in its lowest singlet state (a A) inserts readily into paraffin CH bonds. In this electronic state the nitrene also abstracts hydrogen atoms from hydrocarbons and undergoes quenching to the ground triplet state. For example, the ratio of these channels is 0.6 0.1 03 in the case of the reaction of NH with ethane ... 

Due to the fast quenching processes, the NH radicals appear in the stable ( Z ) electronic ground state or in the metastable ( A) electronic state, which in the case of a large excess of reactant (e.g., in C2H6(l)) reacts before it is quenched to the ground state (see p. 127). 

Removal of the Iti electron from NH(X S ) leads to the ionic ground state X removal of the 3a electron to the excited ionic states a A B A, and C Only a few experimental data for the first, third, and fourth ionization potentials Ej of gaseous NH are available. Resonance-enhanced multiphoton ionization (REMPI) of NH coupled with photoelectron spectroscopy (PES) yielded the most accurate results so far [1] and confirmed the values for the first E, obtained by electron-impact mass spectrometry (EIMS) [2] and by He I PES of NH [3]. Values for the second and third Ej to be observed in the He I PES of NH were predicted [3] from the optical emission spectra of NH [4]. Adiabatic and vertical Ej s (in eV) are compared in the following table ... 

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