F. Stuhl

The 193 (and 248) nm photolysis of HN3: Formation and internal energy distributions of the NH (a 1Δ, b 1∑+, A 3Π, and c 1Π) states

The UV photolysis of HN3 at 193 nm was investigated in detail in the bulk phase at 300 K. NH radicals in the X, a, b, A, and c states were found to be formed with quantum yields ≤0.0019, 0.4, 0.017, 0.000 15, and 0.000 61, respectively. Relative rotational and vibrational populations were measured for all states except for NH(X). Average translational energies were estimated for NH(a,v=0 and 1) and NH(b,v=0). The 248 nm photolysis of HN3 was reinvestigated with respect to processes forming NH radicals other than NH(a). The observed energy distributions differ for both laser wavelengths and for high and low lying NH states. The distribution can be better described by a simple impulsive than by a statistical model. Some conclusions are drawn concerning the upper HN3 potential surfaces involved.

Detection of NH (X p3Σ−) by resonance fluorescence in the pulsed vacuum uv photolysis of NH3 and its application to reactions of NH radicals

Abstract Using resonance fluorescence the kinetics of NH (X p3 Σ − ) was studied in the pulsed vacuum UV photolysis of mixtures of NH 3 and NO. The rate constant of the reaction NH + NO was determined to be (4.7 ± 1.2) × 10 p−11 cm p3 molecule p−1 s p−1 .

Generation of NH(a 1Δ) in the 193 nm photolysis of ammonia

We report the observation of NH(a 1Δ) formation in a one‐photon process during the ArF laser (193 nm) photolysis of ammonia. We have used laser‐induced fluorescence to detect NH(a) and ND(a) and report the rotational and vibrational populations generated. We have also reinvestigated the generation of NH(a) in the 248 nm photolysis of HN3 and find substantial vibrational excitation in contrast to the results of previous workers. We give estimates for the quantum yields for NH(a) and ND(a) in the photolysis of ammonia based on comparison with the yield in 248 nm photolysis of HN3.

Detection and quenching of NH(b 1Σ+) in the pulsed vacuum UV photolysis of NH3

Abstract The kinetics of NH(b 1 Σ + ) was studied in the pulsed vacuum UV photolysis of NH 3 using the emission at 4707.1 A from the forbidden transition NH(b 1 Σ + − X 3 Σ − ). Absolute rate constants were determined for the quenching by NH 3 and Ar.

Two-photon formation of NH/ND(A 3Π) in the 193 nm photolysis of ammonia. II. Photolysis of NH2

Abstract The formation ofNH(A 3Π) in the photolysis of NH2‡ ( X 2B1), internally excited ground state amidogen prepared by the 1 nm photnlysis of NH3, has been studied. The alignment of the NH(A 3Π) was determined from the polarization of the NH (A 3Π→,X 3Σ−) fluorescence. The populations of the Λ-doublet states are found to be unequal with preferential population of the “antisymmetric” Λ-doublet. It is concluded that (i) the parent NH2 ( X 2B1) is rotationally excited, (ii) the dissociation i the NH2(2 2A1) state and (iii) the high rotational excitation of the NH(A3Π) results from the rotational excitation of the NH2‡( X 2B1). NH (A 3Π) formed in the 193 nm photolyses of CH3NH2 and N2H4 and in the 172 nm photolysi generated by mechanisms similar to that in the 193 nm photolysis of NH3.

NH(a1Δ → X3Σ−) emission from the gas-phase photolysis of HN3

Abstract During ArF laser (193 nm) photolysis of HN 3 , a spectrum of the emission from the highly forbidden NH(a 1 Δ → X 3 Σ − ) transition has

Predissociation of the NH/ND(c 1Π,v’,J’) states

The predissociation of the electronically excited NH/ND(c 1Π) state was studied by accurate lifetime measurements. The pulsed generation of single rotational states of different vibrational levels was performed by dye laser excitation in the NH/ND c 1Π–a 1Δ band system. The lifetime of ND(c,v’=0,J’=1) is 500±10 ns. This value is close to the purely radiative lifetime of the rotationless state. The lifetime increases steadily up to J’=15. The lifetime of ND(c,v’=1) is 235 ns for very low J’ and decreases for higher J’. These shorter lifetimes are due to predissociation by the repulsive 1 5Σ− state. Similarly, the NH(c,v’=0) state is predissociated for all J’, having an average lifetime of 463 ns for J’=1–7. For J’>7, the lifetime decreases steadily. The lifetime of NH(c,v’=1) is dominated by predissociation, with values near 65 ns for J’=1–6. These experimental results are found to be in very good agreement with an ab initio electronic structure calculation of the radiative and predissociative decay rates.

Two-photon formation of NH/ND(A3Π) in the 193 nm photolysis of ammonia. I. Mechanism and identification of the intermediate species

Abstract The mechanism for formation of the A 3 Π states of NH and ND in the 193 nm photolyses of NH 3 and ND 3 has been investigated using two ArF excimer lasers in pump—probe experiments. The mechanism is shown to be sequential photolysis involving a long-living intermediate species. Rate constants have been measured for the quenching of these intermediates by their respective parent molecules and several other collision partners; the extrapolated zero pressure lifetimes are found to be > 100 μs. Fluorescences of the NH 2 and ND 2 excited states generated in these photolysis systems were also observed and found to decay bioexponentially. Rate constants were measured for the quenching of both the fast and slow components and the species emitting the longer-living component were found to be similar to the intermediates. Arguments are presented in favor of identifying the intermediates as internally excited NH 2 (∼X) and ND 2 (∼X).

Lifetime and deactivation of NH and ND (A 3Π, v=0 and 1)

A reinvestigation of the lifetime of NH(A,v=0) using ArF laser photolysis of NH3 to produce NH(A) is reported. In addition, the lifetime of NH(A,v=1) and ND(A,v=0 and 1) have been investigated. The lifetime of NH(A,v=0,J=8–12) is 424±8 ns and that for NH(A,v=1,J=4–6) is 461±10 ns. The lifetimes of protonated and deuterated A 3Π imidogen are the same within 5% suggesting no predissociation of the low rotational levels. Rate constants for quenching of NH/ND(A,v=0) and for deactivation of NH/ND(A,v=1) for a few selected collision partners have been measured and no isotope effect is found. The results of this work supersede those reported previously from this laboratory as the latter contain a small uncorrected systematic error.

Collisional quenching at 300 K: CH(A 2Δ) by H2O, NH3 and other gases and NH(A 3Π) by H2O

Abstract The rate constants for collisional quenching of CH(A 2Δ, ν = O) with a 300 K thermal rotational population by H2O, NH3, (CH3)2CO, Ar, CO2, CO, N2O, O2, N2, H2, CH4 and C2H6 as well as that for quenching of NH (A 3Π, ν = 0) by H2O were measured. These rate constants allow comparison with high-temperature values for several colliders of importance in combustion systems.