Fragment energy and vector correlations in the overtone-pumped dissociation of HN3 X̃ 1A′

Abstract

NH stretching overtone and combination states in HN3 X̃ 1A′ were excited by IR–visible double resonance pumping and by direct overtone pumping in the range 6ν1 (17 670 cm−1) to 7ν1 (20 070 cm−1). NH fragments in the a 1Δ and X 3Σ− states were detected by laser induced fluorescence with sub-Doppler resolution to determine branching ratios, correlated fragment rotational state and kinetic energy distributions, and fragment vector correlations. The spin-forbidden triplet channel was accessible to all states excited, while the threshold for the singlet channel was determined to lie in the range 18 190 to 18 755 cm−1. The measured energy release places limits on the HN–NN bond energy, and the heights of barriers to reaction. The barrier in the singlet exit channel is at least 540 cm−1. The singlet channel accessed by 7ν1 dissociation is characterized by a Boltzmann-like NH rotational distribution (〈J NH〉≊3.5), highly excited N2 rotations (〈JN2〉 ≥ 20), and total translational energy release peaked away from zero (〈ET〉≊1350 cm−1). Vector correlations and Λ-doublet propensities indicate that nonplanar dissociation processes influence the NH rotations, but become less important for higher NH rotational states. The principal correlations are a strong positive recoil anisotropy (β≊0.6), a weak positive v–J correlation (βvJ≊0.17), and a JNH-dependent Λ-doublet propensity. A model using parent vibrational motion projected onto fragment rotation is suggested to explain these observations. The triplet channel exhibits similar NH and N2 rotational state distributions, with most of the available energy (substantially greater than in the singlet channel) appearing as fragment kinetic energy.

Photodissociation Dynamics

“…A similar effect is seen in the NH( 1 Δ) fragment from HN 3 photodissociation . More recently, the J · v and other vector correlations have been obtained from a variety of LIF/Doppler measurements, including H 2 from H 2 CO photodissociation, OH from CH 3 OOH and HONO 2 , triplet and singlet NH from HN 3 , CN from CH 3 SCN and NCCN, and NO from a series of nitrites and from CH 3 CCl(NO)CH 3 …”

“…In recent years, there has been some development of new detection schemes for photofragments. For example, LIF detection has been implemented for NH(X 3 Σ - ), enabling the study of both singlet and triplet NH from HN 3 and HNF . The photodissociation of propionaldehyde (CH 3 CH 2 CHO) has been investigated through rotationally resolved LIF of the HCO product …”

Quenching and relaxation of vibrational levels of imidogen (NH/ND)(a1.DELTA.,v)

“…IR dissociation studies of HN3 show in the range of the 5yt to 6vi overtoneN-H pumping that ground state ( 3 ~) radicals are formed due to extensive mixing of vibrational states.28 At higher energies, IR photodissociation spectra show a transition from discrete to continuous structure in the range of the sixth to seventh overtone of the N-H stretch vibration.29 This can be understood by an increasing coupling of the various vibrational modes and the increasing rate of dissociation. In this energy range (17 670-20 070 cm-1), the dissociation product changes completely from ( 3 ") to NH(a'A) , 29 In the reverse process, the colliding NH(a) + N2 pair has to overcome a small barrier.…”

Scalar and angular correlations in CF3NO photodissociation: statistical and nonstatistical channels

“…16,17,21 Measurements of photoproduct trajectory -v-j angular correlations of a wide range of systems have demonstrated the power of vector correlation measurements in understanding reaction mechanisms. 16,20,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] Both semiclassical [40][41][42] and fully quantum mechanical [43][44][45][46][47] theories have been used to extract vector correlations from experimental data. In this paper we use the widely adopted semiclassical approach of Dixon,48 in which the correlated angular distributions of photofragment velocity, v, and angular momentum, j, are represented as an expansion in a basis set of bipolar harmonics.…”