Photodissociation dynamics of NO2 at moderately high energy (λ=309.1 nm; Eavail=7222 cm−1)

Abstract

The dynamics of NO2 dissociation at 309.1 nm have been explored by examining the nascent distribution of NO rotational, vibrational, spin–orbit, and lambda-doublet states. The NO fragment is produced with a monotonically decreasing vibrational distribution over the energetically accessible vibrational states (υ=0–3), and nonstatistical rotational distributions within each vibrational manifold. The distribution within υ=0 and 1 is strongly peaked near J=25.5 with a fairly narrow spread, the distribution within υ=2 is fairly flat, terminating at the limit of available energy, and the υ=3 distribution is oscillatory, also terminating at the limit of available energy. The 2Π1/2 spin–orbit state is more strongly populated than the 2Π3/2 state by a factor of 1.9 for every vibrational state. The differences in lambda-doublet populations are, in general, minor; each Λ state being roughly equally populated, although oscillations are again evident. These results are discussed in relation to results at similar available energy at room temperature and in the free jet at different available energies. It is found that the results are intermediate between the previous data at low excess energy and at high available energy, the distributions showing aspects of both regimes. From the data it is inferred that the dissociation dynamics of NO2 vary continuously from a regime where phase space theory considerations with quantum overtones dominate the product state distributions to the regime where dynamics on the exit channel determine the distributions.

Dependence of the nascent vibrational distribution of NO(v) on the photolysis wavelength of NO2 in the range λ = 266–327 nm measured by time-resolved Fourier transform infrared emission

“…Zacharias et al 7 observed a distribution in NO (v = 0-4) which showed little variation in the first four vibrational levels. These results were subsequently contradicted by Knepp et al 10 who photolysed a jet-cooled (T o 10 K) sample of NO 2 at 309.1 nm and found a monotonically decreasing population, which reflected a partitioning of energy significantly closer to statistical than the results of Zacharias et al 7 Doughty et al 11 obtained a vibrational distribution for NO (v = 1-3), using time resolved FTIR emission spectroscopy, which showed little significant variation in nascent populations for the first three vibrationally excited levels in reasonable agreement with Zacharias et al 7 Brouard et al 12 used the rotational distributions measured by Zacharias et al 7 and Knepp et al 10 to obtain nascent distributions for NO (v = 0-3) based on their O( 3 P) speed distributions from REMPI (2 + 1) velocity map ion imaging of the 308 nm photolysis of NO 2 . Using both data sets they present populations that are inverted at NO (v = 1) with vibrational populations inferred up to NO (v = 3).…”

Some remarks on the photodynamics of NO2

“…65,67,68,74 The fragment rotational distributions tend to unimodal profiles which peak with moderate degrees of fragment rotational excitation as the excess energy in the (1) 2 B 2 state is increased. The photoproduct internal state distributions have been measured right across the first electronic absorption feature in a variety of experiments 46,[74][75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90][91][92][93] which with the observation of interference or stepwise changes in dissociation timescale with excitation energy 59,60,62,94,95 provide support for the dissociation mechanism outlined above.…”

Fully Quantum-State Resolved Study of NO2 Photodissociation: Correlated NO(2.PI..OMEGA., .nu. = 0,J,.LAMBDA.) + O(3Pj) Distributions

“…near DO is modest (<1 per cm-1). [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Since in addition the number of product channels is relatively small, at least at low excess energies I?, quantum state-specific effects are expected. Above DO the spectrum of NO2, which remains highly structured, can be described in terms of coherently overlapping resonance^.^^^,^^ The state-selected resonance widths gradually increase with I?,7 as the dissociation lifetime decreases from a few picosecond just above DO to 1 ps at E?…”