The orientation parameter, β20(21), for the NO(X²Π, v = 0) fragment is found to be essentially the same for the three quantum states, N = 29, 30 and 40 with a value of 0.24 ± 0.04 (2σ), as probed via (1 + 1′) REMPI, on the A²Σ⁺ (v = 0) ← X²Π_Ω (v = 0) transition.The O₂ (a ¹Δ_gv = 0 J = 20) fragment shows a markedly larger orientation parameter, for example β20(21) = 0.55 ± 0.04, measured on the S(20) transition.

We use several (2 + 1) REMPI transitions which involve excitation of the two photon d ¹Π_g(v = 2) ←← a ¹Δ_g (v = 0) transition,¹³ and show that by measurement of both alignment and orientation we can confirm the assignments in the highly perturbed rotationally resolved spectrum: spectroscopy through dynamics.

For prompt dissociation of a non-rotating and non-vibrating NO₂ molecule, where the transition moment μ for the ²B₂–²A₁ band lies in the molecular plane parallel to the line joining the two oxygen atoms, we would expect J to be perpendicular to both μ and v, i.e. the correlations with J to be close to their perpendicular limits, and this is seen to be the case in Table 1.

The present value of 0.64 ± 0.03 for N = 40 at a photolysis wavelength of 320 nm and a sample temperature of 150 K is consistent with previous observations.

Our value of β00(22) = −0.42 ± 0.06 is in keeping with previously published results which show v and J to be dominantly perpendicular^8,21,25 The value of β20(02) = −0.30 ± 0.05 is within experimental error the same as that of Brouard et al. (−0.23 ± 0.02) for the same line at 308 nm,⁸ but as we would again expect the μ–J correlation to be dependent upon fragment translational energy and parent internal energy, the agreement may be fortuitously dependent upon the cancelling of these two effects in the two studies (room temperature sample, 308 nm photolysis and probing N = 29 at room temperature⁸ compared with 320 nm photolysis and probing N = 40 at 150 K for the present data).

The best fits to the difference profiles are shown, and yield β20(21) = 0.26 ± 0.04 and 0.22 ± 0.04 for N = 30 and 29 respectively.

For the P₂₂ + Q₁₂(N = 40) transition we find β20(21) = 0.25 ± 0.04.

We note that in this (1 + 1′) REMPI process the difference profiles are expected to be equal in magnitude but opposite in sign for R and P branches, and this is observed in Fig. 1a and 1b.

We find the β20(21) moment to be insensitive to the three quantum states probed, with a state averaged value of 0.24 ± 0.04.

Saturation of the first excitation step would lead to an apparently reduced magnitude of the moments measured, but the good agreement between our values of β20(21) and the alignment moments compared with those of previous work again suggests that we avoid saturation.

The S and R branches shown in Fig. 2 however have J values around 20 which are highly populated in the photolysis of ozone at 270 nm,²⁹ and the lines in the nascent spectrum are not markedly overlapped by other transitions.

Although populations are difficult to extract from the highly perturbed REMPI spectra, the data of Fig. 2 do show an intensity alternation, with depletion of the intensities of lines with odd J. For example, in Fig. 2 the intensity of the S(19) line is considerably below that of the neighbouring S(20) and S(18) lines, whereas for a REMPI spectrum taken under conditions where the population alternation is absent (for example, Fig. 6 of ^{ref. 13}) the S(19) intensity is higher than those of its neighbours.

The lines have been assigned as S(20) and R(20) transitions respectively¹³ and we again show first the results of experiments with linearly polarised photolysis and probe beams.

Fits to the data with the assumption that the lines are identified as S(20) and R(20) respectively return a common and physically reasonable value of ca. −0.26 for the β00(22) bipolar moment, indicating the velocity and angular momentum of the diatomic photofragment are dominantly perpendicular (although not as marked as in the case of NO₂).

These moments have a particularly marked effect on the R branch transition of a (2 + 1) REMPI process, as reflected in the flatter profile near x = 0 in Fig. 3b compared with the shape of the S branch in Fig. 3a, and their values were consistent with the lower order moments shown in Table 2.

If we neglect the moments with k₂ = 4, we find for example values of β00(22) = −0.16 ± 0.2 and −0.21 ± 0.08 for the R(20) and S(20) lines respectively, and a comparison of these values with the data of Table 2 indicates the large effect for the R branch, together with a change in the error limits for the less affected S branch.

The resultant energy resolution for these data at J = 20 can be estimated as corresponding to ΔJ = ±3, by no means of high spectroscopic quality, but useful again for confirmation of the assignment.

Differences between profiles taken with left and right circularly polarised light should now show the same sign for S and R branches (the dominant q₁ term in eqn. (2) is the same sign for the two branches), as experimentally observed, and in contrast to the linearly polarised example shown in Fig. 3.

Here we measure correlations on the O(24) transition at 321.810 nm and observe a change in sign of the difference profiles compared with the S branch, confirming the assignment of Morrill et al.¹³

For example, on the strong feature at 321.451 nm in Fig. 2, noted as a P branch head, we measure a value of β00(22) = −0.07 ± 0.07 when it is analysed as a pure P branch transition, a result which is incompatible with the values of ca. −0.27 obtained earlier.

The ratio q₁/q₃ in eqn. (2) is 4.5, which in combination with the premultiplier factors means that the effect of the higher moment terms is negligible (for example, the coefficient of β20(23) is only 6% of that for β20(21)).

Extraction of β20(21) for this and similarly for the O(24) line is therefore straightforward and yields values of 0.55 ± 0.04 and 0.60 ± 0.04 respectively.

For the R(20) line the magnitude of the coefficient of β20(23) is now 30% of that for β20(21), and we should not neglect higher terms.

For example, if we set β20(21) equal to 0.57, the average value found from the S and O branch measurements, and vary both β20(23) and β20(43) in fitting the sum and difference profiles of the R branch line we find values of −0.32 and +0.80 respectively.

If we neglect the higher moments we obtain a value of β20(21) = 0.65 ± 0.02, close to our value when higher moments are taken into account, and illustrating the cancelling effect.

In the limiting case when μ and v are perpendicular to J, then we can express⁸β20(23)/β20(43) = −√15/8 = −0.48, and this is close to our observed ratio of −0.40.

Our value for the S(20) line, 0.55, is approximately twice those observed from the photolysis of NO₂ but of the same sign, indicating again the dominance of the recoil effect on the central atom in determining the sense of the product rotation.

The magnitude of the impulse delivered by the departing O atom is approximately the same for the photolysis of both parent molecules, with the departing NO and O₂ fragments both having speeds of approximately 1000 ms⁻¹.

Photolysis of a thermal sample of NO₂ at 320 nm produces NO(X ²Π) fragments whose average orientation parameter, β20(21), is 0.24 ± 0.04.

The orientation parameter is found to be essentially independent of the quantum state probed and our results are in excellent agreement with complementary investigations.

Studies on the O₂ (a ¹Δ_g) fragment produced from photolysis of O₃ at 270 nm show that this fragment is also oriented but now the orientation is markedly larger with β20(21) ≈ 0.6.