Measurements of molecular fragment alignment and orientation in the UV photodissociation of NO2 and O3

REMPI-TOF studies of the UV photodissociation of NO2 and O3 at 320 nm and 270 nm respectively have allowed both the orientation and alignment parameters of the molecular photofragments to be determined.

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) ← X2ΠΩ (v = 0) transition.The O2 (a 1Δgv = 0 J = 20) fragment shows a markedly larger orientation parameter, for example β20(21) = 0.55 ± 0.04, measured on the S(20) transition.

These orientation parameters and the accompanying alignment parameters which describe the fragments' angular momentum polarisation have been used to confirm spectral assignments within the highly perturbed (2 + 1) d 1Πg(v = 2) ←← a 1Δg (v = 0) REMPI spectrum of O2.


Angular momentum polarisation of both atomic and molecular photofragments is a direct consequence of the nature of the potential energy surface upon which the bond cleavage occurs, and experimental studies of photodissociation have increasingly focused on measuring the distributions of photofragment angular momentum vectors J relative to other vector quantities such as the fragment relative velocity and the parent transition moment.1–5

These distributions of angular momentum are characterised by a set of moments, βKQ(k1k2), averaged over the photofragment ensemble,6 and can be divided into two classes, namely those corresponding to the alignment and to the orientation of J (those of even and odd rank k2 respectively).

The former can be probed with linearly polarised light, and have been the subject of many investigations into the alignment of product angular momentum with respect to a reference axis.1–5

An example is in the moment β00(22) which relates the products' velocity v and angular momentum J and is equal to the expectation value of the second order Legendre polynomial, 〈P2(cosα)〉, where α is the angle between the vectors.

However, such measurements give us no insight into the sense of this angular momentum.

For example, they may inform us that a molecular photofragment is rotating in the xz-plane with its velocity related to that plane, say in the positive x direction, but we cannot tell if the angular momentum vector is pointing in the positive or negative y direction: we can measure alignment but not orientation.

The use of circularly polarised light overcomes this difficulty and allows orientation parameters to be measured.

Such experiments on the orientation of molecular fragments7–12 are far less numerous than their alignment counterparts, yet the information they yield is essential for a full understanding of the state or states involved in the photodissociation.

In the work presented here we first employ both linearly and circularly polarised light to probe, via (1 + 1′) resonance enhanced multiphoton ionisation (REMPI), the NO photofragment from NO2 photolysis at 320 nm.

We demonstrate the applicability of time of flight (TOF) spectrometry to extract alignment and orientation parameters on this test system, and compare our orientation results with those previously obtained by Brouard et al8. using laser induced fluorescence (LIF), and Nesterov and Cline9 using ion imaging.

Secondly, we apply the same method to measure alignment and orientation parameters for the O2(a 1Δgv = 0 J = 20) fragment from the 270 nm photolysis of ozone.

We use several (2 + 1) REMPI transitions which involve excitation of the two photon d 1Πg(v = 2) ←← a 1Δg (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 the photolysis of NO2, pulsed radiation at 320 nm was provided by the frequency doubled output of a Nd:YAG pumped tuneable dye laser [Spectra Physics GCR-200 + LAS LDL 2051], while the counter-propagating pulsed probe radiation around 225 nm was from the frequency doubled output of a XeCl excimer pumped dye laser [Lambda Physik EMG 101 MSC + Lambda Physik FL2002], exciting individual rotational lines of the NO A 2Σ+ (v = 0) ← X2ΠΩ (v = 0) band.

The beams were focussed to overlap in the source region of a modified Wiley McLaren TOF mass-spectrometer, into which a room temperature sample of NO2, at a backing pressure of around 120 Torr, was injected via a pulsed valve.

The rotational temperature of the molecular beam was measured to be 150 ± 10 K from the REMPI spectrum of NO expanded through the nozzle under the same conditions.

The laser pulses were overlapped temporally as well as spatially to allow a (1 + 1′) REMPI detection scheme whereby both the photolysis and ionising radiation were provided by the 320 nm wavelength light.

The probe light at 226 nm was kept at low intensities to avoid saturation of the strong one-photon resonant transition,14,15 and the absence of saturation was confirmed by the invariance of the TOF profiles with laser intensity.

For the studies on ozone the excimer pumped system was used as the 270 nm photolysis laser, with the YAG pumped system providing the photons near 320 nm for the (2 + 1) REMPI of the O2 a 1Δg (v = 0) product.

Ozone was introduced through the pulsed valve into the mass spectrometer from a room temperature bulb containing approximately 100 Torr of a 1∶1 mixture of ozone and O2.

The space-focussed ions were detected by the TOF system following the REMPI pulse as previously described.16

The alignment moments were extracted using linearly polarised pump and probe sources.

The polarisation of one laser was fixed with its electric vector at a predetermined angle to the TOF axis, using a polarizing crystal.

The counter-propagating second laser beam was switched between two values of this angle, θ, normally 0 and 90° using a photoelastic modulator [Hinds Instruments Inc. PEM-90].

The extraction of the orientation parameters required a circularly polarised probe beam, which was again provided by the photoelastic modulator.

Here the helicity of the probe beam was switched, with the electric vector of the photolysis beam held at 45° to the TOF axis.

Weak single-laser signals were subtracted from the two-laser signal, and all profiles were power normalised by monitoring the intensity of back reflections of the uv light.

For extraction of the alignment moments, the average of around 1000 time of flight profiles per geometry was found to give a sufficient signal-to-noise ratio.

For the photolysis of NO2 the comparatively small differences between the profiles generated with switched left and right circularly polarised light meant that averages of around 15 000 profiles were required for experiments to determine the orientation moment.

The greater effect on the time of flight profile of the orientation parameter in a (2 + 1) REMPI process compared to that for a (1 + 1′) scheme meant that it was unnecessary to average more than 2000 profiles to extract moments in the ozone experiments.


The intensity and shape of the time of flight profile depends upon both geometric parameters (such as the photolysis and probe lasers' polarisations relative to the time of flight axis and the spectroscopic transition being used) and dynamical parameters (the distributions of the vectors μ, the parent molecule transition moment, v, the fragments' relative velocity and J, the diatomic angular momentum).

The profile may be described by the expression: where gi are functions of geometric parameters (relative directions of the laser beams and TOF axis), line strength factors (we refer to the latter as qi) and moments, βKQ(k1k2), Pi(x) are ith order Legendre polynomials, and the variable x = vz/v0, where vz is the projection of the fragment velocity along the TOF axis, and v0 its speed.

The index i takes values that are determined by the polarisations of the pump and probe lasers and the number of photons used in the REMPI process.

The line strength factors, qi, are calculated using the formalism derived by Kummel, Sitz and Zare (KSZ).17

The values that we seek are βKQ(k1k2), the moments of the distribution, ie the ensemble averaged values of the distributions of the vector quantities.

The most important of the bipolar moments of even rank are β00(22), β20(20) and β20(02), describing the vJ, μv and μJ pair correlations respectively, and these can be found by analysing the TOF lineshapes taken with judicious choices of angles of the electric vectors of the linearly polarised photolysis and REMPI lasers relative to the TOF axis.18–20

There are also higher order moments which affect the shape of the time of flight profile, such as β20(22) which represents the triple vector correlation between μ, v and J, and their extraction and significance is discussed later.

Maximum sensitivity to the orientation moment is obtained by photolysing with linearly polarised light at an angle of 45° to the TOF axis and taking the difference between TOF profiles with left and right circularly polarised probe radiation.

We give explicitly our expression for Δg(x), the difference between left and right circularly polarised signals as a function of the parameter xg(x) can be quantitatively related to the distribution of ion arrival times, our experimental observations.

We note that Δg(x) is for the difference of normalised arrival times: in practice we find that the time integrated left and right circularly polarised signals are equal to within experimental error (4%), giving confidence in the successful operation of the photoelastic modulator.

Our expression for Δg(x) derived from the KSZ formalism17 is identical to that derived by Cline et al.19

In particular we are able to obtain the orientation moment β20(21), which describes the preferred sense of rotation of the diatomic fragment and is defined as β20(21) = −2〈sin θt cos θt sin θr sin ϕr〉 where (θt,ϕt) and (θr,ϕr) are the polar angles which define the photofragment velocity and angular momentum vectors respectively in the body fixed frame.6,8,9

In this frame the fragment recoil velocity lies in the xz plane and the z axis is parallel to the transition dipole moment μ.

Thus, in the case where θt and θr are less than 90°, a positive value for β20(21) indicates that fragments' angular momentum is oriented preferentially along the −y direction.

Again higher order odd rank moments appear in eqn. (2), and we discuss their influence later.

Results and discussion

Photodissociation of NO2 at 320 nm

First, we present data on the nascent NO fragment from the photodissociation of NO2 at 320 nm recorded using (1 + 1′) REMPI via the A state of NO.

Although our emphasis is on the orientation of the NO fragment we discuss first the alignment parameters obtained for photolysis at 320 nm with the linearly polarised probe beam tuned to the P22 + Q12(N = 40) transition in the A 2Σ+ (v = 0) ← X 2Π3/2 (v = 0) band.

Table 1 shows the results.

For prompt dissociation of a non-rotating and non-vibrating NO2 molecule, where the transition moment μ for the 2B22A1 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.

If the ground state geometry of NO2 is preserved in the promptly dissociating excited state we expect the μv correlation β20(20) = 0.88.

Many previous studies have measured β20(20), and have shown that its value is reduced from this limit for the rotating molecule, and depends upon fragment speed (and thus dissociation wavelength).21

In LIF experiments probing NO (v = 0, N = 29) from a thermal sample of NO2 Brouard et al8. found β20(20) = 0.61 ± 0.02 at a photolysis wavelength of 308 nm, whilst earlier experiments by Baker et al. in this group reported β20(20) = 0.26 ± 0.14 for photolysis at 355 nm, where the fragment speed is approximately halved.21

Baker et al. also measured the NO (v = 0, N = 15) fragment and found that β20(20) increased to 0.41 ± 0.06, reflecting the greater energy available for product translation.21

When the same N = 15 line was probed in photolysis of a jet cooled (10 K) NO2 sample, β20(20) increased to 0.61 ± 0.15,21 the same value as that reported by Hradil et al. in ion imaging experiments carried out on a 15 K sample.22

These authors noted that their result, the average over several rotational states, was substantially larger than values previously measured23–25 for samples close to room temperature.

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.

There are fewer previous measurements of other pair correlations.

Our value of β00(22) = −0.42 ± 0.06 is in keeping with previously published results which show v and J to be dominantly perpendicular8,21,25 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 μJ 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).

We finally note the higher moments in Table 1.

β20(22), the triple vector correlation, was found simultaneously with β00(22)from the sum and difference of profiles taken with magic angle photolysis and switching the 226 nm beam between θ = 0 and 90°.

For the β20(42) term (which describes a higher order correlation between μ, v and J), profiles were analysed with the photolysis laser polarised parallel to the TOF axis and with θ = 0 and 90°: these show an increased sensitivity to this moment.

The results are consistent with the pair correlation values of Table 1 and these, together with the pair correlations discussed above serve to give confidence in the analysis procedures employed in these TOF studies.

We now describe the results of experiments with circularly polarised probe radiation to determine the orientation moment β20(21).

Three lines were chosen for study, namely the P22 + Q12(N = 40) line discussed above, together with the R11 + Q21 (N = 30) and P11 (N = 29) features.

In Figs 1a and 1b we show the sum and difference profiles for left and right circularly polarised 226 nm probe radiation for the R11 + Q21 (N = 30) and P11 (N = 29) lines, with the 320 nm photolysis laser polarisation at 45° to the TOF axis.

Sums and differences are shown to give an idea of the magnitude of the difference between the signals with the two circular polarisations, of the order of 5% at the line centres.

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.

Q branches are insensitive to the fragment helicity at these high values of N, and hence the contribution of the Q21 and Q12 sub-bands to the N = 30 and N = 40 lines does not affect the difference profiles providing the R or P branch contribution is appropriately normalised.9,19

The difference profiles should be zero for the geometries where the photolysis beam electric vector is at 0° or 90° to the TOF axis,9,26 and an example confirming this is shown in Fig. 1c for the P22 + Q12 (N = 40) transition and θ = 90°.

The contributions of the higher order β20(23) and β20(43)terms to the difference profile were neglected in our analysis, as the value of q3 in eqn. (2) is zero for a (1 + 1′) REMPI process when the ionisation efficiency is assumed to be independent of fragment alignment.4

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.

This is in excellent agreement with the results of ion imaging studies by Nesterov et al9. at 355 nm in which the NO2 sample was cooled in a molecular beam, (β20(21) = 0.2 for levels probed with the P11 (N = 36) and R21 (N = 21) lines) and with the LIF studies by Brouard et al. at 308 nm8 which employed a room temperature sample and measured β20(21) = 0.22 ± 0.04 for the same P11 (N = 29) transition as measured here.

It appears that the orientation is little affected by parent angular momentum, as the three studies used markedly different NO2 rotational temperatures.

We note that in the work by Cline the A state of NO was ionised using a third colour whereas in this work, we use the photolysis laser to ionise NO from the intermediate A state.

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 results are qualitatively in accord with the expectation that the impulse given by the departing O atom acts on the N end of NO, causing rotation with J pointing in the −y direction8,9.

Photodissociation of O3 at 270 nm

Having established the consistency of the results obtained from our experimental method with those measured previously, we now turn to the photolysis of ozone.

At a photolysis wavelength of 270 nm the dominant dissociation channel produces O2 in the low lying first excited state, a 1Δg,27,28 and Fig. 2 shows a section of the (2 + 1) d 1Πg(v = 2) ←← a 1Δg (v = 0) REMPI spectrum of nascent O2 around 320 nm, with assignments from the recent analysis of Morrill et al.13

The d 1Πg(v = 2) upper level is perturbed by curve crossings with the nearby II 1Πg valence state,13 and this results in a highly irregular band structure, with branches split into a number of spectrally distinct sections.

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.

Photodissociation at 270 nm in the Hartley band proceeds by two spin allowed routes forming O2(a 1Δg) + O(1D) and O2(X 3Σg) + O(3P) in the ratio 0.9∶0.1 respectively.27–29

The curve crossing on the excited state potential that leads to formation of the X 3Σg state of O2 is allowed only for odd J states.

This results in a selective depletion of species with odd J in the rotational distribution of the O2(a 1Δg) product, a result first demonstrated in the CARS experiments of Valentini and co-workers.29,30

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.

A similar effect is seen in the R and in the (more congested) O branch of Fig. 2.

We focus first on the two (2 + 1) REMPI transitions shown in Fig. 2 at the probe laser wavelengths of 320.499 nm and 320.821 nm.

Our wavelength scale for the transitions is in accord with the energies given by Morrill et al13. to within the precision of our laser wavelength step size (0.002 nm).

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.

Table 2 collects data for the two transitions taken (as for NO2) with appropriate combinations of photolysis and probe polarisations, and shows good agreement amongst the values of the alignment moments for the two probe transitions.

The constancy of such data confirms the assignments of Morrill et al13. for these two lines, and we illustrate this confirmation in the analysis of data in Figs. 3a and 3b.

Here we see the effect of switching the polarisation of the REMPI laser from θ = 0 to 90°, whilst holding the photolysis laser at the magic angle of 54.7° to the TOF flight axis.

This experimental geometry is sensitive mainly to the bipolar moment β00(22), i.e. the vJ correlation.18

We see that the difference profiles are of opposite sign in the two cases, exactly as expected for S and R branch transitions, with a smaller difference for the R branch as expected from the relative values of the line strength factors qi.

Probing on an S branch transition will therefore yield a higher sensitivity to the vJ correlation moment β00(22), as seen in the slightly lower error limit of Table 2.

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).

The higher order terms listed in Table 2 were obtained in a similar way to that described for NO2 photolysis.

In addition however it was necessary to involve moments βKQ(k1k2) with the index k2 = 4.

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.

We also note that the widths of the profiles are consistent with these assignments: the width depends upon the translational energy release in the photolysis, and its value gives an estimate of the O2 internal energy.

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.

The other alignment moments in Table 2 for J = 20 are consistent with a parallel transition followed by prompt dissociation, as for NO2.

Our reported values for the μv correlation, β20(20), are in broad agreement with previously published data on the molecular fragment from photofragment translational spectroscopy (0.8 ± 0.1, 275 − 295 nm)31 and time-of-flight studies within this group32 (0.65 ± 0.07 at 290 nm) and on the atomic fragment via photofragment spectroscopy (0.50 ± 0.05, 274 nm),27 VUV LIF (0.56 ± 0.06, 266 nm)33 and ion imaging (0.75 ± 0.08, 265 nm; 0.80 ± 0.08, 275 nm).34

The variation in magnitude of β20(20) within these studies can again be attributed to varying rotational temperatures of the parent O3 molecule.22,33

Qualitative indications of perpendicular alignment of v and J vectors in the Hartley band dissociation of ozone have been reported in our previous TOF measurements,32 and in the ion imaging experiments by Suits et al.35

In Table 2 we also include measurements on the O(24) line at 321.810 nm, which also appears to be isolated in Fig. 2 (we are unable to study O(20) because of spectral congestion).

Once again the analysis as an O branch line yields similar alignment parameters to those for the S and R branch lines.

Measurements of these vector correlations at different wavelengths in the Hartley band, as a function of J, and on transitions in the (1,0) as well as the (2,0) d 1Πg ←← a 1Δg band will be described elsewhere:36 in this work we concentrate only on even values of J for photolysis at 270 nm.

We now turn our attention to the orientation parameter β20(21).

Fig. 4 shows measurements on the S(20) and R(20) features of Fig. 2, again with the electric vector of the photolysis laser at 45° to the TOF axis and with circularly polarised probe radiation.

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.

We also note the magnitude of the difference is considerably larger than for the NO case of Fig. 1.

Although part of this is because of a larger value of β20(21) (see below), we note that the orientation effect is much more easily observed using a (2 + 1) REMPI scheme, as the geometric factors, qi, for probing the orientation moment β20(21) are considerably greater than for (1 + 1′) REMPI.

Lineshapes observed with linearly polarised light allows straightforward differentiation between branches with different values of |ΔJ|, i.e. the S and R branches shown in Fig. 3.

With linearly polarised light however it is not straightforward to distinguish between O and S branch transitions as the sensitivity to branches with |ΔJ| = 2 is the same.

They may be differentiated when circular polarisation is employed, as shown in Fig. 4c.

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

Although the present measurements are on lines where the assignments are robust, we suggest that the use of this “spectroscopy through dynamics” method of determining the magnitude and sign of ΔJ and also the magnitude of J (within rather wide error bars) will be useful in the assignment of the more perturbed and overlapped regions of the spectrum.

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.

Part of this discrepancy may be explained by overlap with O branch features at this wavelength,13 as the two branches in linear polarisation will have difference spectra which have opposite signs.

We now consider the values of the β20(21) bipolar moment, and turn first to the S(20) transition.

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.

However we find that our returned values of β20(21) are relatively insensitive to the absolute magnitudes the two higher terms in eqn. (2), because their coefficients are of opposite sign and their effects tend to cancel.

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.

We should note however that although we are insensitive to the absolute values of these higher moments, we are sensitive to their ratio.

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.

We see good agreement with the values of the orientation moment β20(21) for the three branches.

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.

The difference in β20(21) values remains to be explained in terms of the appropriate angles described in eqn. 3 and we hope that measurements such as these will stimulate trajectory calculations on the latest ab initio surfaces37.


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 1Δg) 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.

We have been able to make use of angular momentum polarisation to confirm spectral assignments of the highly perturbed O2 d 1Πg(v = 2) ←← a 1Δg (v = 0) REMPI spectrum.