STEREODYNAMICAL STUDIES OF VELOCITY ALIGNED PHOTOFRAGMENTS

The state resolved stereodynamics of bimolecular reactions can be probed using velocity aligned 
photofragments as reagents, and polarised, Doppler resolved laser detection techniques for the products. 
The new strategy and its application to the reaction O(1D)


INTRODUCTION
Polarised laser probing allows unprecedented levels of reagent state specification/ selection and product state resolution and, with very narrow line lasers, of product velocity resolution via Doppler resolved spectroscopic techniques. 1-4Such methods, first initiated for the stereodynamics of photodissociation, have now been extended to create a new experimental strategy for probing the stereodynamics of bi-molecular reactions.5-7 Polarised photodissociation is used to generate superthermal "beams" of velocity aligned photofragments and the state-to-state stereodynamics of their subsequent collisions are probed at high resolution and sensitivity in a "simple" bulb experiment.The new strategy both complements and amplifies the more conven- tional molecular beam scattering technique.Doppler resolution of the polarised laser induced fluorescence (or ionisation) spectrum of the nascent bi-molecular collision products can allow determination of their quantum state distributions, scalar pair correlations and angular vector correlations, referenced initially to the laboratory frame, but after a simple laboratory to centre of mass transformation, to the bi-molecular collision frame.
The highly exothermic reaction O(1D) + N20-NO + NO, AEo 341 kJ mo1-1 (1) which follows the photodissociation of N20 at 193 nm leads to the generation of rotationally excited and aligned NO molecules.The excitation into all vibrational levels up to and beyond the thermo-chemical limit of VNO 16 establishes the -Present address: Departamento di Quimica Fisica, Universidad de Barcelona, 08028, Barcelona, Spain.
primary generation of superthermal, velocity aligned O(aD) atoms. 17The current status of our understanding of the reaction follows a brief outline of the basic theory for velocity aligned photofragment dynamics.
2. THEORY Translation Alignment.Figure 1 shows the Newton diagram for the collision of a superthermal velocity aligned atom A with a stationary target molecule BC.The LAB frame is defined by the polarisation vector of the incident photolysis beam ep Z.The CM collision frame is defined by the collision velocity vector k z v, the photofragment recoil velocity; for a stationary target, v k vc, the centre of mass velocity.
The LAB angular distribution of the b/molecular reaction products, AB, C, separating with a relative velocity k' is 5 The coefficient B, reflecting their LAB translational anisotropy is related to/3, the reagent translational anisotropy, via the equation B fi< P2(l} 1}') > 5 < P2(}p I}') > (3)   Determination of the CM translational alignment (P2(I !')) simply involves laboratory frame measurement of the product and reagent translational anisotro- pies.This can be done by using Doppler resolved laser induced fluorescence (or ionisation) spectroscopy, to probe the reaction products and the recasting of Eq. ( 2) in terms of the spectral lineshape function 2 g(9) 1 + flP2(p" i.)P20CD) (4) k. represents the probe laser wave vector and 2D, the relative displacement from the line centre, is defined as (9-9) c 9-9o ,D '0 v AgD Rotational alignment.The classical angular distribution of reaction product rotational vectors, j' is' where A fi(P2(l" j')) 5(P2(p" l')} (7)   In terms of the rotational alignment parameter A ), conventionally defined as 2'8 in the high j limit.
In practice, estimation of the translational anisotropy and rotational alignment parameters B and A via simulation of the experimental Doppler profiles, requires several levels of convolution, to accommodate the probe laser line profile, the thermal motion of the precursor and target molecules and any spread in the superthermal reagent speeds introduced by the dynamics of the primary photodis- sociation process. 6her vector correlations.Doppler resolved probing of photoninitiated bi-molecular collision/reaction products can reveal several additional correlations.As with direct photodissociation a total of nine bipolar moments are required to characterise fully the correlated angular distributions of j' and k' about ep, in a linearly polarised pump-probe experiment. 2 Provided the reagent translational anisotropy is known, the distributions of j' and k' can be referenced to the k vector rather than ep or the transition dipole/, and the Doppler profiles can be analysed to obtain the full set of bipolar moments. 2 Note however, the important correlation k', j', reflecting angle bending torques at the transition state, is invariant to the LAB CM transfor- mation.which proceeds with near unit collision efficiency to generate NO molecules in a very broad spectrum of rovibrational states up to and including VNo 18. 7 Fully dispersed LIF spectra of NO have been recorded under both single collision and partially relaxed conditions, via the , (v < 12) and /3(10 < v < 18) band systems.Full assignment of the populated levels is a daunting task, because of the very broad and dense rovibrational product state distributions (see Figure 2) but the task is now largely complete.Apart from those in the level v 0 (vide infra) they are all endowed with high levels of rotational excitation (see Figure 2).Population of the most highly excited molecules, i.e. those generated in v 18 two quanta above the thermochemi- cal limit, require a collision energy (EcM) < 70 kJ mo1-1.This can only be introduced via the translational excitation of the reagent O(ID) atoms generated in the primary photo-dissociation step.
N20 h-hv(193 nm)--Nz(v, J) + O(aD) If the N2 were formed predominantly in vN 0, for (EcM) < 70 kJ tool -1 energy conservation would require the primary excitation of the N2 into rotational levels J < 90, accounting for <55% of the available energy.? The analogous photodissociation of the isoelectronic molecule HN3 generates N2 in v2 0 with ---50% of the available energy in rotation. 9,a Molecules generated in Vyo 0 are rotationally cold (mean rotational temperatures Tr 320 K--see Figure 3) and translationally unexcited (Doppler with A,D 0.044 cm-l--equivalent to a LAB kinetic energy --110 cm-a--see Figure 4).NO molecules in levels v > 1 are rotationally highly excited; e.g.those in v 1 have a rotational temperature, TI ---5500 K, those in higher vibrational levels appear to be even hotter.They also carry higher translational excitation (see Figure 4).These data allow estimates of the mean energy disposals and by energy conservation, of the scalar pair correlations summarised in Table 1.
The exceptionally cold distribution in NO(v 0) requires that it be partnered by NO molecules carrying very high levels of internal excitation, predominantly in the "super-excited" levels 16 < Vyo < 18.The narrow spread in the Doppler profiles of NO(v 0), reflects a narrow spread in the internal energy of the excited partner; the broad distribution over internal rovibrational levels in NO(16 < v < 18) must reflect the spread of collision energies in the hot atom reaction.A "near-stripping" mechanism for the channel leading to NO(v 0) would accord with the behaviour observed.
The NO molecules produced in levels v 1-3 must also be accompanied by highly excited partners but the dramatic jump in their rotational and translational energies suggests the operation of an alternative dynamical pathway.High internal excitation in both NO partners suggests the operation of a short-lived complex mechanism: this view is reinforced by the absence of any measureable translational vector corre- lation, (P2(I 1') ) 0 (see below).direct measurement of the discussion dynamics by Huber's group gives (Eint (N2)) 58%, and /3[O(D)] +0.48 (P.Felder, B.-M. Haas and R. Huber, private communication).Vector correlations.The LAB rotational alignments, A)(LAB) for NO(v 0) are shown in Figure 5; despite the (inevitable) scatter, they are all negative, with A% (LAB) -0.1.Assuming the product alignment (P2(l.')) > 0, the photofragment alignment must be positive, with 2 > (/3) > 1/2.-If the cold ("old") NO(v 0) were a spectator in a pure stripping reaction, no alignment would be expected; there must be some residual interaction between the two NO species at the transition state.The negative alignment could be introduced by reagent orbital angular momentum transfer.
The NO molecules excited into intermediate vibrational levels also have larger Doppler widths but their resolved parallel and perpendicular detection profiles are identical within experimental precision (see Figure 4(d)) implying a near isotropic angular distribution, with (P2(I !')) 0, and the intermediacy of a short-lived collision complex in the channels leading to vibrationally excited NO pairs.

Figure
Figure I Schematic Newton diagram for the collision of superthermal velocity aligned atoms, with a stationary target molecule.

Figure 5
Figure 5 Rotational alignments for NO(v 0) generated in the reaction of velocity aligned O(1D) with N20.