J. P. Simons

M. Brouard

P. A. Enriquez

R. Sayos

S. P. Duxon

Crossref

Laser Chem. Vol. 11, pp. 265-272
Reprints available directly from the Publisher
Photocopying permitted by license only
1991 Harwood Academic Publishers GmbH
Printed in Singapore
STEREODYNAMICAL STUDIES OF VELOCITY
ALIGNED PHOTOFRAGMENTS
M. BROUARD, S. P. DUXON, P. A. ENRIQUEZ, R. SAYOS?
and J. P. SIMONS
Chemistry Department, The University, Nottingham NG7 2RD, UK
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) + N20 NO + NO are outlined.
KEY WORDS: Stereodynamics, vector correlations, photodissociation.
1. 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. 1-4 Such 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.
265
266 M. BROUARD ET AL.
primary generation of superthermal, velocity aligned O(aD) atoms. 17 The 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’ is5
I(p. 1’) 1 + BP2(tp. 1’) (2)
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)
Figure I Schematic Newton diagram for the collision of superthermal velocity aligned atoms, with a
stationary target molecule.
STEREODYNAMICAL STUDIES 267
in terms of the spectral lineshape function2
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’
I(p. j’) 1 + AP2(p" j’) (6)
where A fi(P2(l" j’)) 5(P2(p" l’)} (7)
In terms of the rotational alignment parameter A), conventionally defined as2’8
A( (LAB) 2(Pz(p. j’) } or A)(CM) 2(P2(1 ’) ) (8)
the alignment coefficient
A--5A)(LAB)-1/3A)(CM) (9)
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. 6
Other 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.
3. STEREODYNAMICS OF THE REACTION O(1D) + N20---> NO + NO
Quantum State Distributions and Scalar Pair Correlations
Photodissociation of N20 at 193 nm initiates the highly exothermic reaction
O(’D) + N20---) NO + NO (10)
In Eqs. (3) and (7) the factor fl/5 may be equated with (P2(p l)}, arising from the LAB CM
transformation.
268 M. BROUARD ET AL.
0.4
0.3
0.2
0.1
II
489 490 491 492
wavelength / nm
Figure 2
(b)
0.8
0.6
0.4
0.2
0
489 490 491 492
wavelength/nm
LIF spectra of NO(v 17) recorded under (a) relaxed, and (b) near single collision conditions.
STEREODYNAMICAL STUDIES 269
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) (11)
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).
270 M. BROUARD ETAL.
0 200 400 600 800
Rotational energy cm-1
(h)
+
"_o
0 5000 10000
Ecm "
Figure 3 Rotational population distributions in NO; (a) v 0; (b) v 1.
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.
A near stripping mechanism requires i- I’ 1, and a LAB translational alignment
B[NO] (/3[O(1D)]) (cf. Eq. (3)). The I,i’ correlation can be estimated from the
Doppler profiles; the rotational lines in the slow moving NO(v 0) are too narrow
for the experimental resolution to reveal any changes in line shape when the
excitation-detection geometry is changed from coaxial to perpendicular but their
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).
STEREODYNAMICAL STUDIES 271
-0.4 -0.2 0 0.2 0.4
-1FREQUENCY SI lllVF cm FREQUENCY SHIFT cm
(b)
_z
-0.4 -0.2 ) 0.2 0.4
-I -IFREQ! IFNCY I llI’ cm FREQUENCY SIIIFT cm
Figure4 Doppler profiles for NO(a) v=0, N=20;(b) v= 1, N=35"(c) v=3, N=31"allCaseA"
(d) v 10, N 26; Case A--circles, Case D--crosses.
Table Energy disposal and pair correlations
NO(v,) E,. cm-’ (En)/cm -1 (ET)tAB/cm -’ NO((v2))
0 0 200 110 16-18
1,876 4,000 690 15
3 5,540 >4,000 1,350 11
10 17,600 not determined 1,280 <4
15 26,145 4,000 700
16 27,600 3-4,000 > 1,600 0
272 M. BROUARD ET AL.
0.4
0.2
(2)
A0 0.0
-0.2
m m-
[] [] []
-0.4
4.5 8.5 12.5 16.5
,J
Figure 5 Rotational alignments for NO(v 0) generated in the reaction of velocity aligned O(1D) with
N20.
forward scattered partners, NO(v 16-18) are endowed with much higher LAB
velocities, VNo > 1.52 km s-1 (for stripping dynamics VNo > 2 VCM 1.8 km s-l).
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.
Acknowledgements
R. Sayos thanks the British Council and the Spanish Ministry of Education and Science for the award of a
Fleming Fellowship, P. A. Enriquez also thanks the Spanish Ministry of Education and Science for
postgraduate research support.
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6. F. Green, G. Hancock and A. J. Orr-Ewing, Faraday Discussion Chem. Soc. 91 (1991) in press; F.
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Stereodynamical Studies of Velocity Aligned Photofragments

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