Detection of ended NO recoil in the 355 nm NO2 photodissociation mechanism

Article abstract of DOI:10.1063/1.479806

Circularly polarized 1+1' REMPI with ion imaging detection was used to probe μ-v-j angular correlations of the NO (X ²Π_1/2,3/2, υ=0) photoproduct from linearly polarized 355 nm photodissociation of NO2 in a molecular beam. Rotational angular momentum orientation was measured for the NO product, with the sense and magnitude of the orientation depending upon the direction of the NO recoil velocity with respect to the polarization vector of the photolysis light. The rotational orientation is shown to originate from the directionality of the torque responsible for NO rotational motion. The experimentally observed sense of rotation in the plane of the NO2 parent molecule is consistent with an effective recoil impulse force applied to the N end of the NO product. A preliminary measure of the bipolar moment associated with this orientation for the j=21.5 and j=36.5 rotational levels gives a value of approximately β₀²(21)=0.2.

Full text of DOI:10.1063/1.479806

Detection of “ended” NO recoil in the 355 nm NO 2 photodissociation mechanism
Vilen K. Nestorov and Joseph I. Cline
Citation: The Journal of Chemical Physics 111, 5287 (1999); doi: 10.1063/1.479806
View online: http://dx.doi.org/10.1063/1.479806
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 111, NUMBER 12
22 SEPTEMBER 1999
COMMUNICATIONS
Detection of ‘‘ended’’ NO recoil in the 355 nm NO photodissociation
2
mechanism
Vilen K. Nestorov and Joseph I. Clinea)
Department of Chemistry and Chemical Physics Program, University of Nevada, Reno, Nevada 89557-0020
͑
Received 15 July 1999; accepted 20 July 1999͒
Circularly polarized 1ϩ1Ј REMPI with ion imaging detection was used to probe ␮–v–j angular
2
correlations of the NO (X ⌸1
, vϭ0) photoproduct from linearly polarized 355 nm
/2,3/2
photodissociation of NO in a molecular beam. Rotational angular momentum orientation was
2
measured for the NO product, with the sense and magnitude of the orientation depending upon the
direction of the NO recoil velocity with respect to the polarization vector of the photolysis light. The
rotational orientation is shown to originate from the directionality of the torque responsible for NO
rotational motion. The experimentally observed sense of rotation in the plane of the NO parent
2
molecule is consistent with an effective recoil impulse force applied to the N end of the NO product.
A preliminary measure of the bipolar moment associated with this orientation for the jϭ21.5 and
2
0
jϭ36.5 rotational levels gives a value of approximately ␤ (21)ϭ0.2. © 1999 American Institute
of Physics. ͓S0021-9606͑99͒01236-2͔
In the study of the angular momenta that characterize
molecular trajectories in a chemical reaction, a molecule
having rotational angular momentum alignment has a pre-
ferred or avoided plane of rotation. Measurements of angular
momentum alignment have become routine. Angular mo-
mentum orientation indicates a preferred sense of rotation
and has only rarely been measured in molecular dynamics
is preferentially applied to one end of the NO product mol-
ecule.
The 355 nm photodissociation of NO2 is described by
2
3
NO ϩh␯→NO͑X ⌸
͒ϩO͑ P0,1,2͒,
1/2,3/2
2
7
–12
and has been the subject of numerous previous studies.
3
The O P product channel predominates and excited rota-
tional and vibrational levels ͑vϭ0,1͒ of both the ⍀ϭ and
spin–orbit states of NO (X ⌸) are observed. Angular
2
1
1
experiments.
2
3
2
2
13
To specify rotational orientation, one must choose an
observation plane. The presence of rotational orientation re-
sults from a net asymmetry of the reaction dynamics in that
observation plane. Zare et al. observed rotational orientation
correlation measurements of the NO fragment trajectory
show there is a preference for jЌv, where j is the NO angular
momentum vector and v is the recoil velocity vector. This
preferred ‘‘cartwheel’’ NO trajectory is expected since the
half-collision recoil dynamics are dominated by forces lying
of N scattered from a crystal surface at non-normal incident
2
2
angles. For this system the observation plane is perpendicu-
in the plane of the three atoms of the NO parent
lar to the surface and contains the incident velocity vector. A
second example is the rotational orientation of CN produced
in the photodissociation of ICN by circular polarized
2
molecule.9
,11,12
In contrast, the experiment described here is sensitive to
the sense of rotation of the NO product in the plane defined
3
–6
light.
In the circularly polarized photodissociation of
by the three atoms of NO . The experimental technique is a
2
achiral molecules the most natural observation plane for
measuring rotational orientation is perpendicular to the
propagation direction of the photolysis light.
Here we report measurements of rotational orientation
for the NO (X, vϭ0) product from the linearly polarized
simple variation of a method described in Ref. 14 for
REMPI-TOF probing of molecular stereodynamics using cir-
cularly polarized probe light. Reference 14 shows how to
quantitatively measure a preferred sense of rotation in the
half-collision scattering plane using an extension of the
widely used semiclassical bipolar harmonic scheme for de-
3
55 nm photodissociation of randomly oriented NO mol-
2
ecules. This result may seem surprising, since the total sys-
tem contains no intrinsic helicity or collective asymmetry.
Nonetheless, rotational orientation is observed when probing
the correlation of NO rotational angular momentum with re-
coil direction. We show below that this NO rotational orien-
tation originates from asymmetry in the photodissociation
1
5,16
scribing ␮–v–j angular correlations.
Within this scheme
2
the spectroscopically measurable ␤ (21) bipolar moment de-
0
scribes the preferred sense of rotation, and the reader is re-
ferred to Ref. 14 for a detailed discussion and interpretation
17
of this special instance of ‘‘false’’ chiral dynamics.
The diagrams in Fig. 1 illustrate how NO rotational ori-
mechanism of individual NO molecules: the recoil impulse
2
entation can appear in the NO fragmentation dynamics. At
2
3
55 nm the absorption transition dipole, ␮, lies in the mo-
a͒Electronic mail: cline@chem.unr.edu
lecular plane and perpendicular to the C axis. The electric
8
2
0021-9606/99/111(12)/5287/4/$15.00
5287
© 1999 American Institute of Physics
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1

5288
J. Chem. Phys., Vol. 111, No. 12, 22 September 1999
V. K. Nestorov and J. I. Cline
and the two possible bonds to be broken. Each of the four
cases has a characteristic NO product recoil direction with
respect to ␮ and a characteristic sense of rotation in the plane
of observation. This NO rotational orientation corresponds to
j directed either into ͑clockwise rotation͒ or out of ͑counter-
clockwise rotation͒ the observation plane.
The full, three-dimensional dynamics, and therefore the
body-fixed distribution of v and j, must have cylindrical
symmetry about ␮. The three-dimensional distribution of the
v–j angular correlation is depicted in Fig. 1͑b͒ by spinning
Fig. 1͑a͒ about the ␮ axis of symmetry. The resultant recoil
velocity sphere in Fig. 1͑b͒ has a radius ͉v͉. The NO angular
momentum vector, j, produced in coincidence with a particu-
lar NO recoil velocity, v, is always tangential to the surface
of the velocity sphere and always orthogonal to ␮. Viewed
from a point above Fig. 1͑b͒, the direction of the j vector
distribution circulates in a clockwise sense in the upper ve-
locity hemisphere, and counterclockwise in the lower hemi-
sphere. This unusual type of ␮–v–j angular correlation cor-
2
0
14
responds to 0Ͻ␤ (21)р1. If the recoil impulse were
instead applied to the O end of the NO product, the sense of
circulation of j in the two hemispheres would reverse, and
2
0
Ϫ1р␤ (21)Ͻ0.
The experiment to detect the endedness of the photodis-
sociation half-collision was conducted in a molecular beam
photofragment ion imaging apparatus similar to those used
elsewhere.1
8–22
A mixture of 10% NO in He was expanded
2
through a pulsed solenoid valve nozzle. The supersonic ex-
pansion passed into the photodissociation chamber through a
skimmer and an aperture in a repeller electrode. The molecu-
lar beam was orthogonally crossed by the laser beams in a
region between the repeller electrode and two aperture
electrodes.23 This stack of electrodes forms an electrostatic
ion lens with a symmetry axis along the propagation direc-
tion of the molecular beam. The potentials applied to the
electrodes were set to the velocity-map focusing condition23
to create an image on the face of a temporally gated multi-
channel plate two-dimensional ion detector. Ion images are
captured by a CCD camera that is synchronized to the pulsed
data acquisition cycle.
The experiment was of the pump-probe type with the
rotational orientation of the NO product probed in coinci-
dence with its recoil velocity by two-color 1ϩ1Ј REMPI
with ion imaging detection. A data acquisition cycle con-
sisted of a sequence of three laser pulses: ͑1͒ a linearly po-
larized photodissociation pulse at 355 nm with an energy of
FIG. 1. Diagram ͑a͒ depicts the four geometrically distinct transition states
for photofragmentation of NO , assuming impulsive recoil between the O
2
atom product and the N end of the NO product. In this half-collision scat-
tering plane, the vertical axis is parallel to ␮, the NO2 photodissociation
transition dipole. The NO products recoiling into the top left and bottom
right quadrants rotate clockwise ͑j directed into the plane͒, and products
recoiling into the top right and bottom left quadrants rotate counterclock-
wise ͑j directed out of the plane͒. Diagram ͑b͒ shows a three-dimensional
representation of the distribution of the NO product trajectories in a frame
attached to the ␮ vector. The distribution is symmetric about the ␮ vector
and the tips of the NO recoil velocity vectors, v, lie on a sphere. The
directions of j are tangent to the surface of the velocity sphere and circulate
2
in opposite senses in the upper and lower hemispheres. A ( ␮ˆ – ⑀ˆ ) angular
d
9
mJ, ͑2͒ a circularly polarized probe laser pulse at ϳ225 nm
with an energy of Ͻ100 ␮J, and ͑3͒ an ionization pulse at
08 nm with a pulse energy of ϳ1 mJ. The helicity of the
distribution of the dynamical objects in ͑b͒ is prepared and probed in the
experimental measurements.
3
circular probe polarization state was alternated between data
vector of the linearly polarized photodissociation laser, ⑀d ,
acquisition cycles using a photoelastic modulator.2
4,25
preferentially excites NO molecules having C axes perpen-
The ϳ225 nm probe pulse was circularly polarized and
tuned to resolved rotational transitions of the NO
2
2
dicular to ⑀ . This is depicted in Fig. 1͑a͒ for a single half-
d
2
2
collision scattering plane. Let us for the moment suppose the
resulting NO product trajectory to be that expected for im-
pulsive recoil of the product O atom from the N end of the
NO product. Within this single half-collision scattering
plane, there are four ‘‘transition states’’ to consider, corre-
A ⌺� X ⌸
(vЈϭ0� vЉϭ0) band. The 308 nm pulse
3
/2,1/2
2
ionized the NO (A ⌺) molecules. for pure P- and R-branch
transitions, the absorption cross section for circularly polar-
ized probe light is sensitive to whether the product molecules
are rotating in a clockwise or counterclockwise sense when
sponding to the two possible orientations of NO in the plane
viewed along the k propagation vector of the probe laser
2
p
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J. Chem. Phys., Vol. 111, No. 12, 22 September 1999
Ended NO recoil
5289
tion laser polarization, ⑀ . We vary ͑1͒ the angle ␪ between
d
D
the photolysis polarization, ⑀ , and the symmetry axis of the
d
apparatus, and ͑2͒ the helicity of the ϳ225 nm probe beam.
The parameter ␤ is used to characterize the probe helicity,
using the convention of Blum.2
6,28
By varying ␪D we can
project NO products with the same sense of rotation from
both the top and bottom halves of the recoil velocity sphere
onto the same region of the imaging detector. It can be
shown1
4,29
that the dependence of the experimental signal on
the probe beam helicity is maximum for tilt angles ␪D
ϭ␲/4 and 3␲/4. The probe helicity dependence vanishes for
␪Dϭ0 and ␲/2, since any augmentation ͑depletion͒ in inten-
sity originating from the top hemisphere is exactly canceled
by a depletion ͑augmentation͒ originating from the lower
hemisphere.
2
0
FIG. 2. Experimental geometry for measurement of ␤ (21) by 1ϩ1Ј
REMPI with velocity-map ion imaging detection. The molecular beam
propagates in the ϩZЉ direction toward the imaging detector and is perpen-
dicularly crossed by laser beams propagating parallel to the YЉ axis. For
simplicity, the velocity-map ion optics are not shown, but the symmetry axis
of the ion lens is parallel to the ZЉ axis. The electric vector of the linearly
In order to provide a compact representation of the ex-
perimental results, we do not show ion images in this Com-
munication, but instead show in Fig. 3 one-dimensional pro-
jections of the image data onto the XЉ axis of Fig. 2. The
one-dimensional projections in Fig. 3 are analogous to
REMPI-TOF transitions and contain the essential features
necessary to immediately identify the endedness of the NO2
photodissociation half-collision. A more complete analysis is
deferred for future publication.30 Figure 3 shows projections
measured with circularly polarized probing on three selected
rotational transitions of the NO product and at four values of
polarized dissociation light, ⑀d is tilted at an angle 0р␪ Ͻ␲ from the ZЉ
D
axis, thereby rotating the dynamical object in Fig. 1͑b͒ in the laboratory
frame. The circularly polarized probe beam propagation vector, k , is in the
p
ϩYЉ direction. The one-dimensional velocity profiles in Fig. 3 are projec-
ϩ
tions of the intensity of the NO velocity-map image onto the XЉ axis.
beam. For clockwise rotation, when viewed along the ϩkp
direction, the P-branch absorption cross section is larger for
right-circularly polarized light26 ͑␤ϭϪ␲/4͒ and the R-branch
absorption cross section is larger for left-circularly polarized
light ͑␤ϭϩ␲/4͒. The polarization dependence for a pure
Q-branch transition is nearly insensitive to the sense of
the photolysis polarization direction, ␪ . For a given probe
D
helicity, the dependence of the shape of the projections on
␪D is a consequence of ordinary photofragment recoil anisot-
ropy and is consistent with the known preference for v ␮
ʈ
2
7
2
2
rotation.
angular correlation arising from the B2� A1 transition of
NO2 at 355 nm.8 The variation in the velocity width of the
projections is due to simple conservation of energy con-
straints.
,9
Figure 2 shows the experimental geometry using the no-
tation from Fig. 1 of Ref. 14. The probe laser propagation
direction, k , is always perpendicular to the photodissocia-
p
FIG. 3. One-dimensional velocity pro-
ϩ
jections of the measured NO recoil
velocity-map image intensity onto the
XЉ axis in Fig. 2. Projections are
shown when probing the NO product
P11(36.5),
Q22ϩR12(33.5),
and
R21(21.5) transitions of the A� X, v
ϭ0 band. The shape of the profiles de-
pends on the photolysis polarization
angle, ␪D , the probe helicity, and the
branch type, as described in the text.
The probe helicity is specified by the
helicity parameter ␤.26
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1

5290
J. Chem. Phys., Vol. 111, No. 12, 22 September 1999
V. K. Nestorov and J. I. Cline
1
Our interest here is in how the velocity projections de-
pend on the probe helicity when probing P- or R-branch
More quantitatively, angular momentum alignment is related to the quad-
rupole moment of the j distribution about a quantization axis. Angular
momentum orientation is related to the dipole moment. In a quantum
mechanical description the terms refer to the properties of the mj distri-
bution; see Refs. 36 and 37.
transitions at ␪ ϭ␲/4 or 3␲/4, the two polarization geom-
D
2
0
etries most sensitive to the ␤ (21) bipolar moment. In these
instances a projection has either a convex or concave peak,
depending on the probe helicity. This results from selective
detection of the clockwise ͑or counterclockwise͒ rotating NO
products concentrated at the center ͑or periphery͒ of the pro-
jection at these two geometries. Any helicity dependence is
2
3
4
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projection. As expected, the probe helicity dependence is
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negligible for the Q ϩR branch transition.27 At the ␪D
6
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12
13
1
1
21
2
3
gives ␤ (21)ϭ0.2, measured by direct fit to imaging
theory for polarized 1ϩ1Ј REMPI probing.
value of ␤ (21) indicates that the N end of the NO product
0
2
9–31
14
The positive
2
0
15
16
receives the recoil impulse.
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complex. The observed sense of rotation is consistent with
NO rotation evolving from a decreasing NO bond angle at
2
2
2
2
1
2
3
the instant of bond rupture. Opposite senses of rotation might
instead be expected for an alternative fragmentation mecha-
nism in which the NO bond angle increases as a N–O bond
24
2
ruptures, in which case the O end of NO would effectively
receive the recoil impulse.
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͑
1996͒.
2
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5
6
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spond to right and left circularly-polarized probe light, respectively, as
defined in Ref. 38.
ciation of NO and the oriented electronic angular momen-
2
2
27
tum of the Cl P atom from the linearly polarized photo-
The spectroscopic branch dependence of circularly polarized probing of
3
/2
3
2,33
(1)
rotational orientation can be understood by examining the h (j) geo-
dissociation of ICl, recently reported by Rakitzis et al.
The experimental manifestation of the orientation in the two
systems is similar,
metrical factor that appears in the relevant absorption intensity expression
in Eq. ͑1͒ of Ref. 14. In the high-j limit, h( ϭ1 for P branches, 0 for Q
branches, and Ϫ1 for R branches; see Table I of Ref. 36.
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1)
6
,34
but has very different mechanistic ori-
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tion arises from quantum mechanical coherence between dis-
sociation channels involving parallel and perpendicular
28
29
V. K. Nestorov, R. Hinchliffe, K. T. Lorenz, R. Uberna, D. W. Chandler,
and J. I. Cline, in preparation.
V. K. Nestorov and J. I. Cline ͑in preparation͒.
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2
30
31
orientation is evident in an intuitive classical picture of the
photodissociation recoil trajectory.
We conclude by noting that measurements of this type of
rotational orientation can provide detailed mechanistic infor-
mation for other systems exhibiting prompt photodissocia-
tion or for bimolecular scattering.35
32
͑
1998͒.
33
T. P. Rakitzis et al., J. Chem. Phys. 110, 3351 ͑1999͒.
34
2
The ␤ (21) bipolar moment of Ref. 14 is identical ͑within a scaling fac-
0
(1)
tor͒ to the Im͓a1
( ,Ќ)͔ term defined by T. P. Rakitzis and R. N. Zare, J.
ʈ
We thank Dr. K. Thomas Lorenz and Dr. David W.
Chandler for numerous contributions and advice in the con-
struction and operation of the ion imaging apparatus at Ne-
vada, and Dr. Gregory E. Hall for helpful discussions. We
thank the National Science Foundation for support of this
research.
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37
38
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1