a
b
Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas,
711 10, Heraklion, Greece. E-mail: [email protected]
Department of Chemistry, University of Crete, 714 09, Heraklion, Greece
COMMUNICATION
Rachel L. Toomesa and Theofanis N. Kitsopoulos*ab
PCCP
Rotationally resolved reaction product imaging using crossed
molecular beams
Received 20th March 2003, Accepted 7th May 2003
First published as an Advance Article on the web 14th May 2003
The hydrogen abstraction reactions of atomic chlorine with
ethane and n-butane are studied in a skimmerless crossed molecular beam experiment by imaging of state-selected products.
The differential cross section for HCl(v ¼ 0, J ¼ 1–5) product
is directly determined from the product velocity map image.
The state selection of products is achieved using (2 + 1) resonance enhanced multiphoton ionization, a method generally not
used in crossed-molecular beam experiments due to sensitivity
problems. This technique opens the way for a plethora of polyatomic reactions (involving more than three atoms) to be studied
with scattering angle and rotational resolution.
The dynamics of chemical reactions can be probed in detail by
experiments that are sensitive to both reactant and product
internal state energy distributions and velocities. The most suitable experiments, which allow unambiguous determination of
state-resolved differential cross-sections, involve crossed molecular beams.1 However, only the product velocity distributions
of the reactions H + D22,3 and O + D24a have been measured
with rotational resolution; both these reactions were studied
using H-atom Rydberg time-of-flight detection, a method that
provides unsurpassed energy resolution but which is restricted
to hydrogen elimination reactions. Only in a recent unprecedented experiment have Liu and coworkers demonstrated that
it is possible to measure the product correlated differential
cross section for the F + CH4 reaction with vibrational and
partial rotational resolution.4b Here we report the first crossed
molecular beam measurements of rotationally state-resolved
differential cross sections for reactions involving more than
three atoms and we present results for the reaction of hot Cl
atoms with ethane and n-butane. Our experimental set-up
yields large signals (at least 103 more than traditional crossed
beam methods) and thus can be extended to a wide range
of chemical systems. Furthermore, these signal levels should
allow detailed rotational anisotropy parameters to be measured, similar to those observed in the remarkable results of
Lorenz et al.5 in their study of the inelastic scattering of Ar
and NO.
Reaction product imaging has been used to investigate a
limited number of reactions6 both with single beam7–9 and
crossed molecular beam set-ups.10–12 In a single beam experiment the reaction is initiated and probed only a few millimetres
from the nozzle orifice in a region of high number density; this
allows state-selective product detection via resonance enhanced multiphoton ionisation (REMPI). The data analysis13,14
requires knowledge of the internal energy distribution of all
products and this makes the scattering information unambiguous only for reactions where one of the two products is an
atom. On the other hand, crossed molecular beam studies
using imaging detection of products have, thus far, been
DOI: 10.1039/b303166g
restricted to the determination of non-state-selective differential cross sections in which an atomic product is probed using
(1 + 1) REMPI or a molecular product is probed by universal
photoionisation. This is because the molecular beams are
skimmed and typical product densities per quantum state
are only 105 cm3 which approaches the sensitivity limit of
state-selective REMPI. In addition, with the exception of
(1 + 1) REMPI, the probe laser must be focused thus creating
a very small interaction volume that yields extremely low count
rates. In their study of the F + CH4 reaction Liu and coworkers were the first to use (2 + 1) REMPI in a crossed molecular
beam experiment to measure state selected differential cross
sections.4b,15 The above shortcomings are circumvented in
the experimental apparatus that is shown schematically in
Fig. 1. In a set-up first demonstrated by Welge and coworkers2
two parallel molecular beams are produced using solenoid
valves which share a custom faceplate that in our case also
constitutes the repeller electrode. The molecular beam carrying
the alkane (R–H) reactant is centred on the time-of-flight axis
while the second beam is centred 19 mm off axis. The off axis
beam comprises about 60% Cl2 (99.8%) in He while the second
beam comprises 75% R–H in He. Two counter propagating
lasers intersect the respective molecular beams perpendicularly
approximately 5–10 mm from the repeller plate surface. A
small percentage of the Cl atoms produced by the photolysis
of Cl2 at 355 nm travel downwards and intersect the R–H
Fig. 1 Apparatus schematic. Counter propagating lasers intersect
pulsed molecular beams. The pump (photolysis) beam produces atomic
Cl that expands outwards and crosses the R–H molecular beam. HCl
product is state-selectively photoionised by the probe laser using
(2 + 1) REMPI. The resulting ions are detected with a 2-D position
sensitive imaging detector after passage through a linear time-of-flight
(TOF) mass spectrometer.
Phys. Chem. Chem. Phys., 2003, 5, 2481–2483
This journal is # The Owner Societies 2003
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Fig. 2 Product HCl(v ¼ 0, J ¼ 1) images from the reaction of Cl
with n-butane as a function of the delay between the photolysis and
probe lasers. The time shown is the time after the 8.5 ms it takes
the Cl atoms to reach the REMPI zone. The arrow indicates the Cl
atom reactant velocity direction.
molecular beam. Reaction ensues and HCl (v ¼ 0, J ¼ 1–5)
product is detected by (2 + 1) REMPI via the Q-branch of
the E(1S+) X(1S+) transition.16 The probe laser pulse is
delayed by 8.5 ms with respect to the photolysis laser pulse,
in order to allow sufficient time for the Cl atoms to reach
the probe laser interaction region. The H35Cl+ ions are accelerated by a velocity-mapping electric field17 and are imaged
using a home built imaging detector. Background images are
obtained using the same procedure as signal scans but with
the photolysis laser blocked.
The product images shown in Fig. 2 show a clear dependence on the time delay between the photolysis and probe
lasers. Specifically, at small delays the detected product is
mostly forward scattered, i.e. most of the intensity is in the
direction of the Cl-atom reactant velocity (see the Newton diagram in Fig. 3), and at longer delays the product distribution
becomes more isotropic. This is because, since both the R–H
beam and the Cl-atom ‘‘ beam ’’ have appreciable cross-sectional areas, there will be a sizeable reaction volume of which
only a small part (the REMPI zone) is probed by the focused
REMPI laser. Consequently, the delay between the lasers must
be increased to allow time for all products to enter the REMPI
zone. We typically step the laser timing over a range of 4 ms,
using increments of 100 ns every 300 laser shots depending
on the product count rates. It may be noted that because we
are operating under velocity mapping conditions,17 reaction
images are dependent only on the velocity (speed and direction) of the HCl product when detected in the REMPI zone
irrespective of where the reaction occurred or where in the
REMPI zone the ionisation occurs.
Fig. 3 Shown are the HCl(v ¼ 0, J ¼ 1 and 5) product images from
the reaction of Cl with n-butane. The Newton diagram for the reaction
is overlaid on the image of the Cl-atom reactant. CM indicates the
position of the centre of mass, the vectors vC12 and vRH represent the
Cl2 and RH molecular beam velocities respectively, vCl and uCl are
the velocities of the Cl atoms in the laboratory and CM frame respectively, uHCl is the velocity of the product HCl in the centre-of-mass
frame and y is the scattering angle.
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Phys. Chem. Chem. Phys., 2003, 5, 2481–2483
The distance between the point where the reactive collision
takes place and the point of entry into the REMPI zone defines
the radius attained by each product HCl Newton sphere. Since
the reaction region is much greater than the REMPI zone, most
of the product Newton spheres will have appreciable radii such
that we detect only a slice of the actual sphere. Newton spheres
created at different points along the direction of the molecular
beam will be sliced at different positions. To analyse our data
we consider the two extreme situations. First, we assume the
experiment measures the complete, unbiased three-dimensional
distribution, i.e. all parts of the Newton spheres are sampled
with equal weighting, in which case the inverse Abel transform
procedure is used to extract scattering information from the
images.6 The second extreme arises when there is preferential
sampling of the central region of the Newton spheres, i.e. there
is slice imaging, in which case direct analysis of the raw data
is possible.18 The two analysis methods give almost identical
angular distributions; the differences are generally less than
5% and never above 10%. The angular and speed distributions
presented in Fig. 4 were obtained by direct integration of the
raw data.6 Both analysis methods require the relative velocity
of the reaction to be parallel to the imaging plane, a condition
that is satisfied if the speeds of the Cl2 and R–H molecular
beams are approximately equal. Using a home built fast ion
gauge placed at different distances from the nozzles, we have
shown that the Cl2 and n-butane molecular beams travel at
approximately the same speed while the ethane molecular beam
is only slightly faster. We estimate the relative velocity of reaction to be less than 4 from the imaging plane thereby justifying
the analysis methods. The photolysis laser wavelength (355 nm)
dictates a 0.24 eV collision energy for the reaction. To calibrate
the speed distributions obtained from the images, we measure
the image of the reactant chlorine as shown in Fig. 3.
From the widths of the Cl speed and angular distributions,
the energy and angular resolution of our experiment are estimated at 15% and 18 respectively.
Fig. 4 The right panel shows the angular distributions while the left
panel shows the centre-of-mass speed distributions for forward scattering (0 –10 ) for the HCl(v ¼ 0,J) product from the reaction of Cl with
ethane (upper section) and n-butane (lower section). The arrows indicate the maximum speed for HCl for the mean collision energy (determined from the Cl calibration images) assuming no energy is
transferred to the internal modes of the alkyl. The maximum speed
indicated for reaction with n-butane corresponds to abstraction of a
secondary H atom.
As examples of typical background-subtracted product
images we show in Fig. 3 the images obtained for HCl(v ¼ 0,
J ¼ 1 and 5) from the reaction of Cl with n-butane. Visual
inspection of the images reveals a strong propensity for scattering in the forward direction for J ¼ 1 that is substantially
reduced for the J ¼ 5 state. Comparison of the intensities in
the backward scattering direction is complicated by residual
background. in this region. The HCl product has a cold rotational distribution and at J ¼ 5 the reactive signal was much
lower than the background. It is thus possible that much of
the intensity in the backward direction in the image for
J ¼ 5 is due to imperfect background subtraction rather than
reactive scattering. The angular distributions for the J ¼ 1 to 5
states of HCl from reaction with both ethane and n-butane are
shown in Fig. 4. The distributions have been truncated at high
scattering angles where the data is less reliable due to residual
background. For both reactions we observe a maximum in the
forward direction (between 0 and 45 ) for J ¼ 1 to J ¼ 4
whereas the distribution is more isotropic for J ¼ 5. The reaction of Cl with ethane has been studied by Kandel et al.19–21 at
the same collision energy as our experiment using single-beam
time-of-flight Doppler measurements. They found a shift
towards backwards scattering above J ¼ 3, however, contrary
to results presented in Fig. 4, they found the reaction to have
broad sideways scattering at low J. To investigate this discrepancy, we have performed single-beam imaging experiments
for this reaction. Using the same analysis method as Kandel
et al., we also find side scattering to be predominant. However,
when both the translational energy and angular distributions
were extracted by Brouard and co-workers, using the recently
developed method of Fourier moment image analysis,14,22
there was good agreement with the crossed molecular beam
results presented here. Hence we conclude that the discrepancy
between the findings of Kandel et al. and ourselves is due to
their assumption of negligible internal energy in the ethyl radical. The centre-of-mass speed distributions for HCl(v ¼ 0,
J ¼ 1 and 5) from the reaction of Cl with ethane (top section)
and n-butane (lower section) scattered at angles 0–10 are also
shown in Fig. 4, along with the maximum speeds predicted if
all available energy is converted into translation using the
average Cl atom speed for calibration. The speed distributions
for HCl from the reaction of Cl with ethane showed little
dependence on scattering angle whereas for the reaction with
n-butane there was a significant shift to lower speeds for higher
scattering angles, for example at J ¼ 1 the average speed is
about 13% lower for scattering at 110 than for forward scattering. The speed distributions and their dependence on scattering angle are in complete accord with those measured for
the reaction of Cl with propane23 and n-pentane24 in crossed
molecular beam experiments by Suits et al. In these experiments there was non-state-selective detection of products and
it was suggested that, at low collision energies, the forwardscattered HCl is associated with abstraction of secondary H
atoms while the sideways/backscattered component results
from abstraction of primary H atoms. Our work with n-butane
has shown that the forward and backscattering components
are due to HCl formed in lower (J < 5) and higher J states
respectively. From the work by Varley and Dagdigian using
selectively deuterated propane25 and isobutane,26 we expect
both primary and secondary H abstraction processes to make
significant contributions to all product HCl J-states. Thus
there seems to be no obvious link between forward and backward scattering and the abstraction of the two types of H
atom. Indeed, the close similarity between the angular distributions for H-abstraction from n-butane and ethane (with only
primary H atoms available) suggests that the nature of the H
atom, secondary or primary, has little influence.
We have demonstrated the use of crossed molecular beams
to measure state resolved differential cross sections for chemical reactions using (2 + 1) REMPI detection and imaging of
reaction products. The technique gives large product-count
rates and can be extended to complex systems of chemical significance; indeed, we have already measured the differential
cross sections for the reaction of Cl atoms with methanol
and dimethyl ether.27 The method offers detailed scattering
information on large systems that we hope will stimulate
extended theoretical/computational efforts to understand chemical reactivity at the most fundamental level.
Acknowledgements
This work is conducted at the Ultraviolet Laser Facility operating at FORTH- (HP, Access to Large Scale Facilities EU
program, Contract No. HPRN-CT-1999-00007) and is also
supported by Network PICNIC HPRN-CT-2002-00183 and
REACTIVES HPRN-CT-9000-0006. TNK also thanks the
joint EU and Hellenic Ministry of Education program Applied
Molecular Spectroscopy (EPEAEK).
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