REVIEW OF SCIENTIFIC INSTRUMENTS
VOLUME 74, NUMBER 4
APRIL 2003
Direct current slice imaging
Dave Townsend, Michael P. Minitti, and Arthur G. Suitsa)
Department of Chemistry, SUNY Stony Brook, Stony Brook, New York 11794 and Chemistry Department,
Brookhaven National Laboratory, Upton, New York 11973
共Received 4 November 2002; accepted 10 December 2002兲
We report a new variation of the velocity map ion imaging method that allows the central section
of the photofragment ion cloud to be recorded exclusively. The relevant speed and angular
distributions for a molecular photodissociation or scattering event may therefore be obtained
without need to utilize inversion methods such as the inverse Abel transform. In contrast to the
recently reported ‘‘slicing’’ technique of Kitsopoulos and co-workers 关C. R. Gebhardt et al., Rev.
Sci. Instrum. 72, 3848 共2001兲兴, our method makes no use of grids or pulsed electric fields which can
distort the photofragment cloud and therefore compromise the resolution of velocity mapping. We
find that by operating a multilens velocity mapping assembly at low voltages, the ion cloud stretches
in the acceleration region owing to the kinetic energy release in the fragments. Furthermore, this
inherent stretching is sufficient to allow the central section of the distribution to be recorded
exclusively by application of a narrow time gate 共⬃40 ns兲 to a position sensitive detector. We have
performed extensive ion trajectory simulations to understand this ‘‘direct current 共dc兲 slice imaging’’
technique, and experimentally we have applied it to the 355 nm dissociation of Cl2 and NO2 as
well-understood test cases. In the Cl2 studies the velocity resolution obtained for the 35Cl fragments
is on the order of ⌬␯/␯⫽2.8% and for the first time we are able to directly observe dissociation via
⫹
state channel whilst imaging the ground state Cl( 2 P 3/2)-atom distribution. For the
the weak B 3 ⌸ 0u
case of NO2 dissociation the internal state distributions of the NO fragment are extracted more
cleanly using slicing than is possible with the Abel inversion and our resolution is sufficient to
resolve some of the NO rotational structure in the kinetic energy release for the first time. Overall,
we find our data to compare very favorably with previously reported results and conclude that dc
slice imaging offers an important, easily implemented refinement to the velocity mapping approach.
We also demonstrate a second dc slice imaging method that records only the central section of an
expanded photofragment distribution by using a probe laser displaced off-axis from the molecular
beam. This approach, which we term ‘‘raster imaging,’’ may be particularly advantageous in
two-color experiments where the probe laser also makes a significant contribution to the initial
photolysis of the molecular species under investigation. © 2003 American Institute of Physics.
关DOI: 10.1063/1.1544053兴
I. INTRODUCTION
quirement that there be an axis of cylindrical symmetry parallel to the imaging plane for reconstruction techniques to be
valid. This imposes clear limitations on the laser polarization
geometries that may be used, particularly in two-color experiments where it is often desirable for the photolysis and
probe laser polarizations to be orthogonal. Cross-polarization
geometries of this type are a prerequisite for studies investigating vector correlations in the angular momentum alignment and/or orientation of the dissociation fragments and it
is an interest in effects of this type that served as the initial
motivation for the work that is presented here. Since the
equatorial slice through the image contains the full angular
and translational energy information, it would be preferable
if only this central section could be recorded directly in the
initial imaging experiment. This idea has recently been addressed by Kitsopoulos and co-workers,6 – 8 who advocate the
application of a pulsed electric field to the expanding cloud
of ionic fragments following a period of field free expansion.
This stretches the distribution along the time-of-flight axis to
the order of several hundred nanoseconds and is sufficient to
allow only the central ‘‘slice’’ to be imaged directly using a
narrow time gate at the 2D position sensitive detector. The
The ion-imaging technique first pioneered by Chandler
and Houston1 and the high resolution, velocity mapping variant demonstrated by Eppink and Parker2 have become standard methods in the study of molecular photodissociation
and reactive scattering.3 By neatly projecting the expanding
photofragment distribution onto a two-dimensional 共2D兲 position sensitive detector, the original three-dimensional 共3D兲
distribution may be reconstructed by means of the Abel inversion or other related techniques.4 This has clear advantages over conventional time-of-flight methods since the full
velocity and angular distributions of the fragments formed in
a dissociation event may be inferred directly from a single
image. While the ion-imaging approach has been applied
successfully to the study of numerous molecular systems,5
the technique has two inherent disadvantages. First, inversion methods introduce artificial noise into the reconstructed
image, especially along the symmetry axis, and this can lead
to a loss of experimental resolution. Second, there is a rea兲
Electronic mail: arthur.suits@sunysb.edu
0034-6748/2003/74(4)/2530/10/$20.00
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© 2003 American Institute of Physics
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Rev. Sci. Instrum., Vol. 74, No. 4, April 2003
need to use the inverse Abel transformation is therefore
eliminated and studies exploiting crossed polarization geometries to investigate angular momentum alignment and/or orientation in the photofragments become feasible.8 The one
drawback with this technique, however, is that in order to
create a field-free expansion region for the nascent fragments
produced during the dissociation event, a fine mesh grid
must be introduced into the ion optics assembly and this
inherently leads to a blurring of the observed image.2 The
high resolution that is possible with the velocity mapping
approach is therefore compromised. In this article we describe a new approach that enables the central slice of the
photofragment distribution to be imaged directly without the
need for pulsed fields. This ‘‘direct current 共dc兲 slicing’’
method therefore requires no grids and, hence, preserves
pure velocity mapping conditions. This in turn maximizes
the resolution attainable for the molecular system of interest.
In order to establish the validity of our approach we present
ion trajectory simulations and experimental studies of the
photodissociation of Cl2 and NO2 at 355 nm as test cases
that have been well studied using both imaging and nonimaging techniques. In the case of Cl2 the atomic photofragments produced provide a good test of overall experimental
resolution and an accurate test of instrument calibration. The
more complex structure exhibited in images recorded following the dissociation of a polyatomic molecule such as NO2 ,
however, provide a much greater insight into the validity of
the slicing approach. Photofragment velocity and angular
distributions reconstructed from dc-sliced images are compared directly with those obtained using conventional velocity mapping in conjunction with the Abel inversion and with
distributions recorded using a time-of-flight core-sampling
technique9 that makes no use of imaging methods.
In Sec. V we also present a variation on the dc slice
imaging method that we term ‘‘raster imaging,’’ and demonstrate it experimentally for the 355 nm dissociation of Cl2 .
This approach is similar in spirit to the ‘‘laser sheet’’ ionization approach of Tonokura and Suzuki,10 however, rather
than attempting to record the entire central section of the
distribution simultaneously using a cylindrical lens, the image is recorded off-axis from the molecular beam in small
sections using a spherical lens that is slowly translated to
move the probe beam through the expanded photofragment
distribution.
II. CONCEPT
A velocity map ion imaging setup typically consists of a
repeller plate and an extractor lens. The critical parameter for
achieving good velocity focusing 共and, hence, good image
resolution兲 is the ratio of the voltage applied to these electrodes. Once this ratio has been established for a given electrode system 共i.e., lens spacing, aperture size, and overall
flight path to the detector兲 then particles of a given mass
possessing any given velocity will be focused onto the same
spot on the detector, irrespective of where they were formed
in the ionization volume. This focusing dramatically reduces
image blurring due to the spatial spread of the molecular
beam. Additionally, unlike conventional ion imaging, no
dc slice imaging
2531
grids are required in the electrode system and the associated
image distortion is also removed. The position of the focal
plane along the time-of-flight axis is the same for singly
charged ions of any mass since the shape of the trajectories is
simply a function of repeller voltage versus kinetic energy
release.2 In order to achieve effective slicing of the expanding ion cloud at the detector it is desirable to construct a
velocity mapping apparatus which, in addition to providing
sharp velocity focusing, also allows for a large degree of
expansion along the time-of-flight axis. i.e., the spread in the
arrival time at the detector, ⌬t, of the expanding photofragment sphere should be on the order of several hundred nanoseconds. Since the application of a pulsed gate to the detector
assembly with a width in the region of 20– 40 ns is perfectly
feasible, the central section of the distribution may then be
recorded independently.
Figure 1共a兲 shows the simulated expansion of an isotropic distribution of 35Cl⫹ atoms with 1 eV translational energy
in a simple ion-optics assembly for which the velocity focusing conditions have been optimized at a repeller voltage, V R ,
of ⫹2500 V. The expansion of the ion cloud is shown in 1 ␮s
divisions, the overall flight path is set at 45 cm and the detector fixed at 80 mm in diameter. Trajectory calculations
were performed with Simion 7.0. In Fig. 1共b兲, V R has been
reduced to ⫹500 V. The associated voltage ratio applied to
the ion lens L 1 is maintained in order to preserve velocitymapping conditions and this scaling of the focusing electrodes with V R will be implicitly assumed throughout the
remainder of this article unless otherwise stated. A second
ion lens, L G , is held at ground and from this point to the
detector the flight path is field-free. It is immediately clear
that at reduced lens voltages the ion cloud is considerably
more expanded along the time-of-flight axis and our simulations show that ⌬t has increased from 40 to 190 ns. Almost
all of this expansion occurs in the region between the repeller and the first ion lens as a result of the reduced potential
difference across this region. In order to continue increasing
this expansion, V R may be decreased further; however, this
approach will quickly run into problems experimentally
since the photofragments will have insufficient kinetic energy to effectively operate a channel plate type detector and
will also become susceptible to distortion from stray fields.
Additionally, since the photofragments continue to expand
perpendicular to the time-of-flight axis the resulting image
will become too large to be accommodated on a detector of
typical size. As a consequence of these practical limitations,
a better strategy for reducing the potential gradient between
the repeller and first ion lens is to increase the potential applied to L 1 relative to V R . In a simple ‘‘two-electrode’’ 共i.e.,
repeller and L 1 ) velocity-mapping scheme, however, this severely compromises the velocity focusing properties of the
ion optics assembly. In order to correct for this, an additional
ion lens, L 2 , must be introduced. Figure 1共c兲 shows the increased temporal width of the ion cloud that now results
from this ‘‘three-electrode’’ scheme when the velocity focusing conditions are fully reoptimized. At V R ⫽⫹500 V, ⌬t is
now increased to 400 ns and, as argued by Kitsopoulos and
co-workers, this is sufficient to implement a slicing approach
experimentally. Figure 2共a兲 shows the predicted variation in
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Rev. Sci. Instrum., Vol. 74, No. 4, April 2003
Townsend, Minitti, and Suits
FIG. 2. 共a兲 Photofragment ion cloud stretching, ⌬t, as a function of repeller
voltage, V R , for four sample atomic masses with 1 eV translational energy.
共b兲 ⌬t as a function of translational energy for four sample atomic masses at
V R ⫽⫹500 V.
FIG. 1. Simulated expansion of the photofragment ion cloud at 1.5 ␮s divisions for 35Cl with 1 eV translational energy. 共a兲 Single focusing lens with
V R ⫽⫹2500 V. 共b兲 Single focusing lens with V R ⫽⫹500 V. 共c兲 Two focusing lenses with V R ⫽⫹500 V.
⌬t as a function of repeller voltage for four sample ion
masses with 1 eV translational energy under velocity focusing conditions. The overall length of the time-of-flight was
set at 1 m. The ratios of the voltages applied the focusing
lenses were L 1 ⬃0.88⫻V R and L 2 ⬃0.78⫻V R . Further details of the electrode assembly may be found in Sec. III.
Figure 2共b兲 shows the extent of the ion cloud stretching for
the same four masses over a range of translational energies at
a fixed repeller voltage, V R ⫽⫹500 V. It is apparent from
Fig. 2 that the ion cloud may generally be expanded along
the time-of-flight axis by several hundred nanoseconds for
the case of relatively heavy 共⭓30 amu兲 and/or energetic
共⭓1.0 eV兲 atomic or molecular photofragments. However,
lighter masses possessing less translational energy will not
be stretched sufficiently to implement an effective high-
resolution slice imaging experiment. It is found that if the
potential applied to L 1 relative to the repeller is increased
much further in an attempt to continue increasing the ion
cloud expansion, tight velocity focusing conditions cannot be
maintained unless the length of the flight path to the detector
is shortened considerably. While in principle the threeelectrode scheme is good enough to allow slicing experiments to be used in the investigation of many photodissociation processes, in order to make the technique more universal
a third ion lens, L 3 , may be introduced into the electrode
assembly. This allows the potential difference between the
repeller and L 1 to be decreased further while still maintaining good velocity focusing and is illustrated in Fig. 3共a兲 for
an isotropic expansion of 16O atoms with 0.25 eV kinetic
energy at a repeller voltage, V R ⫽⫹350 V. The spread in
arrival time at the detector is now expanded to 340 ns and
this compares very favorably with the 205 ns that may be
obtained in a three-electrode arrangement at the same repeller voltage, as shown for comparison in Fig. 3共b兲. Figure 3共c兲
shows a plot of the predicted stretching as a function of
translational energy for 16O, and 1 H masses at V R ⫽
⫹350 V. Converting the x axis to a velocity scale produces a
linear plot. It can be seen that a ‘‘four-electrode’’ scheme has
the potential to greatly expand the range of systems to which
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Rev. Sci. Instrum., Vol. 74, No. 4, April 2003
dc slice imaging
2533
FIG. 4. Schematic representation of the experimental setup for velocity map
dc slice imaging the 355 nm dissociation of Cl2 and NO2 . A cut-through
section of the ion optics assembly is shown for clarity.
FIG. 3. Simulated expansion of the photofragment ion cloud at 1.0 ␮s divisions for 16O with 0.25 eV translational energy. 共a兲 Three focusing lenses
with V R ⫽⫹350 V. 共b兲 Two focusing lenses with V R ⫽⫹350 V. 共c兲 Variation in stretching with three focusing lenses at a function of fragment kinetic
energy for two sample atomic masses.
slicing may be practically applied. Within the confines of
systems typically studied in photodissociation imaging experiments only atomic hydrogen fragments are now realistically beyond the scope of this proposed technique. It should
also be noted that the addition of L 3 now allows a limited
degree of control over the expanding photofragment ion
cloud: While the temporal stretching along the time-of-flight
axis may be controlled by reducing the potential difference
between the repeller and L 1 , the spatial magnification of the
image may be adjusted to a significant extent using L 2 . The
voltage applied to L 3 may then be used to restore the velocity focusing conditions of the assembly and it is therefore
possible control the degree to which the ion cloud is
stretched while still maximizing the dimensions of the image
on a detector of any given size under velocity mapping conditions. The spatial magnification, N, of the image is defined
by the quantity R/R ⬘ , where R is the radius of the velocity
focused image and R ⬘ is the radius of the image that would
be predicted purely on the basis of photofragment kinetic
energy and flight time 共i.e., when N⫽1.0). For the test case
presented here it is found that N may be readily adjusted over
a range between ⬃1.0 and ⬃1.4 while preserving tight velocity focusing of the ion cloud. It is also found that by
reducing the overall flight path to the detector the image
magnification may be scaled down to as low as N⬃0.8, although it is still not possible to increase N to above a value
of ⬃1.4. Additionally, as with conventional velocity mapping, at a given set of ion optics voltages N is constant for
photofragments of all masses at all energies. On extracting
the velocity distribution only a simple linear correction to
account for the image magnification need therefore be applied. In principle, the multiple lens approach to stretching
the ion cloud used in conjunction with narrow gating at the
detector has the potential to enable high resolution imaging
of almost all atomic and molecular photofragments by recording only the central section of the expanded distribution.
Finally, it should be noted that dc slicing of this type may in
principle be implemented in any existing conventional velocity mapping apparatus with only a small amount of modification. Since essentially all of the stretching observed occurs
in the region between the repeller and the first ion lens, a
short flight path and small detector are sufficient to implement experiments of this type. Although the detection efficiencies are low at the lower repeller voltages employed
here, the use of an additional magnifying lens as suggested
by Vrakking et al.11 may allow expansion of an image obtained with higher repeller voltages to match the detector
scaling for the system of interest. We note that the conditions
chosen here in the simulations are those that most closely
match our particular experimental setup, and do not necessarily represent requirements of the technique. Finally, the
short pulses necessary for gating are easily achieved with
commercial high voltage pulsers and no special cabling, as
discussed further in the following section.
III. EXPERIMENT
A. Vacuum system and laser sources
The overall experimental setup is shown schematically
in Fig. 4. The vacuum apparatus is of a rectangular design
fashioned from aluminum, with separate differentially
pumped source and main chamber sections. Each chamber is
pumped by a separate magnetic bearing compound turbomolecular pump 共Osaka Vacuum兲. A gas sample containing 5%
Cl2 or NO2 seeded in argon is introduced into the source
chamber via a pulsed nozzle 共General Valve, Parker Iota One
Driver兲 operating at 10 Hz with a pulse width of 160 ␮s and
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2534
Rev. Sci. Instrum., Vol. 74, No. 4, April 2003
Townsend, Minitti, and Suits
a backing pressure of 20 psig. After passing through a skimmer 共1.0 mm兲 and a collimator 共1.0⫻0.3 mm兲, the molecular
beam enters into the region between the repeller electrode
and the first ion lens, L 1 , and is intersected at right angles by
two counter-propagating laser beams. The photolysis laser
共355 nm兲 is generated by the third harmonic of a seeded
Nd:yttritium–aluminum–garnet 共YAG兲 laser 共Spectra Physics GCR-190兲. After passing through a Glan–Taylor prism to
clean up the polarization and then a ␭/2 wave plate, this
beam was focused into the interaction region using a 30 cm
lens. The photolysis laser power was approximately 2.5 mJ/
pulse. The probe laser is provided by frequency tripling the
output of a dye laser 共Continuum Jaguar兲 pumped at 532 nm
by a second Nd:YAG 共Quanta Ray GCR-5兲. After passing
through a ␭/2 wave plate the beam was focused into the
chamber using a 30 cm lens. The atomic photofragments
produced from the 355 nm dissociation of Cl2 and NO2 were
probed using the following 共2⫹1兲 REMPI schemes:
2h ␯
h␯
Cl 3p 5 共 2 P 3/2兲 → Cl 4p 共 4 P 1/2兲 → Cl⫹
2h ␯
h␯
Cl 3p 5 共 2 P 3/2兲 → Cl 4p 共 2 D 3/2兲 → Cl⫹
2h ␯
239.92 nm,
共1a兲
h␯
O 2p 3 共 3 P 2 兲 → O 3p 共 3 P 2,1,0 兲 → O⫹
235.39 nm,
共1b兲
225.59 nm. 共2兲
Typical output power for the tripled dye laser beam was
⬃0.7 mJ/pulse when probing both the Cl-atom 共LDS 722
dye兲 and O-atom 共DCM/LDS 698 dye兲 transitions. The dye
laser was calibrated with a wave meter 共Coherent WaveMaster兲. Background pressures with the molecular beam turned
on were ⬃2⫻10⫺5 and ⬃6⫻10⫺7 Torr in the source and
main chambers, respectively.
B. Ion optics and detection
Following ionization, the atomic fragments are accelerated out of the interaction region by the potential difference
between the repeller and the first ion lens. The overall lens
setup, complete with lens spacing and aperture dimensions is
shown in detail in Fig. 5. Once the ions pass through the final
lens element, there is an effective field-free drift region to the
detector. Additional electrodes held at ground provide shielding as the ions pass through the main body of the chamber
before entering a 70 cm flight tube. The overall length of the
flight path from the laser interaction region to the detector
was 105 cm. The detector itself consists of a pair of 80 mm
microchannel plates 共MCPs兲 coupled to a P47 phosphor
screen held at 6 kV 共Burle Electro-Optics兲. The front of the
MCP assembly is held at ground and the back plate may be
used to ‘‘gate’’ photofragments of a specific mass by application of a high voltage pulse 共⬃⫹2.5 kV/⫹1 kV bias兲 at the
relevant time using a commercial pulser 共DEI PVX-4140兲. If
this pulse is sufficiently narrow then the central section of an
expanding sphere of photofragments may be sampled exclusively, as discussed in Sec. II. The timing of the gate pulse
with respect to the firing of the probe laser is accurately
controlled using a delay generator 共BNC 555兲. The resulting
FIG. 5. Scaled schematic of the ion lens assembly 共all units in millimeters兲.
image is then recorded using a charge coupled device camera
共Mintron 2821e, 512⫻480 pixels兲 in conjunction with a PC
which allows real time centroiding analysis of the data12,13
using a video integrator 共McLaren Research TM-1000CV兲.
Additionally, the output from a photomultiplier tube
共Hamamatsu HC124兲 may be displayed on an oscilloscope to
provide time-of-flight measurements and allow for accurate
mass selectivity. An important point to note is that the pulser
employed in the setup described here is not designed to produce a narrow, fast rising pulse on the order of 20– 40 ns
wide. However, a pulse on the order of 2.5 kV with a width
of ⬃140 ns displays an inherent rise time 共⬃50 ns兲 and decay 共50–75 ns兲. Since the gain on the MCP assembly is
nonlinear as a function of applied voltage, by setting a suitable signal detection threshold, an effective gate of around
40 ns width may be established. Such a threshold may be
easily set within the acquisition parameters of our data collection software. In principle this narrow gating scheme may
therefore be implemented with any moderately fast pulser
and the overall setup may be easily adapted from that employed in conventional velocity mapping.
IV. RESULTS
A. Cl2
Photofragmentation of Cl2 at 355 nm has been well
studied14,15 and is known to be a predominantly perpendicular transition 共␤⬃⫺1兲 that occurs almost exclusively on the
C 1 ⌸ 1u state potential energy surface to yield ground state
Cl( 2 P 3/2)-atom products with ⬃1.0 eV translational energy.
⫹
state
A very weak 共⬃1%兲 parallel transition via the B 3 ⌸ 0u
2
2
yielding Cl( P 3/2)⫹Cl* ( P 1/2) is also present. Figure 6
shows the 35Cl⫹ and 37Cl⫹ ion time-of-flight distributions
recorded in our apparatus following 355 nm photolysis and
subsequent 共2⫹1兲 REMPI probing the ground state
4 p( 4 P 1/2)←3 p 5 ( 2 P 3/2) transition. The probe laser wavelength was positioned in the center of the Doppler profile and
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Rev. Sci. Instrum., Vol. 74, No. 4, April 2003
FIG. 6. Time-of-flight profile of 35Cl⫹ and 37Cl⫹ recorded in our apparatus
following 355 nm photodissociation of Cl2 at a repeller voltage, V R , of
⫹650 V.
great care was taken to ensure no saturation of the MCP/
phosphor screen detector assembly and photomultiplier tube.
The photolysis and probe lasers were both polarized vertically. The repeller voltage was set at ⫹650 V with L 1
⫽0.882⫻V R , L 2 ⫽0.775⫻V R and L 3 grounded. As discussed in Sec. II, these settings in no way maximize the
stretching of the ion cloud, however, in this instance such
large expansion is not desirable since any overlap in the arrival time of the 35Cl⫹ and 37Cl⫹ isotopes will clearly compromise the quality of the experimental image. Under these
conditions the spread in arrival time at the detector of each
isotopic fragment is successfully expanded to a width of
around 400 ns and, as expected, the peak shape is typical of
a perpendicular dissociation. Figure 7共a兲 shows the equivalent velocity map image recorded using a narrow 共40 ns兲
detector gate set to capture only the central section of the
35 ⫹
Cl time-of-flight distribution. The center is easily found
by stepping the gate position through the image until the
observed distribution reaches its maximum diameter, as has
been elegantly demonstrated by Kitsopoulos and co-workers
in a pulsed slice imaging apparatus.6 Figure 7共b兲 shows the
same image recorded with a 500 ns gate that encompasses
the entire 35Cl⫹ peak. In both instances the probe laser was
tuned over the Doppler width of the Cl fragments while the
images were accumulated and the polarization of the photolysis and probe lasers was vertical. Figure 7共c兲 shows the
central section of this distribution that results from reconstruction using Vrakking’s iterative reconstruction
technique.5 Both the sliced and unsliced images are round 共to
an accuracy of better than 1%兲, confirming that our ion lens
assembly does not exhibit any astigmatism effects at relatively low values of V R . As would be expected from compressing the entire Newton sphere of photofragments into a
single image, the inner region of the unsliced image is less
‘‘clean’’ than in the sliced example. This blurring is removed
in the inverted image that reconstructs the central section of
the 3D distribution and, with the exception of the artificial
noise that has been introduced into the image along the symmetry axis, the sliced and Abel inverted images appear very
similar although the sliced data is somewhat sharper. In order
dc slice imaging
2535
to examine this in more detail, the speed distribution for the
Cl( 2 P 3/2) fragments may be reconstructed by radially integrating the experimental data between 0 and 2␲ following
application of an r sin ␪ weighting to each pixel, as described
by Kitsopoulos and confirmed by our own simulations. The
result is shown in Fig. 7共d兲 for both the sliced and reconstructed images. The velocity resolution, ⌬␯/␯, is found to be
2.8% and 5% in the two cases, respectively, and this provides
a clear demonstration that the dc slicing approach to velocity
mapping can yield superior resolution than that attainable
with reconstruction methods. A resolution of 2.8% in the
sliced image is extremely good in the general context of
velocity map ion imaging experiments, and is significantly
better than that obtained in previous studies of Cl2 at 355 nm
under both conventional velocity mapping15 共⬃8%兲 and
slicing8 共⬃10%兲 conditions. It should also be remembered
that for the case of Cl2 and 35Cl image we observe is subject
to a degree of isotopic blurring since the undetected cofragment may be either 35Cl or 37Cl. This leads to an inherent
limiting resolution of 1.7% and the presence of the less abundant 37Cl will slightly exaggerate the fast side of the velocity
distribution, as is indeed observed. We estimate the blurring
of our images that arises purely from the velocity spread of
the molecular beam to be ⌬␯/␯⬃2.2%. This compares favorably with many ‘‘state-of-the-art’’ velocity-mapping
experiments12,16 and demonstrates that the velocity focusing
conditions of the ion optics are well preserved under conditions optimized for dc slicing. In addition to the dominant
perpendicular transition, a weak inner ring characteristic of a
parallel transition is clearly visible. The Cl( 2 P 3/2) photofragment translational energy distribution obtained from integrating the experimental images in the two quadrants bisecting
the symmetry axis shows this feature in more detail, as can
be seem in Fig. 7共e兲. This is dissociation via the weak
⫹
state channel yielding Cl( 2 P 3/2)⫹Cl* ( 2 P 1/2) fragB 3 ⌸ 0u
ments and its appearance provides another clear demonstration of the resolution attainable with the dc slicing method,
since it has not been resolved in previous studies imaging the
ground state Cl( 2 P 3/2) atom distribution. From our data we
are able to directly extract a Cl*/Cl spin-orbit branching ratio
of ⬃1.6%. This is in good agreement with the recent imaging
study of Samartzis et al.,15 in which the Cl( 2 P 3/2) and
Cl* ( 2 P 1/2) fragments were studied individually.
B. NO2
At 355 nm the primary dissociation pathway in NO2 is
known to proceed via excitation to the short-lived 2 B 2 state
and yields NO⫹O( 3 P J ) with an anisotropy parameter of
␤⬃1.2.17 The spin-orbit branching ratio in the O atom predominantly favors production of ground state O( 3 P 2 )
(⬃80%) 9,18 and the NO cofragment is found to exhibit significant vibrational excitation, with the relative populations
of ␯⫽0 and ␯⫽1 on the order of ⬃0.6 and ⬃0.4,
respectively.19 At 355 nm there is insufficient energy available for the formation of ␯⫽2. In both vibrational levels
formed, the rotational distribution is bimodal and this is attributed to a change in the O–N–O bond angle upon making
the transition to the 2 B 2 potential energy surface. Figure 8共a兲
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2536
Rev. Sci. Instrum., Vol. 74, No. 4, April 2003
Townsend, Minitti, and Suits
FIG. 7. Images of 35Cl( 2 P 3/2) from Cl2 photodissociation at 355 nm. 共a兲 dc sliced image. 共b兲 Unsliced image. 共c兲 Reconstruction of the unsliced image. 共d兲
35
Cl velocity distribution from sliced image and reconstructed unsliced image. 共e兲 Energy distribution from portion 共0°– 60°兲 of sliced image.
shows the central slice of the O( 3 P 2 ) distribution probed
using 共2⫹1兲 REMPI following NO2 photolysis at 355 nm
using a four-electrode scheme. The gate width at the detector
was set to 40 ns and the repeller voltage was ⫹300 V with
L 1 ⫽0.93⫻V R , L 2 ⫽0.81⫻V R , and L 3 ⫽0.75⫻V R . Both
the photolysis and probe lasers were polarized vertically and
the probe laser was tuned over the Doppler profile of the
O-atom fragments during image acquisition. Figure 8共b兲
shows the same distribution recorded with a 500 ns gate that
sampled the entire O-atom distribution simultaneously and
Fig. 8共c兲 is the corresponding reconstructed image again obtained using the iterative reconstruction method.5 As for the
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Rev. Sci. Instrum., Vol. 74, No. 4, April 2003
dc slice imaging
2537
FIG. 8. Images of O( 3 P 2 ) from NO2 photodissociation at 355 nm. 共a兲 dc sliced image 共b兲 Unsliced image. 共c兲 Reconstruction of the unsliced image. 共d兲 NO
photofragment translational energy distributions from sliced image, reconstructed unsliced image, and core sampling data of Liu and co-workers 共adapted with
permission from Ref. 11兲.
Cl2 data, it can be seen that all three images broadly show
the same structure but that the five distinct rings that are
clearly visible in the central region of the sliced image are
difficult to distinguish from noise in the reconstruction and
the associated kinetic energy distribution. This illustrates the
superior resolution attainable with the dc slicing method
compared with conventional velocity mapping schemes.
Once again, both the sliced and unsliced images are round
共to an accuracy of better than 1%兲, confirming that even at a
repeller setting of ⫹300 V the images do not exhibit any
significant distortion. The NO photofragment translational
energy distributions extracted from Figs. 8共a兲 and 8共c兲 are
displayed in Fig. 8共d兲 along with the equivalent distribution
recorded by Liu and co-workers using a time-of-flight core
sampling approach that makes no use of imaging methods.9
Bimodal rotational distributions are clearly visible in both
␯⫽0 and ␯⫽1 and for the case of our sliced data the five low
energy peaks are assigned to NO 共␯⫽1, N⫽19– 24). The
overall agreement between the three data sets is excellent
indicating that effective slicing is possible even for very low
energy photofragments. Additionally, application of an
r sin ␪ weighting to the sliced data yields the correct velocity
and energy distributions for all photofragment energies.
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2538
Rev. Sci. Instrum., Vol. 74, No. 4, April 2003
Townsend, Minitti, and Suits
FIG. 9. Schematic illustration of the raster imaging technique.
V. RASTER IMAGING
The raster imaging approach to velocity mapping is applicable in two-color photolysis-probe or scattering experiments. The probe laser is displaced downstream from the
photolysis interaction zone and displaced in time with respect to the dissociation event. By slowly moving the probe
laser position in a vertical direction an effective ionization
volume may be defined that is width limited only by the
dimension of the laser focus. If there is sufficient offset between the photolysis and probe laser such that the photofragment cloud has expanded to a size that significantly exceeds
this width then only the central section of the distribution is
sampled. This is illustrated in Fig. 9. Note that in contrast to
the dc slicing approach described previously in this article,
with the rastering method there is no need for a narrow detector gate as expansion of the neutral photofragment cloud
in conjunction with the narrow laser focus is exclusively
responsible for the slicing effect. Figure 10共a兲 demonstrates
how the center stripe of the photofragment sphere may be
built up in sections by slowly varying the vertical displacement of the of the probe laser in 0.381 mm steps for the case
of Cl2 dissociation at 355 nm and subsequent 共2⫹1兲 REMPI
via the 4p( 2 D 3/2)←3 p 5 ( 2 P 3/2) transition. The complete image, recorded using 0.127 mm steps, is shown in Fig. 10共b兲.
The probe laser was delayed in time by 1 ␮s with respect to
the photolysis beam and was then offset by ⬃2 mm along the
molecular beam axis until the widest part of the image section was observed. The probe laser beam was then sequentially repositioned in the vertical direction using a lens each
time the Doppler profile was scanned. On comparing the
raster image with that obtained using the slicing approach
described previously 共Fig. 7兲, it is clear that the two experimental schemes are comparable in terms of resolution. Following reconstruction of the speed distribution we find the
resolution to be ⌬␯/␯⫽3%. Although slightly less good than
our initial dc imaging approach, this is still sufficient to
⫹
clearly resolve the minor dissociation channel via the B 3 ⌸ 0u
state and therefore also compares favorably with our unsliced
FIG. 10. 共a兲 Composite image of 35Cl( 2 P 3/2) from Cl2 photodissociation at
355 nm recorded using the raster imaging technique illustrating acquisition
in sections. 共b兲 Overall image showing central slice of photofragment distribution.
data and previous imaging studies at 355 nm.15 An obvious
drawback with this approach compared to the dc imaging
method is that in order to build up a complete image of the
central slice, the probe laser must be scanned over the Doppler width for every vertical offset position and this greatly
increases the data acquisition time. In contrast to the dc imaging method, however, most of the interaction between the
probe laser and the photofragment cloud occurs off-axis from
the molecular beam. Any background contribution to the final image that arises exclusively from the probe laser is
therefore greatly reduced. This makes the raster imaging approach particularly appealing for reactive scattering studies
where background interference from probe of the beam itself
may be particularly problematic.
ACKNOWLEDGMENTS
The authors would like to acknowledge valuable assistance from M. Kim, W. Li, S. K. Lee, and P. Hallock. This
work was supported by the National Science Foundation un-
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Rev. Sci. Instrum., Vol. 74, No. 4, April 2003
der Award No. 0102174. Acknowledgment is also made to
the donors of the Petroleum Research Fund, administered by
the ACS, for partial support of this research.
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