205:
three-dimensional object that we wish to detect. Since the ions created should be of the same mass, they will all be accelerated uniformly toward the detector. It takes very little time for the whole three-dimensional object to be crushed into the detector, so the position of an ion on the detector relative to the center position is given simply by v Δt, where v is its velocity and Δt is the time between when the ions were made and when they hit the detector. The image is thus a two-dimensional projection of the desired three-dimensional velocity distribution. Fortunately, for systems with an axis of cylindrical symmetry parallel to the surface of the detector, the three-dimensional distribution may be recovered from the two-dimensional projection by the use of the inverse
240:
of the molecular beam, photolysis laser, and ionization laser is, say 1 mm x 1 mm x 1 mm, then the spot for an ion moving with a single velocity would still span 1mm x 1mm at the detector. This dimension is much larger than the limit of a channel width (10 μm) and is substantial compared to the radius of a typical detector (25 mm). Without some further improvement, the velocity resolution for a position-sensing apparatus would be limited to about one part in twenty-five. Eppink and Parker found a way around this limit. Their version of the product imaging technique is called velocity map imaging.
61:
internal energies and velocities of all products can be specified. Product imaging approaches this goal by determining the three-dimensional velocity distribution of one state-selected product of the reaction. For a reaction producing two products, because the speed of the unobserved sibling product is related to that of the measured product through conservation of momentum and energy, the internal state of the sibling can often be inferred.
334:
159:
276:, realized that further increase in resolution could be gained if one carefully analyzed the results of each spot detected by the CCD camera. Under the microchannel plate amplification typical in most laboratories, each such spot was 5-10 pixels in diameter. By programming a microprocessor to examine each of up to 200 spots per laser shot to determine the center of the distribution of each spot, Chang
289:
switch, one is able to select the central part of the ion cloud (Newton sphere). This central slice has the full velocity and angular distribution. A reconstruction by mathematical methods is not necessary. (D. Townsend, S. K. Lee and A. G. Suits, “Orbital polarization from DC slice imaging: S(1D) alignment in the photodissociation of ethylene sulfide,” Chem. Phys., 301, 197 (2004).)
264:
measure the three-dimensional product momentum vector distribution without having to rely on mathematical reconstruction methods which require the investigated systems to be cylindrically symmetric. Later, velocity mapping was added to 3D imaging. 3D techniques have been used to characterize several elementary photodissociation processes and bimolecular chemical reactions.
197:
ejected for each one that hits the wall, the channels act as individual particle multipliers. At the far end of the plates approximately 10 electrons leave the channel for each ion that entered. Importantly, they exit from a spot right behind where the ion entered. The electrons are then accelerated to a phosphor screen, and the spots of light are recorded with a gated
146:(Σ), indicating that both the atom and molecule are in their ground electronic state. The image above has no information on this channel, since only the O(D) is probed. However, by tuning the ionization laser to the REMPI wavelength of O(P) one finds a completely different image that provides information about the internal energy distribution of O
263:
Chichinin, Einfeld, Maul, and
Gericke replaced the phosphor screen by a time-resolving delay line anode in order to be able to measure all three components of the initial product momentum vector simultaneously for each individual product particle arriving at the detector. This technique allows one to
247:
to accelerate the ions toward the detector. When the voltages are properly adjusted, this lens has the advantage that it focuses ions with the same velocity to a single spot on the detector regardless where the ion was created. This technique thus overcomes the blurring caused by the finite overlap
239:
A major improvement to the product imaging technique was achieved by Eppink and Parker. A difficulty that limits the resolution in the position-sensing version is that the spot on the detector is no smaller than the cross-sectional area of the ions excited. For example, if the volume of interaction
302:
were perhaps the first to propose a "photoionization microscope". They realized that trajectories of an electron emitted from an atom in different directions may intersect again at a large distance from the atom and create an interference pattern. They proposed building an apparatus to observe the
191:
fragment, its position at any time following the photolysis is nearly the same as it would have been as a neutral. The advantage of converting it to an ion is that, by repelling it with a set of grids (represented by the vertical solid lines in the figure), one can project it onto a two-dimensional
134:
products recoil predominantly along this polarization axis. But there is more detail as well. A close examination shows that the peak in the angular distribution is not actually exactly at the north or south pole, but rather at an angle of about 45 degrees. This has to do with the polarization of
60:
demand the simultaneous measurement of a particle's speed and angular direction; the most demanding require the measurement of this velocity in coincidence with internal energy. Studies of molecular reactions, energy transfer processes and photodissociation can only be understood completely if the
288:
DC slice imaging is a developed version of traditional velocity map imaging technique which was developed in Suits group. In DC slicing, the ion cloud is allowed to expand by a weaker field in the ionization region. By this the arrival time is expanded to several hundred ns. By a fast transistor
97:
vibrations, the less will be available for the recoil. The O(1D) atom's REMPI, combined with the product imaging technique, yields an image that can be used to calculate the O(1D) three-dimensional velocity distribution. A slice through this cylindrically symmetric distribution is shown in the
204:
In this position-sensing version of product imaging, the position of the ions as they hit the detector is recorded. One can imagine the ions produced by the dissociation and ionization lasers as expanding outward from the center-of-mass with a particular distribution of velocities. It is this
225:
varies as the reciprocal of the square root of the ion mass, or the arrival time is proportional to the square root of the ion mass. In a perfect experiment, the ionization laser would ionize only the products of the dissociation, and those only in a particular internal energy state. But the
196:
consisting of two glass discs with closely packed open channels (several micrometres in diameter). A high voltage is placed across the plates. As an ion hits inside a channel, it ejects secondary electrons that are then accelerated into the walls of the channel. Since multiple electrons are
212:
A final advantage of the technique should also be mentioned: ions of different masses arrive at the detector at different times. This differential arises because each ion is accelerated to the same total energy, E, as it traverses the electric field, but the acceleration speed,
226:
ionization laser, and perhaps the photolysis laser, can create ions from other material, such as pump oil or other impurities. The ability to selectively detect a single mass by gating the detector electronically is thus an important advantage in reducing noise.
297:
Product imaging of positive ions formed by REMPI detection is only one of the areas where charged particle imaging has become useful. Another area was in the detection of electrons. The first ideas along these lines seem to have an early history. Demkov
209:. The cylindrical axis is the axis containing the polarization direction of the dissociating light. It is important to note that the image is taken in the center-of-mass frame; no transformation, other than from time to speed, is needed.
186:
product. Both lasers are pulsed, and the ionization laser is fired at a delay short enough that the products have not moved appreciably. Because ejection of an electron by the ionization laser does not change the recoil velocity of the
307:
eventually realized such a "microscope" and used it to study the photodetachment of Br. It was Helm and co-workers, however, who were the first to create an electron imaging apparatus. The instrument is an improvement on previous
312:
spectrometers in that it provides information on all energies and all angles of the photoelectrons for each shot of the laser. Helm and his co-workers have now used this technique to investigate the ionization of Xe, Ne,
201:(CCD) camera. The image collected from each pulse of the lasers is then sent to a computer, and the results of many thousands of laser pulses are accumulated to provide an image such as the one for ozone shown previously.
598:
Chichinin, A. I.; Einfeld, T. S.; Maul, C.; Gericke, K.-H. (2002), "Three-dimensional imaging technique for direct observation of the complete velocity distribution of state-selected photodissociation products",
126:
direction of the light that dissociated the ozone. Ozone molecules that absorb the polarized light are those in a particular alignment distribution, with a line connecting the end oxygen atoms in O
564:
Eppink, A. T. J. B.; Parker, D. H. (1997), "Velocity map imaging of ions and electrons using electrostatic lenses: Application in photoelectron and photofragment ion imaging of molecular oxygen",
122:
Note that the angular distribution of the O(D) is not uniform – more of the atoms fly toward the north or south pole than to the equator. In this case, the north-south axis is parallel to the
77:) dissociates following ultraviolet excitation to yield an oxygen atom and an oxygen molecule. Although there are (at least) two possible channels, the principle products are O(D) and O
93:(Δ) vibrationally to a maximum level of v = 3, and to provide some energy to the recoil velocity between the two fragments. Of course, the more energy that is used to excite the O
40:). The first experiment using photofragment ion imaging was performed by David W Chandler and Paul L Houston in 1987 on the phototodissociation dynamics of methyl iodide (
635:
Kauczok, S.; Gödecke, N.; Chichinin, A. I.; Maul, C.; Gericke, K.-H. (2009), "3D velocity map imaging: Set-up and resolution improvement compared to 3D ion imaging",
252:
905:
Suzuki, T.; Wang, L.; Kohguchi, H. (1999), "Femtosecond time-resolved photoelectron imaging on ultrafast electronic dephasing in an isolated molecule",
424:
Chandler, David W.; Houston, Paul L. (1987), "Two-dimensional imaging of state-selected photodissociation products detected by multiphoton ionization",
721:
Chang, B-Y.; Hoetzlein, R. C.; Mueller, J. A.; Geiser, J. D.; Houston, P. L. (1998), "Improved 2D Product
Imaging: The Real-Time Ion-Counting Method",
89:
for further explanation). At a wavelength of 266 nm, the photon has enough energy to dissociate ozone to these two products, to excite the O
983:
Blanchet, V.; Stolow, A. (1998), "Nonadiabatic dynamics in polyatomic systems studied by femtosecond time-resolved photoelectron spectroscopy",
968:
28:
is an experimental technique for making measurements of the velocity of product molecules or particles following a chemical reaction or the
106:
to O2(1) production at vibrational levels v = 0, 1, 2, and 3. The ring corresponding to v = 0 is the outer one, since production of the O
389:
522:
Geiser, J. D.; Dylewski, S. M.; Mueller, J. A.; Wilson, R. J.; Houston, P. L.; Toumi, R. (2000), "The
Vibrational Distribution of O
102:
would arrive at the center of the figure. Note that there are four rings, corresponding to four main groups of O(D) speeds. These
280:
were able to further increase the velocity resolution to the equivalent of one pixel out of the 256-pixel radius of the CCD chip.
863:
Helm, H.; Bjerre, N.; Dyer, M. J.; Heustis, D. L.; Saeed, M. (1993), "Images of photoelectrons formed in intense laser fields",
484:
Dylewski, S. M.; Geiser, J. D.; Houston, P. L. (2001), "The energy distribution, angular distribution, and alignment of the O(D
939:
Hayden, C. C.; Stolow, A. (2000), "Non-adiabatic
Dynamics Studied by Femtosecond Time-Resolved Photoelectron Spectroscopy",
142:
There are other dissociation channels available to ozone following excitation at this wavelength. One produces O(P) and O
1022:
139:
of this atom (which has 2 units) is aligned relative to the velocity of recoil. More detail can be found elsewhere.
679:
Chichinin, A. I.; Kauczok, S.; Gericke, K.-H.; Maul, C. (2009), "Imaging chemical reactions - 3D velocity mapping",
309:
1027:
36:, to record the arrival positions of state-selected ions created by resonantly enhanced multi-photon ionization (
103:
130:
roughly parallel to the polarization. Because the ozone dissociates more rapidly than it rotates, the O and O
198:
86:
820:
Blondel, C.; Delsart, C.; Dulieu, F.; Valli, C. (1 February 1999). "Photodetachment microscopy of O".
251:
In addition to ion imaging, velocity map imaging is also used for electron kinetic energy analysis in
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114:(Δ). Thus, the product imaging technique immediately shows the vibrational distribution of the O
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81:(Δ); that is, both the atom and the molecule are in their first excited electronic state (see
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of the ions are imaged onto a two-dimensional detector. A photolysis laser dissociates
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530:) produced in the Photodissociation of Ozone between 226 and 240 and at 266 nm",
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Blondel, C.; Delsart, C.; Dulieu, F. (1996), "The
Photodetachment Microscope",
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Houston, Paul L. (1987), "Vector correlations in photodissociation dynamics",
32:
of a parent molecule. The method uses a two-dimensional detector, usually a
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110:(Δ) in this level leaves the most energy for recoil between the O(D) and O
488:) fragment from the photodissociation of ozone between 235 and 305 nm",
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the laser that ionizes the O(D), and can be analyzed to show that the
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and ionization to follow excited state dynamics in larger molecules.
179:
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70:
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Demkov, Yu. N.; Kondratovich, V. D.; Ostrovskii, V. N. (1981),
328:
98:
figure, where an O(D) atom that has zero velocity in the
345:
182:to ionize a particular vibrational level of the CH
253:photoelectron photoion coincidence spectroscopy
243:Velocity map imaging is based on the use of an
230:Improvements to the Product Imaging Technique
69:A simple example illustrates the principle.
8:
166:In the original product imaging paper, the
18:Experimental velocity measurement technique
943:, Advanced Series in Physical Chemistry,
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620:
369:
406:
396:
178:I), while an ionization laser is used
162:Schematic of Product Imaging Apparatus
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192:detector. The detector is a double
259:Three-Dimensional (3D) Ion Imaging
248:of the laser and molecular beams.
14:
332:
822:The European Physical Journal D
384:, Cambridge University Press,
1:
382:Imaging in Molecular Dynamics
154:The Product Imaging Technique
885:10.1103/PhysRevLett.70.3221
800:10.1103/PhysRevLett.77.3755
56:Many problems in molecular
1044:
961:10.1142/9789812813473_0003
378:Whitaker, Benjamin J (ed.)
303:predicted rings. Blondel
701:10.1080/01442350903235045
22:Photofragment ion imaging
643:(8): 083301–083301–10,
319:femtosecond excitation
163:
941:Adv. Ser. Phys. Chem.
842:10.1007/s100530050246
681:Int. Rev. Phys. Chem.
199:charge-coupled device
161:
87:molecular term symbol
235:Velocity Map Imaging
217:, varies as E = ½ mv
100:center-of-mass frame
24:or, more generally,
997:1998JChPh.108.4371B
953:2000AdSPC..10...91H
919:1999JChPh.111.4859S
877:1993PhRvL..70.3221H
834:1999EPJD....5..207B
792:1996PhRvL..77.3755B
735:1998RScI...69.1665C
693:2009IRPC...28..607C
649:2009RScI...80h3301K
613:2002RScI...73.1856C
578:1997RScI...68.3477E
544:2000JChPh.112.1279G
502:2001JChPh.115.7460D
472:10.1021/j100305a003
438:1987JChPh..87.1445C
325:Coincidence Imaging
1023:Physical chemistry
723:Rev. Sci. Instrum.
637:Rev. Sci. Instrum.
601:Rev. Sci. Instrum.
566:Rev. Sci. Instrum.
409:has generic name (
245:electrostatic lens
194:microchannel plate
164:
83:atomic term symbol
34:microchannel plate
991:(11): 4371–4374,
970:978-981-02-3892-6
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622:10.1063/1.1453505
586:10.1063/1.1148310
510:10.1063/1.1405439
496:(16): 7460–7473,
466:(21): 5388–5397,
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353:April 2017
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