A system for Coulomb explosion imaging of small molecules at the Weizmann Institute

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH

Nuclear Instruments and Methods in Physics Research A329 (1993) 440-452 North-Holland

Section A

A system for Coulomb explosion imaging of small molecules at the Weizmann Institute D. Kella a, M. Algranati a, H. Feldman a, O. Heber a, H. Kovner R. Naaman b, D. Zajfman a, J. Zajfman a and Z. Vager a

a,

E. Malkin

a,

E. Miklazky

a,

Department of Nuclear Physics, Weizmann Institute of Science, Rehouot, 76100 Israel 6 Department of Chemical Physics, Weizmann Institute, Rehouot, 76100, Israel

Received 10 December 1992

A description of a system for molecular structure imaging at the Weizmann Institute is presented. A novel method of controlled laser beam photo detachment inside the high voltage terminal of a tandem accelerator, enabling the study of neutral fast molecules by the Coulomb Explosion Imaging technique, is described. Also, a new type of three dimension multiparticle detector is presented.

1. Introduction The Coulomb Explosion Imaging (CEI) [1] technique has evolved over the past several years into a method for determining molecular structure. Using this technique, it has been demonstrated that it is possible to measure the nuclear density functions for a broad variety of polyatomic molecules [2-14] . The method provides all the information for the correlated positions of atoms in molecules. The technique involves foil induced dissociation by electron stripping of fast molecular beams (typically 2 to 3% of speed of light; see fig. 1). The resultant ions begin to Coulomb explode, converting the initial potential energy stored in the stripped ionic system into kinetic energy . For a molecule containing N atoms, measurement of the 3N velocity components after the explosion provides information on the 3N spatial components of the nuclear coordinates of the original molecule . Since the time scale for electron stripping in the very thin foil is much shorter than characteristic time scales for vibrational and rotational motions of the molecule, the Coulomb explosion of each individual molecule gives a "snapshot" of the-relative positions of the nuclei within that molecule . Combining such measurements for a large ensemble of molecules yields the fully correlated many particle density distribution describing the nuclear coordinates within the molecules of that ensemble . In the present paper, we present the first detailed publication on the state of the art of the system used

"Micro scale" Ca 1rrrrrrrrrrrr rrrrrrrrrl I1,1,1,1,1,I,I,I,I,I,I,L "I,I,I,1 1 1,1,1,1,1,1, IDIlih ;1 ;1 ;IDI~1;I11 ;1 ;I ;1~IDill ;l;l;l;l;ll1

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Detector Fig. 1. Schematic diagram of the Coulomb explosion imaging method .

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D. Kella et al. / Coulomb explosion imaging of small molecules for these measurements. The main component of the setup is the Weizmann Institute Pelletron 14UD tandem accelerator . Other major features are the ability to produce cold or hot negative molecular ions, and a pulsed laser photodetachment system which allows selective neutralization of accelerated ions for Coulomb explosion imaging . Also, a description of a new type of multiparticle detector used in this system is presented . It allows the simultaneous measurement of position and time of molecular fragments with a spatial resolution of - 0 .1 mm and - 100 ps time accuracy.

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ion pulses are further accelerated by an additional 90 kV and mass selected by a 90° magnet (Magnet 1) . The resulting pulses of ions are chopped again (Chopper 2) to a time width r of 100-400 ns . The negative ion pulses are then injected into the 14UD tandem Pelletron accelerator and are accelerated toward the high voltage region (HV terminal) to an energy of 6 to 12 MeV. The time width T is selected in such a manner that the corresponding length of an accelerated ion pulse is smaller than the 3 m of the field free region at the HV terminal . When the negative ions reach the HV terminal, a pulse from the laser is fired down the accelerator tube in order to photodetach the extra electron . The neutralized molecules drift through the second part of the tandem accelerator toward Magnet 2, which is used to purge any charged ion left in the neutral beam . A set of slits mounted upstream of Magnet 2 collimates the beam to a rate of about one molecule per pulse . The neutral molecules are then electron stripped by passing through a thin Formvar foil, and the fragment atomic ions emerging from the foil repel each other via their Coulomb interaction, a process called the "Coulomb explosion" . The fragments are then charge and mass separated by Magnet 3 and collected on a multi particle position and time sensitive detector . The time and

2. Experimental setup 2.1 . General description The experimental setup of the CEI at the Weizmann Institute is shown in fig . 2 . A negative molecular ion beam is generated either by a cesium sputter source (Hiconex 834) or by a laser vaporization source for the production of "cold" molecules (details of this source will be published elsewhere [15]) . The beam is extracted by a + 10 to 17 kV potential and chopped by an electrostatic deflector (Chopper 1) to produce pulses of 1-3 Vs duration at a repetition rate of 25 Hz . The

Detector Fig. 2 . The Coulomb explosion imaging setup .

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D. Kella et al. / Coulomb explosion imaging of small molecules FOIL

position pulses from the detector are digitized and analyzed by a dedicated computer. 2.2 . Laser photodetachment The conventional methods of electron stripping from projectiles in the HV terminal of a tandem accelerator via gas or thin foil strippers is not suitable for the CEI method . In the case of foil stripping, the molecules would simply break up, and the gas stripping method is also unsuitable since the collisions tend to insert energy into internal degrees of freedom of the molecule in an uncontrolled way . In order to control the process of electron stripping, a new method has been developed, based on laser photodetachment inside the tandem accelerator [16,17] . The laser beam from a Nd : YAG or Nd : YAG dye lasers (Quantel Datachrome 5000 system) or Excimer pumped dye laser (Lambda Physics LPX315 + LPD3000) is introduced into the accelerator through a window mounted on top of Magnet 1, (see fig . 2) and is aimed colinearly with the accelerated beam . The distance between the window and the HV terminal stripping region is about 30 m . The laser pulse (- 10 to 30 ns long) is timed relative to the ion pulse so that they overlap only in the HV terminal (see fig . 2). At the other end of the accelerator, a movable prism can deflect the laser light to an energy meter (Precision RJP 375) which is used for aligning the laser beam through the accelerator . In order for the laser neutralization process to be efficient, the ratio between the photodetachment probability (PP) and the neutralization probability due to residual gas collisions (Pg ) in the high voltage terminal has to be large . Assuming a cross section for collisional neutralization of _ 10 -15 to 10 -16 cm2 and the HV terminal vacuum better than _ 10 -7 Torr, the probability for a molecule to be stripped along the 3 m flight path is Pg _ 10 -3 to 10 -4 . On the other hand, the typical photodetachment cross section at a wavelength of 532 nm (Nd :YAG second harmonic) is _ 10 -16 to 10 -18 cm2 . A typical laser output is 5 to 100 mJ per pulse . For the lower limit of 5 mJ, the probability for photodetachment is pp _ 1 to 10 -2 . It is important to point out that although in an ideal case Pp/Pg >> 1, when the power output of the dye laser is very weak or at photon energies close to thresholds, where the photodetachment cross section drops considerably, the two processes may compete, and measurements with and without lasers are needed for better accuracy. 2 .3 . Detection chamber The neutralized molecules emerging from the accelerator are collimated by a set of slits, and impinge on a very thin (less than 1 wg/cm2, - 100 t1) Formvar foil

MAGNET III BELLOWS AND ROTATEABLE JOINT

DETECTORS Fig . 3 . The detection chamber . [18,19,20] . After the Coulomb explosion is initiated by the foil, the fragments drift into the detection chamber (fig. 3) in a cone of approximately 10-20 mrad . The charge separation is made using a 30 cm wide magnet (Magnet 3) which is located 15 cm below the stripping foil (see fig . 3). Since the distance between the various exploding fragments in the magnetic field region is of the order of few mm, one can assume that all the fragments are affected by the same field . The magnet deflects the different fragments according to their charge and mass, and by setting the angle of deflection of the different charge states to be larger than the Coulomb explosion cone, the identity of each fragment can be easily deduced from its position on the detector . The lower part of the chamber is rotatable in a plane perpendicular to the field of Magnet 3 and is made of two parallel 6 in . tubes with a detector at each end . The angle of rotation of the detectors and the magnetic field can be changed in such a way that the desired charge states will fall on the detectors . The distance from the stripping foil to the detectors is 2192 mm . 2.4. Detector A basic part of the CEI system is the detector which enables three dimensional imaging of multiparticle

D. Kella et al. / Coulomb explosion imaging of small molecules

events . After the dissociation in the thin target, the velocities of the fragments can be described as follows. Typical Coulomb explosion velocities of - 50 eV/amu are added vectorially to the beam velocity, which is typically - 250 keV/amu. The relative velocity change from the initial center of mass velocity is thus 50/250 x 10 3 = 0.014. Hence, good time and position resolution are needed for the detector . It can easily be seen that the larger the distance between the foil and the detector system, the smaller is the constraint on the absolute time and position accuracy of the detector . In the past, a system of individual solid state detectors was used to scan the multidimensional coincidence space [21] . Later, multipiee position and time gas detectors were developed [22,23] and used at the Argonne National Laboratory Coulomb explosion system . In this system, the flight path between the target and the detectors is 6 m. The fact that, in the present case, the beam has a defined duty cycle (25 Hz) allowed a different approach to the detection system . It is based on a micro channel plate which transforms the impact

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of each fragment into a scintillation on a phosphor screen and an electronic signal on wires for timing . The screen is then imaged by a CCD video camera and read out digitally. The excellent time resolution possible from this system allows the reduction of the explosion path length to about 2 m. The simplicity of the two dimensional position analysis is another attractive feature of this new detector system . For each molecular fragment, the detector extracts the time and position for up to 6 particles simultaneously. It is made of four different layers (fig . 4). The first layer is a foil used as an ion-electrons converting stage, ejecting a few electrons for each particle hit. The second layer is an electron multiplier microchannel plate (MCP) in a chevron assembly . The third layer is a multi wire anode consisting of 48 independent anodes for fast timing, and the last layer is a phosphor screen emitting visible photons for position imaging. The operations of the different layers will be described in more detail in the following. The first stage is made of an aluminized Mylar foil 1 .5 win thick, the aluminum coating facing the MCP.

ALUMINIZED MI LAM

PHOSPHOR SCREEN AND WIRES

WINDOW

Fig. 4. Schematic diagram of the different detector layers .

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Without additional special coating, this stage generates about 5 to 20 electrons for each ion hit (depending on the stopping power). Higher amplification can be achieved by coating the aluminized side with CsI . A typical CsI coating of 3000 Â results in electron multiplication of 50 to 100 electrons for each ion hit. The first foil increases the efficiency of single ion detection from - 50% for a bare MCP ion detector to - 100% for this foil-MCP assembly . A voltage difference of 200 V is maintained between the aluminized foil and the MCP in order to extract the electrons with good focusing properties and optimal kinetic energy for the second multiplication stage. The second layer, located 2 mm from the first one, is an electron multiplier consisting of two micro channel plates in a chevron assembly . Two types of MCP dimension are used : a 42 mm active diameter and a 77 mm active diameter (Hamamatsu models F2225 and F2226). A typical potential of about 1700 V is maintained between the top and the bottom planes of the MCP. An additional electron multiplication of about 10 ° to 10 5 is achieved at this stage, and more amplification can be obtained by increasing the MCP voltage up to 2000 V. As a result of these two first amplification stages, for each fragment hitting the detector, a bunch of _ 10 6 fast and well focused electrons is created with a time width of - 1 ns . The third layer is located 3 mm after the MCP and consists of an array of independent thin (50 ~Lm diame-

ter) conductive wires used as fast anodes, welded to a common printed board. The distance between wire is 0 .83 mm for the small detector (48 wires) and 0.80 mm for the larger one (96 wires) . Since the bottom part of the MCP is about 2000 V below ground potential, each electron bunch is accelerated towards the wire array, which are coupled through an impedance of 82 bZ to the emitter of grounded base transistors . This electron bunch induces a fast signal on the wire array, which is used as the timing signal for each fragment hit. The focusing conditions are adjusted so that each electron bunch can induce signals on several wires simultaneously. A typical histogram for the number of wires which produce a signal for a single ion hit (i .e . a monoatomic beam), is shown in fig. 5. It can be seen that an average of 3 wires produce signals per ion hit. By using a weighted mean of all the wire signals in the final timing analysis, optimal time resolution is obtained . Another timing signal which serves as a common signal for all the fragments from an event, is taken out through a capacitor from the last stage of the MCP. The anode wires are located 0.2 mm from an isolated P-20 phosphor screen . Each electron bunch hitting the screen generates a light spot of - 1 mm diameter. These are recorded by a CCD camera (Javelin Electronics model JE-7242X) looking at the screen through a sealed window . The video output of the camera is connected to the data acquisition system and

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Fig. 5. Number of wires hit for events with a single fragment on the detector .

D. Kella et al. / Coulomb explosion imaging of small molecules

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Fig. 6. The distribution of number of pixels per event. The initial beam was OZ at 12 MeV. The distribution shown includes single and coincidence counts on the detector . also produces the master clock for the system timing

frame (the camera produces interleaved output). The

(see fig. 2) . The CCD output information is made of 625

x 215

position resolution is as good as one pixel and with

pixels with their intensities for each half

standard lens magnification, this corresponds to 100

E 55 E 50 45 40 35 30 25 0 15 10 L ' 20

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Fig. 7. Two dimensional contour plot for the distribution of accumulated fragments from the Coulomb explosion of BZ. The distribution shown is for the coincidence of two B +q where q = 3 (right side) and q = 4 (left side). The beam energy was 12 MeV.

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D. Kella et al. / Coulomb explosion imaging of small molecules FAST PULSE

48

FROM MCP

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a two dimensional picture of the accumulated fragments from the Coulomb explosion of B Z , as seen on the phosphor screen . Two circles can be seen which correspond to different charge states as separated by Magnet 3 . In summary, each multiparticle hit on the detector results in fast timing signals coming from the wires and position signals from the CCD output . All this information is transferred to the data acquisition system for accurate 3D imaging of each event (see section 3). 2.5. Detector electronics

'STOP" DRIVERS

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"LIP FLOPS TRIGGERED ON DECAY

-rLIP FLOPS TRIGGERED 4 ON RISE

CONSTANT

-CONSTANT-

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CURRENT GENERATORS

CHARGE ADC X96

Fig . 8 . Block diagram of time to charge conversion system of the detector . wm in the detector plane . The number of pixels with intensities above the CCD noise for each ion hit is typically - 60 . An example of a distribution for the number of pixels that are generated by the fragmentation of a 12 MeV beam of O z molecules, is shown in fig . 6 . Two peaks can be seen . The first one corresponds to single ion events (when the second ion did not hit the detector due to a different charge state), while the second peak is from double ion hits . The number of pixels depends mostly on the focusing condition, the lens position and magnification . Fig. 7 shows

The most limiting factor for the overall resolution of the three dimensional image is the accuracy of the time measurement : A resolution of 100 ps in time is equivalent to an error of - 1 mm in position, which represents a few percent for a typical event where the distances between the fragments on the detector are of the order of a few centimeters . In order to accurately process timing information coming from the wires of the detector, a special electronic set-up combined with a Time to Digital Converter (TDC) system was built (see figs . 8 and 9) . An ion hitting the detector creates two types of signals : The first, coming directly from the last stage of the MCP (see section 2 .4) is used, after appropriate delay, as a common stop (STOP) for all (i .e . all fragments) timing measurements . The second type is the group of signals induced on the anode wires . These signals are amplified and reshaped for optimum timing determination . A common threshold is set for all the wire outputs using fast discriminators ; the crossings of this threshold result in the initialization of two (one for cross on rise and the other for cross on fall) timing signals for each wire . The threshold is set so that they are above the noise and the signals cross it at the point of steepest ascent . Each crossing of the threshold enables a constant current source into a charge ADC . The constant current is LINE ------RECEIVER

____-------___-------____----------- -, FF

CONSTANT CURRENT GEN

CHARGE A/D

FF

CONSTANT CURRENT GEN

CHARGE A/D

DISCRIM INNTOR

---ADISCRIM LEVEL ' STOP" LOGIC DETECTOR ELECTRONICS

MAIN ELECTRONICS IN "CAMAC" CRATE

Fig . 9. Operational diagram of one channel of the time to charge system .

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D. Kella et al. / Coulomb explosion imaging of small molecules

E 40 20 0 -20 -40 -60 -80 -100 -120

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70 nsec Fig. 10. (a): Output signal from a wire of the detector and (b): The same signal after reshaping and amplification . stopped by the STOP signal from the MCP which is common to all wires. Measurement of both rise and fall of each timing signal enables pulse height correction by software (see section 3 .3). The whole system

can be considered as an effective multichannel digital constant fraction discriminator. A more detailed description of this system follows. Each detector wire is matched to a grounded base

~2 80 z

0 U

70 60 50 40 30 20 10

PS Fig. 11 . Typical time resolution for a single wire . The solid line drawn through the histogram is a Gaussian fit with FWHM = 140 ps .

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D. Kella et al. / Coulomb explosion imaging of small molecules

transistor amplifier for minimum reflections, which is coupled to a home made preamplifier-shaper (see fig. 9) . This stage is followed by an emitter follower and a second amplifier with a shorted cable differentiation network. The output is an almost symmetrically shaped pulse, with 3 ns rise and fall time and 10 ns width with an amplitude of more than 1 V (see fig. 10). The timing is provided by a fast pulse discriminator (LeCroy MVL407 400 MHz, 4 channel voltage comparator) . An externally variable stabilized voltage control is supplied as a common threshold voltage to all discriminators . If the amplified wire pulse is above the threshold then the corresponding discriminator is set on at a time tt and set off at a time t 2 . The accurate measurement of these two times (t i and t2 ) provides the information needed for the extraction of the ion arrival time and the height of the pulse. The fast complementary ECL outputs from the discriminators are sent through long (3 m) twisted pair flat cables to the main electronic chassis. Because fast electronics is needed, all the ICs are of the ECL family . Signals from the twisted pairs are fed into a line receivers with differential input and complementary output . An array of flip flops are set at both i t and t 2 of each discriminator pulse. The reset of all the flip

flops is done simultaneously by the STOP signal, which serves as a reference for all the wire timing . The stop signal originates from the MCP in the form of a fast positive pulse, as the result of the first hit. This pulse is amplified and shaped before being introduced into a constant fraction discriminator (CFD-Tennelec TC 455) . After a proper delay, the signal is fed into the ECL STOP drivers which reset the flip flops. If rather than a number of fragments, only a single ion hit the detector, then the STOP signal timing is accurately related to the time of arrival of this ion. Such events are used for the calibration of the wires parameters which are used later for the extraction of time difference between the individual hits own different wires, independently of the individual pulse heights. The flip flop output signals enable constant current generators, injecting charge into charge sensitive ADCs (CAEN, on CAMC crate) . The charges in the ADC channels are proportional to the time differences Tl --t, - i s or T2 --- t 2 -ts , where is is the time of the common STOP . After appropriate processing of Tl and T2 using calibration parameters (see section 3.3), the FWHM resolution per wire is approximately 140 ps (see fig. 11). Taking into account that between 2 and 3

CAMA FIELD

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Fig. 12 . The timing sequence of the CEI experiment . The lower part of the figure is an expanded view of the time sequence as control by the TIMER (see text), relative to the camera field signal

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D. Kella et al. / Coulomb explosion imaging of small molecules wires participate in the measurement of each fragment, the overall time resolution per fragment is about 80 to 100 ps (FWHM).

3. Control, data acquisition and analysis

3.1 . Synchronization An accurate time sequence, starting at the ion source chopper and ending at the ADC gates and CCD camera frame initialization, is essential for the proper synchronization of this special CEI setup . It is preferable to choose the starting point of such a sequence to be in phase with the 50 Hz main power frequency . This avoids the problem of random noise pick-up from the main power lines, and can easily be achieved by using the time sequence of the CCD camera as the start signal for the sequence of accurately delayed pulses . A home built timing unit (TIMER) generates a series of outputs corresponding to the different delays needed between the different stages of the experiment . The unit is based on a 40 MHz (25 ns steps) crystal clock, 20 bits counter and 6 comparator units each with 20 bit (maximal delay - 26 ms) . The timer is remotely controlled by an RS232 connection driven by the data acquisition computer . The timer is used for controlling the following devices (see figs . 2 and 12) : Chopper 1, Chopper 2, up to

three lasers, the clearing and gating of the ADCs in the detection system, and automatically matches the CCD frame signal . The delays between the different components of the system are chosen so that the ion pulses are synchronized with the two choppers and overlapped with the laser pulses exactly in the HV terminal . The result is a pulsed neutral molecular beam of 100-400 ns duration with a well defined kinetic energy which arrives at the detector in coincidence with the 1 ws gating of the ADCs .

3.2. Data acquisition The functions of controlling the different devices of the experiment, acquiring data and storing it, creating and displaying the histograms needed to monitor the experiment, analyzing and processing the data are distributed between two computer systems (see fig . 13) . The "Real Time" Computer (RTC) is a single board VME bus computer Motorola MVME174, connected to an ethernet network. This system controls the various devices of the set up such as the TIMER (through one of its RS232 ports), the clear and readout of the ADCs of the detector through the CAMAC crate controller and the setting and reading of the frame threshold suppressor (the video information of the detector, see below) via the VME bus . The storage of event data is done (after minimal processing) on a 600 Mbyte disk which is backed up by an Exabyte tape (2 .3 Gbyte per

CLEAR & GATE FROM TIMER

SYNCH FROM CAMERA TO TIMER

DETECTOR

CAMAC CRATE

Fig . 13 . Schematic diagram of the data acquisition system .

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D. Kella et al. / Coulomb explosion imaging of small molecules

tape) . The RTC creates the different histograms needed for monitoring the experiment, but no data analysis is made at this point. The video signal arriving from the detection system is analyzed by a home built Frame Threshold Suppressor (FTS). This VME bus device analyzes the video signal by digitizing it with a 10 MHz, 8 bit ADC and comparing it with a preset digital threshold. When a pixel amplitude passes the threshold, both its amplitude and its position (row and line) are stored in the internal 8 Kbyte memory of the ITS which can be read later by the RTC. This filtering leaves approximately 10 to 100 pixels per hit on the detector (see fig. 6), reducing the amount of relevant data from 1/8 Mbyte for the full frame to a few hundred bytes. Another function of the FTS is the synchronization of the RTC with the video cycle . When the FTS has finished analyzing a frame it sends an interrupt signal to the RTC which starts the data read out cycle. Both display of the monitor histograms and analysis of the data are done on a VAX3500 system . The histogram files are transferred from the RTC through the network using standard TCP/IP-FTP . In turn they are processed and displayed by using the CERN written packages PAW and HBOOK. The raw data is read, after some preprocessing by the RTC, in a similar way into the VAX disk for data analysis . 3.3 . Data handling

The final velocities Vz , VY and VZ of the fragments of a molecule are measured by the detector as distances between hits in the XY plane (parallel to the detector plane) and the time differences between them. The velocities in the XY plane are extracted by multiplying the beam velocity by the ratio between these distances and the flight path from the stripping foil to the detector. In a similar manner the velocity in the Z direction is found by multiplying the beam velocity with the ratio between the time difference between hits and the time of flight from the foil to the detector. In order to extract the final velocities V., VY and Vz for each fragment, two data sets have to be handled and matched . The first set is composed of the coordinates (x, y) and amplitude of each camera pixel, after the digitized hardware filtering from the CCD camera. A computer program recognizes the different fragment positions (X, Y) by clustering the pixels . The second set of data results from the timing information . This information consists of two data points, Tl and T2, for each wire hit. In order to find the exact time of arrival T of a fragment on the detector, a calibration process is done before the experiment using single hit events from the beam. For each wire i, a graph of W = Ti - Tz vs Ti is plotted . For a perfectly symmetric pulse shape and idealized discriminators, this would result in a

straight line with slope = 1 . In practice, the data set is best fitted with a polynomial F, usually of the second order . The function F is used for the correction of the time due to pulse height differences, such that T, = T,' + F,(W). In order to get an absolute calibration of the ADCs for each wire, a similar measurement is done with a known additional delay of the STOP signal. Finally, the time information (T) and the spatial information (X, Y) is combined to give the full three dimensional image of each molecule. 4. An example: Coulomb explosion of C 3 The C3 molecule is an interesting case since it is a very floppy molecule . The electronic ground state is known to be linear with a bending frequency of 63 cm-1 , which results in large bending angle amplitudes. The C3 anions were created using the Cs sputter source or the supersonic expansion laser vaporization source . They were injected into the Pelletron accelerator and accelerated to an energy of 12 MeV . The second harmonic of a Nd :YAG laser (2.33 eV) was used for the photodetachment (the threshold is 2 .1 eV [24]). The neutralized C3 molecules drift through the second part of the accelerator, and Coulomb explode after passing through the 1 wg/cm2 Formvar target . As a result, three fragments are created per molecule, each with a specific charge state . The data was analyzed in terms of the following symmetric coordinates: SZ = (E12

- E23/~,

S3 = (2E13 - E23 - E12)/v, where E,, are the relative kinetic energy between atom i and j. Because of the identity of all three carbons, all possible permutations of i and j are included. In these coordinates, the origin (S2 = S3 = 0) represents a triangular structure, while the presence of three symmetric maxima represent a linear structure . Fig. 14 shows the data in the (S2 , S3) coordinates. In fig . 14a, the ions were created using the Cs sputter source, while in fig. 14b they were produced using the cold ion source [15]. As expected, the supersonic expansion source generates a more linear configuration than the Cs sputter ion source . This is due to the fact that a highly vibrationally excited linear molecule such as C 3 spends most of its time in a nonlinear configuration. It is important to point out that since the ion produced in the ion source is C3 , the photodetachment process, at an energy which is - 0.2 eV above the threshold, populates a series of vibrational state of the neutral cluster, even if the negative ion is in the vibrational ground state . This is the main reason for the central peak in the "cold" data of fig. 14b. A simulation of the process,

D. Kella et al. / Coulomb explosion imaging of small molecules

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itself, and is thus a very versatile tool in the field of molecular physics. Applications with heavy clusters of 100 atoms are also planned.

Acknowledgements We wish to thank N. Altstein and B. Rosenvaser for their technical help, and L.J . Levinson for his valuable advice on the design of the data acquisition system . This work was partially supported by the US-Israel Binational Science Foundation, by the Minerva Fund, Munich, Germany, and by the Basic Research Foundation administered by the Israel Academy of Science and Humanities .

References

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Fig. 14. Contour plot for C3 presented in symmetric coordinates (see text) (a) using the Cs sputter source, (b) using the supersonic expansion source . In these coordinates, the central peak in (a) represent a triangular shape, while the three symmetric peaks seen m (b) represent a linear configuration . using a Monte Carlo code [20] shows that the data in fig. 14b is mainly produced from ground state C3 . 5. Conclusions The Coulomb Explosion Imaging system described here is now being used for the determination of the structure of small carbon clusters. It is planned to use this system for small heteronuclear molecules such as X  H, where X = C, N, and O, and n, m = 1 to 6, as well as other interesting cases . This system also allows the determination of the photodetachment threshold

[1] Z. Vager, R. Naaman and E.P. Kanter, Science 244 (1989) 426. [2] I. Plesser, Z. Vager and R. Naaman, Phys . Rev. Lett . 56 (1986) 1559 . [3] M. Algranati, H. Feldman, D. Kella, E. Malkin, E. Miklazky, R. Naaman, Z. Vager and J. Zajfman, J. Phys . Chem 90 (1989) 4617 . [4] H. Feldman, D. Kella, E. Malkin, Z. Vager, J. Zajfman and R. Naaman, J. Chem. Soc. Faraday Trans. 86 (1990) 2469 . Z. Vager, in : The Structure of Small Molecules and Ions, eds. R. Naaman, Z. Vager (Plenum, New York, 1987) p. 105. [6] Z. Vager, E.P. Kanter, G. Both, P.J . Cooney, A. Faibis, W. Koenig, B.J . Zabransky and D. Zajfman, Phys . Rev . Lett . 57 (1986) 2793 . E.P . Kanter, Z. Vager, G. Both and D. Zajfman, J. Chem . Phys . 85 (1986) 7487 . [8] A. Faibis, E.P. Kanter, L.M. Track, E. Bakke and B.J . Zabransky, J. Phys . Chem . 91 (1987) 6445 . [9] Z. Vager and E.P. Kanter, J. Phys . Chem . 93 (1989) 7745 . [10] D. Zajfman, Z. Vager, R. Naaman, R.E . Mitchell, E.P . Kanter, T. Graber and A. Belkacem, J. Chem . Phys . 94 (1991) 6377 . [11] D. Zajfman, A. Belkacem, T. Graber, E.P . Kanter, R.E . Mitchell, R. Naaman, Z. Vager and B .J . Zabransky, J. Chem. Phys . 94 (1991) 2543 . [12] A. Belkacem, E.P . Kanter, R.E . Mitchell, Z. Vager and B.J . Zabransky, Phys. Rev. Lett . 63 (1989) 2555 . [13] D. Zajfman, E.P . Kanter, Z. Vager and J. Zajfman, Phys . Rev. A 43 (1991) 1608 . [14] O. Heber, D. Kella, R. Naaman and Z. Vager, to be published. [15] D. Majer, O. Heber, D. Kella, D. Zajfman, Z. Vager and R. Naaman, to be published. [16] H. Kovner, A. Faibis, R. Naaman and Z. Vager, in : The Structure of Small Molecules and Ions, eds. R. Naaman, Z . Vager (Plenum, New York, 1987) p. 113. [17] D. Kella, MSc Thesis, Weizmann Inst . of Science (1989) .

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[18] G. Both, E.P . Kanter, Z. Vager, B.J . Zabransky and D. Zajfman, Rev. Sci. Instr. 58 (1987) 424. [19] D. Zajfman, G. Both, E.P . Kanter and Z. Vager, Phys . Rev. A41 (1990) 2482. [20] D. Zajfman, T. Graber, E.P . Kanter and Z. Vager, Phys . Rev. A46 (1992) 194. [21] See, for example, D.S . Gemmell, Chem . Rev. 80 (1980) 301 and references therein.

[22] W. Koenig, A. Faibis, E.P . Kanter, Z. Vager and B.J. Zabransky, Nucl. Instr. and Meth. B10/11 (1985) 259. [23] A. Belkacem, A. Faibis, E.P . Kanter, W. Koenig, R.E . Mitchell, Z. Vager and B.J . Zabransky, Rev . Sci. Instr. 61 (1990) 946. [24] D. Zajfman, H. Feldman, O. Heber, D. Kella, D. Majer, Z. Vager and R. Naaman, Science 258 (1992) 1129 .