Imaging in Chemical Dynamics - ACS Publications - American

Chapter 21

State-to-State Dissociation of Molecular Ions D. Zajfman

Downloaded by COLUMBIA UNIV on September 20, 2012 | http://pubs.acs.org Publication Date: October 18, 2000 | doi: 10.1021/bk-2001-0770.ch021

Department of Particle Physics, Weizmann Institute of Science, Rehovot, 76100, Israel

Abstract

State selective measurement of the dissociation of molec ular ions is made possible using molecular fragment imaging technique. We present a specific example which involve the simplest molecular ion HD , and its dissociation by low en ergy electron impact. The technique demonstrates that it is possible to follow the reaction path of the dissociation process of each initial vibrational state, as long as the pop ulation of these states are known. The population is mea sured using the Coulomb Explosion Imaging technique, and the measurement takes place in a storage ring, to allow for change of the vibrational population as a function of storage time. +

Introduction The last 20 years have seen tremendous progress in our understanding of elementary chemical reactions. Molecular-beam experiments have provided very detailed information about the dynamics of inelastic and reactive colli sions [1]. Supersonic expansion, leading to a strong reduction of the temper ature relevant for the molecular level population, has been widely exploited

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© 2001 American Chemical Society

In Imaging in Chemical Dynamics; Suits, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.


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in experiments dealing with neutral molecules in their vibrational ground state. Much greater experimental difficulties are encountered in producing molecules and molecular ions in well defined excited states and studies of their collision dynamics, important in such areas as high-temperature com bustion media or atmospheric processes under nonequilibrium conditions [2], are scarce. Although the production of beams of molecular ions in a well defined vi brational state has been shown to be possible (more specifically for neutral molecular beams) using complex laser pumping systems, it is a technological challenge to develop such techniques for any molecular ion, since the method usually requires a well suited set of electronic states so that the vibrational population transfer can be done efficiently. In the following we shall demon strate a different technique for measuring vibrational-state specific reaction rates, with a specific application to the dissociative recombination (DR) pro cess of H D [3]. It will be shown that the technique is not limited to D R reaction, but can be applied to a large variety of reactions. The methods presented here are based on advanced molecular imaging techniques and we will also present some new development in the field of three dimensional imaging. In the following Sections, the basic method for measuring state selected reaction rates, with emphasis on the D R process of H D and the storage ring environment is explained. Then a description of the three-dimensional detection techniques used in these experiments will be be given followed by the results of the Coulomb explosion imaging as well as of the D R for H D . Prom these results, the vibrational state-selective D R rate coefficients can be extracted. The last Section present some future thoughts about future experiments. +

+

+

The Dissociative Recombination of H D

+

Dissociative recombination (DR) [4] of molecular ions with free electrons is an elementary reactive collision strongly sensitive to vibrational excita tion. In many partly ionized gas-phase environments, the process removes charged particles and produces neutral fragments carrying considerable k i netic energy and often also internal excitation. It is therefore very important in astrophysics and planetary science; for example, the D R of Û 2 molecules is responsible for the production of the so-called green-light emission (airglow) in the earth ionosphere [5,6]. For a molecular ion A B in an initial vibrational state v, the D R reaction is described as [7,8]: +

+


352 +

AB (v)

+ e~ -* A{n) + B{ri)

(1)

1

where η and n are the final quantum states of the fragments A and Β respectively. The D R reaction is usually characterized by its rate coefficient a(E ), where E is the kinetic energy of the electron in reaction (1). For many years, the main problem in laboratory study of this process has been the extreme sensitivity of D R to the initial vibrational state of the molecular ion. During the last few years, the heavy-ion storage ring technique has been used to produce infrared active molecular ion beam in their ground vibrational state [9]. In this technique, a vibrationally excited molecular ion beam is generated by a standard (hot) ion source, and injected into the storage ring where it is stored for a time which is long enough to allow for complete vibrational relaxation through infrared transitions between the various vibrational states. Rotational cooling has also been demonstrated using this technique, the limit here being the blackbody radiation of the storage ring walls (300 K ) [10]. After full relaxation is obtained, the beam is merged with an intense, cold electron beam (see Fig. 1), which is produced within an electron cooler device [11] at a velocity similar to that of the ion beam. Due to the kinematical transformation between the energies in the laboratory frame of reference to the center of mass frame of reference, a strong reduction is obtained in energy spread, and resolution down to few meV are usually obtained in the study of D R reactions. The heavy-ion storage ring technique has led to many breakthroughs in the field of D R , and has considerably enhanced the theoretical understanding of this process


e

e

[9]. A n important step forward for the storage-ring techniques, would be the possibility to measure the D R cross section also for individual excited vibra tional states, i.e., not only for the ground state. A reaction well suited for initiating these studies is the D R of H D with low-energy electrons (experi mental energy spread ~10 meV). H D is the simplest molecular ion subject to vibrational relaxation by infrared emission. Its D R with low-energy elec trons can be depicted as +

+

+

HD (u)

+ e~ ->

H{ls) + D(nl) or H(nl)+D(ls)

(2)

where nl denotes the orbital of the electron in the atomic fragments. Starting with the capture of an electron by the molecular ion, a rearrangement of the whole electronic cloud leads to a transfer of kinetic energy to the dissociating nuclei. The D R from lower lying vibrational states ν of H D predominantly +


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Χ

starts by electron capture in the Σ + doubly excited, dissociating state of neutral H D (Fig. 2). This state crosses all the vibrational levels of H D at different locations. The recombination rate therefore strongly depends on the vibrational excitation of the ion. For higher vibrational excitation, an additional effect involving the next H dissociative curve and higher dissociative Rydberg states can be expected. It is important to point out that the as well as the Tl doubly excited states cross an infinite number of Rydberg states (shown in the inset in Fig. 2) and that the dissociating flux coming from these dissociating states can branches to these Rydberg levels, producing different degrees of electronic excitation in the product, depending either on the initial vibrational state or on the initial electron energy. Fig. 2 also illustrates that measurement of the kinetic energy release Ek identifies both the initial and the final state of the reaction. +

s

g

3


g

The main ideas behind the experimental technique is to measure, on one hand, the final states of the D R reaction for a beam of vibrationally excited H D , and on the other hand, to measure the vibrational population of the beam using the Coulomb Explosion Imaging (CEI) technique. Both experiments require to measure the kinetic energy release of the dissociation fragments. The experiments were carried out at the Max-Planck-Institut fur Kernphysik, Heidelberg, Germany using the Test Storage Ring (TSR) (see Fig. 1 [11]. A 2.0-MeV H D beam produced by a standard Penning ion source was injected into the T S R ; typically 10 particles circulated in the ring with a lifetime of ~ 10 s. The vibrational cooling time of H D is about 300 ms, depending on the initial vibrational excitation created in the ion source. The circulating beam was merged with a 3.5-cm diameter, quasimonochromatic electron beam over a length of 1.5 m, providing electrons at a typical den sity of 2 x l 0 c m " and a temperature of 10 meV in the comoving reference frame. Recombination fragments produced in this interaction region were detected as a function of the storage time t using an imaging microchannel plate located 6 m downstream [12], yielding the state-specific recombina tion rates r (t). The level populations N {t) were measured as a function of storage time from injection (t = 0) to complete relaxation (t > 300 ms), by extracting a part of the beam from the ring toward the C E I setup (see Fig. 1). The combination of these results then yields the D R rate coefficient OL for molecular ions in a vibrational state ν as +

+

7

+

6

3

v

v

V

a

v

=

Kr {t)IN {t) v

v


(3)


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F I G . 1. Schematic drawing of the T S R storage ring, including the ex traction towards the dedicated beam line for the Coulomb explosion imaging (CEI) of the molecular ions.

1.0

0.5

0.5

1.0

1.5

1.5

2.0

2.5

2.0

2.5

3.0

3.0

Internuclear Distance (Â)

F I G . 2. Potential energy curves for HD+ and H D . For HD+ only the ground electronic state is shown together with the position of its vibrational levels and some corresponding vibrational wave-functions squared. For H D , some of the Rydberg states (Qi and Q2 series) are shown.


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with a normalization constant Κ independent of t and v. The experiment aims to determine rate coefficients relative to a „ , therefore absolute nor malizations of r (t) and N (i) are not required. = 0

v

v

Three-dimensional Molecular Fragments Imaging As pointed out in the previous section, in order to measure the final state for the D R of H D , it is necessary to measure the kinetic energy release of the D R fragments, as well as the kinetic energy of the fragments after the Coulomb explosion imaging. Because of the fast velocity of the beam, the detector must have very good spatial resolution (better than ΙΟΟμιη) and very good time resolution ( « 100 ps). The detector must also be able to measure simultaneously, i.e., with no dead-time at all, the position and time of impact of few particles. For the detection and amplification of the original signal, most of the detectors use standard micro-channel plates ( M C P ) with various schemes of anodes. As the requirement is to measure the position and time of impact of all the fragments simultaneously, the standard technique of resistive anode [13,14] is useless as it allows the measurement of the time and position of a single particle only. Hence, different techniques have been developed, such as the segmented anode [15,16], the crossed-wire anode [17], and direct imaging using phosphor screen and video techniques together with pick-up wires [18]. Very often, the detector geometry (and/or the anode geometry) have been adapted to a specific experimental situation or problem.


+

In the present experiments, the basic imaging technique is based on the use of C C D video camera to measure the positions of impact, providing a two-dimensional image of the dissociation event, together with a time pick up technique, which measure the time differences between the impact of the different fragments. Two different pick-up techniques were used.

The C E I detector The C E I detector consist of a two-stage microchannel plate with a C s l coated, aluminized mylar foil located in front of the detector, in order to achieve nearly 100% detection efficiency on the M C P . Behind the M C P the amplified electron pulse generated by the particle impact is accelerated onto a P-20 phosphor layer that resides on top of a 17 μιη thin kapton foil. The electrons impact creates visible light spot which are imaged by a C C D cam era, which, after digitization (see below), yield the transverse (x parallel and In Imaging in Chemical Dynamics; Suits, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.


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y perpendicular to the magnet deflection plane) coordinates of the parti cle impact positions. The video signal from each of the two cameras is fed into a Frame Threshold Suppressor (FTS), a V M E module developed at the Weizmann Institute of Science [18] which digitizes each video frame by a 10 M H z , 8bit A D C and produce a list of pixels whose intensity exceeds a programmable threshold. Under typical condition, the pixel diameter of the C C D correspond to 200 μτη on the phosphor screen surface. Since the impact position of a particle is taken as an intensity weighted average over 20-50 pixels, the resolution in the transverse coordinates amounts to ~ 100 μια . The time difference between the various particle impact is measured using strip electrodes which are printed on the kapton foil located on the phosphor screen. These strips are use to pick up the fast timing signals induced for each bunch of electrons hitting the phosphor screen [19]. The printed circuit has been photochemically created from a 17 μπι thin copper coating on top of the kapton foil, and is made of 93 wires (100/xm wide, 1.0 mm apart) for the rectangular detector and 64 wires (1.85 mm apart) for the circular detector; the wire orientation is vertical, i . e. perpendicular to the deflection plane of the post deflect or. The detector described in ref. [18], used freely stretched thin wires (50 μπι diameter) in front of the phosphor screen instead of the printed wires behind the phosphor screen in the present setup. It was found that this new last configuration produces stronger signal than in the previous design. In order to avoid efficiency losses when two particles hit the same anode wire, the circular detector has 32 additional anode wires (3.75 mm apart) on the back side of the kapton foil at right angle to the wires on the front side (parallel to the deflection plane of the postdeflector). The time resolution achieved with this setup can be measured as the uncertainty of the time difference between a start and a stop signal from the same particle, neglecting the small contribution from the drift time spread in the M C P . This measurement yield a value (averaged over the wires) of 130 ps ( F W H M ) with a one standard deviation spread of 30 ps.

The D R imaging detector Fig. 3 shows a schematic view of this new multi-particle 3-D imaging detector. The particles are detected by a 40 mm diameter two-stage M C P , and, subject to the detection efficiency of the M C P , each impact produces a light spot of ~ 1-1.5 mm diameter on a P-20 phosphor screen located behind the M C P . The spatial position of each impact is extracted by digitizing



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Camac contrôler Veto

F I G . 3. Schematic drawing of the 3-D imaging detector setup. The in set on the upper left corner shows the geometry of the anode strips in the photomultiplier. P M T : Photomultiplier, C C D : Charge Coupled Device cam era, C F D : Constant Fraction Discriminator, Q - A D C : Charge Analog Digital Converter, C B D : C A M A C Branch Driver, F T S : Frame Threshold Suppres sor.


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the images formed on the phosphor screen using a standard C C D camera, focused on the phosphor screen through a vacuum window (as for the C E I detector). The video output is digitized by a fast frame grabber device which is read out by a V M E computer [18]. Upon analysis, the 2-D position of each impact is determined using a peak finding procedure. The time of arrival of each impact on the surface of the detector is obtained using a multi-anode P M T (Hamamatsu Model R5900U-00-L16), located outside the vacuum chamber. The image of the phosphor screen is focused on the P M T using stan dard lens optics. The present multi-anode P M T has 16 independent anodes (shaped as narrow strips, or wires), each of area of 16x0.8 m m , separated by 0.2 mm dead space between each pair. Other geometries are available such as square pads (4x4 or 8 x 8 independent pads). If each light spot cre ated on the phosphor screen by the impact of a single particle is focused on a different anode, the time of impact of a few fragments can be measured simultaneously by analyzing the 16 outputs of the P M T . For the sake of clarification, we emphasize again that both the 2-D position and the time of all impacts are measured simultaneously. In order to obtain the position and the relative time of arrival of each fragment on the detector surface for one single molecular dissociation (event) at a time, the MCP-phosphor screen assembly is operated in the so-called trigger mode [20,21]. In this mode of operation, the potential of the phosphor screen, which is used to accelerate the electrons from the M C P output plate to the screen itself is lowered to the M C P potential (fall time of about 1 ^s).as soon as an "event" is detected on the screen. The trigger for this is the logical " O R " signal generated by the constant fraction discriminators connected to the anode outputs (see Fig. 3). The high-voltage is turned back on as soon as the video output has been stored in the frame grabber. This ensures that only one event is digitized at a time, and that the measured coordinates (space and time), as obtained from the C C D and the P M T respectively, can be correlated to a single dissociation event. 2

+

Dissociative Recombination of H D ( i / = 0 ) +

In this section we present the results for the D R of H D ( z / = 0) for elec tron energy in the range 0

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