‘Magnetic bottle’ spectrometer as a versatile tool for laser photoelectron spectroscopy

Journal of Electron Spectroscopy and Related Phenomena 112 (2000) 151–162 www.elsevier.nl / locate / elspec

‘Magnetic bottle’ spectrometer as a versatile tool for laser photoelectron spectroscopy a,c a a, b c A.M. Rijs , E.H.G. Backus , C.A. de Lange *, N.P.C. Westwood , M.H.M. Janssen a b

Laboratory for Physical Chemistry, University of Amsterdam, Nieuwe Achtergracht 127 – 129, 1018 WS Amsterdam, The Netherlands Guelph-Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario, N1 G 2 WI Canada c Laser Centre and Department of Chemistry, Vrije Universiteit, de Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

Abstract This paper discusses the use of ‘magnetic bottle’ spectrometers in laser photoelectron spectroscopy. For electron detection both normal time-of-flight methods as well as zero-kinetic energy electron detection with almost laser-limited resolution can be employed. Photofragmentation processes can be monitored via time-of-flight ion detection. A ‘magnetic bottle’ spectrometer can be successfully combined with a molecular beam. To illustrate some of the features, results for the photodissociation of OCS are presented. Highly rotationally excited CO molecules, and S atoms in their excited 1 D 2 state are produced. Rotationally resolved photoelectron spectroscopy of CO is the key to a better understanding of the dynamics of the photoionisation process.  2000 Elsevier Science B.V. All rights reserved. Keywords: Rotationally resolved photoelectron spectroscopy; Multiphoton absorption; Photodissociation of OCS; Rotational excitation; Magnetic bottle

1. Introduction Conventional one-photon photoelectron spectroscopy has since its introduction in the 1960s played an increasingly significant role in chemistry [1–8]. Much of the importance of the technique lies in the fact that it can make molecular orbitals experimentally ‘visible’ for the first time, thus providing the much-needed link between a conceptually simple spectroscopic experiment and electronic structure theory. In such experiments the excitation source usually consists of a HeI gas discharge lamp producing a continuous flux of non-tunable photons of 21.2 eV. It is important to realise that, with this technique, *Corresponding author. Tel. / fax:131-20-525-6994. E-mail address: [email protected] (C.A. de Lange).

detailed information about the molecular ground state and the accessible ionic states, rather than of excited states, is obtained. The limitations associated with the conventional approach have in later years been mostly removed with the advent of lasers possessing high pulse energies in the visible and ultraviolet portion of the spectrum. Employing such new excitation sources the experimentalist can now go beyond the realm of one-photon methods and utilise processes involving the simultaneous absorption of more than one photon, a non-linear process ¨ predicted in 1931 by Goppert-Mayer [9]. These developments have led to resonance-enhanced multiphoton ionisation spectroscopy, either with massresolved ion detection, or with kinetic-energy resolved photoelectron detection. With this technique a number of photons is absorbed and an atomic or

0368-2048 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 00 )00209-7

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molecular excited state is accessed. The optical selection rules associated with multiphoton absorption are strict, and do depend on the number (n) of absorbed photons. This provides for a degree of flexibility, removing many of the limitations which plague one-photon absorption spectroscopy, and providing, for example, access to hitherto unobserved states of g symmetry, formally forbidden in one-photon selection rules from the ground state of many stable species with inversion symmetry. The tunability and monochromaticity of the laser allow for a wide range of excited states to be reached in this first step. In the second step one or more (m) additional photons are absorbed, and ionisation takes place. This step is again subject to optical selection rules, but these do not form a serious constraint on the final state reached, since the outgoing electron can take away the required excess energy and angular momentum. A feature to be emphasised is that with the absorption of a sufficient number of photons essentially every molecule can be photoionised. Through these (n 1 m) multiphoton absorption methods the inherent power of photoelectron spectroscopy can now be directed at an enormous variety of excited states. Indeed, one can regard the ionising step as the photoelectron spectroscopy of excited states, with the advantage of selecting the initial state symmetry, and the initial vibrational level v and rotational level J. The chemical relevance and importance of the method is clearly beyond question. The occurrence of the ionisation process can be monitored either by relatively simple mass-resolved ion detection [10,11], or by the experimentally much more demanding kinetic-energy (KE) resolved photoelectron spectroscopy (PES) [12–18]. Since the excited state is used as a stepping stone in the two-step process, the technique is called resonanceenhanced multiphoton ionisation (REMPI). When kinetic energy resolved photoelectron detection is employed, the acronym REMPI–PES is employed. With the latter method considerations of energy and momentum conservation allow the experimentalist to determine the internal energies of the ions formed in the two-step (excitation plus ionisation) process. Because the excited states are mapped efficiently and with few restrictions onto an ionic state which is often well known, the REMPI–PES method is very general and highly suitable for the detailed observa-

tion of such excited states. It has been argued, and demonstrated in many places, that the determination of ion internal energies (electronic, vibrational, rotational) is a true asset of the method, and that the use of REMPI–PES, also often termed laser photoelectron spectroscopy, offers very significant advantages for the study of molecular excited states which are not easily matched in its broad applicability by other techniques. When lasers pulsed at a certain repetition frequency are utilised as excitation sources, time-offlight (TOF) methods with their inherent multiplex advantage for the detection of the ions and / or electrons become a natural choice. As with any spectroscopic method the key issues are sensitivity and spectral resolution, which are clearly interconnected, and can be traded against each other to some extent. With a continuous excitation source the use of hemispherical analysers has been widespread. The main limitation with such a set-up is the very low collection efficiency of ,1% when it comes to electron detection. When stable molecules are studied the low collection efficiency is usually not insurmountable, but when attention is focused on shortlived gas-phase intermediates which tend to occur in low concentrations, the experimentalist needs all the possible signal. As we shall see in the following, a TOF ‘magnetic bottle’ spectrometer can be developed from the original design [19] into a superior and flexible tool for laser photoelectron spectroscopy. Clearly laser photoelectron spectroscopy with KE electron detection is more informative than monitoring just the presence of ions. Electron detection with high sensitivity and resolution is therefore the main mode of operation of the ‘magnetic bottle’ spectrometer. In both conventional PES and normal laser photoelectron spectroscopy the limiting factor in reaching an optimal spectral resolution is in the detailed construction and operation of the electron spectrometer. A crucial issue in this context is the extent to which critical surfaces close to electron trajectories can be protected from chemical contamination and the associated charging effects. With the recent advent of zero-kinetic energy electron detection with pulsed-field ionisation (ZEKE-PFI), a photoelectron spectroscopic method with essentially laser-limited resolution has been developed [20–23].

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As will be shown, the ‘magnetic bottle’ spectrometer can be operated in the ZEKE-PFI mode also. The fact that both normal laser photoelectron spectroscopy and ZEKE-PFI can be carried out in the same machine under identical experimental conditions is a very useful feature of our experimental setup at the University of Amsterdam. With the high pulse energies required in multiphoton absorption, some degree of photodissociation of the parent molecule is an almost unavoidable consequence. In order to understand and maximise the spectroscopic information obtained, it is important to be able to monitor which fragments are present in the spectrometer. To this end the ‘magnetic bottle’ spectrometer can be used in a TOF ion detection mode also. The mass resolution achieved in the mass range up to |260 a.m.u is sufficient to keep track of the fragmentation processes in small to medium-sized molecules. Finally, under appropriate circumstances the use of jet-cooled samples can be important. It is eminently feasible to interface a ‘magnetic bottle’ spectrometer with a pulsed molecular beam.

2. Experimental

2.1. Design considerations The idea of incorporating an axially symmetric inhomogeneous magnetic field in a photoelectron spectrometer was first realised successfully in a

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practical design by Kruit and Read [19]. A diverging magnetic field is used to collect photoelectrons produced in a small photoionisation volume. A lower magnetic field is used to guide the electrons into a flight tube for TOF analysis (Fig. 1). The name ‘magnetic bottle’ arises from the shape of the magnetic field. The electrons are detected at the end of the flight tube by two microchannel plates (MCPs) chevronned together. The trajectories of the electrons with a velocity component in the direction of the detector are made parallel by means of an inhomogeneous magnetic field which decreases from 1 T at the laser focus to 10 23 T in the 50 cm long flight tube. In this way a 50% collection efficiency (acceptance angle 2p sr) is achieved for electron energies from 0 to 10 eV, independent of the polarisation of the excitation light. The parallelisation occurs in the first few millimeters of the trajectories, which is a distance small compared to the length of the flight tube. A TOF method thus provides a suitable technique to measure the KEs of the electrons ejected in the interaction process between an atom or molecule and the pulsed laser radiation. The shape of the inhomogeneous magnetic field in the ionisation region (generated by a water cooled coil powered by a stabilised power supply) is very critical, and the transition to the much lower homogeneous field in the flight tube (generated by a separately powered coil wound around it) depend critically on the shape of the pole faces. A series of careful measurements of the magnetic field (using a Hall probe) on the axis

Fig. 1. Schematic of ‘magnetic bottle’ spectrometer showing the key functional elements.

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of the instrument as a function of the distance from the ionisation point into the flight tube is required to determine how to machine the pole pieces into optimal shape. In front of each pole face an insulated electrode or grid (not shown in Fig. 1) is placed to which an accelerating or decelerating voltage can be applied. The best resolution in a photoelectron spectrum is obtained for slow electrons. The electrons can be retarded in the first part of their trajectories just after they have been parallelised. A photoelectron spectrum with equal energy resolution for all electron energies can then be constructed by combining the high-resolution parts of the TOF spectra recorded with various retarding voltages. The earth’s magnetic field in the ionisation region and flight tube of the spectrometer, deleterious to the experiment, is compensated by the use of two perpendicular pairs of Helmholtz coils. In the third direction the magnetic field in the flight tube dominates, and compensation of this component of the earth magnetic field is superfluous. In our laboratory much research has been carried out on chemically relevant short-lived molecules and highly reactive radicals in the gas phase. Such species have a tendency to react not only with themselves, but also with critical surfaces in the electron spectrometer. In order to adapt the ‘magnetic bottle’ for the purpose of such studies, the original design of Kruit and Read has been modified in several crucial respects. First, the vacuum properties of the original design had to be improved. The pumping efficiency of the ionisation chamber was increased by enlarging the pumping ports as much as possible. The ionisation region and the flight tube are connected through a small hole in one of the pole faces with an area of only a few square millimeters through which photoelectrons enter the flight tube. In order to create a sufficient mean free path for the electrons in the flight tube, separate pumping was installed in this part of the spectrometer. Secondly, the pole faces were carefully coated with colloidal graphite (DAG 580, Acheson Colloids) in order to protect them from chemical attack and to eliminate electric charges which might otherwise build up locally on the pole faces. Thirdly, much effort has been invested in an overall design which can be

easily dismantled and reassembled for cleaning purposes. Finally, in order to minimise the risk of electrical discharge and breakdown, the individual MCPs were separated by a distance somewhat larger than usual. The homogeneous magnetic field in the flight tube at the position of the channel plates prevents the possible loss of electrons. Two separate ‘magnetic bottle’ spectrometers are operational in our laboratory. The main differences are in the sample inlet systems. The gas sample can be introduced into the ionisation region either via an effusive flow or via a pulsed molecular beam. In the first spectrometer [24,25] an effusive flow is introduced through a Pyrex sample inlet tube with a 13-mm outer diameter. The ionisation chamber is pumped by a 170 l / s oil diffusion pump (Balzers, DIF 170) complete with a baffle, backed by a Leybold Heraeus LH 16B rotary pump. In the vacuum line between these pumps a liquid nitrogen cooled trap is positioned, which is essential for the protection of the rotary pump. Both the ionisation chamber and the pole faces are regularly treated with colloidal graphite. The flight tube is pumped through a large port by a 450 l / s oil-lubricated turbomolecular pump (LH Turbovac 450) backed by a LH 25 B rotary pump. The pressure in the flight tube is monitored with a high vacuum ionisation gauge coupled to a LH Combivac IT 230 pressure indicator. Since the gauge produces unwanted electrons when operating, it is switched off during actual spectroscopic measurements. When no gas is introduced into the spectrometer and both the diffusion and turbomolecular pumps are operating, the pressure in the flight tube is typically 2?10 27 mbar. Under experimental operating conditions, pressures in the flight tube are of the order of 1?10 26 mbar. The pressure in the ionisation chamber is not measured, but estimated to be about 300 times higher (|10 23 mbar). To maintain the spectrometer performance at the required level, the oil of all pumps is changed frequently, and the spectrometer is exposed to dry nitrogen whenever the pumps are stopped. In the second spectrometer [26] a pulsed supersonic expansion is generated using a General Valve Iota One system in an additional chamber which is pumped by an Edwards Diffstak 2000 oil diffusion pump backed by an Edwards E2 M40 rotary pump.

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With a backing pressure of about 2 bar, a pulse duration of typically 280 ms at a repetition rate of 30 Hz is commonly employed. Under these conditions the pressure in the expansion chamber is about 6? 10 25 mbar, measured with a Penning gauge (Edwards CP25-K). Without gas input the pressure is about 1?10 27 mbar. A home-built delay pulse generator is used to control the timing of the gas pulse relative to the laser pulse. Nozzle diameters between 0.1 and 1 mm are employed. The expansion chamber is connected to the ionisation chamber through a Beam Dynamics skimmer with a diameter of 100 mm. The ionisation chamber is pumped by a Balzers TPH 170 turbomolecular pump backed by an LH Trivac D16B rotary pump. The flight tube is evacuated by an LH Turbovac 450 turbomolecular pump also backed by an LH Trivac D16B, leading to a typical pressure of 1?10 27 mbar with or without sample input. In both spectrometers the electrons are detected by a pair of MCPs (Hamamatsu F1054). After detection the MCP output current is amplified (Stanford Research 445), and sampled and stored in a 500 MHz digital oscilloscope (Tektronix TDS 540, 2GS / s) which is interfaced to a computer (Intel 80486 DX2 66 MHz). An extensive software package has been developed to control the laser system and the ‘magnetic bottle’ spectrometer, and to collect, store, and analyse spectra.

2.2. Time-of-flight electron detection and performance To locate the excited states, which are used as intermediate stepping stones in the multiphoton ionisation process, an excitation spectrum or wavelength spectrum is recorded. The wavelength is scanned in a stepwise manner through the energy region of interest and at every step a TOF spectrum is obtained, usually by counting all the electrons regardless of their KEs. The spectral resolution in the wavelength spectra is determined by the spectral resolution of the excitation laser, which is typically a few tenths of a wavenumber when a ns laser system is used. Such wavelength spectra are very similar to excitation spectra obtained by ion current measurement involving the collection of all the ions present.

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When chemical species are present in the ionisation region of the spectrometer with sufficiently different ionisation energies, a more sophisticated approach can be taken. The excitation spectra corresponding to each species can now be collected separately by collecting electrons only in a number of appropriate time windows. By tuning the laser wavelength to one of the resonances in the wavelength spectrum, the ejected electrons can be analysed according to their KEs. A photoelectron spectrum is obtained by converting the resulting TOF information into a linear energy scale. The first step in this conversion process is the choice of the energy resolution step. This choice defines a time window in the TOF spectrum, because only for relatively slow electrons do the time steps defined by the digital oscilloscope correspond to an energy difference less than or equal to the resolution step. If high resolution in the photoelectron spectrum is required, the time window employed must be narrow, and most of the TOF spectrum is then discarded. On the other hand, when resolution is deemed less important, a larger portion of the TOF information can be included. Equal energy resolution for all KEs throughout the entire photoelectron spectrum can be achieved by accelerating or decelerating fast electrons to a sufficient extent to bring them into the time window compatible with the preset energy resolution step. The required retarding voltage is increased in a stepwise manner, and after every increase new TOF data are collected. Visual control of the time-to-energy conversion process after each data acquisition step is possible, since both the TOF spectrum at a particular voltage and the photoelectron spectrum are displayed every time the value of the retarding voltage is changed. Representative values for the resolution in our photoelectron spectra are about 8 meV for KEs of 0.6 V, and 15 meV for KEs of 2.3 eV. It would appear that increasing the length of the flight tube from the current 50 cm would have a positive effect on the spectral resolution. In practice this is only true to a limited extent. We have experimented with a flight tube of 1 m in length, but found that the resolution improves less than that expected and hoped for. Spectrometer contamination and the associated charging effects are probably the limiting factors in

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obtaining optimal resolution in our photoelectron spectra.

2.3. Flight time to energy conversion and calibration When one wishes to distinguish between electrons that only differ a few meV in KEs, this is best achieved for slow electrons in the TOF spectrum. In order to obtain good resolution for faster electrons, they must be decelerated into the high-resolution part of the spectrum. By combining the high-resolution data of a series of experiments carried out at different retarding voltages, the overall photoelectron spectrum with constant resolution throughout the spectrum is obtained. A sensible calibration procedure is crucial in this context. The electron energies are expected to follow from L S D F]] T 1T G

m E5 ] 2e

2

1Vg 1Vc

(1)

d

where E is the electron energy in eV, L the length of the flight tube in meters, m the electron mass in kg, e the electron charge in C, T the flight time in s, T d the trigger delay in s, Vg the applied retarding voltage and Vc a measure for possible residual voltages. In practice the deviations from this equation can be as large as 0.1 eV. We therefore use the following phenomenological power series expression instead p

q

O ]Tb 1V O c T n n

E5

n50

g

n

n

p, q 5 1 2 4

(2)

n50

This is abbreviated to E 5 d(T ) 1 a(T ) Vg

(3)

The flight times T associated with two electron signals of calibration gases whose energies are accurately known from the literature, Elit,1 and Elit,2 , are measured as a function of the retarding voltages Vg,1 and Vg,2 . The two data lists enable the determination of the coefficients, b n and c n , in the following way. Using the abbreviated notation

Vg,1 Elit,2 2Vg,2 Elit,i d (T ) 5 ]]]]]] Vg,1 2Vg,2

(5a)

Elit,1 2 Elit.2 a (T ) 5 ]]]] Vg,1 2Vg,2

(5b)

A least-squares fitting procedure for d and a is used to determine the fitting coefficients b n and c n , which then allow the conversion from TOF to energy spectra using Eq. (2). With this equation and the two data lists, Elit,1 and Elit,2 are recalculated in order to see how well the calibration procedure has worked. When deviations are less than about 2 meV the calibration is considered to be quite good. The calibration procedure works best when narrow calibration signals can be found which occur close to and on either side of the unknown spectral features whose position must be determined. When the region in which the unknown peaks occur cannot be bridged by convenient calibration signals, the experimentalist has to resort to extrapolation.

2.4. Time-of-flight ion detection and performance The ‘magnetic bottle’ spectrometer can be used in modes of operation other than electron TOF. Moreover, switching from one mode to the next is a matter of only minutes. The instrument can be used as a TOF mass spectrometer by utilising the grids mounted on the pole faces. Typical voltages of 1140 and 120 V, respectively, are applied to the grid on the pole face farthest away from the detector, and on the grid on the other pole face. The front end of the first MCP is kept at 23000 V. The signal-tonoise ratio attainable with ion detection is somewhat worse than when recording the electron signal. When the applied voltages are optimized, a mass resolution M /DM of |260 can be achieved in this spectrometer. The TOF mass scale can be calibrated by employing noble gases with known masses.

Elit,1 5 d (T ) 1 a (T ) Vg,1

(4a)

2.5. ZEKE-PFI and performance

Elit,2 5 d (T ) 1 a (T ) Vg,2

(4b)

The grids mounted on, and insulated from, both pole faces can be used for the application of static and pulsed electric fields in the photoionisation

the following expressions for d and a are obtained

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region. Although the ‘magnetic bottle’ spectrometer is by no means comparable to a dedicated ZEKE-PFI machine, the grids still allow for the possibility of ZEKE-PFI, albeit with a resolution of |5 cm 21 rather than a few tenths of a wavenumber. In the multiphoton absorption step the intermediate state is reached, while in the second step, which generally requires a different laser colour, high-n Rydberg states which act as long-lived reservoir states are accessed. There is a certain probability that additional photons are absorbed during the same laser pulse, leading to ionisation and the production of prompt electrons. In order to sweep out these prompt electrons the grid nearest the flight tube is grounded, while a negative variable bias (20.5 to 22.5 V), corresponding to an electric field strength of 2.5 to 12.5 V/ cm is applied to the other grid. After an appropriate delay ranging from 5 to 200 ns a fast electric pulse of |20 V/ cm is switched on to fieldionise the reservoir states, and electrons with virtually zero kinetic energy are formed. Although in normal ZEKE-PFI spectrometers considerably longer delay times are usually used, such delays proved impossible in the ‘magnetic bottle’. The tight focusing employed, and the spatial restrictions on the homogeneity of the magnetic field limit the time, which the high-n Rydberg states spend in the ionisation region. As the result of a relatively fast decay of the high-n Rydberg states on a time scale of |50 ns, the delay time has to be chosen such that a compromise is reached between distinguishing prompt and PFI electrons, while retaining enough PFI electrons for a sufficient signal-to-noise ratio. Finally, this pulse accelerates the ZEKE-PFI electrons towards the MCP detector. In spite of the shortcomings of the ‘magnetic bottle’ for performing ZEKE-PFI experiments, the best energy resolution achieved so far is about 4 cm 21 , which is a considerable improvement compared to the 80–120 cm 21 typical in conventional PES. The fact that electron TOF and ZEKE-PFI can be carried out in the same instrument is very useful. It allows the experimentalist to first explore the crude spectral features with conventional electron TOF spectroscopy, and then to use the ZEKE-PFI capability to ‘zoom in’ on interesting features in the spectrum. Illustrative examples of ZEKE-PFI performed in the ‘magnetic bottle’ are found in the literature [26–28].

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2.6. Laser system The ‘magnetic bottle’ spectrometers can be used in conjunction with a large variety of laser systems. In some cases we have carried out time-resolved photoelectron spectroscopy on a picosecond time scale employing a two-colour pump-probe scheme. For a description of this picosecond laser system we refer to Ref. [29]. Commonly an excimer-pumped dye laser system is employed. The excimer laser (Lumonics HyperEx-460) is used with a XeCl filling, producing pulsed radiation with a fixed wavelength of 308 nm and a pulse duration of |10 ns. The system is operated at a repetition rate of 30 Hz and typical pulse energies of 200 mJ / pulse. The radiation generated by the excimer laser is used to pump either one or two dye lasers (Lumonics HyperDye-300 and 500). Various dye solutions, each with a maximum conversion efficiency between 8 and 15%, allow for the generation of continuously tunable radiation from |330 to |900 nm. The resulting dye laser output with a bandwidth of |0.08 cm 21 (HD-500) is vertically polarised. For an absolute calibration of the dye laser wavelength use is made of the optogalvanic effect. The dye laser radiation is passed through a hollow cathode discharge lamp filled with neon gas (Hamamatsu L233-24 NB, Cr / Ne). As the dye laser is scanned from 600 to 630 nm several optogalvanic transitions are recorded. They can be compared with well-known Ne transitions. The method provides an absolute wavelength calibration of |2 pm. The linearity and reproducibility of the scan control are sufficient to maintain an overall accuracy of 30 pm. To extend the accessible wavelength range further to the blue, the techniques of second (SHG) and third (THG) harmonic generation are employed. Both dye lasers can be frequency doubled (Lumonics HyperTrak-1000 and INRAD II autotracker) with a conversion efficiency of about 10% using angletuned KDP, KD*P, and BBO non-linear crystals. The resulting frequency-doubled light with a bandwidth of |0.2 cm 21 is horizontally polarised. The polarisation direction can be changed by means of a homemade Fresnel rhomb or l / 2 wave plates. The laser light is focused into the ionisation region of the ‘magnetic bottle’ spectrometer by plano-convex quartz lenses with a focal length of 25 mm

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located on adjustable mounts on both sides of the spectrometer. Maximum average power densities obtained in the focus (beam waist) of about 10 mm diameter are in the order of 10 10 –10 12 W/ cm 2 . In a two-colour experiment, or in a photolysis-probe experiment, the two beams enter the spectrometer from opposite sides. The timing of the two pulses is checked with a fast silicon photodiode (HP50824203).

3. Results and discussion The use and applications of the magnetic bottle for the investigation of the spectroscopy and dynamics of small transient and stable molecules has been documented extensively over the years by the Amsterdam group [18]. The following examples will consider applications of the ‘magnetic bottle’ spectrometer for an investigation of the dissociation of triatomic molecules, classic systems for understanding the abundance of photodissociation processes. The 16 valence electron systems CO 2 , N 2 O, OCS, and CS 2 have been studied extensively by conventional absorption spectroscopy [30] and exhibit electronic absorption from the linear X 1 S 1 ground state to either linear or bent excited states. Rydberg states of all four triatomics, CO 2 [31], N 2 O [32], OCS [33,34] and CS 2 [34,35], have been investigated by the Amsterdam group. In circumstances where bent states are involved, photodissociation can lead to the formation of CO, O, N 2 , S and CS fragments with the branching ratios for the product and state distributions strongly dependent upon laser wavelength. The example considered here is that of carbonyl sulphide, OCS, a molecule abundant in the interstellar medium [36], and relevant to interstellar photochemistry. At around 230 nm the dominant photodissociation channel following excitation in the ˜ 1 S 1 absorp(formally one-photon forbidden) 1 D←X tion system is to CO (X 1 S 1 , v050) and S ( 1 D 2 ). As the fragments separate a strong torque is exerted on the separating CO fragment, leading to strong rotational excitation. In the photodissociation process the C–O bond length appears to be only a spectator, leading to a vibrationless product, where the distribution of the N0 states is bimodal with one peak maximum at N0|50 and the second at N0|62. Here

N0 signifies the end-over-end angular momentum exclusive of spin associated with the ground state. Previous work has probed the fragment channels employing laser induced fluorescence [37,38] or ion imaging [39], also combined with (211) REMPI [40,41]. It was concluded that the dissociation takes place via near-degenerate A0 and A9 states, each contributing to the observed rotational distribution. Given the sensitivity of the ‘magnetic bottle’ spectrometer, its ability to monitor the nature of the photoproducts with REMPI excitation scans and ion detection, and its capability of probing dynamics with rotationally resolved measurements, photodissociation of OCS seemed an appropriate molecular system for investigation. Previous REMPI measurements on the Rydberg excited states of OCS [33,34] had also noted the fragmentation and bimodal rotational distributions of CO in ion-detected wavelength scans, but REMPI–PES experiments have not, to our knowledge, been made on the nascent CO produced in this process. In principle, electron KE measurements with ionic state rotational resolution can provide information on the exchange of angular momentum between the departing electron and the ion core and, in the present case, insights into the photoionisation dynamics. Opportunities for rotationally resolved photoelectron spectroscopy are limited, despite the relatively high resolution of the ‘magnetic bottle’ spectrometer compared to conventional electron spectrometers. Typically one requires light diatomics with large rotational constants and the ability to record from high N9 lines of the intermediate state. Here N9 is the angular momentum of the intermediate state exclusive of spin. The optical selection rules dictate that in the ion N 1 values similar to N9 are accessed. The rotational spacings in the photoelectron spectrum go as 2B 1 N 1 , where B 1 is the ionic rotational constant, and N 1 signifies the end-over-end rotation of the ion. For example, rotationally resolved photoelectron spectra have been obtained from the f 1 P, g 1 D and h 1 S 1 states of the 1 NH radical [42]. Transitions to the ion with B e 5 15.689 cm 21 [43] were measured from intermediate states with rotational quantum numbers as low as N959–16. In the present case, the rotational constant of CO 1 (X 2 S 1 , v 1 50) is not especially favourable, 1.9677 cm 21 [44]. Nonetheless, with the high rotational excitation extant in the present experiment,

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there is the opportunity to measure to very high N9 (.70) with the concomitant possibility of observing rotational resolution in the electron KE scans. Fig. 2 provides the raw ion TOF spectrum obtained from the 229.99 nm (43478.49 cm 21 ) photolysis of OCS (Matheson). Photodissociation of OCS at this wavelength leads to the neutrals, CO (X 1 S 1 ) and S ( 1 D 2 ), which are subsequently ionised by multiphoton absorption. Typical flight times for these ions are 16.9 ms for CO 1 and 18.0 ms for S 1 . Mass calibration was performed using xenon. The two strong peaks at m /z528 and 32 correspond to the photodissociation products 12 C 16 O 1 and 32 S 1 , with a weak adjacent peak corresponding to the 13 C isotopomer. C 1 , C 21 , and O 1 are also observed. In principle, excitation scans for both CO and S could be performed by residing in these different mass channels, and recording the ion yields as a function of wavelength. Indeed this is often done when isotopically resolved wavelength scans are required, e.g. for 35 Cl and 37 Cl containing radicals such as SCl [45]. However, as explained above, excitation scans are normally obtained with electron detection, as this is the most sensitive mode of operation. Employing electron detection Fig. 3a shows the

Fig. 2. Time-of-flight mass spectrum of the ions produced in the photodissociation of OCS at 229.99 nm, where both CO(X 1 S 1 ) and S( 1 D 2 ) can be resonantly ionised.

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excitation scan from 86910 to 87040 cm 21 in the region of the strong Q branch of the two-photon accessible B 1 S 1 (v950) state of nascent CO produced by photodissociation of OCS. The same laser is used for both photodissociation and REMPI–PES detection. This spectrum clearly shows a well-resolved bimodal rotational distribution, extending from about N9537, through the two main maxima (N9550, and N9|62). Also a Q branch structure is observed near the origin of the electronic transition. At higher laser power this distribution has been followed all the way up to N9 5 87. The much weaker O and S branches were not searched for in the present experiments, since most intensity is carried by the scalar (T 00 ) component of the twophoton transition probability. They have, however been seen by other (211) MPI investigations via the B 1 S 1 state [33,46,47]. Although detection of all electrons is often used, the present spectrum was obtained by monitoring a defined KE window (corresponding to a flight time range of 807–852 ns) appropriate to that for ionisation of CO. The N9 numbering of the rotational transitions was obtained using known rotational constants (B and D) for the B 1 S 1 and X 1 S 1 states [48–50]. Simultaneously, but in another KE window (flight time range, 963–1051 ns), the same wavelength scan picked out the 7f atomic resonances (Fig. 3b). These are located in the ionisation continuum above the first ionisation energy and arise from a two-photon excitation from the S ( 1 D 2 ) photodissociation product which is 21 3 9238.6 cm above the PJ50,1,2 ground state [51]. Electron KE scans were obtained from various high N9 transitions of the Q branch of the intermediate state. Fig. 4 illustrates such a scan taken from the Q(84) transition at 87063.2 cm 21 , where rotational resolution in the ion (CO 1 (X 2 S 1 , v 1 5 0)) is clearly observed. Several features of this kinetic energy scan can be noted. First, there is no evidence for a transition into v 1 51, indicating that the ionising transition is highly diagonal, i.e. Dv 5 0; this is not unexpected given the similarity between ˚ [48]) and the the B 1 S 1 Rydberg state (r e 5 1.1197 A 2 1 ˚ [52]). This ground state ion (X S , r e 5 1.1151 A, contrasts with recent REMPI–PES on CO via the B state where non Franck–Condon behaviour is observed, especially for ionisation via v950 [53]. In the present case, due to the high rotational levels

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Fig. 3. Excitation scans employing two discrete electron energy windows for simultaneous recording of (a) the two-photon accessible Q branch of the B 1 S 1 ←← X 1 S 1 transition in the nascent CO formed by photodissociation of OCS and (b) the two-photon accessible ( 2 D 3 / 2,5 / 2 )7f←← 1 D 2 transitions of the metastable S atom, both formed by photodissociation of OCS.

accessed in the ion, we are exploring a different region of the continuum, whereas in the earlier work, the non-diagonal result may be due to the presence of superexcited states. This clearly has to be explored further. Secondly, the full-width-half-maximum (FWHM) for the strong component is 17 meV, permitting clear resolution of the N 1 levels, separated by |41 meV. The rotational ion distribution produced upon ionisation shows that some 60% of the intensity has gone into N 1 583. The measured electron KE of this level, |2.14 eV, concurs with an overall three-photon process, the present degree of rotational excitation and an ionisation energy for CO of 14.0136 eV [44]. For ionisation of such Rydberg levels the ion rotational distributions are governed by the propensity rule, DN 1 l5odd, where DN 5 N 1 2 N9 is the change of rotational quantum number and l

is the partial-wave component of the photoelectron orbital [54,55]. In the present case the Rydberg state is a 3ss so we expect a p (l51) partial wave. In the photoelectron spectrum the odd and even DN have comparable intensities suggesting some p participation in the Rydberg state. The N 1 distribution is not symmetrical, with an additional feature at 2.02 eV. The corresponding feature on the other side of the main peak is probably missing due to the poor counting statistics. In summary, we have shown how a ‘magnetic bottle’ spectrometer is an excellent and flexible tool for laser photoelectron spectroscopy. In the present study an application to photodissociation of OCS is discussed in which rotationally hot CO is produced. Despite the small rotational constant of the CO 1 ion, the appreciable sensitivity and resolution of our

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Fig. 4. Rotationally resolved photoelectron spectrum obtained by a one-photon ionisation from the Q(84) level of the intermediate B 1 S 1 (v9 5 0) state of rotationally excited CO.

spectrometer allow for a rotationally resolved PES study. A detailed theoretical analysis of these and associated results is in progress.

Acknowledgements AMR thanks the Holland Research School for Molecular Chemistry for a Ph.D. scholarship. CAdL and NPCW gratefully acknowledge NATO for a collaborative research grant (No. CRG930183).

References [1] D.W. Turner, A.D. Baker, C. Baker, C.R. Brundle, Molecular Photoelectron Spectroscopy, Wiley, London, 1970.

[2] A.D. Baker, D. Betteridge, Photoelectron Spectroscopy, Pergamon Press, Oxford, 1972. [3] J.H.D. Eland, Photoelectron Spectroscopy, Butterworths, London, 1974. [4] J.W. Rabalais, Principles of Ultraviolet Photoelectron Spectroscopy, Wiley, New York, 1977. [5] R.E. Ballard, Photoelectron Spectroscopy and Molecular Orbital Theory, Adam Hilger, Bristol, 1978. [6] J. Berkowitz, Photoabsorption, Photoionization and Photoelectron Spectroscopy, Academic Press, New York, 1979. [7] K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki, S. Iwata, Handbook of He(I) Photoelectron Spectroscopy of Fundamental Organic Molecules, Halsted, New York, 1981. [8] P.K. Gosh, Chemical Analysis, in: P.J. Elving, J.D. Winefordner (Eds.), Introduction to Photoelectron Spectroscopy, Wiley, New York, 1983, p. 67. ¨ [9] M. Goppert-Mayer, Ann. Phys. 9 (1931) 273. [10] G.S. Voronov, N.B. Delone, Sov. Phys. JETP 23 (1966) 54. [11] S.H. Lin, Y. Fujimura, H.J. Neusser, E.W. Schlag, Multiphoton Spectroscopy of Molecules, Academic Press, New York, 1984. [12] J.P. Reilly, Israel J. Chem. 24 (1984) 266.

162

A.M. Rijs et al. / Journal of Electron Spectroscopy and Related Phenomena 112 (2000) 151 – 162

[13] K. Kimura, Adv. Chem. Phys. 60 (1985) 161. [14] K. Kimura, Int. Rev. Phys. Chem. 6 (1987) 195. [15] P.M. Dehmer, J.L. Dehmer, S.T. Pratt, Comm. At. Mol. Phys. 19 (1987) 205. [16] S.T. Pratt, P.M. Dehmer, J.L. Dehmer, in: S.H. Lin (Ed.), Advances in Multiphoton Processes and Spectroscopy, Vol. 4, World Scientific Press, Singapore, 1988. [17] R.N. Compton, J.C. Miller, in: D.K. Evans (Ed.), Laser Applications in Physical Chemistry, Marcel Dekker, New York, 1989, p. 121. [18] C.A. de Lange, J. Chem. Soc., Faraday Trans. 94 (1998) 3409. [19] P. Kruit, F.H. Read, J. Phys. E 16 (1983) 313. ¨ [20] K. Muller-Dethlefs, M. Sander, E.W. Schlag, Chem. Phys. Lett. 112 (1984) 291. ¨ [21] K. Muller-Dethlefs, E.W. Schlag, Ann. Rev. Phys. Chem. 42 (1991) 109. ¨ [22] K. Muller-Dethlefs, E.W. Schlag, E.R. Grant, K. Wang, B.V. McKoy, Adv. Chem. Phys. 90 (1995) 1. [23] E.W. Schlag, ZEKE Spectroscopy: the Linnett Lectures, Cambridge University Press, Cambridge, 1998. [24] B.G. Koenders, D.M. Wieringa, K.E. Drabe, C.A. de Lange, Chem. Phys. 118 (1987) 113. [25] C.A. de Lange, in: I. Powis, T. Baer, C.Y. Ng (Eds.), High Resolution Laser Photoionization and Photoelectron Studies, Wiley, New York, 1995, p. 195. [26] C.R. Scheper, W.J. Buma, C.A. de Lange, J. Elec. Spec. Relat. Phenom. 97 (1998) 147. [27] J.B. Milan, W.J. Buma, C.A. de Lange, J. Chem. Phys. 104 (1996) 521. [28] N.P.L. Wales, W.J. Buma, C.A. de Lange, H. Lefebvre-Brion, K. Wang, V. McKoy, J. Chem. Phys. 104 (1996) 4911. [29] M.R. Dobber, W.J. Buma, C.A. de Lange, J. Phys. Chem. 99 (1995) 1671. [30] J.W. Rabalais, J.M. MacDonald, V. Scherr, S.P. McGlynn, Chem. Rev. 71 (1971) 73. [31] M.R. Dobber, W.J. Buma, C.A. de Lange, J. Chem. Phys. 101 (1994) 9303. [32] C.R. Scheper, J. Kuijt, W.J. Buma, C.A. de Lange, J. Chem. Phys. 109 (1998) 7844. [33] R.A. Morgan, A.J. Orr-Ewing, D. Ascenzi, M.N.R. Ashfold, W.J. Buma, C.R. Scheper, C.A. de Lange, J. Chem. Phys. 105 (1996) 2141.

[34] R.A. Morgan, M.A. Baldwin, D. Ascenzi, A.J. Orr-Ewing, M.N.R. Ashfold, W.J. Buma, J.B. Milan, C.R. Scheper, C.A. de Lange, Intl. J. Mass Spec. Ion. Proc. 159 (1996) 1. [35] R.A. Morgan, M.A. Baldwin, A.J. Orr-Ewing, M.N.R. Ashfold, W.J. Buma, J.B. Milan, C.A. de Lange, J. Chem. Phys. 104 (1996) 6117. [36] A. Dalgarno, J.H. Black, Rept. Progr. Phys. 39 (1976) 573. [37] N. Sivakumar, I. Burak, W.-Y. Cheung, P.L. Houston, J.W. Hepburn, J. Phys. Chem. 89 (1985) 3609. [38] N. Sivakumar, G.E. Hall, P.L. Houston, J.W. Hepburn, I. Burak, J. Chem. Phys. 88 (1988) 3692. [39] T. Suzuki, H. Katayanagi, S. Nanbu, M. Aoyagi, J. Chem. Phys. 109 (1998) 5778. [40] Y. Sato, Y. Matsumi, M. Kawasaki, K. Tsukiyama, R. Bersohn, J. Phys. Chem. 99 (1995) 16307. [41] Z.H. Kim, A.J. Alexander, R.N. Zare, J. Phys. Chem. 103 (1999) 10144. [42] K. Wang, J.A. Stephens, V. McKoy, E. de Beer, C.A. de Lange, N.P.C. Westwood, J. Chem. Phys. 97 (1992) 211. [43] K. Kawaguchi, T. Amano, J. Chem. Phys. 88 (1988) 4584. [44] M. Evans, C.Y. Ng, J. Chem. Phys. 111 (1999) 8879. [45] J.D. Howe, M.N.R. Ashfold, R.A. Morgan, C.M. Western, W.J. Buma, J.B. Milan, C.A. de Lange, J. Chem. Soc. Faraday Trans. 91 (1995) 773. [46] P.J.H. Tjossem, K.C. Smyth, J. Chem. Phys. 91 (1989) 2041. [47] S. Wurm, P. Feulner, D. Menzel, J. Chem. Phys. 105 (1996) 6673. [48] M. Eidelsberg, J.-Y. Roncin, A. Le Floch, F. Launay, C. Letzelter, J. Rostas, J. Mol. Spectrosc. 121 (1987) 309. [49] C.R. Pollock, F.R. Petersen, D.A. Jennings, J.S. Wells, J. Mol. Spectrosc. 99 (1983) 357. [50] A. Le Floch, Mol. Phys. 72 (1991) 133. [51] C.E. Moore, Atomic Energy Levels, NBS Circ. 467, Vol. 1, Natl. Bur. Std., Washington, 1949. [52] K.P. Huber, G. Herzberg, in: Molecular Spectra and Molecular Structure, Vol. 4: Constants of Diatomic Molecules, Van Nostrand Reinhold, New York, 1978. [53] G. Sha, D. Proch, Ch. Rose, K.L. Kompa, J. Chem. Phys. 99 (1993) 4334. [54] J. Xie, R.N. Zare, J. Chem. Phys. 93 (1990) 3033. [55] S.N. Dixit, V. McKoy, Chem. Phys. Lett. 128 (1986) 49.