Threshold photoionization and ZEKE spectroscopy: a historical perspective

Journal of Electron Spectroscopy and Related Phenomena, 66 (1993) 139-149 0368-2048/93/$06.00 0 1993 ~ ElsevierScience Publishers B.V. All rights reserved

Threshold photoionization historical perspective’

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and ZEKE spectroscopy: a

E.W. Schlagap*, W.B. Peatmanb, K. Miiller-Dethlefsa ‘Institut fir Physikulische Chemie, Technische Universitiit Mtinchen, Lichtenbergstrafie 4, D-85747 Garching, Germany bBESSY, Berliner Elektronenspeicherringfir Synchrotronstrahlung, Lentzallee 100, D-14195 Berlin, Germany (First received 6 February 1993; in final form 15 June 1993)

The new ZEKE spectroscopy is described, and its development is presented and related to the older threshold electron spectroscopy. This is compared to the field of photoelectron spectroscopy. Resultsfrom the different techniques are compared.

Photoionization of molecular ions has become one of the standard techniques leading to an understanding of the structure and spectra of ionized species, an understanding indispensable to any modem chemical laboratory [1,2]. Early work by Lossing and Tanaka [3] used vacuum UV radiation for generating molecular ions in a mass spectrometer. Before that in 1954 the work of Watanabe [4] led to the first accurate directly measured ionization energies (IEs) (measured by photoionization) although Price in early spectroscopic work [5] similarly obtained IEs when Rydberg series were seen. In fact, photoionization goes back to ion pair production in 1932 in the work of Terenin and Popov

PI. This was essentially the state before the signal advancements in the field of the early 1960s. Here it was seen that photon excess energies produced a host of emitted electrons which gave information on molecular energy levels [7,8]. It was this recognition in the pioneering experiments by Turner and ‘Dedicated to Professor David Turner for his services to electron spectroscopy. * Corresponding author. SSDI 0368-2048(93)01837-5

his school [8,23] that showed that a vacuum UV resonance light source (He 21 eV line) with an electron monochromator produced an entirely new form of spectroscopy which can well be said to have revolutionized our understanding of molecular ions, the technique of photoelectron spectroscopy (PES). In a different series of experiments Siegbahn [9] applied the study of X-ray photoelectrons in a beautiful manner to core level excitations in molecules, leading to electron spectroscopy for chemical analysis (ESCA). Our own contribution to the study of molecular ions stands on a somewhat different footing and started with experiments carried out in the late 1960s at Northwestern University. We were motivated by the idea of an absolute energy assignment of vibrational energy levels, and eventually rotational levels. In photoelectron spectra light of a fixed energy, typically the 584.& line of He is employed, thus leading to ionization of all levels below this energy of 21 eV. The individual levels are then sorted by measuring the kinetic energies of the emitted electrons. This spectrum of the photoelectrons emitted, the photoelectron spec-

E. W. Schlag ei al./J. Electron Spectrosc. Relat. Phenom. 66 (1993) 139-149

140

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Fig. 1,Photoelectron measurements. Threshold experiments on the left hand side, excited with one-photon excitation or resonant or non-resonant multiphoton ionization. Photoelectron spectroscopy on the right hand side, typically employing the sharp He I resonance lamp at 21 eV, “back-titrating” the electrons from the various molecular states and again showing onephoton or multiphoton excitation, typically with less excess energy.

is then assigned to the various possible electronic states of the molecule under study (Fig. 1, right hand side). The technique involves the measurement of electron energies in an electron monochromator, or time-of-flight electron analyzer, many designs of such instruments having been published in the

trum,

last 30 years [lo]. The puzzling features that all of these instruments have in common are that, (a) they require careful energy calibrations which must be repeated often, as this value is subject to substantial drift, and (b) the resolution, even for modern instruments, is limited typically to some IO-12meV (lOOcmP’), which is a good value and is rarely exceeded, except in special cases. These effects are probably due to uncontrollable contact potentials which must naturally affect any electron energy determination. Our initial experiments were designed to invert these arguments by focussing only on electrons emitted at the threshold of an ionic eigenstate (Fig. 1, left hand side). At this photon energy many types of eIectrons are produced due to previous spectroscopic transitions still contributing to the cross section, but the presence of the new ionic eigenstate is unique in that it produces electrons with no kinetic energy, and hence a new threshold signal. As the energy is increased further the total cross section remains (Fig. 2, left hand side), but the threshold signal disappears (Fig. 2, right hand side); hence, instead of the typical staircase func-

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Fig. 2. Yield and threshold spectra. The left hand side shows the successive increase of the signal with energy, always adding to the previous signal (some autoionizing structure). The right hand side shows the cut-off produced if only threshold electrons arc detected.

ng8A

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Fig. 3. Threshold electron spectrum of NO+. This is a trace of the original spectrum from the 1969 Thesis of Peaunan [IT].

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tion in the total electron yield (ionization efficiency) curve, we now once more obtain sharp transitions only at the onset of ionic eigenstates. These threshold electrons could, in principle, also be detected by setting the electron monochromator near zero [l 11,but this has the same difficulties of drift and bandwidth mentioned above as it is a special variant of PES. In the early work at Northwestern we constructed a device for measuring threshold electrons that made no use of the energy of the electron, and in fact gave an absolute spectroscopically accurate energy even if surface potentials and allied effects were involved, these latter only possibly affecting the intensity. Hence this threshold electron analyzer produced not only peaks as in a normal spectrum but only produced them at the correct energy (Fig. 3) (provided one has a reliable light source) as in any normal spectroscopic experiment. This device employed a different property of threshold electrons, namely that they have a poor steradiancy. In the idealized picture electrons at threshold do not move and

-

hence do not spread out; a small forward field will cause them to travel at no loss down a very long channel penetrating a small hole at the end of this channel. In contrast all hotter electrons have a component of velocity perpendicular to this motion, causing them to miss the hole at the end of the channel and hence detection. We named this a steradiancy analyzer [17, ISa]. This device then transmits only threshold electrons, without relying on the energetics of these electrons, so only the velocity vectors matter. Hence this device cannot falsify the energy of the onset of the detection of an eigenstate of the ion and the worst any surface potential can do is to modulate the intensity, but never the position on the energy scale. Hence this device measures absolute energies as derived from the light source; in the early days this was a source from a high pressure argon discharge into a VW monochromator (Figs. 4, 5), but today it is a laser in multiphoton absorption or a synchrotron light source. In this way this technique was an absolute energy analyzer since it depended on the steradiency properties of the

REIMAGIN’G

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PHOTON BEAM /

ACCELERATOR AND DRIFT TUBE

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Fig. 4. Diagram of the threshold electron-ion coincidence spectrometer from the 1969 Thesis of Peatman [la.

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E. W. Schlag ef al./J. Electron Spectrosc. Ralal. Phenom. 66 (1993) 139-149

MONOCHROMATOR

LIGHT

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Fig. 5. Electron coincidence apparatus from the 1969 Thesis of Peatman [17] showing the high pressure Ar source, the VW monochromator, the cassegrain VW refocussing optics and the threshold electron spectrometer.

photoelectrons at threshold, not their energy. After our original work on NO we also applied this to CH31 [lSb]. As our experience accumulated it became clear that a small amount of in-line electrons would penetrate the analyzer, and, if hot electrons were involved, produce an unwanted background, particularly if strong autoionizing states which are always present i.e. superexcited states of the neutral, were excited. By setting a proper detection gate these hot electrons can be eliminated [12]. Another method is to employ a slightly curved path in the analyzer [13]. A number of variants of this device have been built over the years, such as multiple pipes [14], or indeed a multichannel plate as a steradiancy analyzer [15]. In addition, Peatman et al. [17,1X]suggested the use of electrons so detected to identify the electronic state of the ions or fragments of the parent ions produced ih a pulsed coincidence setup (Fig. 4) along the lines developed by Wiley and McLaren in their time-of-flight mass spectrometer [ 161. Since each ionic state would be identified by the generation of electrons of zero kinetic energy, i.e. by detection of identical species for all transitions,

Ar : ION -YIELD

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Ar : ZEKE - PES

Fig. 6. The ion spectrum of argon. (a) Total ion yield curve. On the left edge is the onset of the ‘P31zion state; the Rydberg series shown converges to the 2PL/2 state. (b) Both ionic states are clearly visible in the ZEICJZspectrum.

E. W. Schlag et al.lJ. Electron Spectrosc. Relat. Phenom. 66 (1993) 139-149

143

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Fig. 7. The ionization spectrum of nitric oxide: (a) threshold electrons to high energy (vibrationally numbered); (b) total ion yield spectrum showing the staircase, with autoionizing structure, at the start and later autoiooization; (c) photoelectron spectrum from Ref. 23.

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the assignments of the coincidence spectra would be unambiguous. A schematic diagram of the experimental arrangement is shown in Fig. 5 [ 17. In Fig. 5 one recognizes immediately the significant effort required by the tunable light sources then available. Such coincidence experiments have been employed more recently as a state selector for the molecular ion [19]. The detection of the threshold electron assured one that the associated ion was in a defined eigenstate. Hence this became a state selector for molecular ions employing any one of many kinds of photoion-photoelectron coincidence techniques (PIPECO). This was employed in a series of experiments principally by Baer [20] to measure reaction rates of molecular ions with defined internal energy, an important series of test cases for the theory of unimolecular reactions (RRKM theory). This technique has become one of the most widely employed precise methods for the study of the decomposition of molecular ions in a mass spectrometer. More recently other techniques, based on multiphoton ionization have been employed. These make use of the particularly sharp Franck-Condon factors of multi-photon absorption to produce molecular ions in the ground state [21] from which they can be pumped with precise energy input. This has even yielded rotationally resolved reaction rate constants of molecular ions [22]. In the most recent variants these autoionizing peaks from neutral can be controlled as shown for the very simple case of atomic argon (Fig. 6). Here one can clearly see in the total yield spectrum the Rydberg series leading to the 2P1,2 state of the ion; in the ZEKE spectrum these peaks disappear and both ion states are now clearly seen, whereas they are nearly totally hidden in the total yield spectrum. Although in a staircase function the ion states can be derived from the separate steps, for the simple case of atomic argon this is no longer possible. Guyon and co-workers [12] in a series of experiments has obtained much useful data by a comparison of threshold curves with total yield curves (PIE) for many molecular ionic species.

These comparisons can be seen extremely well by comparing the various experimental techniques for nitric oxide. In Fig. 7(c) one can see the five peaks in the PE spectrum from the book of Turner et al. [23]. In Fig. 7(b) we see the staircase curve of the total yield spectrum with rich autoionizing structure at higher energies. In Fig. 7(a) clear vibrational structure up to V+ = 26 is seen. This most clearly demonstrates the different information obtained from the three techniques. In the ensuing years there have been many applications of threshold techniques in comparison with photoion efficiency curves [12] and kinetic measurements of ions selected in threshold coincidence [19,20]. From threshold photoelectron to ZEKEspectroscopy The situation at the beginning of the 1980s concerning the threshold photoelectron technique can be summarized as follows: (i) The absolute energy measurement of the steradiancy analyzer found many applications, particularly in coincidence experiments. (ii) The bandwidth was improved with modern light sources by a factor of 5-10. (iii) Multi-photon experiments with low photoelectron energies often improved the resolution of PES measurements, particularly in time-of-flight analyzers. With the advent of pulsed tunable dye lasers, two-colour excitation via a resonant intermediate state (resonance-enhanced multi-photon ionization, REMPI) became a modern method for mass spectrometry 1241. REMPI-PES (also termed “excited state PES”) with, for instance, time-offlight photoelectron spectroscopy and selection of a specific vibronic level in an intermediate state directly influences the vibrational activity in the cation through different Franck-Condon factors. This adds an additional feature to photoelectron spectroscopy and some further improvement in photoelectron energy resolution has been achieved by REMPI-PES [25].

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ZEKE - PES : method I

ZEKE - electrons:

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Fig. 8. ZEKE and near-ZEKE electrons. Schematic diagram showing the separation of free ZEKE and near ZEKE electrons of 0.1 meV kinetic energy. The initially small energy difference of O.lmeV is time-amplified. Note also the 100% transmission for the ZEKE electrons and the reduced transmission due to the steradiancy effect for the near-ZEKE electrons.

A promising development seems to be REMPI coupled with TPES. However, one drawback to TPES, the partial contamination of the signal by (non-zero energy) electrons from autoionizing states still has to be overcome_ The principal advance came in 1984 when the completely resolved rotational structure, at threshold, of the NO+ ion was observed in the first true ZEKE experiment (see Fig. 10 below). This advance came from the delayed pulsed field extraction principle: ionization under field-free conditions is followed by a delayed extraction pulse, thus only collecting those electrons specific to the ionic eigenstate, the ZEKE electrons and rejecting all other electrons from unspecific channels, the near-ZEKE electrons. The signature of a ZEKE electron is its zero velocity. Hence one (possible) way to achieve true ZEKE detection comes from the basic idea that the distinction between a zero-kinetic-energy electron and a non-zero-kinetic-energy electron is its velocity [26]. This basic principle is outlined in Fig. 8. Assume the production of an electron of exactly zero kinetic energy (hence ZEKE) is produced together with electrons of, for example, 0.1 meV under field-free conditions. After a delay of 1 ps (0.1 meV corresponds to 6mm ,I&~ velocity) after ionization the near-ZEKE electrons will have separated by r = 6mm from the ZEKE electron. After this field-free delay time, a pulsed field is applied to extract the electrons into the drift region

with time-of-flight detection at the end. The pulsed field extraction of the practically separated electrons hence leads to a strong “time-amplification” of an initially small energy difference into a large time-offlight difference [26]. In contrast to atoms (see Fig. 9) the lifetime behaviour of molecular Rydberg states is very different. Molecular Rydberg states can decay via predissociation or other intermolecular processes

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0

1

il.El 10

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Fig. 9. Threshold region. Left hand side: i.e. adiabatic (field-free) ionization energy; a, above threshold; b, long-lived high-n Rydberg states; c, short-lived (predistiating) Rydberg states. Right hand aide: approximate lifetime curves for Rydberg states of(i) atoms, (ii) molecules following an n3 decay law (solid line) and of states of molecules with a lifetime enhanced by field-induced 1and n mixing [47l.

E. W. Schlag et al./J. Electron S’ctrosc.

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which are not open to atoms. Hence for molecular systems one finds a very short lifetime for those Rydberg states about 5cm-i below threshold. However, in the region very close to threshold (w5cm-‘), marked b in Fig. 9, the lifetime of the highly excited Rydberg states (n > 150) can be exceedingly large. Lifetimes up to tens of microseconds have been observed and it seems that the usual T a n3 is not valid in this region. Instead, the lifetimes of these Rydberg states even seem to be strongly enhanced compared to the T oc n3 law, possibly due to I and n mixing by the small field present during the excitation [47]. Hence an alternative ZEKE detection scheme comes from the pulsed field ionization of long lived Rydberg states of high principal quantum

11

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I

10.0

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Relat, Phenom. 66 (1993) 139-149

number n > 150 [27]. Pulsed field ionization of these Rydberg states produces the same information about the ionic state as detection of an electron at or just above threshold since the cross-sections have to be continuous through threshold [28]. Also detection of either the electron or the ion produced by pulsed field ionization is equivalent. The first experiment in 1984 produced rotational resolution at the onset of ionization, and hence the rotational structure of the NO+ ion at threshold [29]. Rotational states which are separated only by 4cn-’ (0.5meV) were completely separated in the initial experiment. To demonstrate this drastic improvement in resolution, a comparison of the first ZEKE spectrum with the PES of NO is shown in Fig. 10. Even with the best resolution demonstrated in time-of-flight (TOF) PES (3 meV [30]) a resolution of these close-lying rotational states at threshold would not be possible by the direct TOF technique; the delayed pulsed field amplifies this small (rotational energy) difference substantially. Further molecules studied were benzene [3l] with resolved rotational resolution of the

A

I

20.0

E in, lcm-‘1 Fig. 10. PESs and ZEKE-PES. Comparison of VUV-PES and the first rotationally resolved ZEKE spectrum of NO. N+ denotes the total angular momentumquantum number (excluding electronic spin).

Fig. 11. Comparison between experiment and calculation. Experimental (left, Ref. 33) and computed (right, Ref. 34(b)) rotationally resolved ZEKE spectra of NO through the A2zf state;(a) NA = 0, (b) N* = 1, (c) NA = 2.

E. W. Schlag et al.{J. Electron Spectrosc. Relat. Phenom. 66 (1993) 139-149

cation and the benzene-argon van der Waals’ complex [32]. In 1987 new results for NO with the A%+ intermediate state [33] became of interest since ab initio methods had by then been developed to calculate rotationally resolved photoionization cross-sections [34]. This first vindication of rotationally resolved ZEKE spectroscopy [35] produced really excellent agreement between theory and experiment as shown in Fig. 11. Other groups started using this technique. Narrow-band laser generated VUV-radiation was used to obtain one photon ZEKE-spectra of oxygen [34], H20 [35], N, and Hz [36]. More recently, studies of NH3 [37] para-difluorobenzene [38], a renewed visit of NO [39] and the extension to hydrogen bonded clusters [40] have been carried out in this laboratory to show the general applicability of the ZEKE technique. Many modifications have been reported in ZEKE experiments. In contrast to the complex extraction region, as published in our original ZEKE work, the extraction geometries have been considerably simplified. Even now experiments are done, albeit with reduced resolution, without using p-metal shields [41]. From rotational resolution of symmetric and asymmetric top molecules to vibrational resolution of large (aromatic) molecules to van der Waals’ and hydrogen bonded molecular clusters, ZEKE spectroscopy has found many applications. New aspects that have recently come up are the study of the dynamics of the intermediate excited electronic states. In that sense the high resolution of the ZEKE technique serves as a double resonance experiment. Very interesting dynamic effects in van der Waals’ clusters have been observed [42]. A very new aspect of the ZEKE-pulsed field ionization technique is to measure photoelectron spectra without the detection of electrons. This technique relies on the detection of the photoion produced by the pulsed field ionization of the high n Rydberg state. The advantage of this would be to achieve mass selectivity in a molecular beam with several species of overlapping absorption bands being present. For para-difluorobenzene and the

147

benzene-argon van der Waals complex the mass analyzed threshold photoionization (MATI) spectra have recently been reported [43]. Another important application of ZEKE spectroscopy has developed for photodetachment studies of molecular anions. ZEKE photodetachment provides the structure of the corresponding neutral species. Objects of study have even included the transitory “activated complex” thus leading to the first direct spectroscopy of such species. Examples are IHI (and other halogen analogues) probed by photodetachment of IHI- [44]. Metal clusters [45] and carbon clusters [46] have also been investigated by ZEKE photodetachment. It should be noted, however, that ZEKE photodetachment requires the detection of the free zero kinetic energy electron according to the pulsed extraction scheme (Fig. 9, region a). Pulsed field ionization of high n Rydberg states is not applicable since such states do not generally exist below the detachment threshold (except for molecules with a very strong dipole moment which may show dipole bound states). Also the threshold laws for detachment have a strong effect on the intensities observable in ZEKE photodetachment; this feature leads to interesting information on the angular momentum of the electron being detached. Conclusion ZEKE spectroscopy, by detection of zero kinetic energy electrons from pulsed field ionization of long-lived high-n Rydberg states or by direct photoionization now provides high-resolution information about a large variety of molecular systems, ions and neutrals. Rotational structure of molecular ions and low frequency vibrations of molecular cluster ions can now be obtained in a rather routine manner. Future research will no doubt include dynamic aspects of molecular clusters. This demonstrates the wide range of information obtainable from electron spectroscopy experiments, starting from initial work on photoionization. We think that the applications of ZEKE thresh-

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old techniques in chemistry and molecular physics are only just emerging [48]. Work on clusters and radicals is still in its infancy. New laser systems will allow the study of previously inaccessible systems.

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