Science Physics Threshold Electron Spectroscopy

Threshold Photoionisation of Atoms and Molecules

George King

School of Physics and Astronomy and Photon Science Institute

University of Manchester, UK.



When you excite a reaction just above its threshold energy, something interesting usually happens and often something unexpected. In the present case the reaction is the photoionisation of an atom or molecule, with the removal of one or more of its electrons. For such threshold measurements, it follows that we must be able to vary the energy of the incident photon. This is where the use of tuneable synchrotron radiation becomes essential. Indeed this tunability opens up many new avenues of experimental investigation in atomic and molecular physics.

Our initial use of synchrotron radiation arose from an interest in the three-body Coulomb problem as described by the Wannier theory; in particular, the photo double ionisation of helium. In this reaction, two very low energy photoelectrons are emitted and the reaction is dominated by the correlations between these two outgoing electrons. The threshold for this reaction is 79 eV and indeed one of the gratings of the toroidal grating monochronomator (TGM) on Beamline 3.3 at the Daresbury SRS was designed specifically to deliver maximum photon flux near to this energy. In fact, our experiments on the photo-double ionisation of helium were some of the first to be undertaken at the SRS and indeed the author was present at the opening of the facility by the Secretary of State for Science and Education, Mark Carlisle on the 7th November 1980.

Following our first experiments in helium, we exploited the techniques of threshold photoelectron spectroscopy to explore a wide range of atomic and molecular systems, as we will describe. For these studies, we exploited the high photon energy range provided by the TGM and the high photon energy resolution provided by the 5m McPherson monochromator. We benefited greatly from the SRS scientific staff at Daresbury including John West, Ian Munro, Michael McDonald and David Holland. We also benefited greatly from international and national collaborators including Mariusz Zubek (Gdansk), Richard Hall (Paris), Lorenzo Avaldi (Rome) and Andrew Yencha (Albany, USA). On a personal note, the author met his future wife, Michele Siggel-King, at Daresbury! She was working on the TGM, while he was working on the adjacent beamline; the 5m McPherson monochromator.

Here we give illustrative examples of how we exploited the advantages of threshold photoelectron techniques together with the high performance of the TGM (Beamline 3.3) and the 5m McPherson monochromator (Beamline 3.2).


Experimental technique

The basis of our experimental technique is to collect and detect very low energy (~ few meV) photoelectrons, i.e. threshold photoelectrons. This technique employs the penetrating field technique [1], which is illustrated schematically in Figure 1.

Figure 1. A computer simulation of the trajectories of 2 meV photoelectrons, illustrating the principle of the penetrating field technique.

An extraction electrode is positioned behind a grounded shielding electrode with respect to the photon-gas beam interaction region. Both electrodes have apertures of ~ 2 mm diameter and the distance between the grounded electrode and the interaction region is ~ 10 mm. The extraction electrode is held at a voltage of typically 100V and its action is to draw out photoelectrons emitted from the interaction region and moreover to draw out preferentially those photoelectrons with very low energy ~ few meV. This is illustrated by Figure 1, which shows the computed trajectories of photoelectrons of 2 meV energy. These low energy photoelectrons are collected over a collection angle of almost 4π sr, i.e. with a collection efficiency close to 100%. This efficiency falls off dramatically with increasing photoelectron energy so that the threshold resolution of the system is also a few meV. More energetic photoelectrons that are emitted within the solid angle subtended by the aperture in the grounded electrode are removed by a 127° electrostatic analyser that is positioned after the extraction stage. This is illustrated in the practical example of a threshold photoelectron spectrometer that is shown in Figure 2, and which is described in detail elsewhere [2].

Figure 2. A threshold photoelectron spectrometer. The purpose of the 127° cylindrical deflector analyser is to filter out energetic photoelectrons that are emitted within the solid angle subtended by the aperture in the shielding electrode.

Note that the extraction field produces a crossover in the electron trajectories that is imaged by the electrostatic lens system onto the entrance slit of the analyser. The resultant transmission function of the threshold spectrometer is illustrated schematically in Figure 3. We see that the spectrometer delivers very high energy resolution and very high efficiency.

Figure 3. Transmission function of the threshold analyser. The width of the function is ~ 2 meV and photoelectrons of energy less than this value are collected with ~ 100 % efficiency.

The SRS beam is crossed with the target gas beam that emanates from a narrow capillary tube and the photon energy is scanned across the region of interest. Whenever the photon energy crosses an ionisation threshold of the target species, threshold photoelectrons are produced, which are extracted and detected. The detected yield of threshold electrons, measured as a function of photon energy, is the threshold photoelectron spectrum.


Threshold photoelectron studies

Threshold photoelectron studies of atoms

We conducted comprehensive threshold studies of the rare gases Ne, Ar, Kr and Xe. The aim of the studies was to observe satellite states of the ions, where one electron is ejected and another is raised to an unfilled orbital e.g. Ar+, 3s23p4nl. These states are of particular interest because they occur only because of electron correlation. What the threshold measurements demonstrated was that satellite excitation at or near threshold occurs entirely through doubly excited neutral states of the target i.e. via a two-step process.

A threshold photoelectron spectrum obtained in neon over the photon energy range 48 – 70 eV is shown in Figure 4, [3].

Figure 4. A threshold photoelectron spectrum of the satellite states of neon. The spectrum illustrates the high sensitivity of the technique and its ability to observe all ionic states over an extended energy range.

The spectrum reveals a rich structure due to the 2s satellite states, many of which were observed for the first time. Also indicated on the figure are the positions of the double ionisation potentials of neon that occur in this energy region. The high intensity and abundance of satellite states compared to the main line, in the threshold energy region demonstrates the different mechanisms that are active near threshold also that electron-electron correlations dominate.

Photo double ionisation of helium

Photo double ionisation of helium became a subject of considerable interest as it is a means of studying the dynamics of two electrons in the field of a doubly charged ion. In the threshold region, these two electrons have only a small amount of energy to share and so are highly correlated. Various theories make predictions for (i) the energy dependence of the ionisation cross section σ (E), where E is the excess energy above threshold, (ii) the way the energy is shared between the two electrons and (iii) the angular behaviour of these electrons. One prediction of the Wannier theory [4] is that the dependence of the ionisation cross section on energy in the region of the double ionisation potential is σ (E) is proportional to E α, with α = 1.056. The threshold photoelectron spectrum we obtained in helium [5] is shown in Figure 5.

Figure 5. Threshold photoelectron spectrum of helium showing a cusp at the double ionisation potential. The cusp is due to the correlated motion of the two outgoing electrons in the vicinity of the He++ core. The red curve in the expanded inset is a fit to the data of the Wannier threshold law with the Wannier exponent α = 1.056

This spectrum clearly reveals the He+*(nl) satellite manifold up to n = 12. Higher levels merge into an almost flat shoulder, which falls off rapidly reaching a sharp minimum at the double ionisation potential. A cusp-like feature occurs at the double ionisation potential and this feature can be seen more clearly in the expanded inset. A measurement of the yield of threshold photoelectrons, with energy between 0 and ΔE, gives a partial cross section. Under the reasonable assumption that the energy partitioning between the two photoelectrons is uniform close to threshold, the partial cross section is proportional to E α-1. Since α is close to unity, this is a very sensitive test of the value of α and much more sensitive than measuring σ (E) directly. The cusp-like feature in the experimental data was fitted to the functional form E α-1, after convolution with a Gaussian function of width 60 meV, due to the energy spread in the incident photon beam. The full curve shown in the inset of Figure 5 is a fit of the data over the energy range  +0.08 < +0.48 eV for the Wannier exponent α = 1.056, and then extrapolated to higher energies. The range of validity of the threshold law would appear, from this figure, to be about 1.5 eV. When the exponent α is allowed to vary in the fitting procedure [5], its value was found to be 1.060 ± 0.007, in good agreement with the theoretically predicted value.

Threshold photoelectron studies of molecules

We also measured high-resolution threshold photoelectron spectra for a wide range of molecules. These include the diatomic molecules H2, N2, and CO; the polyatomic molecules CF4, C2H2, SF6, BrCN, and C6H6; the halogens F2, Cl2, Br2, I2; the inter-halogens ICl and IBr; the hydrogen halides HF/DF, HCl/DCl, HBr/DBr and HF/DF; simple organic molecules such as formic and acetic acid; simple aldehydes acetaldehyde, acrolein and molecules of biological interest including isoxazole and tetrahydrofuran.

We saw above for the case of the rare gases that close to threshold, excited states of their ions, i.e. satellite states are excited through indirect processes. In an analogous way, the threshold photoelectron spectra of molecules usually show a wealth of ionic structure due to their indirect excitation via highly excited states of the neutral molecule. Indeed many more ionic states are observed than in conventional (above threshold) photoelectron spectra. Vibrational progressions up to very high values of vibrational quantum number ν are observed, so that the data provide highly accurate values of molecular vibration constants, series limits and even rotational constants in light molecules. The highest resolution we achieved was 0.6 meV in a threshold photoelectron spectrum of HF [6]. In addition, the very high collection efficiency of the threshold spectrometer meant that low target densities could be used, enabling corrosive gases to be studied with high resolution and statistical accuracy.

The biological molecules are of interest because of radiation damage that molecules such as DNA may suffer when exposed to ionising radiation. Regardless of the nature of the incident radiation, a final step in the damage process is the production of low energy electrons that may lead to bond breaking in the DNA molecule. Hence, it is useful to investigate the threshold photoionisation of biological molecules, in which low-energy photoelectrons are produced.

To illustrate the application of threshold photoelectron spectroscopy to molecules, and the high quality of the data obtained, Figure 6 shows the threshold photoelectron spectrum obtained for the inter-halogen ICl [7].

Figure 6. Thresold photoelelctron spectrum of the ICl+ (X 2Π3/2,1/2) band systems at an energy resolution of ~ 2 meV. Notice the observation of long vibrational progressions; up to ν+ = 18 for the X 2Π1/2 band system.

It is interesting to note that there were very few previous experimental investigations of this molecule and these were of modest resolution. This illustrates the robustness of the threshold technique that performs with high performance even for aggressive gases. The figure shows the ICl+ (X2Πi) band system at a resolution of 2 meV. Numerous well-resolved vibrational bands are observed in both spin-orbit components, and in the first few vibrational levels in the X2Π3/2 system, partially resolved rotational-branch profiles can be clearly seen. The vibrational structure was analysed to yield accurate values of vibrational constants, internuclear distances and ionisation potentials.

The ability of the threshold technique to excite essentially all ionic states of a target and observe them with very high sensitivity over an extended energy range is again demonstrated by the threshold photoelectron spectrum obtained in O2. This is shown in Figure 7, [8]. Notice the very wide energy range, 12 – 50 eV, and the numerous ionic states that are observed. The energies of these states are measured with high precision and such spectra continue to provide a valuable reference source.

Figure 7. Threshold photoelectron spectrum of O2, over the extended energy range 12 – 50 eV


Photo double ionisation

Valuable information about the double ionisation process can be deduced by detecting just one of the ejected photoelectrons, as described above. However, more complete information is obtained by detecting both of the ejected electrons in a (γ, 2e) coincidence measurement. In such a measurement, the two ejected photoelectrons are selected in both energy and angle of ejection by the use of two independent energy analysers and they are detected in coincidence.

Figure 8. A schematic diagram of the experimental arrangement for making (γ, 2e) coincidence measurements between the two ejected photoelectrons in a photo double ionisation reaction. One of the analysers is fixed in position, while the other is rotated about the interaction region.

A schematic diagram of the experimental arrangement for such coincidence measurements is shown in Figure 8 and a practical realisation of this is shown in Figure 9. One of the ejected electrons is detected at a fixed angle of ejection with a stationary analyser.

Figure 9. A practical realisation of a photoelectron- photoelectron coincidence spectrometer

The other ejected electron is detected by an analyser that can be rotated about the interaction region. The yield of true coincidences, measured as a function of the angle of the rotated analyser, is a fully differential measurement and is called a triple-differential cross section (TDCS); measured at a particular value of excess energy E.

Helium is the simplest target to choose since photo double ionisation results in two electrons proceeding from a bare core and involves only purely Coulombic interactions. We made the first measurements of the TDCS in helium at very low values of excess energy E (0.6 < E < 2 eV), for both equal and unequal energy sharing between the two outgoing electrons. In addition, and also for the first time, we determined the relative magnitudes of the various TDCS in this region of excess energy. We compared these data with the predictions of the Wannier model and the 3C model of Maulbetsch and Briggs [9]. An example of our data [10] is shown in Figure 10, where the photon beam is directed out of the paper.

Figure 10. Measured TDCS in helium for the condition of equal sharing at 0.6, 1 and 2 eV excess energy. The experimental data are compared with the Wannier model (broken curve) and the calculations (full curve) of Maulbetsch and Briggs

One electron is detected at the fixed angle of ejection; along e1 to the right of the figures. The Broken curve is the prediction of the Wannier model while the full curve is the prediction of the 3C model. The measurements reveal that there is little change in the shape of the TDCS even though the excess energy changes by a factor of 3 and further that the shapes are better produced by the 3C model. The measured relative magnitudes of the TDCS appear to indicate a departure from the standard Wannier model at the largest excess energy studied; E = 2 eV.

Threshold photoelectrons coincidence (TPESCO) spectroscopy

We saw above that a threshold photoelectron spectrum is obtained by measuring the yield of threshold photoelectrons as the energy of the incident photon beam is scanned. Suppose now that we scan the photon energy across the double ionisation potential of the target. In close proximity to threshold, photo double ionization produces two threshold photoelectrons. These can be collected using two separate analysers and measured in coincidence.  A true coincidence signal is the signature of a double ionization event, and may be recorded when the scanned photon energy passes through the threshold of a double-ion state. The yield of true coincidences with respect to photon energy is then a threshold photoelectrons coincidence (TPEsCO) spectrum and is the double-ionisation analogue of the single-ionisation process. This is illustrated schematically in Figure 11 for the case of a rare gas atom. A TPEsCO spectrum maps out the doubly-charged states of the target.

Figure 11. Schematic representation of a TPEsCO measurement. When the energy of the incident photon beam crosses a double ionisation threshold, two photoelectrons are ejected. The measured yield of coincidences between these two electrons with respect to photon energy is the TPEsCO spectrum. This maps out the energies of double charged states and their cross sections at their respective thresholds

We developed the TPEsCO experimental technique at the Daresbury SRS. The two analysers were placed on opposite sides of the target region as shown in Figure 12.

Figure 12. Threshold photoelectrons coincidence spectrometer for the simultaneous detection of two threshold photoelectrons

The collection modes of the two analysers were set up by balancing the potentials on the two extraction electrodes so that roughly half the ejected photoelectrons were collected by one of the analysers and the other half by the second analyser. This arrangement is illustrated schematically in Figure 13, together with the variation in the potential across the interaction region.

Figure 13. The collection modes of the two analysers shown in Figure 11 are set up by balancing the potentials on the two extraction electrodes. This figure shows the variation in the potential across the interaction region.

There is a shallow saddle in this variation at the interaction region and the potential gradient is sufficiently small that the effect on the energy resolution of the technique is minimal. The threshold energy resolution of the two analysers is typically less that 20 meV and so the heights of the observed peaks correspond to the relative cross sections for excitation of doubly charged states within 20 meV of their respective thresholds. Since the two analysers collect threshold photoelectrons over a solid angle of approximately 2π sr, the spectra obtained do not provide any angular information.

TPEsCO studies in the rare gases

We measured TPEsCO spectra in the rare gases: Ne, Ar, Kr and Xe. A TPEsCO spectrum obtained for Ne++ is shown in Figure 14 over the photon energy range 60 to 100 eV [11]. The energy range includes the doubly charges states np4 (3P, 1D and 1S) and nsnp5 (3P and 1P). The energy resolution allows not only the separation of the double-ion states but also separation of the spin-orbit components of the triplet state. This is shown in the inset of Figure 14.

Figure 14.  TPEsCO spectrum obtained for Ne++ over the photon energy range 60 - 100 eV. The energy range includes the doubly charged states np4 (3P, 1D and 1S) and nsnp5 (3P and 1P).

The measured 3P1 / 3P2 and 3P0 / 3P2 intensity ratios are close to the expected statistical values. The most important result of the TPEsCO spectra in all the rare gases is that all possible ionic states are observed. Furthermore the unfavoured ones are excited with intensities that are not very different from the favoured ones. In general the 3P ion states do have the largest relative intensity as expected from a propensity rule. The exception is neon where the 1D peak has the largest intensity.

TPEsCO studies in molecules

Strictly speaking, doubly charged states (dications) of molecules are not stable. However, the binding energy produced by the exchange energy between the electrons can shield the Coulomb repulsion of the two atomic ions and produce a local potential minimum. This forms a barrier that inhibits dissociation and is able to support vibrational levels temporarily. Very little precise information had been obtained about the electronic and vibrational states of dications. This was because high-resolution techniques, such as ion beam laser spectroscopy were applicable only over a limited energy range, while those that can cover a broad energy range, such as ion yield measurements suffered from a lack of energy resolution. And in some important molecular cases, the double-ionisation potential was uncertain. This situation changed with the application of the TPEsCO technique, which combines high detection efficiency and resolution and can be used over extended photon energy ranges.

We measured TPEsCO spectra for a wide range of molecules. These include the diatomics N2, CO, O2, NO, HCl/DCl, Cl2, HBr/DBr, HI; the triatomics  CO2, H2O, OCS and the polyatomics FC4 and C6H6. An example of a TPEsCO spectrum, for the ground state of O2++, is shown in Figure 15, [12]. 19 vibrational levels can be seen in the spectrum.

Figure 15. TPEsCO spectrum for the ground electronic state of O2++, showing 19 vibrational levels

These were fitted to the energy levels of an anharmonic oscillator to deduce the molecular parameters of the state, ωe , ωexe and ωeye. The absolute energy T0 of the state was also deduced, with an uncertainty that was only limited by the energy calibration (20 meV) of the photon beam. This provided an accurate measurement of the double ionisation potential of O2, which was found to be 36.13 ± 0.02 eV. Interestingly, this is 0.5 eV lower than the previously accepted value. The difference is explained by the fact that the first two vibrational levels have low intensity and were not observed in previous studies.



We used threshold photoelectron spectroscopy in conjunction with the Daresbury SRS to investigate a range of physical processes in a wide range of atomic and molecular systems. We exploited the high energy resolution and sensitivity provided by this technique and the high resolution and flux provided by the monochromators at beamlines 3.2 (5m) and 3.3 (TGM). A number of notable experimental firsts were made. (We also performed experiments in above-threshold photoionisation studies and developed various innovative experimental techniques, but these are not described here.) We performed our initial experiments at the SRS when it first came on stream and our final experiments during the last days of the facility. In all over 90 publications in leading journals resulted from our SRS work and 15 students obtained their PhD degrees. Enduring scientific collaborations were forged that resulted in our group working at overseas synchrotron facilities in Italy (Elettra), the US (Advanced Light Source) and France (Super Aco) to extend our experimental investigations. And, of course, the author met his future wife!

Some members of the research team, working on a beamline at the Daresbury SRS. They are from left to right, Lorenzo Avaldi, Richard Hall, George King, Mariusz Zubek, Grant Dawber and Kate Ellis


PhD student Paula Bolognesi and Manchester University technician Alan Venables sitting on the back of a hire van that we used to transport our apparatus from Manchester University to Daresbury. Paula now has a permanent position at CNR-IMIP (Rome)


PhD student Siu Yin Truong and post doc Antonio Juarez Reyes, adjusting an angular-resolving photoelectron, coincidence spectrometer. Siu Yin now works for Waters Ltd, manufacturers of scientific equipment, while Antonio has a permanent position at Universidad Nacional Autónoma de México



We gratefully acknowledge the technical staff at Daresbury and also at the School of Physics & Astronomy at the University of Manchester, whose skills and expertise made our experiments possible. These include Alan Venables and David Coleman (Manchester) and Albert Beckett, David Fance, Ian Maxwell and Marion Hayes (Daresbury).



[1] A Study of threshold photoionisation and photon-double-ionisation in helium

G C King, M Zubek, P M Rutter, F H Read, A A MacDowell, J B West and D M P Holland J. Phys. B: At. Mol. Opt. Phys. 21 L403 (1988)


[2] A penetrating field electron/ion coincidence spectrometer for use in photoionization studies

R I Hall, A McConkey, K Ellis, G Dawber, L Avaldi, M A MacDonald and G C King,  Meas. Sci. Technol. 3 316 (1992)


[3] Near threshold study of the neon photoelectron satellites

R I Hall, G Dawber, K Ellis, M Zubek, L Avaldi and G C King.  J. Phys. B: At. Mol. Opt. Phys. 24 4133 (1991)


[4] The Threshold Law for Single Ionization of Atoms or Ions by Electrons

G H Wannier. Phys. Rev. 90 817 (1953)


[5] Photoionisation phenomena near the double ionisation threshold of helium

R I Hall, L Avaldi, G Dawber, M Zubek, K Ellis and G C King. J. Phys. B: At. Mol. Opt. Phys. 24 115 (1991)


[6] Threshold photoelectron spectroscopy of HF and DF in the outer valence ionization region

A Y Yencha, A J Cormack, R Donovan, A Hopkirk and G C King. J. Phys. B: At. Mol. Opt. Phys. 32 2539 (1999)


[7] Threshold photoelectron spectroscopy of iodine monochloride

A J Yencha, M C A Lopes and G C King, Chem Phys Letts. 325 559 (2000)


[8] High resolution threshold photoelectron and photoion spectroscopy of oxygen in the 12-50 eV photon energy range

K Ellis, R I Hall, L Avaldi, G Dawber, A McConkey, L Andric and G C King J. Phys. B: At. Mol. Opt. Phys. 27 3415 (1994)


[9] Double photoionization in the case of unequal energy sharing

F Maulbetsch and J Briggs. J.P hys. B: At. Mol. Opt. Phys. 27 4095 (1994)


[10] Near threshold TDCS for photo-double ionization of helium

G Dawber, L Avaldi, A G McConkey, H Rojas, M MacDonald and G C King J. Phys. B: At. Mol. Opt. Phys. 28 L271 (1995)


[11] A study of Ne2+ and Ar2+ satellite states observed by threshold photoelectrons coincidence (TPESCO) spectroscopy

L Avaldi, G Dawber, N Gulley, H Rojas, G C King, R I Hall, M Stuhec and M Zitnik J. Phys. B: Atom. Mol. Opt. Phys. 30 5197 (1997)


[12] Vibrational structure of the O2 ground state observed by threshold photoelectron coincidence spectroscopy

R I Hall, G Dawber, A McConkey, M A MacDonald and G C King Phys. Rev. Letts. 68 2751 (1992)