Science Physics NIST/Daresbury collaboration

The NBS (NIST)-Daresbury Collaboration in Photoelectron Spectroscopy


Albert C Parr



The National Bureau of Standards (NBS) 180 MeV synchrotron was fully dedicated to synchrotron radiation research in 1967; in 1974 it became a dedicated storage ring in order to improve its usefulness for a variety of photo-physics experiments. The NBS electron storage ring synchrotron radiation source was called the Synchrotron Ultraviolet Radiation Facility (SURF). It underwent a series of modifications and upgrades in the early 1970s and was renamed SURF-II.  SURF-II was formally commissioned in 1974 as a storage ring operating at about 240 MeV and stored beam currents of about 10 mA. Subsequent modifications during the 1980s raised the stored energy to about 283 MeV and higher stored beam currents.

At about the same time as SURF-II was being commissioned, the staff at NBS designed and constructed a new beamline which featured a 2m normal incident monochromator which produced monochromatic light from about 40 nm to greater than 120 nm[1].

Figure 1. 2-m normal incident monochromator at SURF-II.
S=storage ring source, LB=light baffle, M= reflection mirror, G= grating, k= Leveling screw, SA=drive arm, C= cam for wavelength drive, DS=drive screw, IP= ion pump, SM= scanning mechanism, E=encoder,GV1,2=gate valves, ES=exit slit and PB= baffle, CP=port for cryopump

The 2 m monochromator is shown schematically in Figure 1.  This wavelength range corrreponds to an energy range of roughly 10 eV to 30 eV and coincides with the ionization range of most atomic and molecular systems.  The availability of this new instrument enabled NBS staff to consider pursuing some new research in photoionization dynamics. The SURF staff had pioneered photoabsorption work in the 1960s with studies of the autoionization features in the rare gases and small molecular systems.  Recent theoretical work on diatomic molecules by Dill et al [2] had made some predictions about resonance effects in the ionization of small molecules when vibrational structure could be resolved in the spectra. This was a new and interesting phenomenon and had not been explored at that time.  At that time there was also interest in exploring the angular distribution of photoelectrons from molecular photoionization as a test of theoretical methodology being used to calculate absorption properties of the molecular systems.

There were several groups at NBS, renamed National Institute of Standards and Technology (NIST) in 1988, who began a collaboration in the 1978-79 time frame. The groups were the  far-uv physics group at SURF-II under the management of   Robert Madden and a group in photochemistry and surface science in the Chemistry Lab which was spearheaded by Roger Stockbauer.  Dave Ederer from the far-uv group was involved in the initial project and he and Roger were the early leaders in the effort.  At the same time there were two guest scientists at NBS who were interested in the project who and became heavily involved in the effort.  One was John West from the Daresbury Lab who was spending a year at NBS while the SRS, the new synchrotron radiation source at Daresbury was being commisioned, and the other was myself, the author of this brief history.  I was on sabbatical leave from the University of Alabama for one year and had a background in photoionization studies using laboratory light sources; I later accepted a position in the far-uv group and continued working in the ARPES project in its various phases. John had experience in synchrotron radiation having worked at the old NINA synchrotron, a predecessor of the SRS, and was a post-doc at Reading University working with Geoff Marr and Keith Codling; this gave him an extensive background in UV science.  Later in the project Keith joined the project and spent a year at NBS participating in the experiment during 1979-80. A crucial component of this program was the participation from the start by Joe Dehmer from Argonne National Lab in Illinois.  He had a spare 2” mean radius hemispherical electron spectrometer that he made available for the construction of the first instrument system for this project.  Joe was one of the major participants in the program, as he not only contributed equipment to build the experiment, he also furnished theoretical insights and suggested many projects for the experimental work. He participated from the onset of the project until it wound down in the mid 1990s.  Later in the project, Pat Dehmer participated in the SRS ARPES effort for a few experiments.

This collaboration between NBS/NIST, Daresbury and Argonne lasted for over a decade and saw the experimental team build an entirely new instrument and in the end take the experiment to the Synchrotron Radiation Source (SRS) at Daresbury where it was used for many years of productive scientific research.


Phase 1: Initial Experimental Efforts

Figure 2. 2 inch mean radius electron spectrometer mounted a flange. The light enters from the left hand side using a capillary (not shown). The interaction of the light occurred in the screened region and the light is monitored with a custom photodetector apparatus also enclosed in the screened area.

The first generation Angular Resolved Photoelectron Spectrometer (ARPES) is shown in Figure 2.  The light enters an interaction zone shielded by a dark screen on the left side of the instrument. A jet of sample gas entered through one of the small flanges mounted on the top vacuum flange and the ejected electron is analyzed by the spectrometer whose entrance optics in this figure are pointing upward from the mounting plate of the copper hemispheres.  The electron optical elements were coated with a graphite material to reduce stray electric fields, which might affect the instrument’s electron energy resolution.  The electron spectrometer rotated about a horizontal axis and is driven by a stepping motor system mounted on the outside of the vacuum chamber and whose rotational motion is fed through the vacuum by a rotatable vacuum feedthrough.  This 2 inch mean radius analyzer had an energy resolution of about 100 meV which limited the measurements to small diatomic molecules whose vibrational spacing was ~ 100meV.  This instrument is described in detail in the literature[3].

The flange holding the electron spectrometer is mounted on a vacuum chamber that is positioned behind the exit slit of the 2m monochromator shown in Figure 3.

Figure 3. First generation electron spectrometer in chamber attached to the 2-m monochromator at SURF-II. The vertical chamber contains the electron spectrometer system shown in Figure 1. The L-shaped large chamber projecting out of the back of the primary vacuum chamber is for the cryopump system used to maintain vacuum conditions in the system compatible with the needs of the SURF-II storage ring. In the foreground are the system electronics and computer control. The large blue structure in the background is the magnets for SURF and the yellow feature is the top coil of the pair of excitation coils used to produce the magnetic field. The 2 m monochromator is mounted on the orange frame and connects to the storage ring tangent point with a small vacuum flange.

The vacuum is maintained by a large cryopump mounted on the L-shaped chamber attached to the main vacuum system.  Since there are no effective windows for the wavelength region used in these studies, the whole system is windowless, and hence the vacuum in the experimental chamber could not negatively impact the high vacuum in the storage ring necessary for long lifetimes of the stored electron beam.  The electron beam in SURF-II is injected with a small microtron accelerator and then the large field magnet of SURF-II is ramped up in magnetic field and thereby accelerates the electrons to the desired energy.  The beam is stored many hours and this injection process only needs to be done several times a day [4, 5].

The conceptual ideas for measurements of molecular process depend upon the measurement of a quantity that is related to the cross section for a particular process.  For a given electronic-vibrational process the differential cross section can be expressed in the following terms [6]:

It is difficult to measure cross sections and, to compare results to theory, it is useful to cast the equation into measurable terms. The number of electrons Nv at some angle is proportional to the differential cross section and the total number of electrons N0v produced by the process, hence we can write:

The branching ratio for a given vibrationally resolved process is the fractional number of electrons in that particular process to the total number of electrons created and is given by the quantities in the brackets on the right hand side of the above equation. The asymmetry parameter and branching ratio are quantities which can be calculated in theoretical models of the photoionization process [2]. Inspection of the equation indicates that by measuring the electron count at two or more angles the branching ratio and asymmetry parameter can be determined.

An example of these types of results are shown in Figure 4, which came as a result of the early work at NBS [7].

Figure 4. Branching ratio of the v=1 to v=0 transitions in the photoionization of N2. The solid circles are NBS data and the solid triangles are data from the literature (see text). The solid line is the theoretical prediction of the branching ratio.

The result shows the branching ratio for the transition to the first excited state, v=1, of  N2+ to the lowest level, v=0, of N2+. The solid circles are NBS data and the solid triangles are from Gardner and Samson [8]. The solid line is from the theoretical work cited here.  The early work included studies on CO, rare gases, O2 and a few small organic molecules like acetylene. This early work is summarized in the article by Dehmer et al [9] and will not be reviewed here.

This project and the updated one discussed in later paragraphs greatly benefited from a series of post-docs and guest scientists from other institutions.  The post-docs included Barry Cole, Steve Southworth, Trish Ferrett, Jonathan Hardis and David Holland.  Barry went on to work at Honeywell, Steve ended up at Argonne National Laboratory after a stay at Los Alamos, Trish teaches at Carleton College, Jonathan was appointed to a staff scientist position at NIST and David returned to work at the Daresbury SRS and collaborated on the project over an extended period. Michele Siggel, a post-doc from the Shirley group at Berkeley, joined the project in the later stages. The UK group, under John West at the SRS, continued to collaborate on the ARPES project at NBS and participated with visits and the data analysis and documentation efforts.


Phase 2:   A new 4-inch mean radius spectrometer system.

The first generation instrument was usually used with an electron energy resolution of about 100meV, which limited the class of molecular systems that could be studied as larger molecules typically have vibrational spacing of much less than 100 meV.  In the early 1980s it was decided to build a new instrument with higher resolution. Joe Dehmer had two 4 inch mean radius hemispherical electron energy analyzers, which could be devoted to this project.  It was decided to use both of them in order to perform simultaneous measurements at two angles and hence greatly decrease the data acquisition time for the studies.  The first rendition of the instrument used single channel electron counters but a revision later on installed position sensitive detectors for greater data gathering speed.

A new chamber was constructed and fitted with 3 layers of magnetic shielding to provide an interaction zone free of magnetic fields.  The chamber is about 92 cm long and has a diameter of about 76 cm.  A schematic of the instrument and chamber is shown in Figure 5.

Figure 5. Schematic of 2nd  generation ARPES apparatus built at NBS/NIST. The diameter of the chamber is 76 cm and its length is 92 cm. A rotary motion feedthrough is mounted on a flange of the left side of the instrument to rotate one of the analyzers and the other analyzer is fixed to look along the horizontal direction. The light to the interaction region is introduce through the glass capillary tube on the right side of the instrument. The capillary tube abuts the exit slit of the monochromator. The gas source is mounted on an xyz manipulator to optimize signal and align the interaction zone. The light intensity and polarization are monitored with the light monitor system, LM in the figure.

A photograph of the electron spectrometer system is shown in Figure 6. The surfaces of the analyzer and associated hardware that defined the regions the electrons moved through were all coated with a graphite material to give a uniform field.  This included the surfaces of the hemispheres that provide the energy analysis fields for the spectrometer system. Not shown on the photo or schematic is graphite coated wire cage that surrounded the interaction region and helped provide a field free region for the area where the electrons were created.  This was necessary to optimize the energy resolution of the analyzer systems.  The analyzers could achieve a demonstrated energy resolution of about 10 meV but were typically operated at a nominal resolution of 50 meV to increase signal and speed of data acquisition.

Figure 6. Photograph of the 2nd generation instrument with the original single channel electron counters and electrostatic exit lens on the monochromator. The light handling mechanism and shielded interaction region are not present.

The electron spectrometer seen in cross section in Fig. 5 is rotatable using a rotary feedthrough on the left side of the chamber.  The electrons are detected with single particle detector electron multipliers or multichannel array devices for enhanced signal.  When the multichannel array was installed the exit electron lens system was removed and the array detector placed where the entrance lens image occurs. The system is described in more detail in the literature [10, 11]. The experimental apparatus was utilized at SURF-II until the late 1980s utilizing NBS/NIST permanent staff and post-docs as mentioned above. Studies were done for a range of small molecules including sulfur dioxide, hydrogen cyanide, methyl cyanide and boron trifluoride.

Move to the Daresbury/SRS

In about 1986, I assumed a position in a different group from the far-uv physics group, which had furnished support for the ARPES effort, and the project was run on a day-to-day basis by post-docs and some assistance from NBS-SURF staff.  The future of the ARPES project at SURF-II was limited since there were no full-time NBS staff assigned to the project and the collaborators at Argonne could devote only a limited amount of time to the project. Roger Stockbauer moved to the Surface Science Division at NBS and had time for only very limited collaboration in the gas phase work as he was responsible for building a surface science beamline at NBS and later at the Brookhaven synchrotron light source. Dave Ederer became involved in other projects at the French synchrotron facility Super-ACO and at NBS and had a decreasing amount of involvement with the ARPES project.

The far-uv physics team at Daresbury, headed by John West, expressed an interest in perhaps moving the experiment to the SRS and utilizing the new 5-m normal incidence monochromator newly installed at the SRS.  There were additional monochromators such as a toroidal grating instrument for shorter wavelengths that could also be utilized at the SRS.  The team at Daresbury included David Holland who joined the staff at Daresbury after his stay at NBS and a year at Argonne.  Others on the team at Daresbury, Marion Hayes and Mike MacDonald, also participated in the project along with post-docs working in the far-uv group project at the SRS. After evaluating the pros and cons of moving the equipment it was decided to move the apparatus to the SRS and funding was located at NIST to ship the apparatus to the UK. Vacuum equipment was available at the SRS but the main chamber, its stand, and electronics were sent to the SRS in about 1989 and the experiment was up and running by 1990-91, helped by NIST staff Jonathan Hardis who worked on the software, and Steve Southworth and Trish Ferrett who helped set up the experiment at Daresbury.  Marion Hayes adapted the software to be compatible with procedures used at the SRS and David Holland and the post-docs were largely responsible for getting the equipment integrated onto the 5-m monochromator.  Joe and I were able to spend some time at the SRS helping to get the experiment running; in my case I was awarded a position as visiting scientist at Aberdeen University and thereby obtained some funding for work in the UK.

The installed ARPES and the initial team are shown in Figure 7.  The figure shows the cylindrical ARPES chamber on the left side connect to the 5-m monochromator, which is behind the people standing.  The experiment ran periodically for about 5 years until the mid 1990s.

Figure 7. The NBS/NIST ARPES apparatus installed on the 5-m normal incidence beamline at the SRS at Daresbury. The ARPES chamber is on the left of the figure with the cylinder partially obstructed by electronics racks. Front row left to right; X. –M. Hu, Marion Hayes, and Michele Siggel. Rear row left to right, Albert Parr, John West and Joe Dehmer,

Some of the original team at NBS, now NIST after a name change in1988, had taken jobs elsewhere. Roger Stockbauer had taken a faculty position at Louisiana State University in the early 1990s and David Ederer had left NIST to join the faculty at Tulane.  They did not participate in the effort at Daresbury SRS or in the last few years at NIST prior to the move.  After the move to the SRS, John West assumed the day-to-day management of the experiment and was helped by Michele Siggel, David Holland and Marion Hayes as well as the technical crew at the SRS laboratory.  Michele had been appointed as a senior research associate at the SRS to participate in the ARPES and other experiments. She joined the permanent staff at the SRS after completion of her post-doc position.

During the course of the work at the SRS scientist from other institutions joined the ARPES experiment for short stays.  This included Kyoshi Ueda from Japan and several scientists from Sweden who collaborated on new carbon monoxide measurements [12, 13]. In most of the experiments the monochromator resolution was a few meV and the electron spectrometer resolution was set to about 50 meV, although the system could achieve an energy resolution of 10 meV if needed. This gave adequate resolution to separate vibrational levels in small molecules and generate vibrationally resolved asymmetry parameters and vibrational branching ratios.  Molecules studied include nitrogen, carbon dioxide, carbon monoxide and hydrogen.  The last joint NIST-SRS collaboration was a study of carbon dioxide in 1996[14].

The original team at NBS/NIST broke up owing to career changes by some of the staff as mentioned above.  I left the far-uv group in the late 1980s and eventually became a division chief in the Optical Technology Division and had little time for the ARPES effort. The equipment remained at the SRS and parts were used for other experiments and eventually became surplus equipment when the SRS ceased operation.  The collaboration lasted basically two decades and was highly productive, yielding 55 publications on photoelectron spectroscopy and the techniques involved; these are listed here.


Units and abbreviations

SRS                 Synchrotron Radiation Source

SURF              Synchrotron Ultraviolet Radiation facility

NBS                National Bureau of Standards

NIST               National Institute of Standards and Technology

ARPES            Angle Resolved Photoelectron Spectrometry

m                     meter

nm                   nanometer

eV                    electron Volt

meV                 milli-electron Volt



1.         Ederer D L, Cole B E and West, J B, A high throughput 2 m normal incidence monochromator for surf-II Nucl. Instrum. and Methods 172 185 (1980)

2.         Dill D, Dehmer J L and Wallace S, Shape-Resonance-Enhanced Nuclear-Motion Effects in Molecular Photoionization Phys. Rev. Letters 43 1005 (1979)

3.         Parr A C, Stockbauer R, Cole B E, Ederer D L, Dehmer J L and West J B, An angle resolved photoelectron spectrometer for atoms and molecules Nucl. Instrum. and Methods 172 357 (1980)

4.         Madden R P, A Status Report on the SURF-II Synchrotron Radiation Facility at NBS Nucl. Instrum. and Methods 172 1 (1980)

5.         Parr A C and Ebner S, SURF-II User Handbook, National Bureau of Standards, Department of Commerce, Gaithersburg MD 1981 p68.

6.         Samson J A R and Gardner J L, Branching Ratios in Photelectron Spectrometry J. Opt. Soc. Am. 62 856 (1972)

7.         West J B, Parr A C, Cole B E, Ederer D L, Stockbauer R and Dehmer J L, Shape-resonance-induced non-Franck-Condon vibrational intensities in 3σg photoionzation of N2 J. Phys. B: Atom. Molec. Phys., 13 L105 (1980)

8.         Gardner J L and Samson J A R, Vibrational intensity distributions for the various electronic states of O2+, N2+ and CO+ produced by photoionization J. Electron Spectrosc. 13 7 (1978)

9.         Dehmer J L, Dill D and Parr A C, Photoionization Dynamics of Small Molecules in Photophysics and Photochemistry in the Vacuum Ultraviolet, S P McGlynn, G L Findley and R H Huebner Eds 1985 p341-408 (D. Reidel: Dordrecht)

10.       Parr A C, Southworth S H, Dehmer J L and Holland D M P, A High Resolution Angle-Resolved Photoelectron Spectrometer System Nucl. Instrum. and Methods 222 221 (1984)

11.       Parr A C, Southworth S H, Dehmer J L and Holland D M P, A Phototoelectron spectrometer for high resolution angular resolved studies Nucl. Instrum. and Methods 208 767 (1983)

12.       Baltzer P, Lundqvist M, Wannberg B, Karlsson L, Larsson, M., Hayes, M.A., West, J.B., Siggel, M.R.F., Parr, A.C., Dehmer, J.L. Inner-valence states of CO+ between 22 eV and 46 eV studied by high resolution photoelectron spectroscopy and ab intitio CI calculations, J. Phys. B: At. Mol. Opt. Phys. 27 4915 (1994)

13.       Ueda K, West J B, Hayes M A, Siggel M R F, Parr A C and Dehmer J L, Photoelectron study of electronic autoionization in rotationally cooled N2: the n=6 member of the Hopfield series J. Phys. B: At. Mol. Opt. Phys. 26 L601 (1993)

14.       West J B, Hayes M A, Siggel M R F, Dehmer J L, Dehmer P M, Parr A C and Hardis J E, Vibrationally resolved photoelectron angular distributions and branching ratios for the carbon dioxide molecule in the 685-795 Å region. J. Chem. Phys. 104 3923 (1996)