NAP-XPS and NAP-UPS
X-ray photoelectron spectroscopy (XPS) and Ultraviolet photoelectron Spectroscopy (UPS) is used to analyze the surface chemistry of a material. XPS spectra are obtained by illuminating the sample surface with monochromatic X-rays and eventually measuring the photo emitted electrons. With XPS the elemental composition and the chemical state of the detected elements can be determined quantitatively in the surface region with an information depth of up to 10 nm.
EnviroESCAs special system design and many NAP-XPS system designs allow for investigations of a large variety of different samples, including insulating samples, gases, liquids and their interfaces that are not accessible with standard XPS systems. The sample to be investigated is the central part of every X-ray Photoelectron Spectroscopy (XPS) system. In a NAP-XPS system or the EnviroESCA the sample needs to be positioned underneath the analyzer nozzle which is the entrance to the analysis section of the machine. Samples can be flat surfaces or really rough three dimensional structures. They can be very tiny with a diameter of just a few hundreds of microns or as large as 10 mm in NAP-XPS and 120 mm in EnviroESCA.
Besides the sample itself and the analyzer an X-ray source is the third part needed to perform an XPS analysis of a surface to gain insights into its chemical composition on the molecular level. The X-rays used in the EnviroESCA are called soft X-ray because of their energy. Their energy is so low, compared to medical X-ray sources that the stainless steel or µ-metal chamber walls of the machine will absorb them completely so that no X-rays will leave the system. The beam of X-rays is focused into a spot on the sample to be analyzed of about 300 µm in diameter defining the analysis region.
X-ray Photoelectron Spectroscopy based on the physical principle of Photoionization. Whenever a photon of high enough energy interacts with an atom in the uppermost surface layers (roughly 10 nm) an electron of a specific energy is being released. The energy is characteristic for the element and the nearest neighbors of the atom in the surface layer. Therefore it is not only possible with XPS to determine the surface concentration of an element down to 0.1 atomic percent but also its binding conditions to other atoms and molecules.
In conventional XPS systems where the analysis region needs to be kept under vacuum an effect occurs that is named surface charging. Imagine a sample that is nonconductive and the impinging X-ray photons continuously removing electrons from the analysis region than the sample will slowly be charged up positively. The escaping electrons will be influenced in their path and energy by the positive charges at the surface. To prevent the surface from charging up conventional XPS systems are equipped with electron and ion guns that can be tuned in a way that additional charges will be brought to the surface region compensating the lost charges due to the photoionization process. A really difficult and time consuming task.
NAP-XPS systems or EnviroESCA are not conventional XPS systems but Ambient Pressure XPS (AP-XPS) systems. Which means it does not require the sample to be kept under vacuum. A low pressure of less than 50 mbar is sufficient. This extremely high working pressure when compared to conventional systems opens up the field of biology, biochemistry, astrobiology, medicine and others. Non vacuum conditions to operate in means that the sample can be outgassing or even be a liquid with a vapor pressure in this range. Mainly it means that there are neutral gas atoms surrounding the sample. Whereby it does not matter if they evaporate from the surface or if one uses the built-in gas delivery system of the to bring gas molecules to the analysis region.
The impinging X-ray photons will also interact with the neutral gas atoms and ionize those generating free positive charged ions and electrons. These free charges act as a charge cloud above the surface layer of the sample. Allowing every escaping electron from the surface to be exchanged against an electron of the charge cloud. We call this intrinsic charge neutralization effect Environmental Charge Compensation. It allows the user to easily acquire high resolution XPS spectra from nearly any surface independent if it is liquid, solid, conductive or nonconductive with ease.
NAP-XPS also works at synchrotron beamlines with variable photon energies. A special beam entrance stage either with differential pumping or a Si3N4-window has to be used in this case.
With special small spot UV-sources with additional differential pumping and a Al-window ín front also UV light (classically He I and He II radiation) can be used to excite electrons under NAP conditions, so that also changes in the valence band structure in pressure of up to 1 mbar can be studies.