Spectroscopy

IR
Almost any compound that have covalent bonds (whether organic or inorganic) absorbs various frequencies of electromagnetic radiation in the IR region. For chemical purposes, the vibrational region of the IR region is 2.5 um to 25 um. When molecules absorb IR, they are excited to a higher energy state. This is a quantized process, meaning a molecule only absorbs selected frequencies of IR (the absorption of IR corresponds to an energy change in the order of 8 to 40 kJ/mole). However, not all bonds in a molecule are capable of absorbing IR, even if the frequency matches that of the bond motion. Only those bonds that have a (electric) dipole moment that changes as a function of time are capable of absorbing IR. So, symmetrical bonds that have identical or nearly identical groups on each end (such as ethylene) will not absorb in the infrared region.

NMR
Any atomic nucleus that has either an odd mass, or odd atomic #, has a quantized spin angular momentum and a magnetic moment. If it had an even mass, such as carbon-12, it has a spin of 0 and is therefore NMR inactive. So for odd masses, in an applied magnetic field, the nucleus is a charged particle, so any moving charges generates a magnetic field of its own. Therefore, the nucleus has a magnetic moment generated by its charge and spin. In a 60-MHz spectrometer, the 0-10 ppm (chemical shift) ranges from 0-600 Hz, and in a 300-MHz spectrometer, the 0-10 ppm ranges from 0-3000 Hz. 60 MHz corresponds to a magnetic field of 14,092 gauss, 300 MHz at 70,459 gauss. An unshielded proton absorbs radiation of 42.6 MHz frequency in a field strength of 1 Tesla (10,000 Gauss), which is also 60.0 MHz in a field strength of 1.41 Tesla. In a NMR chart, the magnetic field increases from left to right, from deshielded protons (downfield) to shielded protons (upfield). Some of the most deshielded protons are those that are attached to carboxylic acids, with resonances at 10-12 ppm.

UV
The wavelengths of UV light absorbed by a molecule are determined by the electronic energy differences between orbitals in the molecule. Sigma bonds are very stable, so the electrons in sigma bonds are usually unaffected by wavelengths of light above 200 nm. Pi bonds have electrons that are more easily excited into higher energy orbitals. As a molecule absorbs energy, an electron is promoted from an occupied orbital to an unoccupied orbital of greater potential energy. The energy differences between electronic levels in most molecules vary from 125 to 650 kJ/mole.

For both fluorescence and phosphorescence, compounds emit radiation at a longer radiation than absorbed (Stokes shift). The 2 ways a particle of a substance can absorb and emit light energy, is through electronic transitions (which occurs at discrete wavelengths), or through changes in kinetic energy (vibration, rotation, translation, etc.).

Raman spectroscopy
The scattered radiation shined from a vibrating molecule at a particular frequency (usually laser), called Rayleigh scattering, +/- the fundamental frequency of a a vibrating mode in a molecule, is Raman scattering. Most of the radiation scattered from the vibrating molecule, has the same frequency before hitting the molecule. For inorganic compounds, the initial radiation is usually a visible red krypton laser of λ = 647 nm. For a mode of vibration to be Raman-active, it must give rise to a change in the polarizability of the molecule (polarizability is the ease with which the electron cloud associated with the molecule is distorted).

For molecules that have a central symmetry, such as CO2 or SF6, have a rule of mutual exclusion. That means vibrations that are IR active, are Raman inactive, or if are Raman active, are IR inactive. That means if we were to deduce a molecule has a center of symmetry, both tests are done and only 1 must have activity.

Orbach relaxation spectrocopy
The temperature dependence of the relaxation rates of paramagnetic ions centers in crystals is measured. This is primarily useed in solid-state physics and material science to study the relaxation processes of paramagnetic defects in crystals.

EPR
Electron paramagnetic resonance or ESR (electron spin resonance) is for materials that have unpaired electrons. The basic concepts of EPR are analogous to that of NMR, except the spins excited are the electrons instead of the atomic nuclei. So, EPR specroscopy is useful for studying metal complexes and organic radicals.

Both NMR and EPR have a Zeeman splitting. For NMR, Zeeman splitting occurs for nuclear spins in the presence of an external magnetic field, whereas for EPR, Zeeman splitting occurs due to the interaction betwee the magnetic moment of an unpaired election and an external magnetic field. The Zeeman splitting is much larger in EPR because the magnetic moment of the electron (μB aka Bohr magneton) is much larger than that of the nuclei (μN aka nuclear magneton).

Transient absorption spectroscopy
Femtosecond transient absorption spectroscopy measures the change in absorbance of certain wavelengths in the excited state of a material through a pulsed laser pump-probe technology. The transient absorption spectrum is generated by subtracting the unexcited ground state spectrum from the excited state spectrum. Therefore, the transient absorption signal will decrease with time as the excited state decays into the ground state. This kind of spectroscopy is good forphotocatalysts, where when it absorbs light, a valence-band electron is excited to the conduction-band forming and exciton.

Photoemission spectroscopy (PES)
Also known as photoelectron spectroscopy, is the energy measurement of electrons emitted from solids, liquids, or gases by the photoelectric effect, in order to determine the binding energies of electrons in the substance. The binding energies (eV) are characteristic of the chemical structure and molecular bonding of the material.

If the photons are from x-rays, are called x-ray photoelectron spectroscopy (XPS). XPS measures the top 10 nm of any surface, or topmost 200 atoms. XPS is a powerful technique because it can show what elements are present, and what other elements they are bonded to.

Photoemission electron microscopy (PEEM)
PEEM is a type of surface-sensitive electron microscopy that uses light (usually UV, x-ray, or synchroton radiation) to generation photoelectrons from a sample and then forms an image from them. So, you shine photons on the sample, and these photos excite electrons in the material above the vacuum level, so electrons are emitted (photoelectric effect). PEEM measures the coefficient indirectly by collecting the emitted secondary electrons generated in the electron cascade that follows the creation of the primary code hole in the absorption process.

This is useful if you want to know where on a surface chemical states vary, and what magnetic domains look like (with spin-polarized PEEM).

Transient XUV spectroscopy is an ultrafast pump-probe technique that uses extreme UV light (XUV) to study how electrons in atoms, molecules, or solids evolve in real time after being excited, down to femtosecond or attosecond scales.. Transient means time-resolved, and the wavelenths are from 10 to 100 nm, photon energies are from 10 to 100 eV. The XUV light ejectrons electrons (photoemission) or is absorbed (transmission/reflectivity). Measuring these signals reveals ultrafast changes in electronic structure (valence bands, conduction bans, core states), charge transfer, excitons (bound electron-hole pairs), and chemical bonding. This is useful in condensed matter physics because it watches how correlated electrons relax in superconductors or Mott insulators.

Second Harmonic Generation (SHG)
2nd Harmonic Generation (aka frequency doubling) is a non-linear optical spectroscopy technique that is used to gain molecular insights into the electrostatics and structure of the electrical double layer. It is the lowest-order wave-wave non-linear interaction that occurs. It is widely used in doubling laser frequencies. An application of this are green laser pointers, which are IR lasers with a frequency-doubling 2nd-harmonic generation step at the end (from 1064 nm source to the green 532 nm laser).

Resonant Inelastic X-ray Scattering (RIXS)
RIXS is an advanced technique that measures low-energy excitations such as phonons, magnons, and d-orbital splitting. While X-ray Absorption Spectra (XAS) is widely used to study the electronic and spin states of materials, it has limitations due to the broadening of peaks caused by the core-hole lifetime. RIXS overcomes this challenge through 2D spectroscopy mapping as a function of incident energy and energy loss, enabling high-resolution measurement of energy excitations.

Mössbauer spectroscopy
Mössbauer spectroscopy measures how gamma rays interact with nuclei in a sample, where tiny shifts in energy can reveal extremely precise information about oxidation states, magnetic environment, chemical bonding, fields at the nucleus, electron density at the nucleus (Fe2+ vs. Fe3+). Iron-57 (57Fe) is the most commonly used nucleus, making this useful for studying iron-containing compounds such as heme proteins, Fe-S clusters, and minerals.

Polariton chemistry.

Polaritons involving molecular vibrations interacting with cavity modes.
In this specific context, polaritons were discovered by the physics departments, but physicists do not like to think of homogeneity, but chemists do. So the 1st polaritons were demonstrated by 2 lab groups in 2015. Note that this is not about exciton polaritons or plasmon-polaritons, which were coined in 1958 by J. J. Hopfield.

Photon + electron-hole pair -> exciton polaritons.
Photon + surface plasmon -> surface plasmon-polaritons.

Polaritons are hybrid particles made up of a photon strongly coupled to an electric dipole. Now there is a 4th way for controlling a chemical reaction: thermochemistry, electrochemistry, photochemistry, then polariton chemistry. But as of 2023, it is stil controversial if this type of polariton will have much to do with enhancing chemical reactions, such as IR-modulated photoluminescence, and, quantum dots having a strong field ionization.

Vibrational strong coupling (VSC) generates hybridized quasiparticles of molecular vibrational and cavity electromagnetic modes, known as molecular vibrational polaritons (MVPs, which was 1st demonstrated in 2015). VSC has arisen as a promising handle to manipulate chemical reactions in condensed phases. Experimental evidence has shown that simply by placing a solution inside an optical cavity, the reaction rate can be either accelerated or decelerated by VSC, and the reaction selectivity can also be altered. Polaritons, as light-matter coupled systems, are examples of how molecular properties can be changed by coupling. Curved mirrors can be used to enhance the coupling strength.

There's an upper and lower polariton, where upper has more donors and lower has more acceptors. Under strong coupling, vibrational energy transfers within or between molecules can be enhanced. It may further impede competing dynamic pathways. Only polaritons can modify chemical dynamics (dark modes behave the same as regular molecules). The intermolecular coupling is mediated by photons, which is delocalized. Polaritons can also reverse intersystem crossing.

QDM
Quantum diamond microscopes use a green light that shines to a nitrogen-vacany diamond, that emits red light. N-vacancy is where a carbon-carbon is replaced with a nitrogen next to a vacancy with a negative charge. (If the vacany was neutral, rather than a negative charge, then it cannot do quantum sensing, and emits more blue light). They were available by about 2019, where researchers like Ronald Walsworth and colleagues published principles and techniques for the technology. A N-vanacy depth of 1 micron, or 100 nm for CVD, is deep enough for fluorescence, and has a band gap of 5.5 eV. The QDM consists of a few electromagnetic coils wrapped around a camera, a small laser, and a sample slide of the diamond sensor. This device is connected to a computer. The N-vacancies degrade at 400 C, as well as below cryogenic temperatures.

News.

7/17/2024 Advanced Photon Source reopens at Argonne National Laboratory in Lemont after being upgraded.

Argonne National Laboratory's newly-upgraded Advanced Photon Source, or APS, is open again for research. The technology is complex, but boiled down, the APS uses bright X-Rays to see really small things in high resolution allowing scientists to observe virtually any material at an atomic level. Director Paul Kearns said "The original APS examined moon rocks, T-Rex bones and mummies."

Jim Kerby, the project director of the Advanced Proton Source upgrade, said the Advanced Photon Source is one of five in the world and is by far the most advanced. The project cost $815 million, with the funding coming from the U.S. Department of Energy via taxpayer money.

Wednesday's ribbon-cutting comes a day after the news of $140 million in federal investment for quantum computing research in Illinois. "I'm particularly excited about how APS is helping build out our quantum computing infrastructure," Illinois Governor Pritzker said. "Over the past few years the General Assembly and I have invested in making Illinois the undisputed global capital for quantum, and Argonne is a critical piece of that vision." More than 5,000 scientists from around the world use the machine every year, and, perhaps most notably, Argonne officials said the technology has directly led to 2 Nobel Prizes in chemistry.