Before a chemical reaction can be induced by light, the light must be absorbed by some of the reactant molecules. Einstein proposed that each reacting molecule must be excited by absorption of one quantum of light. This principle is not easy to test, however, since secondary reactions occurring after the primary reaction may cause the destruction of many more of the molecules of the absorbing substance. In addition, excited molecules may lose energy by such processes as fluorescence or by undergoing deactivating collisions. In consequence the yield of reaction products may only be a fraction of that calculated from Einstein s law. The law is, however, considered to hold for the primary act of light absorption.  [c.149]

An s orbital is spherically symmetrical and can contain a maximum of two electrons with opposed spins. A p orbital has a solid figure-of-eight shape there are three equivalent p orbitals for each principal quantum number they correspond to the three axes of rectangular coordinates.  [c.152]

The d and f orbitals have more complex shapes there are five equivalent d orbitals and seven equivalent f orbitals for each principal quantum number, each orbital containing a maximum of 2 electrons with opposed spins.  [c.152]

The above definitions must be qualified by stating that for principal quantum number I there are only s orbitals for principal quantum number 2 there are only s and p orbitals for principal quantum number 3 there are only s, p and d orbitals for higher principal quantum numbers there are s, p, d and f orbitals.  [c.152]

In an electron spin resonance spectrometer, transitions between the two states are brought about by the application of the quantum of energy hv which is equal to g H. The resonance condition is defined when hv = g H and this is achieved experimentally by varying H keeping the frequency (v) constant. Esr spectroscopy is used extensively in chemistry in the identification and elucidation of structures of radicals.  [c.152]

I being the nuclear spin quantum number.  [c.197]

Pauli exclusion principle In any atom no two electrons can have all four quantum numbers the same. See exclusion principle.  [c.297]

The SQUID (Superconducting Quantum Interference Device) is a magnetic flux sensor with an ultimate flux resolution, based on the superconducting Josephson effect. Combined with a flux antenna a field resolution better than 1 pT is possible. The SQUID sensor has to be cooled and special care has to be taken to reduce the influence of background fields on the field resolution. It has been shown that a field resolution of about 20 ff can be obtained in a magnetically well shielded environment (laboratory) with a HTS-SQUID. For non destructive testing the system has to be mobile and has to be operated at locations with typical background fields in the order of 100 pT. The influence of these mainly homogeneous fields can be reduced by applying flux antennas with a differential nature. In this manner, a field gradient resolution of about 1 pT/cm x/Hz could be achieved without magnetic shielding.  [c.297]

To realize an automatic evaluation system, it would be desirable to also suppress geometrically caused signals as well, so that only the actual defect signals are obtained. Several approaches have already been made which are also to be implemented as part of a SQUID research project (SQUID = Super Conducting Quantum Interference Device).  [c.310]

It can be observed from the Figure 1 that the sensitivity of I.I. system is quite low at lower thicknesses and improves as the thicknesses increase. Further the sensitivity is low in case of as observed images compared to processed images. This can be attributed to the quantum fluctuations in the number of photons received and also to the electronic and screen noise. Integration of the images reduces this noise by a factor of N where N is the number of frames. Another observation of interest from the experiment was that if the orientation of the wires was horizontal there was a decrease in the observed sensitivity. It can be observed from the contrast response curves that the response for defect detection is better in magnified modes compared to normal mode of the II tube. Further, it can be observed that the vertical resolution is better compared to horizontal which is in line with prediction by the sensitivity curves.  [c.446]

Although direct coupling of a camera to a scintillator can give acceptable results one of its major drawback is the degradation of the quantum noise mainly related to the low transmission of the optics. The following schematics summarizes the particles flux (photons and electrons) across the different stages of the detector  [c.595]

Due to the conversion process an absorbed photon give rise to less than one electron generated in the CCD. This phenomenon, also called a "quantum sink" shows that the detector is degrading the S/N ratio of the image. The quality of an image being mainly limited by the quantum noise of the absorbed gamma this effect is very important.  [c.596]

If an intensifier, such as the 85 mm presented here, is now replacing the screen, a relative gain of the order of x50 is obtained which results in a conversion factor of 1 to 7.5 (1 incident X photon --> 7.5 electrons). This conversion efficiency not only resolves the quantum sink problem but also increases the light level significantly to compensate for the low gamma fluxes obtained from radioactive sources.  [c.596]

At low energies, this effect is even more important although the photon flux is generally higher. The conversion for a 20 keV X-ray photon is now 1 to 3 electrons (absorption efficiency close to 100% at these energies) which is the beginning of quantum noise degradation (a ratio of 1 to 10 is necessary to avoid any degradation).  [c.596]

After the primary step in a photochemical reaction, the secondary processes may be quite complicated, e.g. when atoms and free radicals are fcrnied. Consequently the quantum yield, i.e. the number of molecules which are caused to react for a single quantum of light absorbed, is only exceptionally equal to exactly unity. E.g. the quantum yield of the decomposition of methyl iodide by u.v. light is only about 10" because some of the free radicals formed re-combine. The quantum yield of the reaction of H2 -f- CI2 is 10 to 10 (and the mixture may explode) because this is a chain reaction.  [c.310]

Tracing the historical development of quantum physics, the author describes the baffling and seemingly lawless world of leptons, hadrons, gluons and quarks and provides a lucid and exciting guide for the layman to the world of infinitesimal particles.  [c.440]

Nuclear magnetic resctnance involves the transitions between energy levels of the fourth quantum number, the spin quantum number, and only certain nuclei whose spin is not zero can be studied by this technique. Atoms having both an even number of protons and neutrons have a zero spin for example, carbon 12, oxygen 16 and silicon 28.  [c.62]

Sandborg, M. and G. Alm-Carlsson, Influence of x-ray energy spectrum, contrasting detail and detector on the signal-to-noise ratio (SNR) and detective quantum efficiency (DQE) in projection radiography. Phys. Med. Biol., 1992. 37(6) p. 1245-1263.  [c.215]

The phosphor has several characteristics which are suitable for obtaining high-quality images with low doses of ionizing radiation high absorption cofficient for electron or X-ray radiation, the spectrum of PSL radiation is in the range where the quantum efficiency of the PMT is high, there is no substantial fading of the stored image for several hours and the residual image can be erased simply by exposing it to a large dose of visible (UV) light. The IP can be used repeatedly.  [c.506]

It was found that that in the case of soft beta and X-ray radiation the IPs behave as an ideal gas counter with the 100% absorption efficiency if they are exposed in the middle of exposure range ( 10 to 10 photons/ pixel area) and that the relative uncertainty in measured intensity is determined primarily by the quantum fluctuations of the incident radiation (1). The thermal neutron absorption efficiency of the present available Gd doped IP-Neutron Detectors (IP-NDs) was found to be 53% and 69%, depending on the thicknes of the doped phosphor layer ( 85pm and 135 pm respectively). No substantial deviation in the IP response with the spatial variation over the surface of the IP was found, when irradiated by the homogeneous field of X-rays or neutrons and deviations were dominated by the incident radiation statistics (1).  [c.507]

See pages that mention the term Qmetidine : [c.9]    [c.14]    [c.45]    [c.48]    [c.48]    [c.50]    [c.146]    [c.148]    [c.152]    [c.152]    [c.152]    [c.152]    [c.152]    [c.152]    [c.158]    [c.170]    [c.246]    [c.310]    [c.310]    [c.311]    [c.327]    [c.337]    [c.337]    [c.337]    [c.369]    [c.369]    [c.369]    [c.371]    [c.425]    [c.444]    [c.595]   
The organic chemistry of drug synthesis Vol.4 (1990) -- [ c.89 , c.95 , c.112 ]