Frequency of absorbed radiation

Fig. 2.3. Schematic variation of absorption coefficient as a function of the frequency of absorbed radiation experimental curve (1) and theoretical curves in Debye approximation (2) and impact approximation (3) (Rocard formula).

V = frequency of absorbed radiation 2. = wavelength of absorbed radiation h = Planck s constant c = velocity of light. 

Radiation energy has to be absorbed before it can initiate reactions. The frequency of absorbed radiation energy depends in part on the chemical structure of the irradiated material. Carbonyl groups in an aldehyde or in a ketone absorb at 187 and 280 - 320 nm, a C-C bond absorbs at 195 nm and in the 230 - 250 nm range the hydroxyl group absorbs at 230 nm and primary C-C-bonds at 135 nm UV light. 

Figure 10-1 Absorption of energy occurs when incident radiation has exactly the right frequency v so that its energy hv equals the energy difference AE between the ground state and the excited state of a molecule [v, frequency of absorbed radiation h (Planck s constant) =...

IR absorption (or Raman shift) is caused by a quantum transition between two energy levels, and so band positions ought to be defined in terms of energy units. However, atoms can be treated as oscillating under the influence of spring-like bonds obeying Hooke s law, so the terms vibration and vibrational frequency are habitually used. The vibrational frequency is considered to be equal to the frequency of absorbed radiation (or frequency difference in the... 

In the process of excitation, the dye molecule absorbs a quantum of uv or visible radiation. The quantum has an energy E = hv, where b is Planck s constant and O is the frequency of the radiation. The higher the frequency of the quantum, the shorter the wavelength X, with u-A = c, where c is the velocity of light in a vacuum. 

The PAS phenomenon involves the selective absorption of modulated IR radiation by the sample. The selectively absorbed frequencies of IR radiation correspond to the fundamental vibrational frequencies of the sample of interest. Once absorbed, the IR radiation is converted to heat and subsequently escapes from the solid sample and heats a boundary layer of gas. Typically, this conversion from modulated IR radiation to heat involves a small temperature increase at the sample surface ( 10 6oC). Since the sample is placed into a closed cavity cell that is filled with a coupling gas (usually helium), the increase in temperature produces pressure changes in the surrounding gas (sound waves). Since the IR radiation is modulated, the pressure changes in the coupling gas occur at the frequency of the modulated light, and so does the acoustic wave. This acoustical wave is detected by a very sensitive microphone, and the subsequent electrical signal is Fourier processed and a spectrum produced. 

When electromagnetic radiation passes through transparent matter, some of it is absorbed. Strong absorption will occur if there is a close match between the frequency of the radiation and the energy of one of the possible electronic or molecular absorption processes characteristic of the medium. A plot of absorbance (A) against wavelength (X) or frequency (v) for a particular material is termed an absorption spectrum. The complexity of the absorption spectrum depends on whether atomic (simple, with a few sharp absorption bands) or molecular (complex, with many broad bands) processes are responsible. 

Emission or absorption spectra are produced when molecules undergo transitions between quantum states that correspond to two different internal energies. The energy difference AE between the states is related to the frequency of the radiation emitted or absorbed by the equation DE = hn. Infrared frequencies in the wavelength range 1-50 mm are associated with molecular vibration and vibration-rotation spectra. 

The energy of the radiation absorbed or emitted is quantized according to Planck s equation (Eqn. 1.1). These quanta are known as photons, the energy of which is proportional to the frequency of the radiation. 

Therefore the frequency of the radiation absorbed or emitted in a transition between the ground state and a fundamental level is (JTJlb — W yh — the pth fundamental frequency. The corresponding wavefunction will be... 

II. The Bohr frequency rule. The frequency of the radiation absorbed by a system and associaled with the transition from an initial stale with energy W to a final stale with energy W% is... 

PHOTOELECTRIC CONSTANT. A quantity equal to hje where h is the Planck constant, and e, the electronic charge, and which multiplied by the frequency of any radiation exciting photoemission gives the potential difference corresponding to the quantum energy absorbed by the escaping photoelectron. 

Different bonds vibrate at different frequencies- When the resonance frequency of the oscillating bond is matched by the frequency of infrared radiation, the 1R energy is absorbed. In IR Spectroscopy, an infrared spectrometer slowly changes the frequency of infrared light shining upon a compound and records the frequencies of absorption in reciprocal centimeters, cm 1 (number of cycles per cm). 

The most important modem system of units is the SI system, which is based around seven primary units time (second, abbreviated s), length (meter, m), temperature (Kelvin, K), mass (kilogram, kg), amount of substance (mole, mol), current (Amperes, A) and luminous intensity (candela, cd). The candela is mainly important for characterizing radiation sources such as light bulbs. Physical artifacts such as the platinum-iridium bar mentioned above no longer define most of the primary units. Instead, most of the definitions rely on fundamental physical properties, which are more readily reproduced. For example, the second is defined in terms of the frequency of microwave radiation that causes atoms of the isotope cesium-133 to absorb energy. This frequency is defined to be 9,192,631,770 cycles per second (Hertz) —in other words, an instrument which counts 9,192,631,770 cycles of this wave will have measured exactly one second. Commercially available cesium clocks use this principle, and are accurate to a few parts in 1014. 

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