Infrared photon

Tokmakoff A and Fayer M D 1995 Homogeneous vibrational dynamics and inhomogeneous broadening in glass-forming liquids infrared photon echo experiments from room temperature to 10 KJ. Chem. Phys. 103 2810-26... 

Tokmakoff A and Fayer M D 1995 Infrared photon echo experiments exploring vibrational dynamics in liquids and glasses Acc. Chem. Res. 28 439—45... 

Quack M, Sutcliffe E, Hackett P A and Rayner D M 1986 Molecular photofragmentation with many infrared photons. Absolute rate parameters from quantum dynamics, statistical mechanics, and direct measurement Faraday Discuss. Chem. Soc. 82 229-40... 

Tokmakoff A and Payer M D 1995 Infrared photon eoho experiments exploring vibrational dynamios in liquids and glasses Accounts Chem. Res. 28 437-45... 

Thermal Transducers Infrared radiation generally does not have sufficient energy to produce a measurable current when using a photon transducer. A thermal transducer, therefore, is used for infrared spectroscopy. The absorption of infrared photons by a thermal transducer increases its temperature, changing one or more of its characteristic properties. The pneumatic transducer, for example. 

The process of dissociation by fhe absorption of infrared photons clearly involves fhe simulfaneous absorption of many photons - of fhe order of 30, depending on fhe dissociation energy and fhe photon energy - and is called multiphofon dissociation. 

Ideal Performance and Cooling Requirements. Eree carriers can be excited by the thermal motion of the crystal lattice (phonons) as well as by photon absorption. These thermally excited carriers determine the magnitude of the dark current,/ and constitute a source of noise that defines the limit of the minimum radiation flux that can be detected. The dark carrier concentration is temperature dependent and decreases exponentially with reciprocal temperature at a rate that is determined by the magnitude of or E for intrinsic or extrinsic material, respectively. Therefore, usually it is necessary to operate infrared photon detectors at reduced temperatures to achieve high sensitivity. The smaller the value of E or E, the lower the temperature must be. 

In the x-ray portion of the spectmm, scientific CCDs have been utilized as imaging spectrometers for astronomical mapping of the sun (45), galactic diffuse x-ray background (46), and other x-ray sources. Additionally, scientific CCDs designed for x-ray detection are also used in the fields of x-ray diffraction, materials analysis, medicine, and dentistry. CCD focal planes designed for infrared photon detection have also been demonstrated in InSb (47) and HgCdTe (48) but are not available commercially. 

Molecules vibrate at fundamental frequencies that are usually in the mid-infrared. Some overtone and combination transitions occur at shorter wavelengths. Because infrared photons have enough energy to excite rotational motions also, the ir spectmm of a gas consists of rovibrational bands in which each vibrational transition is accompanied by numerous simultaneous rotational transitions. In condensed phases the rotational stmcture is suppressed, but the vibrational frequencies remain highly specific, and information on the molecular environment can often be deduced from hnewidths, frequency shifts, and additional spectral stmcture owing to phonon (thermal acoustic mode) and lattice effects. 

UV-Vis spectroscopy may also provide valuable information if small molecules are studied. However, the photochemical sensitivity of many sulfur-containing molecules may trigger changes in the composition of the sample during irradiation. For instance, this phenomenon has been observed in Raman spectroscopy using the blue or green hnes of an argon ion laser which sometimes decompose sensitive sulfur samples with formation of Sg [2, 3]. Reliable spectra are obtained with the red hnes of a krypton ion or a He-Ne laser as well as with the infrared photons of a Nd YAG laser. 

To conduct electricity, a semiconductor must be provided with energy that is at feast equal to its band gap. In a photoconductor, this energy must come from photons. Thus, we need to compare the band gaps with the energy of the infrared photons. 

Perhaps the first evidence for the breakdown of the Born-Oppenheimer approximation for adsorbates at metal surfaces arose from the study of infrared reflection-absorption line-widths of adsorbates on metals, a topic that has been reviewed by Hoffmann.17 In the simplest case, one considers the mechanism of vibrational relaxation operative for a diatomic molecule that has absorbed an infrared photon exciting it to its first vibrationally-excited state. Although the interpretation of spectral line-broadening experiments is always fraught with problems associated with distinguishing... 

Absorption of infrared photons identification of lowest energy pathway, 35,36f... 

Direct visualization of femtosecond filamentation is crucial to understanding the phenomenon. As the energy of a single infrared photon is much too small to effect an electronic transition, one has to take recourse to multiphoton absorption induced fluorescence to come up with a scheme to directly visualize filamentation in condensed media. One such scheme that has been successfully implemented involves the use of a crystal of barium fluoride, a material that is known to be very good scintillator [38]. 

The oxide CeC>2 doped with approximately 1% Er3+ exhibits up-conversion involving only one active ion. The Er3+ ions substitute for Ce4+ to form a low concentration of Erte defects randomly distributed within the oxide matrix. Irradiation with near-infrared photons with a wavelength of 785 nm excites the Er3+ ions from the 4Ii5/2 ground state to the 4I9/2 level, that is, a GSA mechanism ... 

Spectroscopic techniques look at the way photons of light are absorbed quantum mechanically. X-ray photons excite inner-shell electrons, ultra-violet and visible-light photons excite outer-shell (valence) electrons. Infrared photons are less energetic, and induce bond vibrations. Microwaves are less energetic still, and induce molecular rotation. Spectroscopic selection rules are analysed from within the context of optical transitions, including charge-transfer interactions The absorbed photon may be subsequently emitted through one of several different pathways, such as fluorescence or phosphorescence. Other photon emission processes, such as incandescence, are also discussed. 

We have now looked at the way photons are absorbed. Photons of UV and visible light cause electrons to promote between orbitals. Infrared photons have less energy, and are incapable of exciting electrons between orbitals, but they do allow excitation between quantized vibrational levels. The absorption of microwaves, which are less energetic still, effects the excitation between quantized rotational levels. 

Figure 9.21 depicts the infrared spectrum of aspirin (VI). The presence of several peaks demonstrates how several infrared photon energies are absorbed while others are not, so the spectrum shows how aspirin is transparent to photons of energy 2000 cm-1 (corresponding to a molar energy of 26.6 kJmor1), but does absorb very strongly at 2900 cm-1. 

Just as the absorption of UV or visible light causes electrons to excite between different electronic quantum states, so absorption of infrared photons causes excitation between allowed vibrational states, and absorbing microwave radiation causes excitation between allowed rotational states in the absorbing molecule. As a crude physical representation, these quantum states correspond to different angular velocities of rotation, so absorption of two photons of microwave radiation by a molecule results in a rotation that is twice as rapid as following absorption of one photon. 

A similar type of azobenzene dendrimers was reported by Jiang and Aida [37]. Interestingly, the aryl groups in such a dendrimer were found to be able to simultaneously absorb five infrared photons and then transfer the energy to the azobenzene core moiety to cause trans/cis isomerizations. 

Luminescence is an emission of ultraviolet, visible or infrared photons from an electronically excited species. The word luminescence, which comes from the Latin (lumen = light) was first introduced as luminescenz by the physicist and science historian Eilhardt Wiedemann in 1888, to describe all those phenomena of light which are not solely conditioned by the rise in temperature , as opposed to incandescence. Luminescence is cold light whereas incandescence is hot light. The various types of luminescence are classified according to the mode of excitation (see Table 1.1). 

W. Wang, B.H. Gu, L. Y. Liang, and W.A. HamUton Fabrication of near-infrared Photonic Crystals Using Highly-Monodispersed Submicrometer Si02 Spheres. J. Phys. Chem. B 107, 12113 (2003). 

Straub M, GuM (2002) Near-infrared photonic crystals with higher-order bandgaps generated by two-photon photopolymerization. Opt Lett 27 1824-1826... 

When formaldehyde absorbs an infrared photon with a wavenumber of 1 251 cm 1 (= 14.97 kJ/mol), the asymmetric bending vibration in Figure 18-12 is stimulated Oscillations of the atoms are increased in amplitude, and the energy of the molecule increases. 

Detectors for visible and ultraviolet radiation rely on incoming photons to eject electrons from a photosensitive surface or to promote electrons from the valence band of silicon to the conduction band. Infrared photons do not have sufficient energy to generate a signal in either kind of detector. Therefore, other kinds of devices are used for infrared detection. 

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