Absorption/emission processes

Scattering spectroscopy measures the light that a sample scatters at a certain wavelength, incident angle, or polarization angle. This technique is somewhat similar to emission spectroscopy, but scattering occurs much more quickly than the absorption/emission process. Raman spectroscopy is the most useful example of this type. 

For the bound-bound absorption-emission processes, the effect of the retardation is negligible, as it is well known and as that may be confirmed numerically in a precise way (see (9.38)-(9.40) later). 

If we take the value of k = 3a/8a suitable to the simple absorption-emission process and if we neglect the terms in a2, one finds again the formulas of Chap. 7. So we have the confirmation that the retardation is quite negligible for these transitions. 

A similar effect can occur In the photon absorption/ emission process. The wave function of the oscillator (exclmer) In the ground vibrational state Is, In coordinate space. 

Schematic illustration of UC processes for ions (a) excited state absorption (ESA) upconversion, (b) energy transfer upconversion (ETU), (c) photon avalanche (PA) upconversion, and (d) energy migration-mediated upconversion. The dotted, dashed-dotted, and full arrows represent energy transfer, nonradiative relaxation, and photon absorption/emission processes, respectively.

To derive these and other conclusions, we need to specify the molecular states 7>. Since we are interested in vibrational spectra associated with coherent electronic absorption-emission processes, we represent the molecular states by products of electronic and vibrational wavefunctions... 

In molecular spectroscopy, absorption/emission processes can be grouped into three... 

The difference between non-equilibrium and equilibrium energies is known as reorganisation energy, which is one of the factors responsible for the symmetric broadening of the absorption / emission bands (yide infra). The difference between the solvation energies of the final and initial states for absorption (emission) processes in various solvents with respect to the isolated phase is the so-called sol-vatochromic (fluorosolvatochromic) shift. 

A laser-induced change in the temperature of an isotropic liquid crystal can modify its refractive index in two ways, very much as in the nematic phase. One is the change in density dp due to thermal expansion. This is the thermal absorptive component discussed before [Eq. (9.18) for p ] this term may be written as (0n/0p) p. The other is the so-called internal temperature change dT which modifies the spectral dependence of the molecular absorption-emission process we may express this contribn-tion as (0n/07)p dT. A pnie density change effect arises from the electrostrictive component p, which contribntes a change in the refractive index by (0u/0p) p . 

When hydrogen is burned up in the nuclear furnace of a star, helium burning takes over, forming carbon, which in turn leads to oxygen, etc. Subsequent emission processes releasing a-particles, equilibrium processes, neutron absorption, proton capture, etc. lead to heavier elements. 

Figure 2.2 (a) Absorption and emission processes between states m and n. (b) Seeding... 

The majority of infrared spectra are obtained by an absorption rather than an emission process and, as a result, the change of signal intensity 1(5) with retardation 5 appears very different from that in Figure 3.13. 

Figure 7.18 shows sets of vibrational energy levels associated with two electronic states between which we shall assume an electronic transition is allowed. The vibrational levels of the upper and lower states are labelled by the quantum numbers v and u", respectively. We shall be discussing absorption as well as emission processes and it will be assumed, unless otherwise stated, that the lower state is the ground state. 

Band gaps in semiconductors can be investigated by other optical methods, such as photoluminescence, cathodoluminescence, photoluminescence excitation spectroscopy, absorption, spectral ellipsometry, photocurrent spectroscopy, and resonant Raman spectroscopy. Photoluminescence and cathodoluminescence involve an emission process and hence can be used to evaluate only features near the fundamental band gap. The other methods are related to the absorption process or its derivative (resonant Raman scattering). Most of these methods require cryogenic temperatures. 

The third common level is often invoked in simplified interpretations of the quantum mechanical theory. In this simplified interpretation, the Raman spectrum is seen as a photon absorption-photon emission process. A molecule in a lower level k absorbs a photon of incident radiation and undergoes a transition to the third common level r. The molecules in r return instantaneously to a lower level n emitting light of frequency differing from the laser frequency by —>< . This is the frequency for the Stokes process. The frequency for the anti-Stokes process would be + < . As the population of an upper level n is less than level k the intensity of the Stokes lines would be expected to be greater than the intensity of the anti-Stokes lines. This approach is inconsistent with the quantum mechanical treatment in which the third common level is introduced as a mathematical expedient and is not involved directly in the scattering process (9). 

Figure 3. Energy diagram for 1064 nm excitation of PuFg(g). The 5f electron states of PuF6 are shown at the left. The solid arrows Indicate photon absorption or emission processes. The wavy arrows indicate nonradiative processes by which excited states of PuF6 are lost. Comparison of observed fluorescence photon yields versus the fluorescence quantum yield expected for the 4550 cm" state indicate that the PuFg state initially populated following 1064 nm excitation may dissociate as shown.

To aid our understanding of absorption and emission processes, Eq. (2.1) can be expanded in terms of electronic, vibronic (vibrational components of an electronic transition), and spin wave functions ... 

Phillips and Schug (24) have suggested that the 390 nm emission, observed when PET is excited with high energy electrons, might be from a triplet state or an excimer. Since the triplet states of both PET and DMT are lower in energy (MSO nm), it is unlikely that the emission is from a triplet state. In addition, excimer formation and emission should not effect the absorption-excitation processes therefore, it is unlikely that the 390 nm emission is from an excimer. 

---