Induced absorption

Jones WT and Stoicheff B P 1964 Inverse Raman spectra induced absorption at optical frequencies Phys. Rev. Lett. 13 657-9... 

Induced absorption, in which the molecular (or atom) M absorbs a quantum of radiation and is excited from m to n ... 

The rate of change of population iV of state n due to induced absorption is given by... 

There are three main EA spectral features in the energy range of band I a derivative-like feature with zero-crossing at 2.29 eV, followed by vibrational features, and an induced absorption band between 2.9 and 3.2 eV. The features below 2.5 eV are the results of a redshifted 1 Bu exciton energy, and its phonon sidebands (Stark shift). These features are more easily observed in EA than in absorp-... 

The induced absorption band at 3 eV does not have any corresponding spectral feature in a(co), indicating that it is most probably due to an even parity state. Such a state would not show up in a(co) since the optical transition IAK - mAg is dipole forbidden. We relate the induced absorption bands to transfer of oscillator strength from the allowed 1AS-+1 (absorption band 1) to the forbidden 1 Ak - mAg transition, caused by the symmetry-breaking external electric field. A similar, smaller band is seen in EA at 3.5 eV, which is attributed to the kAg state. The kAg state has a weaker polarizability than the mAg, related to a weaker coupling to the lower 1 Bu state. 

Figure 7-25. Doping induced absorption of u-6T at low (solid line) and high (dolled line) doping concentrations.

Figure 8-5. Transmission difference spectra of m-LPPP films at 7=77 K excited at 3.2 eV for various pump-probe delays. The inset zooms out the low energy region for 0 ps (solid line) and 400 ps (dashed line) delay. Doping induced absorption (D1A) data are also shown for comparison (from Ref. (251 with permission).

The final remark of this section concerns the polaronic transition of m-LPPP around 1.9 eV, where we can observe P2 with its vibronic replica P3 at 2.1 eV. In Figure 9-20 we show this polaronic absorption in m-LPPP as detected by photoin-duced absorption (a), chaige-induced absorption in conventional light-emitting devices (b), and chemical redox-reaction (c). Only under pholoexcilation, which creates both neutral and charged species, the triplet signal at 1.3 eV is also observed. 

The excellent agreement between the TSC and P1A results has two implications. First, since the TSC method probes the product of mobility and carrier density, while the P1A probes only the carrier density, there seems to be no dominant influence of temperature on the carrier mobility. This was also found in other conjugated polymers like /ra/ry-polyacetylene [19, 36]. Second, photoconductivity (observed via the thermal release of photoexcited and trapped earners) and photo-induced absorption probe the same charged entity [36, 37J. 

The irradiation-induced absorption coefficient // may be a better way of evaluating the radiation hardness and it is expressed as ... 

Vo) in the crystal. (Vo) can catch electrons to form F and centers. (Pb) is also able to attract electrons while (Vb)" can trap holes to give rise to color centers. They vdll make a contribution to the X-ray irradiation-induced absorption. Of course, the charge balance of the crystal is kept by charge compensation among these defects. Regretfully, the detailed characterization of these defects is too difficult to cover here and further experiments need to be performed. 

Fig. 21.13 Irradiation-induced absorption spectra of the BaBPOs crystal (a) after irradiation by X-rays for 15 min (b) after annealing at 150°C for 1 hour of the X-ray irradiated sample.

Fig. 7.10 Ni Mossbauer spectra of LaNi5 samples after various treatments (a) no hydrogen exposure, (b) activated in hydrogen, (c) after 1,584 thermally induced absorption-desorption cycles, as described in the text. The solid lines are least-squares fits to a single Lorentzian in (a) and (b). (c) was fitted with a single Lorentzian (representing nonmagnetic nickel atoms) and a 12-line hyperfine spectrum (from [20])...

L. Frommhold, Collision Induced Absorption in Gases, 1st edition, Cambridge Monographs on Atomic, Molecular, and Chemical Physics, Vol. 2 Cambridge University Press, England, 1993. 

Let us consider a molecule and two of its energy levels E) and f 2- The Einstein coefficients are defined as follows (Scheme B2.2) Bn is the induced absorption coefficient, B2i is the induced emission coefficient and A21 is the spontaneous emission coefficient. 

TA1 >LE I Dimerization induced absorption band shifts (cm ) for some porphyrins... 

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