Mean frequency absorption

SYNERGISTIC EFFECTS IN TWO-PHOTON ABSORPTION THE QUANTUM ELECTRODYNAMICS OF BIMOLECULAR MEAN-FREQUENCY ABSORPTION... 

Following theoretical developments (Andrews and Harlow 1983, 1984a), bimolecular mean-frequency absorption was next discovered by researchers working on the photodynamics of charge transfer reactions. For example, Ku et al. (1983) performed studies on gaseous mixtures of xenon and chlorine passed through a laser fluorescence cell. The proposed reaction mechanism... 

Bimolecular mean-frequency absorption is a nonlinear process which results entirely from the effects of molecular proximity, although wavefunction... 

As mentioned above, there are four specific cases of bimolecular mean frequency absorption that are of special interest. These are distinguished by the type of mechanism (cooperative or distributive) involved and whether the photons absorbed have the same or different frequencies. The latter condition is in most cases determined by whether a single laser beam or two laser beams are employed for the excitation. We consider first the single-beam cases. 

The former relation, Eq. (4.5), indicates the fact that this cooperative process again has the characteristics of mean-frequency absorption here, however, it is the molecular excitation frequency which equals the mean of the two photon frequencies. 

For two-frequency synergistic absorption, the vaiious resonance possibilities are also summarized in Table VIII. For cooperative mean-frequency absorption, there are again three distinct possibilities for resimance enhancement of the process, as shown by the lowest three broken lines in Fig. 11. If the centers involved in the process possess energy levels close i o ( q + ftcui) or ( q + 0)2), then although the molecular tensors or S° ((U2) are resonantly... 

In assessing the wider significance of mean-frequency absorption for flash photolytic experiments based on USLS radiation, perhaps the most important factor to consider is the enormously wide range of possibilities for synergistic absorption leading to the simultaneous excitation of more than one excited state. For example, with a s ngle-component sample, it should be possible to observe the process... 

In conclusion, it is worth reiterating that the anomalous absorption effects described here may be manifest in any experiments that employ sufficiently high-intensity broadband radiation. To this extent, anomalies may be observable in experiments not specifically involving USES light. In particular, the continued advances in techniques of laser pulse compression have now resulted in the production of femtosecond pulses only a few optical cycles in duration (Knox et al. 1985 Brito Cruz et al. 1987 Fork et al. 1987) which necessarily have a very broad frequency spread, as the time/energy uncertainty principle shows. Thus, mean-frequency absorption may have a wider role to play in the absorption of femtosecond pulses. If this is correct, it raises further questions over the suitablity of absorption-based techniques for their characterization. 

Synergistic Effects In Two-Photon Absorption The Quantum Electrodynamics Of Bimolecular Mean-Frequency Absorption 39... 

Resonance in the sense used here means that the radio-frequency absorption takes place at specified resonance frequencies. However, you will see that almost all of the forms of spectroscopy we discuss in this book involve resonance absorption in the same sense. 

Mean Frequencies/Amplitudes and Corresponding Density Distributions. Form of Rotational Absorption Band... 

In Section VILA a strongly idealized picture was described. The dielectric response of an oscillating nonrigid dipole was found in terms of collective vibrations of two charged particles. Now a more specific picture pertinent to an idealized water structure will be considered. Namely, we shall briefly consider thermal motions of a dipole as (i) pure rotations in Fig. 56b and (ii) pure translations in Fig. 58a. Item (i) presents the major interest for us, since we would like to roughly estimate on the basis of a molecular dynamics form of the absorption band stipulated by rotation of a dipole. Of course, even in terms of a simplified scheme, the internal rotations of a molecule should also be accompanied by its translations, so the Figs. 56a and 56b should somehow interfere. However, in Section IX.B.l we for simplicity will neglect this interference. This assumption approximately holds, since, as will be shown in Section IX.B.2, the mean frequencies of these two types of motion substantially differ. 

The second process concerns the resonance-like absorption of radiation, when the ac frequency is close to the mean frequency of dipole oscillations. In liquids such a resonance usually rather degrades due to strong collisions of a dipole with the surrounding medium. These collisions influence the full energy of a particle. We suggest that the particle s velocities have Boltzmann distribution. In our theory... 

It has been shown that FM techniques are capable of measuring absorption signals down to the shot-noise limit (caused by the number fluctuations for the discrete photons), wliich frequently means, that absorptive signals of 1 part in 10 or 10 are detectable. Frequency modulation spectroscopy is described and discussed in [10.85]. 

So far as rule 2 is concerned, since AJ is conventionally taken to refer to J -J", where J is the quantum number of the upper state and J" that of the lower state of the transition, AJ = — 1 has no physical meaning (although it emerges from the quantum mechanics). It is commonly, but incorrectly, thought that AJ = +1 and AJ = — 1 refer to absorption and emission, respectively in fact AJ = +1 applies to both. Transition wavenumbers or frequencies are given by... 

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