Photon-molecule interaction

The strength of a photon—molecule interaction is deterrnined by the frequency-dependent cross section 0 (v), expressed in cm for absorption and related to a(y) in equation 1 or by the differential cross section (k5(y) jin units of cm /sr for scattering (14). The latter specifies the likelihood that active species scatter some portion of the incident laser fluence (photons /cm ) into a viewing soHd angle, AQ, measured in steradians (Fig. 1). The cross sections can be expressed as in equation 5 ... 

The next step is to consider tire cross-sections of the absorption of radiation by the diatomic halogen molecules in order to decide if the relative effects result from the efficiency of the radiation photon-molecule interactions. These are reflected in the dissociation cross-sections of tlrese interactions. 

Several books and review chapters devoted to the field of ion-neutral reactions in the gas phase have appeared in recent years, la 8, j,k some of which are concerned at least in part with the special topic of interest for the present review chapter—namely, the role of excited states in such interactions. The present review attempts to present a comprehensive survey of the latter subject, and the processes to be discussed include those in which an excited ion interacts with a ground-state neutral, interaction of an excited neutral with a ground-state ion, and on-neutral interactions that produce excited ionic products or excited neutral products. Reactions in which ions are produced by reaction of an excited neutral species with another neutral, for example, Penning ionization, are not included in the present chapter. For a recent review of this topic, the reader is referred to the article by Rundel and Stebbings.1 Electron-molecule interactions and photon-molecule interactions are discussed here only as they relate to the production of ions in excited states, which can then be reacted with neutral species. 

Fig. 5. Some of the possible consequences of a photon-molecule interaction. The lengths of the upward-pointing arrows are proportional to the frequencies of the incoming light while the lengths of the downward-pointing arrows are proportional to the frequency of the scattered (or in the case of fluorescence, emitted) light. The vibrational quantum numbers in the upper and lower electronic states are v and v" respectively. The energy spacing v" between the lower state vibrational levels is equal to From [2].

The principal aim of this study is to explore and to check the possibility of an uniform description of the dynamical properties of excited molecules, ranging from the free diatomic to the large molecular systems. Our attention is then directed to the intermediate case, which have characteristics significantly different from the familiar limiting cases. From this point of view, our presentation may be considered as complementary to the most recent reviews in the field, dealing mainly with systems in the statistical limit (Freed, 1967a Avouris et al., 1977) or with the quantum description of photon-molecule interaction applied to simpler molecular models (Mukamel and Jortner, 1977). 

In a typical experiment, a beam of photon wave packets is emitted from a collimated source. Thereafter the experimenter has no control over the evolution of the system, until the photons scattered by the molecular target reach his detector. In practice, source and detector are located sufficiently far from the target, so that photon-molecule interaction can be neglected during the processes of photon emission and detection. 

Before closing this section, some additional remarks may be useful. We first recall that, for simplicity, only the fc> states defined by Eq. (54) have been considered to derive the form of the density operators for the system before the photon-molecule interaction process. A generalization accounting for nonnegligible initial populations of vibrational sublevels of the molecular ground state is straightforward by making the replacement... 

Fig. 2. Photon molecule interaction processes. (A) Normal Raman scattering, (B) discrete resonance Raman scattering, (C) continuum resonance Raman scattering. All these processes are amenable to direct scattering experiments generally, only (B) can be easily studied by time-resolved observation.

As seen above, the general treatment of the photon-molecule interaction expands the transition operator as a series with electric and magnetic multipole contributions ... 

The main cost of this enlianced time resolution compared to fluorescence upconversion, however, is the aforementioned problem of time ordering of the photons that arrive from the pump and probe pulses. Wlien the probe pulse either precedes or trails the arrival of the pump pulse by a time interval that is significantly longer than the pulse duration, the action of the probe and pump pulses on the populations resident in the various resonant states is nnambiguous. When the pump and probe pulses temporally overlap in tlie sample, however, all possible time orderings of field-molecule interactions contribute to the response and complicate the interpretation. Double-sided Feymuan diagrams, which provide a pictorial view of the density matrix s time evolution under the action of the laser pulses, can be used to detenuine the various contributions to the sample response [125]. 

Ionization cross-section. A measure of the probability that a given ionization process will occur when an atom or molecule interacts with an electron or a photon. 

The interaction between radiation and a mass of gas can be treated as a collision problem. In this case the number of photon-molecule collisions, Nx, in unit length of an absorbing gas is given by... 

As molecular dipoles vibrate, they emit photons which excite vibrations in nearby molecules. In turn, these molecules emit photons which interact with the initiating molecule. In this way, the molecules interact by exchanging photons. Again there are two modes. In one case, the vibrations of the molecules occur in phase with one another. In the second case, they interact out of phase. The energy of the system is lower when the vibrations are in phase, so this case creates attractions between the molecules, while the out-of-phase case creates repulsions. Since the energy of the in phase case is lower, the net effect is attraction. 

While dense IS regions are generally well-shielded against high-energy photons, the interaction between molecular ions and photons is relevant to the question of the survival of such ions in the diffuse interstellar medium, where UV irradiation might be expected to be a powerful destructive force. Such effects are, of course, important also for the fate of neutral molecules in the diffuse interstellar radiation field, but UV photo absorption by molecules of moderate size is often more likely to lead to photoionization (itself an important topic, but not covered herein) than to photodissociation. 

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