Quasi-resonant processes

Doktorov A. B. The impact approximation in the theory of bimolecular quasi-resonant process, Physica A 90, 109-36 (1978). 

E. Effective Hamiltonian for Two-Photon Quasi-Resonant Processes in Atoms ... 

Yet another type of quasi-resonant processes corresponds to the so-called intramultiplet mixing, i.e. to transitions between fine-structure components of... 

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

A) During the luultiphoton excitation of molecular vibrations witli IR lasers, many (typically 10-50) photons are absorbed in a quasi-resonant stepwise process until the absorbed energy is suflFicient to initiate a unimolecular reaction, dissociation, or isomerization, usually in the electronic ground state. 

B) The multiphoton excitation of electronic levels of atoms and molecules with visible or UV radiation generally leads to ionization. The mechanism is generally a combination of direct, Goeppert-Mayer, and quasi-resonant stepwise processes. Since ionization often requires only two or tln-ee photons, this type of multiphoton excitation is used for spectroscopic purposes in combination with mass-spectrometric detection of ions. 

B2.5.351 after multiphoton excitation via the CF stretching vibration at 1070 cm. More than 17 photons are needed to break the C-I bond, a typical value in IR laser chemistry. Contributions from direct absorption (i) are insignificant, so that the process almost exclusively follows the quasi-resonant mechanism (iii), which can be treated by generalized first-order kinetics. As an example, figure B2.5.15 illustrates the fonnation of I atoms (upper trace) during excitation with the pulse sequence of a mode-coupled CO2 laser (lower trace). In addition to the mtensity, /, the fluence, F, of radiation is a very important parameter in IR laser chemistry (and more generally in nuiltiphoton excitation) ... 

Nevertheless, the one-electron approach does have its deHciencies, and we believe that a major theoretical effort must now be devoted to improving on it. This is not only in order to obtain better quantitative results but, perhaps more importantly, to develop a framework which can encompass all types of charge-transfer processes, including Auger and quasi-resonant ones. To do so is likely to require the use of many-electron multi-configurational wavefunctions. There have been some attempts along these lines and we have indicated, in detail, how such a theory might be developed. The few many-electron calculations which have been made do differ qualitatively from the one-electron results for the same systems and, clearly, further calculations on other systems are required. 

Processes that are resonant at zero held (i.e., with a atomic Bohr frequency that is an integer multiple of the laser frequency) can be investigated through an effective Hamiltonian of the model constructed from a multilevel atom driven by a quasi-resonant pulsed and chirped radiation held (referred to as a pump held). If one considers an w-photon process between the considered atomic states 1) and 2) (of respective energy E and Ef), one can construct an effective Hamiltonian with the two dressed states 11 0) (dressed with 0 photon) and 2 —n) (dressed with n photons) coupled by the w-photon Rabi frequency (2(f) (of order n with respect to the held amplitude and that we assume real and positive) and a dynamical Stark shift of the energies. It reads in the two-photon RWA [see Section III.E and the Hamiltonian (190)], where we assume 12 real and positive for simplicity,... 

The adiabatic passage induced by two delayed laser pulses, the well-known process of STIRAP [69], produces a population transfer in A systems (see Fig. 7a). The pump field couples the transition 1-2, and the Stokes field couples the transition 2-3. It is known that, with the initial population in state 11), a complete population transfer is achieved with delayed pulses, either (i) with a so-called counterintuitive temporal sequence (Stokes pulse before pump) for various detunings as identified in Refs. 73 and 74 or (ii) with two-photon resonant (or quasi-resonant) pulses but far from the one-photon resonance with the intermediate state 2), for any pulse sequence (demonstrated in the approximation of adiabatic elimination of the intermediate state [75]). Here we analyze the STIRAP process through the topology of the associated surfaces of eigenenergies as functions of the two field amplitudes. Our results are also valid for ladder and V systems. 

The topological analysis thus shows that with two quasi-resonant delayed lasers it is not possible to end in a superposition of states between the lowest states 1) and 13) in a robust way. We can remark that in Ref. 76, it has been shown that one can create such a superposition—however, in a nonrobust way but still by adiabatic passage, by modifying the end of the STIRAP process (with the counterintuitive sequence), maintaining a fixed ratio of Stokes and pump pulse amplitudes. 

We study processes with two fields of different carrier frequencies coi and (02 which act in resonance (or in quasi-resonance) on the same atomic transition, which are referred to as bichromatic processes. They induce dynamical resonances in the system due to the beat frequency... 

Hence the CO-molecules seem to migrate from their primary places to their final adsorption locations, which may be in the slit-like micropores. Here they often will experience strong electrostatic interactions with dipoles of the active groups of the AC. This will lead to a neutralization of the local dipoles and electric fields. Thus the capacitance spectra decrease again with this diffusion based secondary adsorption process of the CO-molecules. Also the quasi-resonance frequencies of the maxima of these curves are shifted to lower frequencies again. This process was completely reversible and reproducible. Similar processes have been observed with different systems of sorbents and sorptive gases, cp. Sect 3.2 and [6.3, 6.13, 6.33]. 

However, the modulational instability is a quasi-resonant interaction process, i.e., wave numbers and frequencies satisfy the following conditions ... 

This example is one of the many relaxation processes attributed to the col-lisional vibrational-translational energy exchange. Without discussing other processes, we shall only mention those most important for the kinetics of nonequilibrium reactions vibrational relaxation of anharmonic oscillators in a heat bath, relaxation of one- and two-component mixture of harmonic oscillators with resonant and quasi-resonant energy exchange and relaxation of anharmonic oscillators [87, 164, 343, 460] (see Section IV. 15). 

Intermolecular Quasi-Resonant Vibrational Energy Exchange (Intermolecular TV Process)... 

A collision of diatomic or polyatomic molecules also involves, along with VT, VR and VRT processes, the exchange of vibrational energy. When the net vibrational energy change in such processes is small, this is referred to as a quasi-resonant intermolecular VV process. 

A simple model of quasi-resonant VV exchange corresponds to linear collisions of two diatomic molecules simulated by harmonic oscillators. When the Massey parameter cotq is large, only processes with minimum energy consumption or energy release can be taken into account, thus neglecting simultaneous excitation or de-excitation of both oscillators. The first-order perturbation treatment gives the probability of one-quantum exchange [339] in the case of repulsive exponential interaction U(E.) exp ( —a/R)... 

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