Excitation of molecules

Faubel M and Toennies J P 1977 Scattering studies of rotational and vibrational excitation of molecules Adv. Atom. Mol. Phys. 13 229... 

Photosensitization is an important alternative to direct excitation of molecules, and this method of excitation usually results in reactions that occur via triplet excited states. If... 

Vibrational and Electronic Excitation of Molecules by Short-Pulse Strong Laser Fields... 

The vibrational and electronic excitation of molecules has received less attention over the years, but the understanding of excitation processes is important for a number of reasons. Thus, in this paper, we will review the various mechanisms leading to vibrational and/or electronic excitation of molecules in strong laser fields. As we do this, it will become clear that exploring these mechanisms (1) reveals new features of the strong field interaction ... 

The simplest and first observation of the excitation of molecules by strong laser fields was the identification with TOF spectroscopy of the following dissociation channel —> I2+ + I°+ [31]. In this example, a short laser... 

The main intermolecular photophysical processes responsible for de-excitation of molecules are presented in Table 4.1. It is interesting to note that most of them... 

According to the model, a perturbation at one site is transmitted to all the other sites, but the key point is that the propagation occurs via all the other molecules as a collective process as if all the molecules were connected by a network of springs. It can be seen that the model stresses the concept, already discussed above, that chemical processes at high pressure cannot be simply considered mono- or bimolecular processes. The response function X representing the collective excitations of molecules in the lattice may be viewed as an effective mechanical susceptibility of a reaction cavity subjected to the mechanical perturbation produced by a chemical reaction. It can be related to measurable properties such as elastic constants, phonon frequencies, and Debye-Waller factors and therefore can in principle be obtained from the knowledge of the crystal structure of the system of interest. A perturbation of chemical nature introduced at one site in the crystal (product molecules of a reactive process, ionized or excited host molecules, etc.) acts on all the surrounding molecules with a distribution of forces in the reaction cavity that can be described as a chemical pressure. 

All the above-mentioned experiments dealt with vibrational excitation of molecules by infrared laser lines. Inelastic collision processes in excited electronic states of molecules can be investigated in a similar way by means of visible or ultraviolet laserlines. 

Electronic excitation of molecules lead to a drastic change of their reactivities. One effect of the excitation is the powerfiil change of the redox properties, a phenomenon which may lead to photoinduced electron transfer (PET) [1-4] The electron-donating as well as the electron-accepting behavior of the excited species are approximately enhanced by excitation energy. This can be explained by means of a simple orbital scheme. By excitation of either the electron donor (D) or the acceptor (A) of a given pair of molecules, the former thermodynamically unfavorable electron transfer process becomes exergonic (A et) (Scheme 1). 

The electron excitation of molecules in nearly all classes of organic compounds can be performed by visible or ultraviolet light (lex > 190 nm, < 6.5 eV). There are a few ex-... 

Excitations of molecules with femtosecond laser pulses lead to excited-state matter wave packets coherently, launching them with such well-defined spatial resolution and coherence in nuclear motions that they evolve like single-molecule trajectories. Both electronically excited and vibrationally excited ground-state species may be studied. The structural change versus time profile of a reaction turns out to be compatible with classical modes of thinking. 

The wavelength of the emitted light depends on the choice of atoms A and B. Besides the principle of excimer formation, the technical parameters for the lamp and discharge are responsible for the quasi-monochromatic character of the emitted spectrum. The most important commercial excimers are formed by electronic excitation of molecules of rare gases (He2, Ne2, Ar2, Kr2, Xe2),... 

Besides these primary reactions, various secondary reactions take place in which ions or excited molecules participate. The final resulf of these three events is that, through the diverse primary and secondary fragmentations, radicals are formed and fhe complefe cascade of reactions triggered by the primary excitation of molecules may take up to several seconds. The energy deposited does not always cause change in the precise position where it was originally deposited, and it can migrate and affect the product yield considerably. 

VMP, a term coined by Crim and coworkers in 1987 [26], has been applied extensively by him and others since then [27-33], However, some VMP studies were carried out even earlier (see Section 2.3). As mentioned earlier, direct photodissociation, Figure 2.1a, involves electronic excitation of vibrationless ground state parent molecules to an excited PES, while in VMP, Figure 2.1b, excitation of molecules initially prepared in a particular vibrational state is involved. Therefore, in VMP, two- or three-color photons are required to induce a double resonant... 

Femtosecond pulse trains offer another control knob to rotational excitation of molecules, namely, the control of its directionality. By applying a series of laser pulses, linearly polarized at an angle with respect to one another, either clockwise or counterclockwise rotation can be initiated in a molecular ensemble [27]. The effect has been experimentally demonstrated with a sequence of only two pulses [28], and longer pulse trains [29, 30]. The latter were dubbed chiral because of the directional rotation of light polarization from pulse to pulse shown in Figure 10.3a. Chiral pulse trains enable control of the sense of molecular rotation... 

S. Zhdanovich, C. Bloomquist, J. Floss, I. Sh. Averbukh, J. W. Hepburn, and V. Milner. Quantum resonances in selective rotational excitation of molecules with a sequence of ultrashort laser pulses. Phys. Rev. Lett., 109(4) 043003 (2012). 

The rate constant for excitation of molecule C to an energy greater than or equal to e (= nhv) is... 

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