One-photon absorption and emission

All nonlinear (electric field) spectroscopies are to be found in all temis of equation (B 1.3.1) except for the first. The latter exclusively accounts for the standard linear spectroscopies—one-photon absorption and emission (Class I) and linear dispersion (Class II). For example, the temi at third order contains by far the majority of the modem Raman spectroscopies (table B 1.3.1 and tableBl.3.2). 

Figure 1. One-exciton band (energy vs wave vector) for a 15-srte lattice described by the Hamiltonian in Eq. (1), with only nearest-neighbor exciton hopping, 7, and constant intermolecular spacing, r = 1. Left and right panels refer to negative and positive 7, respectively. The arrows mark one-photon absorption and emission processes. The emission process for 7 > 0 is forbidden...

Up to now, we have been primarily concerned with one-photon absorption and emission processes, whose probability amplitudes are given by the first-order term... 

This chapter is devoted to computational spectroscopy approaches for the study of one-photon absorption and emission (OPA and OPE, respectively) spectra in solution by means of focused models [1-4] coupling a quantum mechanical (QM) description of the solute (mostly by methods rooted into the density... 

Computation of vibronic spectra within time-independent and time-dependent approaches are described in more detail in Ref [270, 271] and references therein. Here, we mainly refer to the integrated procedure set within the Born-Oppenheimer and harmonic approximations (along with Eckart conditions), which is described in detail in Refs. [133, 272, 273]. This approach can be applied to one-photon absorption and emission (OPA/OPE), electronic circular... 

Returning to the kinetie equations that govern the time evolution of the populations of two levels eonneeted by photon absorption and emission, and adding in the term needed for spontaneous emission, one finds (with the initial level being of the lower energy) ... 

Ordinary STIRAP is only sensitive to the energy levels and the magnitudes of transition-dipole coupling matrix elements between them. These quantities are identical for enantiomers. Its insensitivity to the phase of the transition-dipole matrix elements renders STIRAP incapable of selecting between enantiomers. Recently we have demonstrated [11] that precisely the lack of inversion center, which characterizes chiral molecules, allows us to combine the weak-field one-and two-photon interference control method [29,54,95,96] with, the strong-field STIRAP to render a phase-sensitive AP method. In this method, which we termed cyclic population transfer (CPT), one forms a STIRAP loop by supplementing the usual STIRAP 1) o 2) <=> 3) two-photon process by a one-photon process 1) <=> 3). The lack of inversion center is essentrat, because one-photon and two-photon processes cannot connect the same states in the presence of an inversion center, where all states have a well defined parity, because a one-photon absorption (or emission) between states 1) and 3) requires that these states have opposite parities, whereas a two-photon process requires that these states have the same parity. 

FITC+FAM (fluorescein derivatives). ECFP, EGFP, EYFP (enhanced—cyan, green and yellow fluorescent protein, respectively, Clontech Laboratories, Inc., USA). Bodipy is a trademark of Invitrogen Ltd., UK. Attoxxxis a trademark of ATID-TEC GmbH, Gennany. A A S, and A J), A are the one- and two-photon absorption and emission maximum, respectively. QY is the fluorescence quantum yield, SS the Stokes shift, e the molar extinction coefficient and Tf the fluorescence lifetime. Further information regarding two-photon measurements can be found In Chapter 3. 

We begin our discussion with an ensemble of identical two-level systems in which the upper and lower state populations are N2 and Ni, respectively. The energy levels are spaced by AE = hv, and the systems are at thermal equilibrium with a radiation energy distribution over light frequencies v given by p(v). It is assumed that only three mechanisms exist for transferring systems between levels 1 and 2 one-photon absorption, spontaneous emission (radiation of a single photon), and stimulated emission (Fig. 8.3). In the latter process, a photon... 

Fluorescence is a process that occurs after excitation of a molecule with light. It involves transitions of the outermost electrons between different electronic states of the molecule, resulting in emission of a photon of lower energy than the previously absorbed photon. This is represented in the Jablonski diagram (see Fig. 6.1). As every molecule has different energy levels, the fluorescent properties vary from one fluorophore to the other. The main characteristics of a fluorescent dye are absorption and emission wavelengths, extinction... 

What has been presented here is a semiclassical theory of TJ 1) quantum electrodynamics. Here the electromagnetic field is treated in a purely classical manner, but where the electromagnetic potential has been normalized to include one photon per some unit volume. Here the absorption and emission of a photon is treated in a purely perturbative manner. Further, the field normalization is done so that each unit volume contains the equivalent of n photons and that the energy is computed accordingly. However, this is not a complete theory, for it is known that the transition probability is proportional to n + 1. So the semiclassical theory is only appropriate when the number of photons is comparatively large. 

The first panel of Figure 5.12 shows the bichromatic control scenario. The sec panel shows the simplest path to the continuum, consisting of one-photon absorpt of CO]. The subsequent panels show the three-photon process to the contir (absorption of a> followed by stimulated emission and reabsorption of coj, ctc ... 

Figure 5.12 Interfering pathways from Et) to the continuum associated with the scenario in Figure 5.11. The frequency and phase of the lasers are co, and (a) Bichromatic control, (b) One-photon absorption, (c) Three-photon process in which initially unpopulated state Ej) is coupled to the continuum at energy E and interferes with one-photon absorption from state ] ,). (d) Same as in (c) but for a five-photon process. Notice that in processes depicted in (c) and (d) the phase

Photoexcitation by photon absorption and subsequent events that lead from one to another state of a molecular entity through radiation and radiationless transitions without any chemical change are called photophysical processes. The processes are classified as radiative and radiationless ones, depending on the photon emission (or absorption) and energy loss without any photon emission according to the kinetic aspects the monomolecular (spontaneous) and bimolecular (quenched) processes are distinguished (see Figure 4.1). 

In the case of NMR spectroscopy we will be concerned only with absorption and emission of rf radiation. Quantum mechanics, the field of physics that deals with energy at the microscopic (atomic) level, allows us to define selection rules that describe the probability for a photon to be absorbed or emitted under a given set of circumstances. But even classical (i.e., pre-quantum-mechanical) physics tells us there is one requirement shared by all forms of absorption and emission spectroscopy For a particle to absorb (or emit) a photon, the particle itself must first be in some sort of uniform periodic motion with a characteristic fixed frequency. Most important, the frequency of that motion must exactly match the frequency of the absorbed (or emitted) photon ... 

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