Single-photon absorption, molecular photonics

Double transitions. In molecular fluids, simultaneous or double transitions occur at sums and differences of the rotational lines, with the absorption of a single photon. Several of such double transitions have been pointed out above, Figs. 3.32 and 3.37. In general terms, one may say that these occur at sums and differences of the rotovibrational (and/or electronic) transition frequencies of the molecules involved, as was explained in the discussions related to Fig. 1.3. 

When deciding to study the dynamics of electronic excitation energy transfer in molecular systems by conventional spectroscopic techniques (in contrast to those based on non-linear properties such as photon echo spectroscopy) one has the choice between time-resolved fluorescence and transient absorption. This choice is not inconsequential because the two techniques do not necessarily monitor the same populations. Fluorescence is a very sensitive technique, in the sense that single photons can be detected. In contrast to transient absorption, it monitors solely excited state populations this is the reason for our choice. But, when dealing with DNA components whose quantum yield is as low as 10-4, [3,30] such experiments are far from trivial. 

The distinction between the various dissociation schemes (with the exception of multiphoton dissociation) is rather artificial from the formal point of view. Common to direct dissociation, predissociation, and unimolecular decay is the possibility of state-specificity, i.e., the dependence of the dissociation on the quantum state of the parent molecule (Manz and Parmenter 1989). The absorption of a single photon uniquely defines the energy in the dissociative state. As we will demonstrate in subsequent chapters, one can treat all three classes of fragmentation with the same basic theoretical tools. However, the underlying molecular dynamics is quite different demanding different interpretation models. 

Absorption of a photon by the purple iill-trans retinal chro-mophore (with a single broad absorption band with a maximum at 568 nm) initiates the reaction sequence BR-/zv K L Ml M2 N -o- N - O BR (7, 8), where each state and substate is well defined by spectroscopic and crystallographic means. Although a kinetic scheme that rigorously fits all data into a linear sequence has not yet been produced, the proton transport mechanism can be understood by the molecular properties of the intermediate states and by their interconversions. 

The absorption lineshape corresponds to the photon-energy dependence of the rate at which the photon is absorbed by the molecule. We consider absorption under conditions where it is a linear process, that is, where the rate at which the molecular system absorbs energy from the radiation field at frequency co is proportional to the radiation intensity (number of photons) at this frequency. Under such conditions it is enough to consider the rate of absorption from a single photon state and to use the... 

Figure 16. The time-ordered diagrams associated with the formation of an appropriate molecular grating for SHG. The two writing beams r2 and r3 populate the upper electronic state via two- and single-photon absorption respectively.

In the two-frequency distributive case, the molecular tensor y °(a)i,o)2) has resonance conditions similar to those for )- As the single-beam case, two of the proposed resonance conditions would be likely to allow the process to be masked by single-photon absorption a third leads to the possibility of conventional two-photon absorption, and a fourth cannot be satisfied if the centers involved are initially in their ground states. The remaining condition E x E — hu>i) if a>i t02, or E x(E — ho)2) if CO2 < ft)i) remains the only truly useful resonance. Naturally, since the energetics of the excitation process are constrained only by a condition on the sum of the photon frequencies, there is a wide scope for choosing laser frequencies specifically with the aim of exploit ng this type of resonance possibility. 

Infrared Absorption is a single-photon process. Here, also, kiR = K 0 applies. Thus, infrared absorption detects only phonons at the F point of the first BZ. In this case, we have oo = L2, where ho) is the quantum energy of the infrared radiation. The frequencies or the wavenumbers of the optical phonons in molecular crystals are of the order of 3 THz or 100 cm" thus the wavelengths of infrared absorption are of the order of 100 /xm. Infrared spectroscopy of phonons in molecular crystals is therefore in fact far-infrared spectroscopy. The symmetry selection rules are complementary to those for Raman scattering for vibrations with u and g states w g transitions are allowed and g g transitions are forbidden. 

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