One photon process

This expression may be interpreted in a very similar spirit to tliat given above for one-photon processes. Now there is a second interaction with the electric field and the subsequent evolution is taken to be on a third surface, with Hamiltonian H. In general, there is also a second-order interaction with the electric field through which returns a portion of the excited-state amplitude to surface a, with subsequent evolution on surface a. The Feymnan diagram for this second-order interaction is shown in figure Al.6.9. 

In the discussion in Section 9.1.6 of harmonic generation of laser radiation we have seen how the high photon density produced by focusing a laser beam into certain crystalline materials may result in doubling, tripling, etc., of the laser frequency. Similarly, if a laser beam of wavenumber Vl is focused into a cell containing a material which is known to absorb at a wavenumber 2vl in an ordinary one-photon process the laser radiation may be absorbed in a two-photon process provided it is allowed by the relevant selection rules. 

The example we consider is the two-photon fluorescence excitation specfrum of 1,4-difluorobenzene, shown in Figure 9.29 and belonging to the >2 point group. The transition between the ground and first singlet excited state is Table A. 3 2 in Appendix A shows that 82 = r(T ) and, therefore, according to Equation (7.122), the electronic transition is allowed as a one-photon process polarized along the y axis which is in-plane and... 

Figure 9.50 Processes involved in obtaining (a) an ultraviolet photoelectron spectrum, (b) a zero kinetic energy photoelectron (ZEKE-PE) spectrum by a one-photon process and (c) a ZEKE-PE spectrum by a two-photon process in which the first photon is resonant with an excited electronic state of the molecule...

For many years, investigations on the electronic structure of organic radical cations in general, and of polyenes in particular, were dominated by PE spectroscopy which represented by far the most copious source of data on this subject. Consequently, attention was focussed mainly on those excited states of radical ions which can be formed by direct photoionization. However, promotion of electrons into virtual MOs of radical cations is also possible, but as the corresponding excited states cannot be attained by a one-photon process from the neutral molecule they do not manifest themselves in PE spectra. On the other hand, they can be reached by electronic excitation of the radical cations, provided that the corresponding transitions are allowed by electric-dipole selection rules. As will be shown in Section III.C, the description of such states requires an extension of the simple models used in Section n, but before going into this, we would like to discuss them in a qualitative way and give a brief account of experimental techniques used to study them. 

The rate of photobleaching of unisotropic dye molecules in solid polymer matrices has been investigated by Kaminov et al. I65a) bleaching rate is linearly proportional to the intensiy of the incident radiation from an argon laser, indicating a one-photon process. 

Figure 3.18 Schematic representation of transition moment integral for monophotonic and biphotonic transitions in naphthalene. (A) Transition forbidden by one photon process (B) Allowed by two photon process.

The two-photon absorption spectroscopy can overcome the symmetry barrier imposed by the selection rule for angular momenta in the one-photon process. Thus, the technique is able to identify and assign molecular and atomic states which are not accessable to one-photon spectroscopy. 

This equation is, of course, well known and often called the Pauli equation. We recognize on the right-hand side the familiar gain and loss terms. The transition probabilities which appear in the Pauli equation correspond to the Born approximation for one-photon processes. For further reference let us summarize the main properties of this weakly coupled approximation. 

The first NeNePo experiments dealt with silver clusters, Ag3, Ags, Ag7, and Ag9, particularly with the first of these. The photodetachment and photoionization were done with a single titanium-sapphire laser producing pulses of approximately 60 fs duration. Doubled in frequency, these could be tuned over a wavelength span from above 420 to below 390 nm. As with the dimer, photodetachment was a one-photon process and photoionization a two-pho-ton process. (The clusters of odd numbers of atoms could be studied this way the even-numbered clusters require at least three photons in the available energy range for photoionization). The interval between pulses could be varied from zero (simultaneous pulses) to 100 ps the two pulses were made to differ in intensity by about a factor of 2, and either could be the leading pulse. 

That the observed resonances for m 0 become narrower and more symmetric as the power is increased can be understood in the following way, using the one-photon process as an example. The coupling matrix element of Eq. (15.2) has two... 

Photodissociation spectra were obtained by monitoring the appearance of ionic photoproducts as a function of the wavelength of light. Shot-to-shot variation of the lasergenerated metal precursor ions made monitoring the photodisappearance of the parent ion impractical. Assuming a one-photon process [17 29]- the photodissociation of AB+, eq. 1, can be described by first-order kinetics, eq. 2,... 

Fig. 3.16) to control product directionality in HD+ dissociation to II -)-1)+ a H+ + D. Here a combination of a one-photon process, induced by the secuiicll , harmonic, and a two-photon process, induced by the fundamental frequency, jwcny used to excite the molecule to a repulsive 2pa state, yielding either the H +1) 01 ... 

Desorption from on-top species Buntin et al. [6] carried out observations on NO desorption from NO-saturated Pt(l 1 1) surfaces in detail, using X = 1064 (hoy = 1.17), 352 (2.33), and 355 nm (3.49 eY) at surface temperatures of 117 and 220 K. State-selective detection using the LIF method combined with the TOF measurement were used. In the present paragraph, the results observed at 220 K are described, because of the desorption from on-top species. Desorption occurs by the one-photon process, for which the desorption cross section is smaller at X = 1064 nm than those at X = 532 and 355 nm. Thus, the threshold energy for the NO desorption is regarded as <1.2 eV. 

Absorption of radiation is a one photon process. Absorption of one photon excites one atom or molecule in primary (initiating) step and all subsequent physical and chemical reactions follow from this excited species... 

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. 

Figure 18.1 One and multi-photcHi transitions one-photon process (a), two-photon single step process (b), three-photcHi single step process (c) three-photon two steps process (cQ...

Fig. 16. Photoreactions at low temperatures. The notation corresponds to the optical absorption lines of the reaction intermediates. An intermolecular chain termination reaction is assumed. The dimer initiation reaction requires two photons (hv). The photoaddition reaction is a one photon process (hv or hv ). The chain termination reactions are most effectively performed by resonant irradiation into the absorption of the DR or AC intermediates...

Perhaps the most promising materials for all-optical write-read-erase switches are systems in which ultraviolet irradiation induces ring opening. One of the most studied systems are spiropyrans in which writing and nondestructive reading are based on successive two-photon and one-photon processes [46, 205 209]. 

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