Single-photon dissociation

Owing to the symmetry property of an optical dipole transition, the data analysis for a photodissociation study is greatly simplified. The center-of-mass differential cross-section for a single-photon, dissociative process can be expressed as38,39... 

Laser-induced decomposition of mixtures of polychlorinated biphenyls in the liquid phase has been investigated, employing radiation from three different excimer lasers (XeCl at 308 nm, KrF at 248 nm and ArF at 193 nm)475. The mixtures can be quantitatively and efficiently destroyed by means of UV radiation at 248 nm. A single-photon dissociation process, which leads to both HC1 elimination and biphenyl bond rupture, is induced by the KrF laser radiation. 

In the following section, two types of photodissociation will be discussed single-photon dissociation and VUV multiphoton dissociation. The examples given are those with which we have worked and so is not a complete list of photodissociation reactions for which internal energies of photofragments have been determined. Several excellent reviews of a more general nature are available in the literature. ... 

NH3. The single photon dissociation of NH3 between 185 and 214 nm has been studied extensively and has been shown to result in predominantly NH2(X 5,)+H. DiStefano et al. have shown that photodissociation of NH3 using a hydrogen discharge lamp results in a weak visible emission which they have assigned as NH2( ... 

As shown in Figure 33a, the S intensity resulting from the 2 -t- 1 REMPI of S( P2) in the 193-nm photodissociation laser F range of (0.5-10) X 10 photons/cm is linear, indicating that the formation of S atoms from thiophene is the result of single photon dissociation events. The S intensity due to 2 -I- 1 REMPI of S( D) is very low. Figure 33b depicts... 

This experiment involves a one-color, three-photon process. That is, the SH and CH3S radicals are first prepared by the single-photon dissociation of HjS, CH3SCH3, and CH3SSCH3 in the wavelength range of 235-240... 

The ionization of ammonia clusters (i.e. multiphoton ionization,33,35,43,70,71 single photon ionization,72-74 electron impact ionization,75 etc.) mainly leads to formation of protonated clusters. For some years there has been a debate about the mechanism of formation of protonated clusters under resonance-enhanced multiphoton ionization conditions, especially regarding the possible alternative sequences of absorption, dissociation, and ionization. Two alternative mechanisms63,64,76,77 have been proposed absorption-ionization-dissociation (AID) and absorption-dissociation-ionization (ADI) mechanisms see Figure 5. 

Photoionization ti me-of-fli ght mass spectrometry is almost exclusively the method used in chemical reaction studies. The mass spectrometers, detectors and electronics are almost identical. A major distinction is the choice of ionizing frequency and intensity. For many stable molecules multi photon ionization allowed for almost unit detection efficiency with controllable fragmentation(20). For cluster systems this has been more difficult because high laser intensities generally cause extensive dissociation of neutrals and ions(21). This has forced the use of single photon ionization. This works very well for low i oni zati on potential metals ( < 7.87 eV) if the intensity is kept fairly low. In fact for most systems the ionizing laser must be attenuated. A few very small... 

The transition to the C B- state of H 0 was achieved by a two photon absorption of KrF laser light near 248 nm (32). The OH(A-X) fluorescence excitation spectrum in the 247.9-248.5 nm range follows the rotational structure of the C B -+ X A transition. However, (i) the OH(A-X) fluorescence spectrum produced by the two photon dissociation of H 0 has a maximum population at N = 14, while single photon absorption near 124 nm generates OH fluorescence spectrum with a maximum population at N = 20 (ii) only absorption to Ka= 1 (and not Ka= 0 where K is the rotational angular momentum about the a axis) of the ClB-L state predissociates into 0H(a2%) + H probably through the B A state. Apparently, the two-photon absorption near 248 nm predominantly populates the c b state, while the single photon process populates the B A near 124 nm. 

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. 

In dilute solutions,. /y = 0=1—, the only possible reaction is the irreversible dissociation of C whose kinetics R(t) = Pe(t) was studied in Section V.D. As was shown there, the long-time behavior of this quantity obeys the power law (3.249). This asymptotic expression represents the very end of the kinetics when, for instance, Pc(f) starting from Pc(0) = 1 is already three or four orders of magnitude smaller. This tail is hardly available for detection even with contemporary single-photon counting. Besides, the question arises as to what the difference is in the precursor time evolution of /y (V) or /y (t) predicted using a number of different theoretical methods. 

TRCIS was first applied to dissociative multiphoton ionization of NO2 at 375.3 nm [129]. This was identified as a three-photon transition to arepulsive surface correlating with NO(C2II) + 0(3P) fragments. The NO(C) was subsequently ionizing by a single photon, yielding NO+ (X1 S+) + e. ... 

Resonantly enhanced two-photon dissociation of Na2 from a bound state of the. ground electronic state occurs [202] by initial excitation to an excited intermediate bound state Em,Jm, Mm). The latter is a superposition of states of the A1 1+ and b3Il electronic curves, a consequence of spin-orbit coupling. The continuum states reached in the two-photon excitation can have either a singlet or a triplet character, but, despite the multitude of electronic states involved in the computation reported J below, the predominant contributions to the products Na(3s) + Na(3p) and Na(3s) + Na(4s) are found to come from the 1 flg and 3 + electronic states, respectively. The resonant character of the two-photon excitation allows tire selection of a Single initial state from a thermal ensemble here results for vt = Ji — 0, where vt,./, denote the vibrational and rotational quantum numbers of the initial state, are stJjseussed. 

Section 8.1 introduces general rules under which this type of control is possible, in Section 8.2, we consider the two-photon dissociation of a single quantum tate of a B-A-B molecule to yield BA+B and B+AB, where B and B are Qantiomers. We demonstrate two results (1) ordinary photodissociation of the BAB molecule with linearly polarized light yields identical cross sections... 

Bocarsly, Pfennig and co workers reported interesting multi electron photoreac tions for the trimeric M" Ptlv M" complexes 25a-c.212 215 In this system, a single photon excitation into the intervalence charge transfer band results dissociation of the trimer into [Pt(NH3)4]2+ and two equivalents of a M111. The initial photoexcited complex is though to dissociate first to a Mm complex and Ptni-Mn intermediate. The latter dimer subsequently undergoes a thermal electron transfer reaction to yield the final products. 

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