Photofragmentation

Photodissociation and photoionization are two aspects of a more general process photofragmentation (Greene and Zare (1982)). Photodissociation of a diatomic molecule 

The properties measured in photofragmentation experiments (photodissociation or photoionization) may be divided into scalar quantities, such as photofragmentation cross sections and branching ratios, and vectorial quantities, such as angular distributions of the photofragments and their alignment or orientation. Moreover, these properties may be measured independently or in coincidence where two or more properties associated with either or both photofragmentation products are measured simultaneously, event-by-event. 

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Kulander K C and Heller E J 1978 Time-dependent formulation of polyatomic photofragmentation application to Hj" J. Chem. Rhys. 69 2439... 

Heller E J 1978 Photofragmentation of symmetric triatomic molecules Time dependent pictured. Chem. Phys. 68 3891... 

Lee Y-Y, Leone S R, Champkin P, Kaltoyannis N and Price S D 1997 Laser photofragmentation and coiiision-induced reactions of sit and siH - Chem. Phys. 106 7981-94... 

Balint-Kurti G G, Dixon R N and Marston C C 1992 Grid methods for solving the Schrodinger equation and time-dependent quantum dynamics of molecular photofragmentation and reactive scattering processes/of. Rev. Phys. Chem. 11 317—44... 

King R A, Allen W D and Schaefer H F III 2000 On apparent quantized transition-state thresholds in the photofragmentation of acetaldehyde J. Chem. Phys. 112 5585-92... 

Faraday Discuss. Chem.. Soc. 1986 Dynamics of molecular photofragmentation. No 82 Faraday Discuss. Chem. Soc. 1995 Unimolecular dynamics. No 112... 

Leone S R 1989 Time-resolved FTIR emission studies of molecular photofragmentation Accounts Chem. Res. 22 139-44... 

Quack M, Sutcliffe E, Hackett P A and Rayner D M 1986 Molecular photofragmentation with many infrared photons. Absolute rate parameters from quantum dynamics, statistical mechanics, and direct measurement Faraday Discuss. Chem. Soc. 82 229-40... 

Tiggesbaumker J, Koller L, Lutz FI O and Meiwes-Broer K FI 1992 Giant resonanoes in silver-oluster photofragmentation Chem. Phys. Lett. 190 42... 

Bloomdield L A, Freeman R R and Brown W L 1985 Photofragmentation of mass-resolved Si jOlusters Phys. Rev. Lett. 54 2246... 

Miohaels C A, Tapalian C, Lin Z, Sevy E and Flynn G W 1995 Superoollisions, photofragmentation and energy transfer in mixtures of pyrazine and oarbon dioxide Faraday Discuss. 102 405-22... 

The alkyl ethers of benzoin undergo dkect photofragmentation upon absorption of uv energy at ca 360 nm to produce two free-radical intermediates. 

Other siinjlai structures undergo the following photofragmentation reartangement decomposition processes ... 

Photofragmentation of chloromethyldiazirine in the presence of several gases was assumed to involve C—N bond cleavage to form a diradical, which undergoes a second C—N bond cleavage to form the carbene (8OMI508OI). 

Doubt (75ZN(B)822) has been cast on a number of claims for the formation of 2-azetin-4-ones from cycloaddition of activated isocyanates to acetylenes (70TL119). The simple 2-azetin-4-one (246) was not isolated or even detected directly at -50 °C in the photofragmentation of compound (245), but indirect evidence for its formation was the isolation of adducts (248 X = MeO, MeNH) in the presence of methanol or methylamine (75TL1335). The most convincing evidence for an isolable 2-azetin-4-one involves treatment of the... 

Singlet phenylnitrene, and hence /V,A -diethyl-3//-azcpin-2-amines, e. g. 102, can be generated by the thermolysis of A,-phenyl-Af,<9-bis(trimcthylsi]yl)hydroxylamine (100) in the presence of dialkylamines the reaction fails, however, with arylamines.210 Photofragmentation of the spiro oxaziridine 101 in diethylamine solution also produces the 3//-azepine 102,2,1 and an oxaziridine intermediate is probably involved in the formation, in low yield (1 %), of azepine 102 by the photolysis of A/,A( -diarylbenzoquinonc diimine A/,A/ -dioxides in benzene/die-thylamine solution.212... 

To determine molecular motions in real time necessitates the application of a time-ordered sequence of (at least) two ultrafast laser pulses to a molecular sample the first pulse provides the starting trigger to initiate a particular process, the break-up of a molecule, for example whilst the second pulse, time-delayed with respect to the first, probes the molecular evolution as a function of time. For isolated molecules in the gas phase, this approach was pioneered by the 1999 Nobel Laureate, A. H. Zewail of the California Institute of Technology. The nature of what is involved is most readily appreciated through an application, illustrated here for the photofragmentation of iodine bromide (IBr). 

A. M. Wodtke and Y. T. Lee, High resolution photofragmentation - Translational spectroscopy , in Molecular Photodissociation Dynamics, eds. M. N. R. Ashfold and J. E. Baggott, Royal Society of Chemistry, London, 1987. 

In the case of 22b-e, the butadienes 24b-d and the hexatriene 24 e, respectively, are also obtained on trapping with cinnamaldehyde, the 1,3-diene 24a is even the sole reaction product. It is quite obvious that the olefins 24 are secondary products of the trapping reaction of 9 arising by photofragmentation of 22. The other product is phenyldioxophosphorane (23) which also numbers among the short-lived compounds of quinquevalent phosphorus with coordination number 3 (see Sect. 3.1). 

The fact that only the vinyl-substituted l,2X5-oxaphosphetanes 22 but not the arylated phosphorus heterocycles 21 undergo photofragmentations is presumably due to the inability of the latter to absorb the light (X > 280 nm) supplied for carbene formation (7- 8) [e.g. 21, Ar = C6H5 e280 200 22b, d e2g0 9000 (in methanol)]I8,20). 

In most cases the photofragmentations were also carried out with the corresponding deuterated isotopomers (see indications in the formula). By this means the structural identification of the new species by comparison between the experimental and calculated spectra became much more reliable. Furthermore it was again possible to register the bands of "complexed" deuterium molecules [16] [23-D2 v = 2994 cm"1 24-D2 v = 3010 / 3005 cm"1]. 

Photofragmentation of Transition Metal Cluster Complexes in the Gas Phase... 

The gas phase photofragmentation of transition metal cluster complexes is discussed. The information available for the gas phase dissociation of... 

CO)1q, Co CCOlpCCHj and 0200113)4 is compared to the well established data base on cluster compounds in condensed phase. The gas phase results are discussed in light of the predictions of electronic structure theory concerning the photofragmentation of these cluster compounds. 

Presently, the gas phase photofragmentation of several transition metal cluster complexes is reviewed. The techniques employed for these gas phase studies rely on sensitive ionization detection and the use of a broad range of excitation energies. 

The information available is discussed in light of the effects of excitation energy and the environment on the photofragmentation process of several transition metal cluster complexes. The photochemical information provides a data base directly relevant to electronic structure theories currently used to understand and predict properties of transition metal complexes (1,18,19). 

In this paper, the photofragmentation of transition metal cluster complexes is discussed. The experimental information presented concerning the gas phase photodissociation of transition metal cluster complexes comes from laser photolysis followed by detection of fragments by ionization (5.). Ion counting techniques are used for detection because they are extremely sensitive and therefore suitable for the study of molecules with very low vapor pressures (6.26.27). In addition, ionization techniques allow the use of mass spectrometry for unambiguous identification of signal carriers. 

Mi CO). The first metal-metal bond to be characterized (35) is the formally single Mn-Mn bond in Mi CO). This compound has often been used as the model for developing electronic structure theories (1.18.36.37). Extremely efficient photofragmentation is responsible for the structureless electronic spectrum and the lack of emission following excitation of this molecule. This spectroscopic deficiency necessitates photofragmentation studies to obtain data to verify theoretical models. Most of the photochemical experiments in the past explored the reactions of the lowest excited singlet state in the near ultraviolet. 

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