Photoionization reactions

An example of one photon-three photon continuous-wave (CW) interference control of the product distribution in the branching photofragmentation and photoionization reactions... 

The photoionization of a molecule to yield an electron and an unfragmented ion may be considered to be the simplest of all photodissociation reactions, and therefore also one of the simplest of the radiationless processes in an isolated molecule. In addition, because the products are charged, a combination of mass spectrometric and photometric data yields information about photoionization reactions not now available for molecular fragmentation reactions. For example, the reaction cross sections for generation of specific charged products and the total photon absorption cross section may be measured and compared, thereby yielding the residual cross section corresponding to radiationless processes other than photoionization. From this information we can deduce some of the consequences of the competition between several radiationless processes in an isolated molecule. 

Interpreting Data Why are photodissociation and photoionization reactions more common in the upper atmosphere than in the lower atmosphere ... 

The intermediacy of a species analogous to (i) (Scheme 1.7) that some described as a resonance hybrid of a radical pair and an ion pair and others as an ultrafast equilibrium between radical and ion pair has been proposed in the photoionization reaction of benzyl derivatives [172, 174-176]. Also, interestingly, to explain the driving force behind the rotational motion that leads to an extremely rapid deactivation of S of TAM cations, Vogel and Rettig [177] proposed that SI promptly forms a biradicaloid charge-transfer state. 

TABLE 18.3 Photoionization Reactions for Four Components of the Atmosphere... 

Investigating Ion-Molecule Reactions by Analyzing Neutral Products Formed in the Radiolysis and Photoionization of Hydrocarbons... 

Here we present new data on various ion-molecule reactions occurring in the radiolysis and photoionization of hydrocarbons. We will also review some earlier findings which will illustrate the kind of information which can be derived from the approach mentioned above. 

Our experimental techniques have been described extensively in earlier papers (2, 13). The gamma ray irradiations were carried out in a 50,000-curie source located at the bottom of a pool. The photoionization experiments were carried out by krypton and argon resonance lamps of high purity. The krypton resonance lamp was provided with a CaF2 window which transmits only the 1236 A. (10 e.v.) line while the radiation from the argon resonance lamp passed through a thin ( 0.3 mm.) LiF window. In the latter case, the resonance lines at 1067 and 1048 A. are transmitted. The intensity of 1048-A. line was about 75% of that of the 1067-A. line. The number of ions produced in both the radiolysis and photoionization experiments was determined by measuring the saturation current across two electrodes. In the radiolysis, the outer wall of a cylindrical stainless steel reaction vessel served as a cathode while a centrally located rod was used as anode. The photoionization apparatus was provided with two parallel plate nickel electrodes which were located at equal distances from the window of the resonance lamp. 

On the other hand, the formation of ethylene was ascribed mainly to the unimolecular decomposition of a neutral excited propane molecule. These interpretations were later confirmed (4) by examining the effect of an applied electrical field on the neutral products in the radiolysis of propane. The yields of those products which were originally ascribed to ion-molecule reactions remained unchanged when the field strength was increased in the saturation current region while the yields of hydrocarbon products, which were ascribed to the decomposition of neutral excited propane molecules, increased several fold because of increased excitation by electron impact. In various recent radiolysis 14,17,18,34) and photoionization studies 26) of hydrocarbons, the origins of products from ion-molecule reactions or neutral excited molecule decompositions have been determined using the applied field technique. However, because of recent advances in vacuum ultraviolet photolysis and ion-molecule reaction kinetics, the technique used in the above studies has become somewhat superfluous. 

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

Methane-to-methanol conversion by gas-phase transition metal oxide cations has been extensively studied by experiment and theory see reviews by Schroder, Schwarz, and co-workers [18, 23, 134, 135] and by Metz [25, 136]. We have used photofragment spectroscopy to study the electronic spectroscopy of FeO" " [47, 137], NiO [25], and PtO [68], as well as the electronic and vibrational spectroscopy of intermediates of the FeO - - CH4 reaction. [45, 136] We have also used photoionization of FeO to characterize low lying, low spin electronic states of FeO [39]. Our results on the iron-containing molecules are presented in this section. 

Photoionization can also access excited electronic states of the ion that are difficult to study by optical methods. The photoionization yield of FeO increases dramatically 0.36 eV above the ionzation energy. This result corresponds to the threshold for producing low spin quartet states of FeO. These states had not been previously observed, as transitions to them are spin forbidden and occur at inconveniently low energy. Because the FeO + CH4 reaction occurs via low spin intermediates, accurately predicting the energies of high and low spin states is critical. 

The cluster reactor is attached to the pulsed cluster source s condensation channel, as shown in Figure 6. (16) To it is attached a high-pressure nozzle from which a helium/hydrocarbon mixture is pulsed into the reactor at a time selected with respect to the production and arrival of the clusters. The effect of turbulent mixing with the reactant pulse perturbs the beam, but clusters and reaction products which survive the travel from the source to the photoionization regime ( 600y sec) and the photoionization process are easily detected. 

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