Excited iodine atoms

As an example, we mention the detection of iodine atoms in their P3/2 ground state with a 3 + 2 multiphoton ionization process at a laser wavelength of 474.3 run. Excited iodine atoms ( Pi/2) can also be detected selectively as the resonance condition is reached at a different laser wavelength of 477.7 run. As an example, figure B2.5.17 hows REMPI iodine atom detection after IR laser photolysis of CF I. This pump-probe experiment involves two, delayed, laser pulses, with a 200 ns IR photolysis pulse and a 10 ns probe pulse, which detects iodine atoms at different times during and after the photolysis pulse. This experiment illustrates a frindamental problem of product detection by multiphoton ionization with its high intensity, the short-wavelength probe laser radiation alone can photolyse the... 

Early work by Rollefson and Booher1 in 1931 and by Goodeve and Taylor2 in 1936 established that gaseons hydrogen iodide displays an absorption continuum from 2000 to 4000 A with a maximum at about 2200 A. It was recognised that at wavelengths below 3100 A excited iodine atoms would be produced and that... 

The proportions of ground-state and (2P ) excited iodine atoms produced in a photolysis using monochromatic radiation can be approximately calculated from Fig. 1. To conserve momentum, essentially all the energy from the primary process in excess of that used in bond dissociation (HI -> H+I) =... 

A very interesting device regarding high output power is the photochemical iodine laser 411) where excited iodine atoms are formed by photodissociation of CH3I. Pulses with 5 MW peak power and 5 nsec duration have been produced which could be further amplified in two stages up to 10 W. Energies of 1 K Joule seem to be attainable 412). 

The rates of reaction of the electronically excited iodine atoms have been compared directly with a number of analogous reactions of the ground state atoms in the following manner. The equilibrium constants K2a, K21, and K22 for ground state reactants and products (since the small Boltzmann populations of excited states may be neglected) in the following processes... 

Agl (g). Jellinek and Rudat2 computed the heat of vaporization from vapor pressure data. From the spectroscopic absorption limit and the assumption that the products of dissociation are a normal silver atom and an excited iodine atom, Franck and Kuhn1 computed a value for the energy of dissociation of gaseous silver iodide which leads to the value Qf= —38 for Agl (g). 

From the spectroscopic side it may be concluded that the same interpretation of the continuous spectrum exhibited by hydrogen-iodide may be adopted as was proposed for non-polar molecules that gaseous hydrogen-iodide dissociates in a single and elementary act after absorption of radiation into a normal hydrogen atom and an excited iodine atom. 

The excited iodine atom produced in reaction (18a) absorbs one or two photons to yield the I+ ion. The Xe pressure in the third harmonic cell is adjusted so that the one-and three-photon signals are approximately equal. Variation of the H2 pressure in the phase-tuning cell produces the sinusoidal variation of the ion signals shown in Fig. 6. Evident in this figure is a phase lag of 150° between the two products, HI+ and I. Also shown is modulation of the signal produced by photoionization of H2S, which provides a reference phase for the HI+ and I+ signals. 

Further kinetic experiments, using electronically excited iodine atoms prepared as described above, have been reported by Hathorn and Husain . They in-... 

The lack of laser action in the photolysis of isopropyl iodide raises intriguing questions. As Husain and Donovan point out, this does not necessarily indicate the absence of population inversion, since under the laser experimental conditions there could instead be an insufficient absolute concentration of I atoms. Spectroscopic studies show that excited iodine atoms are produced from isopropyl iodide photodissociation, but at lower relative concentrations than for n-propyl iodide under similar conditions. Since the two propyl iodides show similar I quenching rates, it would appear most likely that a decreased I /I ratio is the reason stimulated emission is not seen. The present experiments, unfortunately, cannot provide a more quantitative explanation. The distinct broadness of the isopropyl iodide distribution in fig. 2 indicates a departure from the methyl- ethyln-propyl trend, and might represent comparable amounts of P and I atom production, with overlapping translational energy distributions, at least when viewed with our present... 

There is evidence that excited iodine atoms or molecules will displace hydrogen atoms from carbon-hydrogen bonds.) Alternatively it has been suggested that hot radicals may be involved in reactions such as... 

Fig. 30. Decay of the population inversion AN = Nj — (1/2) Ni and of the concentration of excited iodine atoms Nj after flash photolysis at 20 Torr of CF3I...

NOCl is separated into Cl and NO radicals when photolysed by 3000—4000 A radiation in the vapour phase. The reaction of excited iodine atoms, produced... 

Figure 2 Experimental results for the production o/I( P ) from the photodissociation o/ICN. The solid curve and circles is the absorption spectrum of ICN, with e the molar extinction coefficient. The short dashed curve and open circles is the product of t x FP ), where 4> is the quantum yield for excited iodine atom formation. The long dashed curve and triangles is E — e (FP ), and thus represents the contribution to the absorption spectrum of dissociation leading to ground-state iodine atom production...

For Xel the excited-state yield is only 16% of its expected value, which implies that other reaction channels dominate. This additional process is the believed to be the production of electronically excited iodine atoms by the reaction... 

An X-ray fluorescence method has been developed for in vivo determination of iodine in thyroid (Aubert et al., 1981 Jonckheer and Deconinck, 1982). This method is based on the irradiation of iodine in the thyroid by o(-rays provided by a -source, such as Am. The excited iodine atoms emit a characteristic X-ray fluorescence radiation, which is proportional to the amount of iodine present in the gland. The reported detection fimit reaches 0.01 mg/ml thyroid, this value is much lower than the iodine concentration in thyroid. The reported radiation dose equivalent is only 60mSv per measurement. This method has been successfully used for the clinical determination of in thyroid (Milakovic et al., 2006 Reiners et al., 1996, 2006 Briancon et al., 1992). An indirect method was also reported to determine of thyroid iodine in vivo (Imanishi et al., 1991), which is based on the relationship of CT attenuation values with iodine concentration in the thyroid. It was reported that the CT value correlated finearly with iodine concentration in thyroid nodules when iodine concentration was higher than 0.02 mg/g. 

We now demonstrate the use of Eq. (105) for the control of the PD of CH3I initially in its electronic ground state XMi to yield a vibrationally excited CH3 and a ground and excited iodine atom... 

Kelley and Rentzepis [297] have recently studied the recombination of iodine atoms in liquid and fluid xenon over times to 150 ps after photolysis. The iodine molecule can be biphotonically dissociated through the state to produce geminate pairs with larger initial separations. Some degree of spin relaxation of excited iodine atoms ( Pi/2) produced by biphotonic excitation may occur and reduce the probability of recombination. There is also evidence that the 11 state of I2 may be collisionally predissociated and that recombination may be more rapid than the rate of vibrational relaxation of the excited 12 state in polyatomic solvents (see also ref. 57). Despite these complications, several workers have attempted to model the time dependence of the recombination (or survival) probability of iodine atom reactions. The simple diffusion equation analysis of recombination probabilities [eqn. 

CN(Jr S+) . o + lifP was discounted since more recent flash photolysis studies had not indicated any significant population of excited iodine atoms. 

In the gain region, the molecular iodine is fully dissociated and a near equilibrium between excited iodine atoms... 

The NCl(a)-I laser hardware appears similar to an HF laser in that the chemistiy is driven by atomic fluorine. The chemical generation of excited iodine atoms is based on the following sequential reaction mechanism ... 

We have labelled these four reaction channels, using F and S to refer to the channels involving fast and slow H atoms reactants respectively, and I and I to refer to the concomitant production of ground-state and spin orbit-excited iodine atoms in the reaction step. Knowing our experimental conditions and the energetics of the reaction, the speed of the H2 products formed via these different reaction channels can be calculated and compared directly with the product speeds observed in our images to identify the reaction channels present. 

This photochemical gas laser was first mentioned by Pimentel (12). Its chemical principle is shown in Fig. 3. Perfluoroisopro-pyliodide (i-C F J) gas is flashed by UV-light resulting in electronically excited iodine atoms their stimulated emission produces laser pulses. The main part of the initial compound is rebuilt after deexitation of iodine atoms and the C F -radicals. 

Without dwelling on the theoretical estimation of the rate constants of these processes, which would require the construction of potential energy surfaces for the initial and final states of the system, [216] consider only certain experimental results obtained for a reaction of an electronically excited iodine atom with a propane molecule, and for reactions of some other species. 

Qearly, these distributions are characteristic of systems where there are fluorine atoms present, but there has to be an alternative explanation to that suggested by Davis et al [14,15] [Reactions (3-5), where IF(B) is produced by colhsional excitation of IF(X) by 02( A)) in order to rationalise our observation that 02( A) is not necessary to produce such distributions. We have suggested [26] that in addition to F atoms, the other key species responsible for IF(B) production is electronically-excited iodine atoms I (2P y2) These can be produced in such systems by a reaction of the form... 

Such a process would favour the formation of high vibrational levels of IF(B) which when partially relaxed, might give a vibrational distribution of the form B. In general, the rates of two-body or three-body recombination reactions would not be sufficient to account for the observed yields of EF(B). One possibility is that an exciplex of the excited iodine atom with the iodide, (I. ..RI), might be formed [32] and the subsequent reaction of this exciplex with a fluorine atom would enhance the overall rate for IF (B)... 

We have shown that for low pressure gas systems (< 2 mbar), it is possible to categorise the mechanism of IF(B) formation by inspecting the form of the IF(B) vibrational state distribution. At these pressures, the effects of vibrational relaxation are minimised and the measured distributions are near to their nascent forms and hence characteristic of their mode of formation. We find that all the systems studied so far can be divided into three broad categories. These are bimolecular chemiluminescent reactions of Fj, creation of IF(B) by fluorine atoms and spin-oibit excited iodine atoms and direct coUisional excitation of BF(X) to IF(B) by an energy rich electronically or vibrationaUy excited species. Such a categorisation should assist the development and identification of promising schemes for a visible chemical IF (B-X) laser. 

The photodissociation of I2 molecule generates one ground-stated Iodine atom and one excited Iodine atom... 

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