Laser irradiation, vibrational excitation

The term desorption is used in contrast to evaporation in cases in which a transition of a molecular or ion from the condensed into the gas phase is assumed to take place under non thermal equilibrium condition. The underlying idea is that at thermal equilibrium, temperatures for an evaporation would lead to a correspondingly high excitation of internal vibrational modes of excitation leading to fraigmentation of the molecule. As mentioned above, several characteristics of the ion spectra (2., 6.) cannot reasonably be fitted to an equilibrium temperature model. These properties seem to be the more pronounced, the higher the laser irradiance (i.e. usually the shorter the pulse) and are best documented for the LAMMA technique. Though metastable decay of ions is observed and will be discussed below, the decay rate for most of the ions is very small and decay... 

The impulse model is applied to the interpretation of experimental results of the rotational and translational energy distributions and is effective for obtaining the properties of the intermediate excited state [28, 68, 69], where the impulse model has widely been used in the desorption process [63-65]. The one-dimensional MGR model shown in Fig. 1 is assumed for discussion, but this assumption does not lose the essence of the phenomena. The adsorbate-substrate system is excited electronically by laser irradiation via the Franck-Condon process. The energy Ek shown in Fig. 1 is the excess energy surpassing the dissociation barrier after breaking the metal-adsorbate bond and delivered to the translational, rotational and vibrational energies of the desorbed free molecule. 

Another technique that shows great promise and is still being developed is laser enrichment, which can be applied to either atoms or molecules (Figure 10.6). The molecular laser process exploits the fact that UFs and UFe molecules have slightly different vibrational frequencies (of the order of 0.5 cm ). The UFe molecules in UFe vapour (supercooled in order to produce sharper absorption bands) are selectively excited with a mneable IR laser, then irradiated with a high-intensity UV laser, whereupon the excited UFe molecules are photodecomposed into UFs (the UFe molecules are unaffected). Under these conditions, the UFs is a solid, and is separated from the UFs (using a sonic impactor). 

The photochemical ligand substitution reaction of la was investigated by ultrafast TR-IR spectroscopy (Fig. 16) 51). An acetonitrile solution of la was irradiated by a 266-nm laser pulse ( 150 fs pulse width). A broad IR absorption band which was attributed to the reaction products in higher vibrational excited states was produced within 1 ps after the laser flash. The broad band sharpened and a vqo peak at 1828 cm of the reaction product was observed in the 50- to 100-ps duration. This time scale is much shorter than the decay of the lowest MLCT excited state (right-hand side of Fig. 16). The TR-IR results indicate that this photochemical reaction proceeds from higher vibrational states or high-energy electronic excited states instead of the lower vibrational excited states of MLCT and thermal accessible states from MLCT such as the LF state. 

GW cm ) pulsed CO2 laser radiation shown to result in broad u.v.-visible chemiluminescence Measurement of time-resolved emission spectra in the 618 2—5 p,m region following CO2 laser irradiation of CDF, and mixtures of CDF3-CHF,. Emission from vibrationally excited CDF, and DF observed, the latter being produced in the IRMPD of CDF,... 

Irradiation also affects the course of more conventional separation processes. Visible and ultraviolet light have been found to affect plutonium solvent extraction by photochemical reduction of the plutonium (12). Although the results vary somewhat with the conditions, generally plutonium(VI) can be reduced to pluto-nium(IV), and plutonium(IV) to plutonium(III). The reduction appears to take place more readily if the uranyl ion is also present, possibly as a result of photochemical reduction of the uranyl ion and subsequent reduction of plutonium by uranium(IV). Light has also been found to break up the unextractable plutonium polymer that forms in solvent extraction systems (7b,c). The effect of vibrational excitation resulting from infrared laser irradiation has been studied for a number of heterogeneous processes, including solvent extraction (13). 

Since we had proposed a similar experiment with irradiation in the ultraviolet-visible absorption band of the uranyl ion (15), we tried to reproduce these results, but without success (16). Collisions in the liquid phase occur so rapidly (about 10l2 s x) that vibrational excitation of the uranyl ions would be dissipated long before any significant fraction of excited uranyl ions could reach the interface and therefore change the distribution between the two phases. Rapid loss of vibrational excitation in relation to other processes is a generic problem for infrared laser effects in any system of condensed phases. However, differences between experimental setups may account for the differences in results,... 

Vibrationally excited [CF30] ions (generated from the dissociation of CF3OOCF3 using an electron beam pulse) were trapped whilst irradiating with infrared light from a CO laser. Multiphoton dissociation results in the following reaction [1034] ... 

Vibrationally excited processes can be applied in the formation of semiconductor films. One of the examples to demonstrate the photoenhanced chemisorption and reaction due to the vibrational excitation is the interaction of SF molecules with silicon (2). In this case, SFg molecules can be chemically activated by multiple photon absorption of CO2 laser either in the gas phase or in the adsorbed state. Deposition of Si on quartz or glass surface can also be stimulated by the decomposition of SiH enhanced by the irradiation of CO2 laser to the gas phase (3). 

Decomposition of H3B,PF3 by irradiation with a CO2 laser (10.6 pm band) is due to vibrational excitation, and not to net rise in temperature. No boron isotope effect was detected. The rate-determining step seems to be the reaction ... 

A number of photoionization and photoexcitation processes are being investigated for isotopic separation, especially of uranium. In one such process UF is irradiated by a laser beam, producing selective vibrational excitation in the molecule (cf. 2.5). By... 

The results obtained from the irradiation at 235 nm of vibrationally excited dichloromethane suggest that there is higher non-adiabaticity for vibrationally excited molecules.Laser irradiation at 355 nm of methylene chloride and methylene bromide brings about ionization.A simple C-Br bond fission is the key step in the photodissociation of methylene dibromide at 248 nm. Ionization of methylene bromide in the 10-24 eV range has been reported. Irradiation of methylene iodide in water has provided evidence for the formation of iodine atoms and protons.Further work on the photochemical behaviour of methylene iodide has reported that vibrationally relaxed CH2l- T reacts with cyclohexene to afford norcarane. ... 

Raman spectroscopy is a spectral measurement based on inelastic scattering of monochromatic radiation. When a molecule is irradiated with an intense monochromatic light (usually a laser source),photons excite the molecule from the ground state to a virtual energy state. The photons are re-emitted when the molecule relaxes. The frequency of the re-emitted photons shifts in comparison with the original monochromatic light frequency. This shift provides information about vibrational, rotational, and other low frequency transitions in molecules. Information from Raman spectroscopy is summarized in Rg. 11.4. 

Although the formidable difficulties associated with isotope separation schemes based on photochemical vibrational excitation plus chemical reaction continue to attract considerable attention, some earlier hopes appear to have been dashed. An experiment performed in 1970 by Mayer et al. has been much quoted. They reported irradiating mixtures of CHjOH and CD3OD in the presence of Bts with lines from an HF laser that are absorbed only by CH3OH. Product analysis indicated the selective depletion of the CH3OH. This observation was interpreted in terms of a selective reaction between vibrationally excited CH3OM and Bra, nnd it appeared to point the way to an economic method for the production of heavy water. However, the results of a careful re-examination of this system have just... 

The idea of the experiment is illustrated in Fig.3. The upper part of the figure shows a simple cell filled with H2O. A slow gas flow is maintained at a constant pressure of about 40mTorr. The cell is irradiated by three different laser pulses, which follow each other within less than 50nsec. First an infrared laser at 2.7iJm is fired to vibrationally excite H2O and to prepare a well defined rotational state in the asymmetric stretch mode. The second laser., an excimer laser at 193nm, dissociates the H2O and the third laser analyses the formation of the OH products by LIF. The infrared excitation of H2O is monitored by the photoacoustic method with the microphone shown in the cell. At the short delays and the low pressures the products are analysed collision free. 

An investigation of the effects of vibrational excitation of HI in promoting the reaction HI + HI — H2 + I2 was reported in 1981 by Horiguchi and Tsuchiya [26]. In these experiments the rates of the reaction were measured in mixtures with carbon dioxide which was excited vibrationally by irradiation with light from a cw laser. An enhancement of reaction rate by a factor of about 2.5 was observed for mixtures with carbon dioxide compared to those without carbon dioxide when both were irradiated. The enhancement was attributed to vibrational excitation of HI through collisional transfers of energy from laser-excited carbon dioxide. The results indicate that vibrational excitation of one or both of the colliding HI molecules promotes the reaction. 

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