Visible photodissociation

UV/Visible Photodissociation. In the case of UV/visible PD the photon energies are high enough to enable direct fragmentation. Previously, UV photodissociation was utilized mainly to study dissociation kinetics of gas-phase ions, but recently it was demonstrated that UV PD could be used for tandem MS of MALDI generated protonated peptides [18]. 

In addition to UV/visible flash photolysis and TRIR spectroscopy, other techniques have been used for the detection of transition metal-noble gas interactions in the gas phase. The interaction of noble gases with transition metal ions has been studied in detail. A series of cationic dimeric species, ML" " (M = V, Cr, Fe, Co, Ni L = Ar, Kr, or Xe), have been detected by mass-spectroscopic methods (55-58). It should be noted that noble gas cations L+ are isoelectronic with halogen atoms, therefore, this series of complexes is not entirely unexpected. The bond dissociation energies of these unstable complexes (Table IV) were determined either from the observed diabatic dissociation thresholds obtained from their visible photodissociation spectra or from the threshold energy for collision-induced dissociation. The bond energies are found to increase linearly with the polarizability of the noble gas. 

As with our CARS experiments which probe O2 E ) formed in the visible photodissociation of ozone, the species we detect in this experiment, HD, is a stable chemical aitity, and not a "transient" in the cotmon sense. However, as for the O2 photofragnent, the HD is a transient, short-lived species in the rotational and vibrational states in which the... 

Y. Chen, L. Hunziker, P. Ludowise, and M. Morgen, Femtosecond Transient Stimulated Emission Pumping Studies of Ozone Visible Photodissociation , J. Chem. Phys. 97, 2149 (1992). 

The easiest method for creating many vibrational excitations is to use convenient pulsed visible or near-UV lasers to pump electronic transitions of molecules which undergo fast nonradiative processes such as internal conversion (e.g. porjDhyrin [64, 65] or near-IR dyes [66, 62, 68 and 62]), photoisomerization (e.g. stilbene [12] or photodissociation (e.g. Hgl2 [8]). Creating a specific vibrational excitation D in a controlled way requires more finesse. The easiest method is to use visible or near-UV pulses to resonantly pump a vibronic transition (e.g. 

We use laser photofragment spectroscopy to study the vibrational and electronic spectroscopy of ions. Our photofragment spectrometer is shown schematically in Eig. 2. Ions are formed by laser ablation of a metal rod, followed by ion molecule reactions, cool in a supersonic expansion and are accelerated into a dual TOE mass spectrometer. When they reach the reflectron, the mass-selected ions of interest are irradiated using one or more lasers operating in the infrared (IR), visible, or UV. Ions that absorb light can photodissociate, producing fragment ions that are mass analyzed and detected. Each of these steps will be discussed in more detail below, with particular emphasis on the ions of interest. 

Vibrationally mediated photodissociation (VMP) can be used to measure the vibrational spectra of small ions, such as V (OCO). Vibrationally mediated photodissociation is a double resonance technique in which a molecule first absorbs an IR photon. Vibrationally excited molecules are then selectively photodissociated following absorption of a second photon in the UV or visible [114—120]. With neutral molecules, VMP experiments are usually used to measure the spectroscopy of regions of the excited-state potential energy surface that are not Franck-Condon accessible from the ground state and to see how different vibrations affect the photodissociation dynamics. In order for VMP to work, there must be some wavelength at which vibrationally excited molecules have an electronic transition and photodissociate, while vibrationally unexcited molecules do not. In practice, this means that the ion has to have a... 

The darkness associated with dense interstellar clouds is caused by dust particles of size =0.1 microns, which are a common ingredient in interstellar and circum-stellar space, taking up perhaps 1% of the mass of interstellar clouds with a fractional number density of 10-12. These particles both scatter and absorb external visible and ultraviolet radiation from stars, protecting molecules in dense clouds from direct photodissociation via external starlight. They are rather less protective in the infrared, and are quite transparent in the microwave.6 The chemical nature of the dust particles is not easy to ascertain compared with the chemical nature of the interstellar gas broad spectral features in the infrared have been interpreted in terms of core-mantle particles, with the cores consisting of two populations, one of silicates and one of carbonaceous, possibly graphitic material. The mantles, which appear to be restricted to dense clouds, are probably a mixture of ices such as water, carbon monoxide, and methanol.7... 

Those organometallic thexi states which have been detected have involved compounds where the quantum yield for photodissociation is very low. Time-resolved uv-visible absorption and emission studies have been made on W(CO)5L and W(CO)4L species (L = acetylpyridine, L = o-phenanthroline) (54), but, as in the case of intermediates, these studies provided lifetimes but no structural information. 

When the H202-loaded TS-1 sample was irradiated with 355-nm light of a Nd YAG laser or the visible emission of a conventional tungsten source, photodissociation of TiOOH was observed (133). The 837 and 3400 cm-1 bands (and the corresponding 180 substitutes) diminished in intensity (Fig. 20). 

As discussed in Chapter 4, N03 only exists in sufficient concentrations to play a role in nighttime chemistry, due to its strong absorption of light in the visible and subsequent photodissociation. 

The photodissociation products of the homonuclear halogens in the visible and ultraviolet are now comparatively well established in view of the detailed spectroscopic studies that have been made. The strongest absorption system observed in this spectral region is associated with a transition to the 3II0u+ state which correlates with X / ) + X(2Pyz). Thus photoexcitation to the continuum associated with this state leads directly to the formation of an excited atom, while excitation to the banded region followed by predissociation will lead only to ground state atoms. 

Photodissociation dynamics [89,90] is one of the most active fields of current research into chemical physics. As well as the scalar attributes of product state distributions, vector correlations between the dissociating parent molecule and its photofragments are now being explored [91-93]. The majority of studies have used one or more visible or ultraviolet photons to excite the molecule to a dissociative electronically excited state, and following dissociation the vibrational, rotational, translational, and fine-structure distributions of the fragments have been measured using a variety of pump-probe laser-based detection techniques (for recent examples see references 94-100). Vibrationally mediated photodissociation, in which one photon... 

Since Br2 photodissociates efficiently at >400nm [94], the photolysis of both CH3CHO and N02 can be avoided by using a visible light source. Also, the Br + N02 reaction was shown to yield negligibly small concentrations of BrN02 and BrONO, presumably due to the photochemical instability of these products. 

At wavelengths greater than 310 nm, the Huggins bands correspond to the limit of the O3 ultraviolet absorption, and in the visible region (410— 850 nm) the Chappuis bands play an important role leading to the 03 photodissociation in the lower part of the atmosphere, troposphere, and lower stratosphere. 

The peak of the O2 photodissociation occurs in the stratosphere (near 35 km for an overhead sun) where the total number of 02 molecules pho-todissociated is of the order of 107 cm-3 sec-1. Below the ozone peak (<25 km) the photodissociation rate decreases rapidly, particularly when the solar zenith angle increases. Below 20 km, the atomic oxygen production becomes very small and there is no atomic oxygen production in the troposphere by the 02 photodissociation. The ozone photodissociation is the result of the absorption of solar radiation in the visible and the ultraviolet ... 

This molecule is ideal for photodissociation dynamics studies, since it has allowed excited states in the visible region of the spectrum which is more easily accessible with tunable lasers. Both products can in principle be detected using the LIF method, though at the present time only the product distribution of the CN radical has been measured, since NO is often present as an impurity. The spectroscopy is also well studied... 

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