Infrared photodissociation

Buck U, Gu X, Lauenstein C and Rudolph A 1988 Infrared photodissociation spectra of size-selected (CHjGH) clusters from / = 2 to 8 J. Phys. Chem. 92 5561... 

The earliest molecular beam infrared experiments on Van der Waals complexes used photodissociation spectroscopy a molecular beam is irradiated witli a tunable infrared laser and tire molecular beam intensity is measured as a function of... 

Dyer R B, Einarsdottir 6, Killough P M, Lopez-Garriga J J and Woodruff W H 1989 Transient binding of photodissociated CO to of eukaryotic cytochrome oxidase at ambient temperature. Direct evidence from time-resolved infrared spectroscopy J. Am. Chem. Soc. Ill 7657-9... 

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. 

Figure 11. Infrared resonance enhanced photodissociation spectrum of V (OCO)5 obtained by monitoring loss of CO2. The antisymmetric stretch of outer-shell CO2 is near 2349 cm (the value in free CO2, indicated by the dashed vertical line). The vibration shifts to 2375 cm for inner-shell CO2.

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... 

In the gas phase, ions may be isolated, and properties such as stability, metal-ligand bond energy, or reactivity determined, full structural characterization is not yet possible. There are no complications due to solvent or crystal packing forces and so the intrinsic properties of the ions may be investigated. The effects of solvation may be probed by studying ions such as [M(solvent) ]+. The spectroscopic investigation of ions has been limited to the photoelectron spectroscopy of anions but other methods such as infrared (IR) photodissociation spectroscopy are now available. 

The laser action originates from electronically excited I atoms. This type of laser is termed a photodissociation laser. Since there are no vibrational and rotational modes in the I atom, the efficiency of I production may be 100%. These systems emit in the infrared region. 

Although, relevant information about ferrous hemeproteins kinetics, dynamics and ligand photodissociation pathways has been obtain, less is known about ferric hemeproteins photophysic processes. Recent studies performed with Hbl-CN and Mb-CN at ultrafast time scale, have suggested that some of the transients intermediaries observed after ferrous complexes ligand photodissociation are observed in ferric Mb and Hbl [7], However, time-resolved infrared data shows that the complex remained six coordinated after photoexcitation. In this work we present ultrafast data on ferric Hbl-NO, HM-N3, HM-H2S and metHbl complexes that suggest a mechanism for the photoinduced reduction of Hbl species. 

There has been a flurry of recent activity in the study of the photodissociation dynamics of this molecule (186,187,188). Hermann and Leone used the infrared luminescence technique with a circular variable filter to determine the IR emission as a function of frequency when this molecule was photolyzed with lasers at 248 and 266 nm. From their results, they were able to show that the umbrella bending V2 mode of the CH3 radical was the only mode of CH3 that was excited in the photofragmentation of CH3I. This is in accord with the idea that the photodissociation of this molecule is unusually simple, and involves primarily the scission of the C-I bond with the simultaneous relaxation of the pyramidal structure of the CH3 part of the molecule into its final planar form. The data are used to obtain a vibrational distribution of the CH3 radical that peaks at v" = 2 and extends all the way out to the v" = 10 level. 

Protonation of fluorobenzene in the gas phase has been studied by infrared photodissociation (IRPD) spectroscopy by Solca and Dopfer.351 F-protonated fluorobenzene was formed in significant amount when protonation was carried out with CH5+. It was found to be the most stable isomer in the gas phase by quantum mechanical calculations [B3LYP/6-311G(2df,2pd) level] separated by a large energy barrier from the four Wheland intermediates. F-protonated fluorobenzene is best described as a weakly bound ion-dipole complex between the phenyl cation and HF. 

Bonn M, Bakker H J, Kleyn A W, Santen R A Van. 1996. Dynamics of infrared photodissociation of methanol clusters in zeolites and in solution. JPhys Chem 100 15301-15304. 

Output from both gated continuous wave and pulsed carbon dioxide lasers has been used to desorb ions from surfaces and then to photodissociate them in a Fourier transform ion cyclotron resonance mass spectrometer. Pulsed C02 laser irradiation was most successful in laser desorption experiments, while a gated continuous wave laser was used for a majority of the successful infrared multiphoton dissociation studies. Fragmentation of ions with m/z values in the range of 400-1500 daltons was induced by infrared multiphoton dissociation. Such photodissociation was successfully coupled with laser desorption for several different classes of compounds. Either two sequential pulses from a pulsed carbon dioxide laser (one for desorption and one for dissociation), or one desorption pulse followed by gated continuous wave irradiation to bring about dissociation was used. 

The extremely wide range of possible dissociation energies necessitates the use of different kinds of light source to break molecular bonds. Van der Waals molecules can be fragmented with single infrared (IR) photons whereas the fission of a chemical bond requires either a single ultraviolet (UV) or many IR photons. The photofragmentation of van der Waals molecules has become a very active field in the last decade and deserves a book in itself (Beswick and Halberstadt 1993). It is a special case of UV photodissociation and can be described by the same theoretical means. In Chapter 12 we will briefly discuss some simple aspects of IR photodissociation in order to elucidate the similarities and the differences to UV photodissociation. 

Suzuki, T., Kanamori, H., and Hirota, E. (1991). Infrared diode laser study of the 248 nm photodissociation of CH3I, J. Chem. Phys. 94, 6607-6619. 

Since Ar has an I value lower than that of F, it is not unreasonable to expect that argon fluorides will be synthesized soon. Recently HArF was detected in a photodissociation study of HF in an Ar environment (see Section 5.8.1). With infrared spectroscopic studies of species such as H40ArF, D40ArF, and H36ArF, it was determined that the bond lengths of H-Ar and F-Ar are 133 and 197 pm, respectively. As of now, He and Ne are the only two elements that do not form any stable compounds. 

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