Photodissociation Lasers

Watson, C.H. Baykut, G. Eyler, J.R. Laser Photodissociation of Gaseous Ions Formed by Laser Desorption. Anal. Chem. 1987,59, 1133-1138. 

The Mg+—CeHe dissociation energy at 0 K was determined to be 134 4 kJmol (1.39 0.10 eV) using collision induced dissociation and 112 kJmoH by laser photodissociation . Using the radiative association kinetics approach to ion cyclotron resonance spectrometry, the value was shown to be the comparable 1.61 eV (155 kJmoU ). It was also shown that the binding of the second benzene to Mg , i.e. the Mg+ (CeHe)—CgHe bond energy, is less than 1.4 eV (135 kJmoU ). 

This paper focuses on special ionization methods such as secondary ion MS (SIMS) (1, 13, 24-28) and ZCf plasma desorption (PD), and on MS/MS methods for characterizing primary ions, such as surface induced dissociation (SID), laser photodissociation, and neutralization of multiply charged ions. A Hadamard transform method for more efficient recording of multiple MS-II spectra is also proposed. 

Laser desorption Fourier transform mass spectrometry (LD-FTMS) results from a series of peptides and polymers are presented. Successful production of molecular ions of peptides with masses up to 2000 amu is demonstrated. The amount of structurally useful fragmentation diminishes rapidly with increasing mass. Preliminary results of laser photodissociation experiments in an attempt to increase the available structural information are also presented. The synthetic biopolymer poly(phenylalanine) is used as a model for higher molecular weight peptides and produces ions approaching m/z 4000. Current instrument resolution limits are demonstrated utilizing a polyethylene-glycol) polymer, with unit mass resolution obtainable to almost 4000 amu. 

Fig. 3. Transient profile of O ( Pj) density obtained by monitoring the vacuum ultraviolet atomic resonance fluorescence signal following laser photodissociation of O3. (a) Photolysis at 308 nm, Pqs = 1 mTorr, Pjjg = 5.7 Tort, 40288 averaged, (b) Photolysis at 248 nm, Pon = f mTorr, Pjj = 9.7 Torr, 8192 shots averaged. The solid lines through the points representing non-linear least squares fit to the data. [Source Greenblatt and Wiesenfeld, 1983].

Laser photodissociation [83] or IRMPD [84], where 193-nm photons from a laser are applied for ion excitation. 

Photodissociation dynamics of alkyl nitrites adsorbed on Mgp2 surfaces and on the Ag(lll) surface have been studied. The laser photodissociation and thermal pyrolysis of poly(glycidyl nitrate) have been investigated. Such high-energy polymers have been proposed for use as binders in solid rocket motors. 

Ab initio calculations gave vibrational wavenumbers for 9 isomers of CNNS.524 A CARS study of the effects of 266 nm. pulsed laser photodissociation of NCNCS showed that the vapour contained both NCNCS and CNCN.525 IR and Raman spectra, with factor group analysis, gave vibrational assignments for crystalline ammonium dicyanamide, NH4[N(CN)2].526 Variable-temperature Raman spectroscopy was used to follow the solid state transformation of NI I4[N(CN)2] into NCN=C(NH2)2.527 Ab initio and/or DFT calculations gave vibrational wavenumbers for CH2=CH-N=C=X (X = O, Se) 528 NN-C(CN)2 529 nitroso-azide, NNN-N=0, and nitro-azide, NNN-N02.530... 

Laser photodissociation of ketene at 230 nm has been investigated in molecular beams. The experimental rovibrational population distribution has been compared to predictions from phase-space theory for the channels leading to CO + CH2(a Ai) and CO + CH2(b Bi). The calculations are not compatible with the latter channel, suggesting that it does not contribute significantly to the dissociation process. The photodissociation of singlet ketene by two-step IR + UV excitation has been studied using state-selective detection of CH2 by laser-induced fluorescence, and the results compared with... 

Since the decomposition process of Cgo is well known and simple, it provides fundamental information on the formation of polyynes from carbon materials. We found that C2 H2 polyynes are produced under irradiation of pulse Nd YAG laser into Cgo particles suspended in hexane or methanol solution at room temperature. Laser photodissociation and photoionization of Cgo in the gas phase leading to neutral and ionic carbon clusters has been studied by O Brien et al. [21] and Lykke and Wurz [22,23]. According to their results C2-I0SS processes are major dissociation... 

UV LASER PHOTODISSOCIATION OF SMALL MOLECULES ON SOLID SURFACES... 

Laser surface chemistry has been used as a basis for many new methods in surface processing, for example, photochemical deposition of metals and photochemical etching of solid substrates, which are potentially useful techniques for the microelectronics industry (1-3). However, molecular dynamical studies of UV photodissociation of adsorbates on solid surfaces have been very scarce (4-7). We have studied UV laser photodissociation of small molecules on solid surfaces using photofragment spectroscopy. 

During this study, we have found that laser intensity is one of the important factors that control laser surface chemistry. At a small laser intensity, molecules adsorbed on solid surfaces dissociate into atoms and radicals. Some of these atoms or radicals react with atoms of the solid substrates. At a large laser intensity, atoms are photoablated from the solid surfaces to react with the molecules adsorbed or in the gas phase. Hence, we describe in this paragraph a) the dynamical study of UV laser photodissociation of halogen or metal-containing molecules on solid surfaces, b) reactions of atoms generated in the photodissociation of an adsorbate with solid surfaces, and c) reactions of molecules in the gas phase with the photoelectrons or metal atoms generated on intense laser irradiation of solid surfaces. 

UV Laser Photodissociation of Small Molecules on Solid Surfaces... 

To obtain information concerning energy disposals, product kinetic and internal energy distributions, and identification of primary product structures resulting from the UV photolysis of organosulfur pollutants, we have performed UV laser photodissociation and photoionization mass spec-trometric studies of a series of sulfur-containing species in recent years [40-45,49,50,54-61]. Ab initio calculations [49,50,55-59,65,66,69] have also been made to compare with the energetic measurements and to interpret the experimental observations. This review mostly summarizes and evaluates the results of these experimental and theoretical studies. Related results obtained by other laboratories are also discussed. 

In recent high-resolution, pulsed-field ionization-photoelectron (PFI-PE) spectroscopic studies of laser-initiated radicals, such as SH [61] and CHjS [54] formed in the laser photodissociation of H2S and CH3SH and CH3SSCH3, respectively, we have demonstrated that the PFI-PE spectroscopic technique is potentially useful for probing nascent state distributions of primary products formed in photochemical or reactive processes. 

Figure 5. Kinematics of a laser photodissociation experiment using a rotating-beam-source apparatus. The detector is fixed on the z axis and the laser is propagating along the y axis (perpendicular to the x-z plane) with its electric field (E) lying on the x-z plane. The molecular beam source rotates on the x-z plane. The diagram shows the relationship y = + e — where y is the angle between the laser...

The second harmonic output of an excimer (Lambda Physik EMG 201 MSC) pumped-dye laser (Lambda Physik FL 3002) is focused into the photoionization region by a 2(X)-mm, focal-length, fused-silica lens. The wavelength calibration is made using the known ZEKE spectrum of atomic sulfur [194], which is produced by the multiphoton laser photodissociation of HjS. 

Laser photodissociation of both reduced PaNiR—CO and PaNiRc di +-CO results in the formation of a conformationally unrelaxed transient that undergoes rapid structural relaxations before rebinding of CO occurs. In the transient species the reduction of heme c by heme... 

The rate constant for NO binding to P450nor(Fe +) to form P450nor(Fe " NO) (reaction (8)) was determined by monitoring the decay in absorbance at 434 nm as rebinding occurred following laser photodissociation of the P450nor(Fe NO) adduct ... 

Dunbar, R.C. Kinetics of low-intensity infrared-laser photodissociation-the thermal-model and application of the Tohnan theorem. J. Chem.Phys. 1991, 95, 2537-2548. 

Hemberger, P.H. Nogar, N.S. Williams, J.D. Cooks, R.G. Syka, J.E.P. Laser photodissociation probe for ion tomography studies in a quadrupole ion-trap mass-spectrometer. Chem. Phys. Lett. 1992,191, 405-410. 

Laser photodissociation sources [9-12] can also be problematical. Because of strong absorption, often characteristic of excited molecules, it is difficult to control the degree of excitation and hence the extent... 

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