Laser-ion Beam Photodissociation Research Articles

Laser ion beam photodissociation studies of model amino acids and peptides

Visible (458-514.5 nm) and uv (333-385 nm) photodissociation of the (M + H){sup +} ions of dinitrophenyl (DNP) derivatized amino acids and peptides is reported. Photoexcitation of the DNP peptides by a visible proton results in fragmentation of the peptide chain with little fragmentation within the chromophore. Conversely, uv photoexcitation of the DNP peptides results in fragmentation of the chromophore as well as the peptide chain, but loss of NO or NO{sub 2} (within the chromophore) often dominates the photofragment ion spectrum. These results are rationalized with particular emphasis on energy-selective dissociation channels of large ionic systems. DNP-leucine and DNP-isoleucine (M + H){sup +} can be differentiated on the basis of photodissociation reactions which yield distonic radical cations. The rate of dissociation of photoexcited ions of DNP peptides is shown to decrease with increasing molecular weight (degrees of freedom). Lastly, comparisons between photodissociation and collision-induced dissociation as a structural probe are presented. 55 refs., 8 figs., 3 tabs.

The utility of a biased activation cell for laser-ion beam photodissociation studies

The use of a biased activation cell for performing photodissociation experiments on a triple sector ion-beam mass spectrometer is described. The objective of this work is to eliminate or reduce background signals from metastable ion and/or residual collision-induced dissociation and thereby increase sensitivities for laser-ion beam photodissociation. Selected photodissociation reactions demonstrate the utility of the biased activation cell. Biasing the activation cell separates a significant fraction (⪢ 85%) of the metastable background from the photodissociation signals, thereby increasing signal-to-noise ratios and shortening data collection times in the photodissociation experiment. In addition, the rates of photodissociating ions can be estimated. The photodissociation mass spectra of the geometric isomers trans,trans-2,4-hexadiene and cis,trans-2,4-hexadiene ions (C 6H + 10 are distinguishable.

Laser-ion beam photodissociation studies of ionic cluster fragments of iron carbonyls: Fex(CO)y+ (x = 1-3; y = 0-6)

Ionic cluster fragments of the type Fe x (CO) y + (x=1-3; y=0-6) are mass resolved by high-resolution mass spectrometry and then photodissociated. The photofragment ions are mass analyzed by an electrostatic analyzer. The ions studied include Fe(CO) y + (y=1-5), Fe 2 (CO) y + (y=0-6), and Fe 3 (CO) y + (y=0-4) derived from Fe 3 (CO) 12 neutral

A photodissociation study of 1,3-butadiene

Laser-ion beam photodissociation is used to study the C4H+6 radical cation formed by electron impact (70 eV nominal energy) ionization of 1,3-butadiene. The kinetic energy release value obtained from C4H+6→C3H+3+CH3 does not agree with the distribution reported by Farrar [J. Chem. Phys. 77, 263 (1982)]. The photodissociation studies of C4H+6 suggest the existence of a long-lived (>0.05×10−6 s) photoexcited C4H+6 ion. Also, results of photodissociation studies of C4H+6 trapped in the electron beam space charge are consistent with the proposed long-lived photoexcited C4H+6 ion. Therefore, it is proposed that the two-photon photodissociation of C4H+6 reported by Baer [J. Chem. Phys. 85, 6361 (1986)] is a sequential two-step process involving a long-lived excited C4H+6 ion.

Differentiating geometric isomers by laser-ion beam photodissociation

The laser-ion beam photodissociation reactions of the cis-1,3-pentadiene and trans-1,3-pentadiene C/sub 5/H/sub 8//sup +/ ions and the cis,trans-2,4-hexadiene and trans,trans-2,4-hexadiene C/sub 6/H/sub 10//sup +/ ions have been investigated. The photodissociation spectra are found to agree qualitatively with the published ion cyclotron resonance (ICR) photodissociation data. Although the C/sub 5/H/sub 8//sup +/ and C/sub 6/H/sub 10//sup +/ geometric isomers exhibit indistinguishable photodissociation spectra and product ion branching ratios, the wavelength-dependent kinetic energy release values are unique for the individual isomers. The significant changes in the photofragment kinetic energy release values for the trans-1,3-pentadiene and trans,trans-2,4-hexadiene ions as a function of photon energy are attributed to isomerization of the photoexcited parent ion prior to dissociation. 28 references, 5 figures, 2 tables.

Laser-ion beam photodissociation studies of C 3H 3Br + and C 3H 3Cl +

The photodissociation reactions of the 3-chloro-1-propyne and 3-bromo-1-propyne radical ions have been studied using a laser-ion beam photodissociation apparatus. The photodissociation spectra reflect the spin-orbit splitting of the ground electronic states of the ion. The C 3H 3Cl + and C 3H 3Br + systems dissociate to yield exclusively C 3H 3 + in the total energy range 12.9–13.7 eV. This reaction channel is assigned to a direct dissociation from a repulsive electronic state. Finally, comparisons between the unimolecular and the photofragment kinetic energy release values suggest a propargyl structure for the C 3H 3 + photofragment ion.