No abstract

Chlorine nitrate photolysis has been investigated with the use of a molecular beam technique. Excitation at both 248 and 193 nanometers led to photodissociation by two pathways, CIONO(2) --> CIO + NO()2 and CIONO(2) --> Cl + NO3, with comparable yields. This experiment provides a direct measurement of the CIO product channel and consequently raises the possibility of an analogous channel in CIO dimer photolysis. Photodissociation of the CIO dimer is a critical step in the catalytic cycle that is presumed to dominate polar stratospheric ozone destruction. A substantial yield of CIO would reduce the efficiency of this cycle.

The photodissociation of dichlorine monoxide (Cl,O) at 308, 248, and 193 nm was studied by photofragment translational energy spectroscopy. The primary channel upon excitation at 308 and 248 nm was Cl-O bond fission with production of ClO+Cl. A fraction of the Cl0 photoproducts also underwent spontaneous secondary dissociation at 248 nm. The center-of-mass translational energy distribution for the ClO+C!l channel at 248 nm appeared to be bimodal with a high energy component that was similar in shape to the 308 mn distribution and a second, low energy component with a maximum close to the threshold for the 2Cl+O(3P) channel. Observation of a bimodal distribution suggests that two pathways with different dissociation dynamics lead to CIO+Cl products. The high product internal energy of the second component raises the possibility that Cl0 is formed in a previously unobserved spin-excited state a 4Z-. Following excitation at 193 nm, a concerted dissociation pathway leading to Cl,+0 was observed in addition to primary Cl-O bond breakage. In both processes, most of the diatomic photofragments were formed with sufficient internal energy that they spontaneously dissociated. The time-of-flight distributions of the Cl,+0 products suggest that these fragments are formed in two different channels Cl,(311)+O(3P) and Cl,(X 'Iz)+-@I).-

No abstract

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry has in recent years significantly advanced the field of polymer analysis. However, the mechanisms of the desorption and ionization processes, and in particular the critical role played by the matrix, remain unclear. In the present work, the usual matrix is replaced with a self-assembled monolayer consisting of a UV absorbing matrix-like compound covalently linked to a gold surface. Analytes such as proteins or oligonucleotides are directly deposited on the covalently modified probe tips and mass analyzed by laser desorption time of flight (TOF) mass spectrometry. Several types of monolayers were investigated and tested for their ability to produce positive and negative analyte ions. Molecular ion signals were obtained for dT10 oligonucleotides and proteins as large as cytochrome C on monolayers of methyl N-(4-mercaptophenyl)carbamate (MMPC). The amenability of this model system to characterization with established physical and chemical methods should help investigate the processes involved in MALDI.