We have investigated the dynamics of low-energy (1−20 eV) electron-induced reactions in condensed thin films of methanol (CH 3 OH) through both electron-stimulated desorption (ESD) and postirradiation temperature-programmed desorption (TPD) experiments conducted under ultrahigh vacuum conditions. Results of ESD experiments, involving a high-sensitivity time-of-flight mass spectrometer, indicate that anion (H − , CH − , CH 2 − , CH 3 − , O − , OH − , and CH 3 O − ) desorption from the methanol thin film at incident electron energies below about 15 eV is dominated by processes initiated by the dissociation of temporary negative ions of methanol formed via electron capture, a resonant process known as dissociative electron attachment (DEA). However, postirradiation TPD investigation of radicals, especially •CH 2 OH and CH 3 O• remaining in the methanol thin film, demonstrates that electron impact excitation, not DEA, is the primary mechanism by which the radical−radical reaction products methoxymethanol (CH 3 OCH 2 OH) and ethylene glycol (HOCH 2 CH 2 OH) are formed. This apparent dichotomy between the results of ESD and postirradiation experiments is attributed to the low DEA cross section for methanol compared to that of species such as halomethanes. Our results suggest that for molecules such as methanol, low-energy electron-induced electronic excitation, rather than DEA, plays a dominant role in ionizing radiation-induced chemical synthesis in environments such as the interstellar medium. ■ INTRODUCTIONBecause of its simple chemical structure, methanol is a prototypical candidate for radiolysis studies of oxygencontaining biomolecules such as DNA. The radiation chemistry of methanol is of particular interest because methanol is exposed to different types of ionizing radiation in varying environments and phases. For example, liquid methanol is used as a solvent in radiation-induced grafting of copolymer composites. 1 The radiation chemistry of condensed methanol is also of astrochemical interest because methanol is found in relatively high abundance in protostar environments. Methanol is thought to be an important precursor in cosmic ices not only to species such as methyl formate (HCOOCH 3 ), ethylene glycol (HOCH 2 CH 2 OH), and dimethyl ether (CH 3 OCH 3 ) but also to many prebiotic species such as simple sugars and amino acids. 2−4 Because of such applications, the high-energy radiolysis of methanol has been extensively studied over a period spanning seven decades. 5−9 More recent advances, however, have demonstrated that studying the interactions of low-energy electrons with condensed matter is essential to obtaining a fundamental understanding of radiation chemistry because the interactions of high-energy radiation, such as cosmic rays (E max ∼ 10 20 eV), with matter produce large numbers of low-energy (<15 eV) secondary electrons, which are thought to initiate radiolysis reactions in the condensed phase. 10,11 In this publication, we investigate the chemistry induced in condensed methanol by such low-energ...

The characteristics of the fluorescence and phosphorescence emission of 2-amino-4 (3H) pteridinone (or pterin) in aqueous solutions are pH dependent. The room temperature fluorescence quantum yield is low and is maximum at pH = 10 (& -0.057). The 77 K phosphorescence emission consists of two overlapping emissions originating from z,n* triplet states. In agreement with low temperature results, the 353 nm laser flash photolysis makes it possible to detect at pH 9.2, two transient triplet absorptions (T, -0.3 ps and T~ -2.3 p). The longer lived triplet is characterized by z 0.20 and eT. (550nm) = 2000 M -' cm-'. It reacts with the solvent forming the semireduced pterin with a quantum yield d R -0.06. The photosensitizing properties of pterin have been studied by laser flash spectroscopy and steady state irradiations. Photoreactions implying singlet oxygen formation are shown to occur. Laser flash spectroscopy indicates that the pterin triplet is reduced by amino acids and nucleic acid bases. Corresponding bimolecular reaction rate constants have been measured.PROPERTIES OF 2-AMINO-4 PTERIDINONE:

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Purpose-Determine experimentally the absolute cross sections (CS) to deposit various amount of energies into DNA bases by low-energy electron (LEE) impact.Materials and methods-Electron energy loss (EEL) spectra of DNA bases are recorded for different LEE impact energies on the molecules deposited at very low coverage on an inert argon (Ar) substrate. Following their normalisation to the effective incident electron current and molecular surface number density, the EEL spectra are then fitted with multiple Gaussian functions in order to delimit the various excitation energy regions. The CS to excite a molecule into its various excitation modes are finally obtained from computing the area under the corresponding Gaussians.Results-The EEL spectra and absolute CS for the electronic excitations of pyrimidine and the DNA bases thymine, adenine, and cytosine by electron impacts below 18 eV are reported for the molecules deposited at about monolayer coverage on a solid Ar substrate.Conclusions-The CS for electronic excitations of DNA bases by LEE impact are found to lie within the 10 −16 -10 −18 cm 2 range. The large value of the total ionisation CS indicates that ionisation of DNA bases by LEE is an important dissipative process via which ionising radiation degrades and is absorbed in DNA.

The 355 nm laser flash photolysis of nalidixic acid at pH 9.2 leads to the formation of the nalidixate anion triplet state (absorption lambda max = 620 nm; 5700 less than or equal to epsilon T less than or equal to 9000 M-1cm-1; 0.6 less than or equal to phi T less than or equal to 1). The first order triplet state decay (kT = 7.7 x 10(3) s-1) is accompanied by a diffusion controlled triplet-triplet annihilation. Oxygen efficiently quenches the triplet state (k = 3.2 x 10(9) M-1s-1). The nalidixate radical dianion (absorption lambda max = 650 nm; epsilon = 3000 M-1cm-1) is produced by the diffusion controlled reductive quenching of the triplet state by tryptophan and tyrosine. The superoxide anion (O2-.) is produced by diffusion controlled reaction of the radical dianion with oxygen. The O2-. is characterized by its reactions with ferricytochrome c and superoxide dismutase. The physiological form of nalidixic acid is thus a good Type I and Type II photosensitizer.