Photoionization mass spectrometric study of the prebiotic species formamide in the 10-20 eV photon energy range.

A photoion mass spectrometry study of the prebiotic species formamide was carried out using synchrotron radiation over the photon energy range 10-20 eV. Photoion yield curves were measured for the parent ion and seven fragment ions. The ionization energy of formamide was determined as IE (1(2)A') = 10.220 +/- 0.005 eV, in agreement with a value obtained by high resolution photoelectron spectroscopy. The adiabatic energy of the first excited state of the ion, 1(2)A'', was revised to 10.55 eV. A comparison of the ionization energies of related formamides, amino acids, and polypeptides provides useful information on the varied effects of methylation and shows that polymerization does not substantially alter the ionization properties of the amino acid monomer units. Assignments of the fragment ions and the pathways of their formation by dissociative photoionization were made on the basis of ion appearance energies in conjunction with thermochemical data and the results of earlier electron impact mass spectral studies. Some of the dissociation pathways are considered to involve coupling between the 1(2)A' ground state and the low-lying 1(2)A'' excited state of the cation. Heats of formation are derived for all ions detected and are compared with literature values where they exist. Formation of the HNCO(+) ion occurs by two separate paths, one involving H(2) loss, the other H + H. In the conclusion a brief discussion is given of some astrophysical implications of these results.

Photoionization of atmospheric aerosol constituents and precursors in the 7-15 eV energy region: experimental and theoretical study.

Photoionization mass spectrometry (PIMS) has been used to study the dissociative ionization of three anthropogenic atmospheric aerosol precursors (o-xylene, 2-methylstyrene, indene) and five of their main atmospheric degradation products (o-tolualdehyde, 2-methylphenol, o-toluic acid, phthalic acid, and phthaldialdehyde). Ionization and fragment appearance energies have been experimentally determined in the 7-15 eV photon energy regime. Moreover, intensive ab inito quantum chemical calculations have been performed to compute the first ionization energies and heats of formation of these compounds (including also phthalic anhydride). Several methods have been used, and the theoretical results are compared to the experimental values with the aim to find the best method to predict thermochemical data for similar molecules. The vacuum-UV fragmentation pathways following photoionization are discussed. The results of this work are important with respect to the analytical chemistry of these compounds since their basic gas phase ion energetics data are mostly unknown. They will help in interpreting real-time mass spectrometric measurements used for the study of organic aerosol formation in smog chambers and in the real atmosphere.

VUV photodissociation of ammonia: a dispersed fluorescence excitation spectral study.

NH2 and NH transitions were identified in dispersed fluorescence spectra obtained by VUV excitation of NH3 at nine different energies between 8.266 and 15.498 eV. Fluorescence excitation (FEX) spectra between 6 and 15 eV were measured at seven specific emission wavelengths for these species. This extends the usual 11.7 eV upper limit of NH3 FEX spectra. Among the results of our work are the following: (i) a clarification of the assignment of the emission carriers; (ii) analysis of the FEX spectra of NH2(A 2A1 --> X 2B1) emission and assignment of the features to members of the npe' <-- 1a"2, nde" <-- 1a"2, nsa'1 <-- 1a"2 and nda'1 <-- 1a"2 Rydberg series transitions of NH3; (iii) new information on photodissociation processes leading to formation of NH2(A 2A1) and of NH in various excited states, in particular concerning the formation of NH (A 3pi) at 8.266 eV by dissociation of the a 3A"2 state of NH3, the existence of a potential barrier of 360 +/- 70 meV in the dissociation to form NH (c 1pi) at 9.85 eV and its formation from a non-planar state above 11.3 eV; (iv) information pertaining to the heat of formation of the NH radical, which we determine as deltaH(f,0) degrees (NH) = 3.70 +/- 0.01 eV, and to the thermochemical limits of the dissociation reactions of NH3.