Irradiation geometries, molecular

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

Today there is an arsenal of experimental physical techniques and computational methods for the determination of molecular geometry [28], The respective precisions are often better than the various operational effects influencing these parameters, such as the consequences of the specificities of the matter/irradiation interaction and of molecular vibrations. 

The implication is that under the conditions of deep UV irradiation, which likely existed on the early earth, selective chemistry may have taken place in favor of species with the shortest excited state lifetimes. Benner and coworkers, for example, have proposed a molecular lexicon of 12 alternate bases that can produce 6 base pairs with virtually identical geometries as the guanine-cytosine (GC) triply hydrogen bonded base pair [1], Several of these alternate bases have been observed as products in simulation experiments that test the feasibility of synthesis of such compounds under primitive conditions [77, 78], This raises the... 

The possibility of ring closure was expected to depend on molecular geometry. Thus, photolysis of n-propyl azide (7) in cyclohexane produced only the imine (8) in 59% yield, isolated as the 2,4-dinitro-phenylhydrazone (9). Irradiation of phenylethyl azide (10) resulted... 

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