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It is now known that tunable dye laser light leads to resonance enhanced photoionization of molecules in a mass spectrometer via multiphoton absorption. In some cases this process also leads to ion fragmentation. Time delayed tandem laser pulses of different wavelengths are employed to study the mechanism of this multiphoton ionization and fragmentation process in benzene. The experimental results strongly indicate that excitation does not proceed up an autoionization ladder of the neutral molecule, rather the multiphoton process is shown to be due to the climbing of two independent ladders, one up to the ionization potential in the neutral molecule, the second in the new species of molecular parent ions produced. Hence multiphoton ionization could also form a new basis of ion spectroscopy.

4857the competition for the intermediate, but still leaves the formation of the intermediate as the rate-determining step.Because of the low concentrations involved and the lower sensitivity of the Spectronic 20 spectrophotometer used, it was difficult to get accurate concentration measurements for nitrite ion in the low pH stoichiometric experiments where hydrolysis is a major contributor. In addition, the time involved in monitoring the S(1V) concentrations to obtain the stoichiometry limits the number of available data points, which decreases the probability of detecting spurious values. For several conditions measurements were taken to compare to a calculated decay curve for NO2-. Because the third term in eq 1 may be neglected at low bisulfite concentrations and the first term is not observed in the current experiments, only the middle term is needed to calculate the expected nitrite ion decay curve. The rate expression then becomes(4) At a pH of 2.8 and initial nitrite and bisulfite ion concentrations of 8.4 X lo-* and 1.7 X M, respectively, the decay curve can be calculated, with the observed stoichiometric ratio for these conditions as 1.5. Integration with a A S I A N ratio of 1.5 yields the expression X 1 [HSO,-]o -1.5[NO,-]O In I= [NO2-]o([HSO3-]0 -1.5[NO,-]o + 1.5[NO2-],]) [HSO,-Io[NO2-1, kz[H+lt (5) where [NO<], is the nitrite concentration at time t . The decay curve as calculated from this result with ( k 2 = 3.8 X lo3 L2/mo12 s)): in addition to the data points from an experiment with the same initial conditions, are shown in Figure 5. The agreement is quite good, which indicates that the rate law for nitrite ion loss is valid even when 50% of the intermediate has undergone hydrolysis. Similar agreement is found under different observed stoichiometries as well. One concludes from this that the same rate-determining step is involved, which in turn lends credence to the proposed reaction scheme. ConclusionsThe results of this study serve to verify the previously reported rate law for the formation of hydroxylaminedisulfonate. No evidence for the bisulfite-independent term (of interest to atmospheric chemists) was seen in the current investigation. Evidence has been presented that an intermediate species (probably ONS03-) is formed which can hydrolyze to form N20 or go on to form HADS.Although this hydrolysis reaction is noted only in very acidic solutions in this study, at lower bisulfite ion concentrations this reaction will become important in even weakly acidic solutions. The ratio of the rate constants for sulfonation and hydrolysis reactions (kA/kB) is 1.7 f 0.5; this indicates that, whenever H+ and HS03-are present in equal concentrations, substantial amounts of N 2 0 will be formed.A time-of-flight mass spectrometer with a laser multiphoton ionization source is employed in studying metastable ions produced in the multiphoton ionization-dissociation process. The use of a. reflecting field gives rise t,o sharp distinguishable mass peaks for the metastable ions. This spectrometer gives short ion...

Two-photon ionization o f benzene molecules in a mass spectrometer is performed w ith a tunable frequency-doubled dye laser. The nonlinear ionization is resonantly enhanced by real interm ediate rovibronic levels in the Si state o f the molecule. Our results show th at stepwise m ulti-photon ionization is a very selective and versatile ionization source for a mass spectrom eter.Photoionization has been a highly selective method for the preparation of polyatomic molecular ions [1]. VUV light, usually of low intensity, must be employed; the produced ion flux has, however, often been insufficient for m any interesting appli cations. Classical methods have employed discharge lamps or, more recently, synchrotron radiation [2]. The former is limited to a few wavelengths a t which sufficient flux is available, whereas the latter requires an immense experimental effort to produce the radiation.Lasers would here be an ideal alternative, but so far no tunable VUV lasers for this purpose have been available [3]. Frequency multiplication in metal vapors can only be employed to produce coherent light over a limited range of energy and is not very efficient [3]. Multi-photon absorption [4] would be a real alternative for large molecules particularly since VUV optics can be avoided. Multi-photon ionization has been performed in alkali metal beams [5] with low ionization potential, but for polyatomic mclscules only in the dense bulk gas phase [6].We here present results obtained in a low density molecular gas, th at is in a molecular beam, carried out directly in the source of a mass spectrometer [7]. For the case of a prototype organic molecule we shall show th a t efficient two-photon ionization is possible with a low power frequency-doubled djre laser, in the dilute gas of a molecular beam. This

A new analytical method for the fast analysis of trace substances in gas mixtures has been developed incorporating a pulsed tunable laser system and a reflection time-of-flight mass spectrometer. The technique is especially designed for the time-resolved detection of air pollutants in the exhaust gas of motor engines. The purpose of this new analytical method is to achieve high time resolution (<100 ms), high sensitivity (down to 1 ppm), high quantitative precision (10%), and applicability to most exhaust emission components. The main difficulties for the analysis arise from the large number of components which show very different and rapidly varying concentrations. For a preliminary list of 25 exhaust emission components, all essential prameters have been determined. First