Time-resolved molecular beam mass

TIME-RESOLVED MOLECULAR BEAM MASS SPECTROMETRY OF TRANSIENT COMBUSTION PHENOMENA... 

ABSTRACT. We describe an apparatus by which the detonation products of an explosive can be identified and whose relative concentrations can be determined quantitatively. These measurements can be made on products that have been formed in less than one microsecond after the passage of the detonation wave. The technique is based on the rapid quenching of chemical reactions by virtue of the free expansion of the products into vacuum. Of course, products that have been formed over a longer period of time and under different pressure/temperature conditions can also be studied. Time resolved molecular-beam mass spectrometry is used, so that whether detonation occurred or not in forming the products can be determined. We describe optical techniques, principally Schlieren photographs, that also confirm detonation. We report measurements made on six standard explosives, PETN, RDX, HMX, HNS, TNT and TATB, and one research explosive, nitric oxide. For none of the standard explosives do we measure product distributions that agree with model predictions based on equilibrium assumptions. A computer model of the free expansion is described briefly and its importance to the interpretation of the data is emphasized. 

The spectroscopic and time-resolved studies of ESPT reactions in gas-phase clusters have provided significant information regarding the proton transfer mechanism between an excited molecule and a cluster of solvent molecules at a molecular level [15]. The resonant two-photon ionization method combined with mass spectrometry was used to measure the S, spectra of various clusters. Phenol (PhOH) and naphthol (NpOH) complexed with a variety of solvents were investigated by using molecular beam mass spectrometry and molecular beam... 

Undoubtedly, the technique most suited to tackle polyatomic multichannel reactions is the crossed molecular beam (CMB) scattering technique with mass spectrometric detection and time-of-flight (TOF) analysis. This technique, based on universal electron-impact (El) ionization coupled with a quadrupole mass filter for mass selection, has been central in the investigation of the dynamics of bimolecular reactions during the past 35 years.1,9-11 El ionization affords, in principle, a universal detection method for all possible reaction products of even a complex reaction exhibiting multiple reaction pathways. Although the technique is not usually able to provide state-resolved information, especially on a polyatomic... 

Other techniques utilize lasers for sample evaporation/pyrolysis and excitation such as laser induced desorption (LID) or laser microprobe mass analysis (LAMMA) (see e g. [1]). Some of the sample introduction procedures in Py-MS enhance the information obtained from Py-MS by the use of time-resolved, temperature-resolved, or modulated molecular beams techniques [10]. In time-resolved procedures, the signal of the MS is recorded in time, and the continuous formation of fragments can be recorded. Temperature-resolved Py-MS allows a separation and ionization of the sample from a platinum/rhodium filament inside the ionization chamber of the mass spectrometer based on a gradual temperature increase [11]. The technique can be used either for polymer or for additives analysis. Attempts to improve selectivity in Py-MS also were done by using a membrane interface between the pyrolyzer and MS [12]. 

Initial work with respect to Fe(CO)5 photodissociation includes the work mentioned previously by Waller [37] and Seder [31] who studied the state-resolved photochemical breakdown. A number of smdies have been reported which looked at the ultrafast (i.e., sub-picosecond) photodissociation dynamics of Fe(CO)5. Examples of such smdies include that of Banares et al. [57], which looked at the photodissocation dynamics of Fe(CO)5 in a molecular beam using femtosecond laser pulses via two-photon pumping at 400 nm followed by non-resonant ionisation at 800 nm. Detection of the photoproducts was by means of a time-of-flight mass-spectrometer. The timescale for the dissociation of the CO ligands was measured, and it was found that Fe(CO)4 was formed after 20 5 fs, Fe(CO) formed after 100 fs, and complete dissociation of the metal and all ligands sometime after 230 fs. 

The major features of the crossed molecular beams apparatus used in these studies have been described elsewhere (21-22). However, several important modifications were made specifically for these studies. The major objectives were to reduce the velocity spread of the reactant beams in order to resolve the product vibrational states as distinct peaks in time of-flight measurements and to reduce the background of mass 20 in the detector, especially near the F atom beam. A scheoiatic top cross sectional view of the experimental arrangement is shown in Figure 2. 

Figure 4.2. Experimental set-up for kinetic studies of SiHj reactions by excimer laser photolysis in a tubular slow-flow reactor with time-resolved detection by molecular beam sampling, near-threshold ionization electron impact mass spectrometry...

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