Combination of Molecular Beam Laser Spectroscopy and Mass Spectrometry

9 Combination of Molecular Beam Laser Spectroscopy and Mass Spectrometry 

The combination of laser- and mass-spectrometry has brought a wealth of new information on the structure and dynamics of molecules [494]. If, for instance a mixture of different isotopomers of a molecular species (these are molecules with different atomic isotopes) is present, the absorption spectra of the different isotopomers might overlap which impedes the analysis of the spectrum. Therefore the separation of these isotopomers by a mass spectrometer will facilitate the unambiguous analysis. The isotope shift of spectral lines gives additional information on the molecular structure. 

For pulsed laser excitation time of flight mass spectrometers are the optimum choice [495]. A schematic sketch of their design is shown in Fig. 4.37. By resonant two-step excitation and ionization in a molecular beam molecular ions are generated which are extracted by electric fields, pass a field-free region and are then acceler- 

The time resolved measurement of the different arrival times of the ions yields the masses m. 

In Fig. 4.38 the time of flight spectrum of the silver Ag2 dimers Ag Ag, i0 Agi09Ag and depicted [496]. When the time window of the detec- 

The combination of pulsed lasers, pulsed molecular beams, and time-of-flight mass spectrometry represents a powerful technique for studying the selective excitation, ionization, and fragmentation of wanted molecules out of a large variety of different molecules or species in a molecular beam [9.93-9.99]. The technique, developed by Boesl et al. [9.93] is illustrated by Fig. 9.29 rotationally and vibrationally cold neutral parent molecules M in a supersonic molecular beam pass through the ion source of a time-of-flight mass spectrometer. A pulsed laser LI forms molecular ions M by resonant enhanced multiphoton ionization. By selecting special intermediate states of M, the molecular ion can often be preferentially prepared in a selected vibrational level. 

In the second step, after a time delay At that is long compared with typical lifetimes of excited states of M but shorter than the time of flight out of the excitation region, a pulsed tunable dye laser L2 excites the molecular ions M from their electronic ground state into selected electronic states of interest. 

The detection of spectroscopic excitation is performed by photofragmentation of (M ) with a pulsed laser L3. For discrimination of the resulting 

If the wavelength A3 of laser L3 is chosen properly, the ions cannot be excited by L3 if they are in their electronic ground state. Therefore, the excitation by L2 can be monitored by L3. 

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