Two-Photon Ionization of Polyatomic Molecules in a Mass Spectrometer

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 many 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, that 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 that efficient two-photon ionization is possible with a low power frequency-doubled djre laser, in the dilute gas of a molecular beam. This