Anal. Chem. 1985, 57, 2911-2917 (6) Tao, Hiroaki; Mlyazaki, Akira; Bansho, Kenji; Umezaki, Yoshimi Anal. Chlm. Acta 1984, 156, 159-168. (7) Hiraide, Masataka; Ito, Tetsumasa; Baba, Masafurnl; Kawaguchi, Hlrcshi; Mizuike, Atsushi Anal. Chem. 1980, 52, 804-807.
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RECEIVED for review June 11,1985. Accepted July 22,1985.
Resonance Enhanced Multiphoton Ionization Spectroscopy for Detection of Azabenzenes in Supersonic Beam Mass Spectrometry Roger Tembreull, Chung Hang Sin, Ho Ming Pang, and David M. Lubman* Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109
The analytlcai lonlzatlon spectroscopy of azabenzenes is investigated in supersonic jets for selectlve detectlon In mass spectrometry. The detection of arabenzenes represents an lnterestlng problem since the lonlzatlon potentials are often too hlgh for direct resonant two-photon lonlzatlon. For this work, we develop an efficient ionlzatlon scheme based on a three-photon process where the first photon excites a molecule to a resonant state, a second photon then excltes the molecule to an upper hlgh lylng “state”, and a third photon Ionizes the molecule. The spectra obtalned reflect the absorption of the n-p* transitlon since the second photon Is resonant with a dense reglon of valence and Rydberg states where there is no sharp structure evldent. Thls method has been applled to aza- and dlarabenzenes with resultlng sharp spectral ilnes for unique detectlon and production of only the molecular ion for mass spectrometry.
In this work, we have studied the use of laser photoionization methods for selective detection of azabenzenes in mass spectrometry. This class of molecules was chosen because of its importance in biological chemistry and their detection presents an interesting challenge in molecular spectroscopy. In previous work, the analytical spectroscopy of various aromatic hydrocarbons was studied ( I , 2). In these systems the transitions studied were a-a* where an electron was excited from a a molecular orbital in the ground state to a a* orbital in an excited electronic state. The azabenzenes bear a close resemblance to benzene and other hydrocarbon analogues since in every case one or more of the benzene -CH functions are replaced by the isoelectronic nitrogen atoms. However, the addition of the N2pu lone pair orbitals adds the possibility of n-a* transitions occurring by excitation of the lone pair electrons in addition to a-a* transitions. The detection of azabenzenes through the n--R* transitions may afford several advantages. The n-r* transitions generally occur in the near-UV and visible and the addition of more N’s to the aromatic ring often serves to shift the absorption to longer wavelength. Thus, this transition can generally be detected in a region of the spectrum easily accessible with current tunable dye lasers. Further, the n-a* transitions are generally sharp, whereas the first a-r* transition may often be diffuse and may not exhibit any sharp bands. The main problem is that study of the n-a* transitions is often limited due to small absorption cross sections and rapid radiationless transitions which provide low quantum yields in fluorescence.
In the case of quinoline, isoquinoline, and many diazanaphthalenes, n-a* bands have not been detectable in the past. Indeed, as the size of the conjugated system increases, one needs more aza substitution to move the n-a* band to longer wavelength than the first a-a* band so that the stronger a-r* transition does not totally overlap it. Thus, in many cases a detection scheme based on the a-a* transition may be preferable to the n-a* and an important aim of this study will be to determine the conditions under which each is appropriate. Herein we will demonstrate the detection of azabenzenes and several derivatives in a supersonic expansion by laserinduced multiphoton ionization. In principle resonant twophoton ionization (R2PI) is the most desirable process for ionization of molecules due to its high efficiency and the ability to efficiently produce soft ionization, i.e., no fragmentation in a mass spectrometer for identification (1-10). In RBPI two photons are used to produce ionization where the first photon excites a molecule to a resonant intermediate state followed by a second photon that ionizes the molecule. The necessary condition for ionization is that the sum of the two photons must be greater than the ionization potential of the molecule. However, this is not possible for study of the n-a* transitions of many azabenzenes since twice the one-photon energy is often less than the IP. Of course, a two-color ionization scheme could in theory be used. However, for molecules with very short (
