Abstract: Some alkyl bromides (CHBr3, CH2Br2, C2H5Br, C2H4Br2) have been studied at 234 nm and 267 nm in the time-of-flight mass spectrometer by means of multiphoton dissociation and ionization. Under the radiation of UV laser, alkyl bromides are photolyzed after absorbing one photon, release Br atoms, absorbing photons continuously, and (2＋1)resonance-enhanced multiphoton ionization of Br atoms takes place. The Br atoms from the dissociation of the bromides are probably at the ground state Br(2P03/2) or at the excitated state Br*(2P01/2),and the branching ratio of Br atom N(Br*)/N(Br) is obtained. We find that the branching ratio at 267 nm is obviously bigger than the result at 234 nm. After discussing the mechanics of the multiphoton process, we conclude that the change tendency of the branching ratio with the wavelength is due to the different dissociation channels.

Key words: Alkyl bromide, REMPI (resonance-enhanced multiphoton ionization), TOF-MS (time-of- flight mass spectrometer), Branching ratio