Resonant IR Photon Induced Dissociation in Organic Molecules with Chain-like Substructures

Photon induced dissociation (PID) of molecules due to action of low intensity IR radiation is the subject of several experimental investigations [1–12]. The interest in this field arose, as information of molecular dissociation by IR photons is important not only for the development of quantum chemistry of organic molecules and for the theory of laser beam interactions with large molecules, but also for practical applications. It was shown experimentally that the PID of molecules consisting of chain-like substructures can be induced resonantly by IR radiation with frequencies which do not coincide with the vibrational frequencies of the diatomic valence groups contained in these molecules [4, 5]. Also, it was shown that IR photon induced dissociation (IR-PID) leads with high probability to fragments consisting of small atomic groups. These groups are mainly the end groups of the chain-like substructures inside the irradiated molecules [6–12]. The abundance of different fragments depends on the photon fluence, on the number of diatomic groups in the chains and on the corresponding dissociation energies [6–12]. In the present work a model for the dissociation of organic molecules with chain-like substructures by IRPID is suggested. The model is based on the excimol theory for excitation and fragmentation of organic molecules, which was worked out previously [13]. In the frame of the excimol theory the collection of internal molecular energy occurs in chain-like molecular substructures consisting of identical diatomic groups with significant dipole momenta. In these chains many collective vibrational states (excimols) with frequency wex are induced by a resonant external periodic field, e.g. IR photons. The excimol energy Eex = hwex is less than the energy E01 = hw01 of the first vibrational state of an isolated diatomic quantum oscillator. This energy difference is caused by the energy Etr required for energy transitions within the chain from one diatomic oscillator to another. These tansitions are due to resonance dipole–dipole interactions between identical molecular dipoles and the transition energy Etr which strongly depends on the dipole moment of the diatomic groups and on the three-dimensional structure of the chain [14, 15]. The excimol lifetime tex is larger than the lifetime t01 of the excitation of an isolated diatomic oscillator in the first vibrational state, because the resonance interaction of the diatomic dipoles within the chain decreases the probability of vibrational relaxation in each dipole. For example, the excimol lifetime tex in a (CH2)n chain is two orders of magnitude higher than the lifetime t01 of the excitation of an isolated CH-group. Resonant excimol excitation can be induced by periodic electromagnetic fields including IR-fields, if the frequency of the field is equal to the excimol frequency. Several excimols can be excited independently in a chain. They are not accumulated in a single diatomic group because of the unharmonic energy spectra of the Resonant IR Photon Induced Dissociation in Organic Molecules with Chain-like Substructures H. Jungclas, L. Schmidt, V. V. Komarov, A. M. Popova, and I. O. Stureiko Department of Chemistry, Philipps-University Marburg, 35032 Marburg, Germany a Institute of Nuclear Physics, Lomonosov State University, Moscow 119899, Russia Reprint requests to Prof. H. J.; Fax: +49-6421-28 62 830; E-mail: [email protected]