Kinetics of low-intensity infrared laser photodissociation. The thermal model and application of the Tolman theorem

Abstract

Infrared multiphoton dissociation by low-power cw lasers under collision-free conditions is analyzed from the point of view of thermal kinetic theory. It is pointed out first that the laser has the same effect on the molecular population as irradiation by an equivalent black-body source, assuming only that the dominant IR emissions of the molecule at steady state are at wavelengths not far from the irradiating IR laser wavelength. Then the dissociation reaction kinetics are considered for this thermal population under the assumption that the reaction perturbs only the high-energy tail of the Boltzmann distribution. Analysis from this point of view leads to a slightly modified version of the Tolman theorem giving the activation energy as a function of temperature. A random walk simulation of the kinetics for dissociation of a model molecular system was carried out and the validity of the thermal kinetic approach was verified by the excellent agreement between the activation energies from the Tolman-theorem analysis and those from the random-walk simulations. The extent of dissociation is strongly dependent on intensity (at constant fluence) under these conditions.

Activation of large lons in FT‐ICR mass spectrometry

“…In the absence of dissociation the IED of ions excited with lowintensity cw infrared radiation is well approximated by a Boltzmann distribution (Dunbar, 1991). The slow nature of ion excitation allows for efficient competition between the absorption and emission of IR photons.…”

“…Estimation of the densities of vibrational states demonstrated that for the dimer the quasi-continuum is reached by absorption of only one photon whereas absorption of four photons is necessary to access the quasi-continuum for the monomer. Infrared multiphoton dissociation (IRMPD) of small polyatomic ions with low-power CW lasers has been studied by a number of groups (Bomse & Beauchamp, 1981a,b;Bomse, Berman, & Beauchamp, 1981;Jasinski et al, 1982, Tumas et al, 1983Baykut et al, 1985;Shin & Beauchamp, 1990;Dunbar, 1991;Uechi & Dunbar, 1992;Dunbar & Zaniewski, 1992;. Early studies have been reviewed by Thorne & Beauchamp (1984).…”

“…If ion dissociation is fast relative to ion activation the thermal distribution is depleted. Similarly to BIRD depletion of the thermal distribution by IRMPD has been modeled using master equation modeling (Dunbar, 1991;Uechi & Dunbar, 1992;Dunbar & Zaniewski, 1992;Jockusch, Paech, & Williams, 2000;Freitas, Hendrickson, & Marshall, 2000;Paech, Jockusch, & Williams, 2002). In master equation modeling, the entire internal energy range is divided in small bins.…”

Infrared multiphoton dissociation of the siderophore enterobactin and its Fe(III) complex. Influence of Fe(III) binding on dissociation kinetics and relative energetics

“…Despite these limitations, relative dissociation energetics have been calculated in comparativeüstudiesü [48].üToüdetermineürelativeüactiva-tion energies (E a,r ), Marshall and coworkers developed [40,ü41]ütheüfocusedüradiationüforügaseousümultiphoton energy-transfer (FRAGMENT) method, in which E a,r is determined from unimolecular dissociation reaction rate constants at various laser intensities. In the FRAGMENT method, the activation energy is determinedüfromütheüfollowingüequationü [38]:…”

“…Similarly, IRMPD can be used for the determination of the activation energy, E a , for gas-phase unimolecular dissociation reactions. This approach was first introduced for small molecules (Ͻ50 atoms) by Dunbar [38,39], and later extended to large biomolecules (Ͼ50 atoms) by Marshall and coworkers [40,41]. These authors renamed the technique as focused radiation for gaseous multiphoton energy-transfer (FRAGMENT).…”