Una Marvet

Femtosecond photoassociation spectroscopy: coherent bond formation

Abstract The method femtosecond photoassociation spectroscopy (FPAS) is introduced and we report on its application to the study of the reaction Hg+ Hg + h ν → Hg 2 . Our results demonstrate coherent bond formation obtained in the femtosecond time scale from free unbound reactants. The product Hg 2 is formed in the D1 u state and shows a high degree of rotational anisotropy induced by the polarization of the binding femtosecond pulse.

Femtosecond observation of a concerted chemical reaction

Abstract A time-resolved study is presented of the photodissociation of methylene iodide (CH 2 I 2 ) to produce a carbene diradical and molecular iodine. This reaction pathway occurs only at high excitation energies (vacuum ultraviolet) and is initiated in this case by a two-photon transition at λ equivalent = 155 nm. Discrimination over competing pathways is achieved by wavelength-selective monitoring of the nascent I 2 dynamics. The results show clear evidence that breakage of the two carbon-iodine bonds and formation of the iodine-iodine bond is concerted. The reaction takes place in less than 100 fs and is characterized by coherent vibrational motion in the iodine molecule.

Femtosecond dynamics of photoinduced molecular detachment from halogenated alkanes. II. Asynchronous concerted elimination of I2 from CH2I2

The photoinduced molecular detachment dynamics of CH2I2 have been investigated with femtosecond time resolution. Upon multiphoton excitation of CH2I2 with 312 nm femtosecond pulses, weak fluorescence in the 260–290 nm region was observed in addition to the I2 fluorescence in the 290–345 nm region studied in the previous paper. The weak fluorescence has also been interpreted as due to emission from I2, where I2 was produced from the photodissociation process CH2I2→CH2+I2*. In order to investigate the detailed dynamics of this reaction, femtosecond time-resolved data have been obtained by selective detection of the I2 fluorescence at 272 and 285 nm. From these transients, it has been found that the dissociation process takes place within the temporal width (50 fs) of the laser pulse and that the I2 photofragments exhibit coherent vibrational motion. The 272 nm transients also exhibit clear, fast decaying rotational anisotropy, quantitative analysis of which reveals a distribution of rather high rotational l...

Femtosecond dynamics of photoinduced molecular detachment from halogenated alkanes. I. Transition state dynamics and product channel coherence

The direct observation of the photoinduced molecular detachment of halogens X2 from halogenated alkanes RCHX2 is presented. Three-photon excitation at 312 nm produces molecular halogens and a carbene; the halogen products are formed predominantly in the D′ state. Femtosecond pump–probe spectroscopy of the reaction reveals a fast (τ<50 fs) dissociation with no evidence of intramolecular vibrational redistribution. This is consistent with a prompt dissociation without intermediates. The experimental results demonstrate vibrational coherence in the halogen product, which requires that the reaction proceed by a concerted mechanism.

Femtosecond concerted elimination of halogen molecules from halogenated alkanes

The concerted dynamics involved in the molecular detachment of halogenated alkanes, i.e., CX2YZ→CX2+YZ (where X=H, F or Cl and Y, Z=I, Br or Cl) have been studied by femtosecond pump–probe spectroscopy. Particular emphasis has been placed on exploring the role of symmetry in the parent molecule. For experiments carried out on CH2ICl, product fluorescence was observed in two regions of the spectrum: at 430 nm, corresponding to the D′→A′ transition, and at 340 nm, attributed to the G→A transition. Differences in the dissociation time for the two pathways can be understood by considering the energy available for fragment recoil. The elimination process was found to be slower for methylene bromide than for methylene iodide, possibly because of the difference in the enthalpy of reaction. When three gem-dibromo compounds (Y, Z=Br, X=H, F or Cl) were compared, they were found to have significantly different dissociation times: 29.7 fs for the fluorinated species, 58.6 fs for the hydrogenated species, and 80.6 fs for the chlorinated species. The difference in the transition state lifetime between the hydrogenated and chlorinated species can be rationalized in terms of changes in the reduced mass, the thermodynamics of the reactions, and the energy partitioning. The fluorinated compound was found to dissociate much faster than predicted by thermodynamic and reduced mass arguments.

Concerted elimination dynamics from highly excited states

Photoinduced molecular detachment has been investigated for methylene iodide (CH2I2) and several related compounds (CH2Br2, CH2Cl2 and CF2Br2). Multiphoton excitation of these molecules at 310 nm gives rise to halogens in the D′ state. Femtosecond pump-probe experiments on the dissociation of these compounds indicate that the process is extremely fast (<60 fs) and proceeds without an intermediate. In addition to the characteristic D′→A′ fluorescence at 342 nm, photodissociation of CH2I2 also produces several fluorescence bands in the 260–290 nm region. The CH2I2 transients show characteristic I2 vibrational coherence. Time resolved data collected by detection at 272 nm also demonstrate clear, fast decaying rotational anisotropy, analysis of which reveals a distribution of rather high rotational levels of I2. Based on analysis of the dissociation time, rotational anisotropy and vibrational coherence, and on the estimated partitioning of energy in the fragments, an I2 concerted molecular detachment mechanism has been proposed.