Laser-induced molecular dissociation

J.L. Lyman Laser-induced molecular dissociation. Applications in isotope separation and related processes, in [Ref.l0.1,p.417]... 

Infrared femtochemistry Vibrationally induced molecular dissociation and its control employing shaped fs MIR laser pulses... 

In this contribution we demonstrate vibrationally induced molecular dissociation in two polyatomic molecules, namely chromium hexacarbonyl and diazomethane (Fig. 1), employing fs mid-infrared laser sources [1,2]. As will be shown below, in both cases the initially excited mode does not lead directly to reaction but provides the doorway to access the right combination of modes. Thus both reactions have three steps (1) Stepwise multiphoton excitation of the doorway mode, (2) sharing of energy with other modes, and (3) formation of... 

The dependence of the dissociation constant kj) and the recombination constants kjt in the reaction (a) on the optical polarization of the atoms A was predicted by Bernheim [65] and Kastler [225] and later demonstrated experimentally [7, 363]. The dependence can be understood from the obvious fact that only hydrogen-like atoms with opposite electronic spins may recombine and form a molecule A2(X1E+). Hence we have kR = kft(l — S2), where S is the degree of electron spin polarization of the atoms. A convenient indicator of dimer formation is provided by the kinetics of the laser-induced molecular fluorescence after switching on magnetic resonance which destroys the polarization of the atoms, as performed by Huber and Weber [201] for a Na — Na2 mixture. The se-... 

The recent experiments on IR lasing of molecules have revealed a new type of molecular photodissociation caused by absorption of a large number of quanta in a time shorter than the interval between collisions (the so-called laser-induced collisionless dissociation). 

The primary photochemical reaction for nitromethane in the gas phase is well supported by experiments to be the dissociation of the C—N bond (equation 98). The picosecond laser-induced fluorescence technique has shown that the ground state NO2 radical is formed in <5 ps with a quantum yield of 0.7 in 264-nm photolysis of nitromethane at low pressure120. The quantum yield of NO2 varies little with wavelength, but the small yields of the excited state NO2 radical increase significantly at 238 nm. In a crossed laser-molecular beam study of nitromethane, it was found that excitation of nitromethane at 266 nm did not yield dissociation products under collision-free conditions121. 

FIGURE 1. Polarization of laser induced fluorescence of 0H(X n) photodissociated from H2O in the 145-185 nm region. The absorption transition moment is perpendicular to the molecular plane corresponding to the transition A P-j-XlA]. The dissociated 0H(x2n) is also in the molecular plane, since the induced fluorescence intensity of OH is preferentially polarized along the Z axis (25) perpendicular to the molecular plane. The OH radical rotates on the H2O plane XY plane) after dissociation. The unpaired p-orbitals of excited H2O and of dissociated OH are perpendicular to the molecular plane. 

Details of the photoexcitation and dissociation of seeded supersonic molecular beams of I2 by 514.5 nm radiation from a CW Ar laser have been provided by Bernstein and co-workers.A number of measurements were made, including LIF, I2 beam attenuation, and I2 TOF distributions as functions of carrier-gas pressure and nozzle temperature. A value for the direct photodissociation cross-section for I2 was determined to be (2.4 0.5) x 10 cm from measurements of the laser-induced beam loss. Use of additional spectroscopic information enabled calculation of the fraction of molecules excited on the basis of a simple model, and comparisons of the degree of excitation for different beam conditions and beam/laser geometries were made. 

Laser photodissociation of ketene at 230 nm has been investigated in molecular beams. The experimental rovibrational population distribution has been compared to predictions from phase-space theory for the channels leading to CO + CH2(a Ai) and CO + CH2(b Bi). The calculations are not compatible with the latter channel, suggesting that it does not contribute significantly to the dissociation process. The photodissociation of singlet ketene by two-step IR + UV excitation has been studied using state-selective detection of CH2 by laser-induced fluorescence, and the results compared with... 

Rate constants for reaction of the CH radical with a number of atomic and molecular collision partners have been reported, with multiple-photon dissociation of suitable precursor molecules using either infrared or ultraviolet " laser radiation used as the pulsed photolysis source, and laser-induced fluorescence near 431 nm employed as a sensitive time-resolved detection method. A similar technique has been used to measure removal rates of CH2 and CDj with... 

H. Abou-Rachid, T. Nguyen-Dang, O. Atabek, Dynamical quenching of laser-induced dissociations of diatomic molecules in intense infrared fields Effects of molecular rotations and misalignments, J. Chem. Phys. 114 (2001) 2197. 

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