Photo fragments

The first part of the review deals with aspects of photodissociation theory and the second, with reactive scattering theory. Three appendix sections are devoted to important technical details of photodissociation theory, namely, the detailed form of the parity-adapted body-fixed scattering wavefunction needed to analyze the asymptotic wavefunction in photodissociation theory, the definition of the initial wavepacket in photodissociation theory and its relationship to the initial bound-state wavepacket, and finally the theory of differential state-specific photo-fragmentation cross sections. Many of the details developed in these appendix sections are also relevant to the theory of reactive scattering. 

Excitations above 193 nm generate A A excited state predominantly but at a < 193 nm about two thirds ofthe OH photo-fragments are formed via. 4 A... 

Postponing the discussion of the purification of racemic mixtures to later sections, we consider in this section the simpler case of controlling the handedness of photo-fragments resulting from the dissociation of a prochiral molecule of the type BAB where B and Bf are enantiomers. 

Figure 38 Experimental CO photo-fragment rotational distributions following the dissociation of H2CO. The two upper traces are for CO(n = 0), while the bottom trace is for CO(n = 1). Reproduced, with permission of the American Institute of Physics, from Ref. 312.

The absorption cross sections of CIONO2 (Figure 4.50) have been measured by Molina and Molina (1979) and by Burkholder et al. (1994). The values are in good agreement and exhibit a temperature variation. The nature of the photo-fragments has been investigated in several laboratory studies. As concluded by JPL (1997), the quantum efficiency i for the following channel... 

Fig. 14. Principle of the specific heat measurement. Size-distribution of photo fragments is observed as the temperature of the primary cluster ion is varied. As the temperature increases, the sizes of the photofragments generated by the n-photon process shift toward lower sizes. Panels (b) and (d) illustrate that the n-photon process at the temperature, Ts, generates photofragments of the same sizes that produced by the (n-l-l)-photon process at the temperature, Tj (< T3). The specific heat, C, is evaluated by C = AQ/AT, where the increase in the internal energy of the cluster, AQ, is the photon energy, hi>, and the temperature rise, AT, is equal to T3-T1.

The spectroscopic study of energy disposal in non-fluorescent photo-fragments was initiated in the early 1960 s by Basco and Norridi when they idiotographed transient u.v. absorption from vibrationally excited NO molecules following the flash photodissociation of CINO and BrNO... 

Photo-excitation of gas-phase ions may result in the photodetachment of an electron rather than photo-fragmentation. Coulombic considerations dictate that this process is more prevalent for anions than for cations. Electron photodetachment action spectroscopy of trapped anions has proved also to be a valuable source of molecular information. In some systems, electron photodetachment and PD compete. The mechanisms for these two processes in large molecules are yet to be understood fully consequently, their branching ratios in specific experimental conditions cannot be predicted as yet. One exciting possibility is the idea of using frequency and phase-shaped pulses to promote selected photochemical pathways. 

Section II of this paper contains the theory of the preparation and decay of a continuum superposition state and a theoretical discussion on the limits of control attained. In section III we present a number of computational results in which we demonstrate how to achieve control over the yield of I vs. I in the photodissociation of a diatomic (FI) and a polyatomic (CHo I) molecule. We also show how a single vibronic state of a photo-fragment can be enhanced or completely turned off. 

Common applications of excimer lasers are their use as pump lasers for tuneable dye lasers, and as light sources in photo-fragmentation experiments (see the chapters in Part 4). 

Figure 25.5 Time evolution of the (COj) photo-fragmentation. Absorption recovery of Ij, as a function of the pump-probe delay. Reproduced from Papanikolas et al, J. Chem. Phys., 1992, 97 7002, with permission of the American Institute of Physics...

Dixon RN. 1986. The determination of the vector correlation between photo-fragment rotational and translational from the analysis of Doppler-broadened spectral line profiles . J. Chem. Phys. 85(4) 1866-1879. 

In traps using buffer-gas cooling, photo fragmentation is an established method [15], where it is applied to polyatomic ions [15,16,85,86], In connection with sympathetic cooling of MgH+ molecular ions, two-photon dissociation was demonstrated and the branching ratio of the two possible dissociation channels. Mg - - H+ and Mg" " + H, was investigated [76]. In all of these experiments, pulsed lasers were employed. Photodissociation of HD+ is described in the following section. 

Photoactivated dissociation processes [40-42] are briefed. Ion photo dissociation time-resolved measurements have been made for a long time. In time-resolved photo dissociation, the rate of dissociation reaction can be measured by monitoring photo fragmentation of ions as a function of time. MS-MS allows selection of the target ion to be dissociated and analysis of the product ions. FTICR or QIT are well suited for time-resolved photo dissociation experiments. The photoactivated dissociation is assumed to occur by a two-step mechanism the photo activation (excitation) of the precursor ions, (A+B)+, and their fragmentations, similar to CID. 

---