Product state distribution electronic

However, with the advent of lasers, the teclmique of laser-induced fluorescence (LIF) has probably become the single most popular means of detennining product-state distributions an early example is the work by Zare and co-workers on Ba + FLT (X= F, Cl, Br, I) reactions [25]. Here, a tunable laser excites an electronic transition of one of the products (the BaX product in this example), and the total fluorescence is detected as a... 

These experiments use the product state distribution technique to allow a qualitative characterization of the competition between multiple electronic states. In contrast to the pathway competition in the molecular channel of formaldehyde (Section V.D), where the correlated product state distributions delineate the two channels quite cleanly, it will likely more often be the case that the product state distribution method allows only qualitative separation, due to overlapping distributions. Nevertheless, such experiments provide critical insight into pathway competition. 

Photodissociation dynamics [89,90] is one of the most active fields of current research into chemical physics. As well as the scalar attributes of product state distributions, vector correlations between the dissociating parent molecule and its photofragments are now being explored [91-93]. The majority of studies have used one or more visible or ultraviolet photons to excite the molecule to a dissociative electronically excited state, and following dissociation the vibrational, rotational, translational, and fine-structure distributions of the fragments have been measured using a variety of pump-probe laser-based detection techniques (for recent examples see references 94-100). Vibrationally mediated photodissociation, in which one photon... 

The product state distributions and the populations of the various chemical and electronic channels belong to the category of so-called scalar properties which are defined without reference to a particular coordinate frame. They have a magnitude but no direction. However, since the electromagnetic field vector E of the photolysis laser defines a specific direction in the laboratory frame, all vectors inherent to a photodissociation process can be measured relative to E. The vectors of interest are ... 

The interpretation of final state distributions following the decay of metastable states is a very interesting topic. If the intermediate complex lives longer than an internal period, the memory on the parent molecule in the electronic ground state will be essentially erased and the product state distributions will no longer reflect the initial wavefunction. As we will show in Section 10.3, they instead reflect the wavefunction in the transition state of the upper electronic state and the dynamics in the exit channel following the transition region. 

Herschbach [58] noted a striking similarity between the recoil energy distribution of Cl atoms in the H + CI2 reaction and that observed in the photodissociation of CI2. This suggests that the electron attachment to the molecule is essentially a vertical process, hence he proposed the DIP extension to the model, which makes the AB repulsion after the electron jump analogous to that encountered in photodissociation experiments. This provided the necessary empirical basis for estimating the parameter of the repulsive interaction. All the mathematical expressions relevant to the model were given by Truhlar and Dixon [62]. Zare and co-workers extended the model to chemiluminescent reactions and a full account of the new model is given in Ref. [81]. It was used to predict successfully the product state distribution in the reaction Ca( So) -I- F2 —> CaF(B ) + F. 

Hankel, M., Balint-Kurti, G.G., and Gray, S.K. (2000) Quantum mechanical calculation of product state distributions for the 0( D)- -H2 —t OH H reaction on the ground electronic state surface. J. Cham. Phys. 113, 9658-9667. 

In the experimental studies of state specific NO2 unimolecular dissociation (Miy-awaki et al., 1993 Hunter et al., 1993 Reid et al., 1994, 1993), NO2 is first vibra-tionally/rotationally cooled to 1 K by supersonic jet expansion. Ultraviolet excitation is then used to excite a NO2 resonance state which is an admixture of the optically active and the ground electronic states. [It should be noted that in the subpicosecond experiments by Ionov et al. (1993a) discussed in section 6.2.3.1, a superposition of resonance states is prepared instead of a single resonance state.] The NO product states are detected by laser-induced fluorescence. Both lifetime and product energy distributions for individual resonances are measured in these experiments. A stepwise increase in the unimolecular rate constant is observed when a new product channel opens. Fluctuations in the product state distributions, depending on the resonance state excited, are observed. The origin of the dynamical results is not clearly understood, but it apparently does not arise from mode specificity, since analyses of... 

The difference from Eq.5 is the additional averaging over final states. Most experiments, in which product state distributions are measured, are done at one particular wavelength. In contrast, the total absorption cross section is typically measured as a function of wavelength. Although the absorption cross section is a higher averaged quantity, it contains important information about electronically excited states. 

Probing product state distributions by multiphoton ionization is one of the most sensitive methods for the analysis of both bimolecular and photofragmentation dynamics. For example, by using REMPI one can measure the rotational state distribution in the N2 fragment produced in the photofragmentation of N2O it was found that the maximum in the rotational state population is near J 70. This reveals that although the ground electronic state is linear, the excited state is bent and thus the recoil from the O atom results in rotational excitation of the N2 molecule... 

Since the discovery of the Ba + chemiluminescence [16] it was found consistently in many reactions of metal atoms with halogen and oxygen containing molecules [1] that electronic product state distributions decrease monotonically with available energy in a manner suggesting qualitatively statistical behaviour and that the electronic inversions necessary for laser action [10] are elusive. QL yields for the reactions of ground state M S) are maximally a few percent (for Ca + I ) and more typically a fraction of a percent [1,17]. This is also typical for the reactions of metal atoms with the oxygen-donors O3, NO2 and N2O [1] and it is appropriate... 

By examining the expression for Q ( equation (B1.16.4)). it should now be clear that the nuclear spin state influences the difference in precessional frequencies and, ultimately, the likelihood of intersystem crossing, tlnough the hyperfme tenn. It is this influence of nuclear spin states on electronic intersystem crossing which will eventually lead to non-equilibrium distributions of nuclear spin states, i.e. spin polarization, in the products of radical reactions, as we shall see below. 

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